Water Quality Control and Treatment Water Treatment

Water Quality Control and
Treatment
Water Treatment
By
Dr. Khamis AL-Mahallawi

Water treatment
• Overview of the Water Treatment Process
• Preliminary Treatment
– Presedimentation
– Aeration
– Primary Sedimentation
– Sedimentation and flotation zones
• Aeration
• Adsorption
• Ion Exchange
• Coagulation and Flocculation
• Filtration
• Membrane Processes / Electro-dialysis
• Nanofiltration and reverse osmosis
• Softening
• Treatment
– Groundwater types and treatment
– Surface water treatment
• Disinfection

Water Treatment Process
abstraction - treatment – transport – storage - distribution

Water treatment processes
Preliminary Treatment
• Preliminary treatment is any physical, chemical or mechanical process used
on water before it undergoes the main treatment process.
• The purpose of preliminary treatment processes is to remove any materials
which will interfere with further treatment.
• Pretreatment may include screening, presedimentation, chemical
addition, flow measurement, and aeration.

Preliminary Treatment / Screens
• The screens are used to remove
rocks, sticks, leaves, and other debris.
• Very small screens can be used to screen out algae
in the water.
• All objects are removed by physical size separation
• Screens on the outside of intakes are often cleaned by
flushing water from the treatment plant backwards
• There are two primary types of screens - bar screens
and wire-mesh screens.
• A bar screen is used to remove large debris. The
spaces between the bars are two to four inches wide.
• A wire-mesh screen is used to remove smaller
debris. The gaps are about half an inch wide.
• Water must be flowing slowly in order to pass
through a wire-mesh screen - velocities should be no
greater than 3.5 inches per second.
A wire-mesh screen
A bar screen

Preliminary Treatment / Presedimentation - Aeration
Presedimentation is to settle out sand, grit, and gravel which will settle rapidly out of the
water without the addition of chemicals at the beginning of the treatment process.
Presedimentation depends on gravity and includes no coagulation and flocculation.
Presedimentation will reduce the load on the coagulation/flocculation basin and on the
sedimentation chamber, as well as reducing the volume of coagulant chemicals required to
treat the water.
Presedimentation basins are useful because raw water entering the plant from a reservoir is
usually more uniform in quality than water entering the plant without such a holding basin
Here in pretreatment, the purpose of sedimentation is to make the chemical treatment phase
of the water treatment process more efficient by removing sediment from the raw water.
In presedimentation basin, activated carbon may be added to the basin for taste, odor, and
color problems, and some chemicals to control the growth of algae.
Aeration removes carbon dioxide and hydrogen sulfide from the water. It also oxidizes
the iron and manganese.

Preliminary Treatment / Monitoring
• Flow Measurement : to adjust chemical feed rates, calculate detention
times, and monitor the amount of water being treated.
• It is also monitored for a variety of characteristics including
pH, turbidity, total alkalinity, temperature, and coliform bacteria.
• The pH and total alkalinity of the water will influence the amount of alkali to
be added and can also influence the flocculation conditions
• The level of turbidity will influence the amount of polymer (coagulant) added
to the water.
• Temperature is also measured since cold water does not floc as well as warm
water and requires the addition of more polymer

Primary Sedimentation
Notes:
• sedimentation may not be necessary in
low turbidity water of less than 10 NTU
• In this case, coagulation and flocculation
are used to produce pinpoint (very
small) floc which is removed from the
water in the filters
• Sedimentation is a treatment process in which the velocity of the water is
lowered below the suspension velocity and the suspended particles settle
out of the water due to gravity.
• The process is also known as settling or clarification
• Settled solids are removed as sludge, and floating solids are removed as scum
• The efficiency or performance of the process is controlled by: detention
time, temperature, tank design, and condition of the equipment.

Primary Sedimentation / Location in the
Treatment Process
• The most common form of sedimentation follows coagulation and flocculation
and precedes filtration.
• This type of sedimentation requires chemical addition (in the
coagulation/flocculation step) and removes the resulting floc from the water.
• sedimentation following coagulation/flocculation is meant to remove most of
the suspended particles in the water before the water reaches the filters,
• Sedimentation at this stage in the treatment process should remove 90% of the
suspended particles from the water, including bacteria.
• The purpose of sedimentation here is to decrease the concentration of
suspended particles in the water, reducing the load on the filters.
• Sedimentation can also occur as part of the pretreatment process, where it is
known as presedimentation.

Types of
sedimentation basins
Rectangular basins: have a variety of advantages -
predictability, cost-effectiveness, and low
maintenance. In addition, rectangular basins are
the least likely to short-circuit, especially if the
length is at least twice the width. A disadvantage
of rectangular basins is the large amount of land
area required.
Double-deck rectangular basins: This type of basin
conserves land area - has higher operation and
maintenance costs.
Square or circular sedimentation basins with
horizontal flow are known as clarifiers. This type
of basin is likely to have short-circuiting problems.
Solids-contact clarifiers , also known as upflow
solids-contact clarifiers or upflow sludge-
blanket clarifiers combine coagulation,
flocculation, and sedimentation within a single
basin. found in packaged plants and in cold
climates where sedimentation must occur indoors

Sedimentation and flotation zones
• All sedimentation basins have four zones -
the inlet zone, the settling zone, the
sludge zone, and the outlet zone.
• In a clarifier, water typically enters the
basin from the center rather than from one
end and flows out to outlets located around
the edges of the basin. But the four zones
can still be found within the clarifier
A rectangular sedimentation basin

Sedimentation and flotation zones/Inlet Zone
Purposes of the inlet zone of a sedimentation basin are
• to distribute the water and to control the water's velocity as it
enters the basin.
• inlet devices act to prevent turbulence of the water.
• The incoming flow must be evenly distributed across the width of
the basin to prevent short-circuiting.
• Short-circuiting is a problematic circumstance in which water
bypasses the normal flow path through the basin and reaches the
outlet in less than the normal detention time.
• If the water velocity is greater than 0.5 ft/sec, then floc in the
water will break up due to agitation ) of the water.

Sedimentation and flotation zones/Inlet Zone
Two types of inlets.
1. The stilling ( ) wall, also known as a perforated
baffle wall , spans the entire basin from top to bottom
and from side to side. Water leaves the inlet and enters
the settling zone of the sedimentation basin by flowing
through the holes evenly spaced across the stilling wall.
2. The second type of inlet allows water to enter the
basin by first flowing through the holes evenly spaced
across the bottom of the channel and then by flowing
under the baffle in front of the channel.
The combination of channel and baffle serves to evenly
distribute the incoming water

Sedimentation and flotation / Settling Zone
• water enters the settling zone where water velocity is
greatly reduced.
• the bulk of floc settling occurs and this zone will
make up the largest volume of the sedimentation
basin.
• For optimal performance, the settling zone requires a
slow, even flow of water.
• The settling zone may be simply a large expanse of
open water. But in some cases, tube settlers and
lamella plates, such as those shown below, are
included in the settling zone.
• Tube settlers and lamella plates
• Water flows up through slanted tubes or along
slanted plates. Floc settles out in the tubes or plates
and drifts back down into the lower portions of the
sedimentation basin.
• Clarified water passes through the tubes or between
the plates and then flows out of the basin.
Actual Area vs.
Effective Area

Why Tube settlers and lamella plates:
To increase the settling efficiency and speed in sedimentation basins. Each tube or plate
functions as a miniature sedimentation basin, greatly increasing the settling area. Tube
settlers and lamella plates are very useful in plants where site area is limited, or to increase
the capacity of shallow basins.
Adding inclined settling surface technology to an existing clarifier can increase water
treatment flow by as much as 75%.

Horizontal Parallel Plate Clarifiers

Vertical Parallel Plate Clarifiers

Traditional Circular Clarifiers
/ Settling Zone

Sedimentation and flotation / Outlet Zone
Outlet Zone is designed to:
• prevent short-circuiting of water in the basin.
• ensure that only well-settled water leaves the basin and enters the filter.
• control the water level in the basin.
• ensure that the water flowing out of the sedimentation basin has the minimum
amount of floc suspended in it.
• A typical outlet zone begins with a baffle in front of the effluent.
• This baffle prevents floating material from escaping the sedimentation basin and
clogging the filters.
• The weirs serve to skim the water evenly off the tank

Sedimentation and flotation / Sludge Zone
• The sludge zone is found across the bottom of the sedimentation basin.
• Velocity should be very slow to prevent resuspension of sludge.
• The tank bottom should slope toward the drains
• Sludge removal by ( automated equipment or manually at least twice per year).
• The best time of cleaning when water demand is low, (April and October).
• Many plants have at least two sedimentation basins so that water can continue
to be treated while one basin is being cleaned, maintained, and inspected.
• If sludge is not removed from enough, the effective volume of the tank will
decrease, reducing the efficiency of sedimentation.
• Sludge built up on the bottom of the tank may become septic (anaerobically).
• Septic sludge may result in taste and odor problems or may float to the top of
the water and become scum or resuspended to be carried over to the filters.

Aeration
Types of Aerators
• air into the water
• water into the air

Aeration Efficiency
Surface contact between air and water
• Smaller bubble size, greater surface contact with water.
• Smaller drop size, greater surface contact with the air.

Aeration• Aeration is the process of bringing water
and air into close contact.
• Aeration is the process to remove
dissolved gases, such as carbon
dioxide, hydrogen sulfide, and to oxidize
dissolved metals such as iron. It can also be
used to remove volatile organic chemicals
(VOC).
It happened by:
• Exposing drops or thin sheets of water to
the air or
• introducing small bubbles of air and letting
them rise through the water.
the aeration is accomplished the desired
results by:
• Sweeping ) or scrubbing action
caused by the turbulence of water and air
mixing together
• Oxidizing certain metals and gases

CHEMICAL SUBSTANCES AFFECTED BY
AERATION
The constituents that are commonly affected by aeration are:
• Volatile organic chemicals, such as benzene, found in
gasoline, or trichloroethylene, dichloroethylene, and
perchloroethylene, examples of solvents are used in dry cleaning
or industrial processes.
• Carbon dioxide
• Hydrogen sulfide (rotten-egg odor)
• Methane (flammable)
• Iron (will stain clothes and fixtures)
• Manganese (black stains)
• Various chemicals causing taste and odor

CHEMICAL SUBSTANCES AFFECTED BY
AERATION / CARBON DIOXIDE
• Surface waters have a low CO2 content ( 0 to 2 mg/l).
• Deep lake or reservoir can have high CO2 content due to the respiration of
microscopic animals and lack of abundant plant growth at the lake bottom.
• aerators remove CO2 by the physical scrubbing or sweeping action caused by
turbulence.
• aeration can reduce the CO2 content to 4.5 mg CO2 /l
• Concentration of CO2 in groundwater are usually higher than in surface
water.
• Water from a deep well normally contains less than 50 mg/l, but a shallow well
can have a much higher level, up to 50 to 300 mg/l.

CARBON DIOXIDE REMOVAL
• The most appropriate treatment for carbon dioxide may be aeration, addition
of an alkali, or a combination of the two
• CO2 gas dissolves easily in water, resulting in carbonic acid:
• H2O + CO2 <===> H2CO3
• CO2 is neutralized through the addition of an alkali (basic, ionic salt), such as
lime (Ca(OH)2) or soda ash (Na2CO3).
• Lime reacts with carbon dioxide, removing the carbon dioxide from the water
as shown below:
• CO2 + Ca(OH)2 <===> CaCO3 + H2O
CO2 above 5 to 15 mg/l in raw water can cause three operating problems:
• It increases the acidity of the water, making it corrosive by forming a “weak”
acid, H2CO3.
• It tends to keep iron in solution, thus making iron removal more difficult.
• It reacts with lime added to soften water, causing an increase in the amount of
lime needed for the softening reaction.

HYDROGEN SULFIDE
• A poisonous gas (Brief exposures--less than 30 minutes in concentrations as low
as 0.03 percent by volume in the air) - rotten-egg odor
• H2S occurs mainly in groundwater supplies.
• It may be caused by the action of iron or sulphur reducing bacteria in the well.
• Occasional disinfection of the well can reduce the bacteria producing the H2S
• H2S in a water supply will disagreeably alter the taste of coffee, tea, and ice.
• H2S is corrosive to piping, tanks, water heaters, and copper alloys that it contacts.
Operational problems:
• Disinfection of the water can become less effective because of the chlorine
demand exerted by the hydrogen sulfide.
– H2S + Cl2 + O2- → S + H2O + 2Cl-
– H2S + 4Cl2 + 4 H2O → H2SO4 + 8 HCl
• There could be corrosion of the piping systems and the water tanks.

H2S Removal by Aeration method
• Hydrogen sulfide is physically removed by agitating ) the water via
bubbling or cascading ) and then separating or "stripping )" the
hydrogen sulfide in a container. H2S + O2 = water (H2O) + elemental sulfur
• Aeration is most effective when hydrogen H2S are lower than 2.0 mg/l.
• At higher concentrations, this method may not remove all of the offensive odor
unless the air is used to oxidize hydrogen sulfide chemically into solid
sulfur, which is then filtered.
• In a typical aeration system, ambient air is introduced into the water using an air
compressor or blower.
• Well-designed aeration tanks maintain a pocket of air in the upper third or upper
half of the tank.
• If the tank does not maintain an air pocket, sulfur odor may return.
• When sulfur levels exceed 10 mg/l, larger aeration tanks, repressurization
systems, chlorination systems, or a combination may be needed.

METHANE
• Methane gas can be found in groundwater.
• It may be formed by the decomposition of organic matter.
• It can be found in water from aquifers that are near natural-gas deposits.
• Methane is a colorless gas that is highly flammable and explosive.
• When mixed with water, methane will make the water taste like garlic.
• The gas is only slightly soluble in water and therefore is easily removed by
the aeration of the water.

IRON AND MANGANESE
• Iron and manganese minerals are found in soil and rock.
• Iron and manganese can dissolve into groundwater as it percolates through the
soil and rock.
• Iron in the ferrous form and manganese in the manganous form are
objectionable.
• more than 0.3 mg/l of iron will cause yellow to reddish-brown stains of
plumbing fixtures or almost anything that it contacts.
• If the concentration exceeds 1 mg/l, the taste of the water will be metallic and
the water may be turbid.
• Manganese even at levels as low as 0.1 mg/l, will cause blackish staining of
fixtures and anything else it contacts.
• If the water contains both iron and manganese, staining could vary from dark
brown to black.
• Consumer complaints are laundry is stained and that the water is red or dirty.
• iron and manganese should not be aerated unless filtration is provided.

Iron and Manganese Removal
• Iron and manganese in well waters occur as soluble ferrous and manganous
bicarbonates.
• In the aeration process, the water is saturated with oxygen to promote the
following reactions
• The oxidation products, ferric hydroxide and manganese dioxide, are insoluble.
• After aeration, they are removed by clarification or filtration
• Occasionally, strong chemical oxidants such as chlorine (Cl2 or potassium
permanganate (KMnO4 may be used following aeration to ensure complete
oxidation
4Fe(HCO3)2 + O2 + 2H2O = 4Fe(OH)3
-
+ 8CO2
ferrous bicarbonate oxygen water ferric hydroxide carbon dioxide
2Mn(HCO3)2 + O2 = 2MnO2 + 4CO2
-
+ 2H2O
manganese
bicarbonate
oxygen manganese
dioxide
carbon
dioxide
water

TASTE AND ODOR & DISSOLVED OXYGEN
TASTE AND ODOR
• Aeration is effective in removing tastes and odors that are caused by volatile
materials
• Volatile materials (e.g Methane and hydrogen sulfide) have low boiling point
and will vaporize very easily.
• Many taste and odor problems in surface water could be caused by oils and by-
products that algae produce.
• Since oils are much less volatile than gases, aeration is only partially effective.
DISSOLVED OXYGEN
• Oxygen is injected into water through aeration to remove the flat taste.
• The amount of oxygen that the water can hold is dependent on the temp.
• The colder the water, the more oxygen the water can hold.
• Water that contains excessive amounts of oxygen can become very corrosive.
• Excessive oxygen can cause air binding of filters.

TYPES OF AERATORS
Aerators fall into two general categories.
• introduce air into the water or water into the air.
• The water-to-air method is designed to produce small drops of
water that fall through the air
• The air-to-water method creates small bubbles of air that are
injected into the water stream.
• All aerators are designed to create a greater amount of contact
between the air and water to enhance the transfer of the gases.

WATER INTO AIRCascade Aerators
• consists of a series of steps that the water
flows over.
• aeration is accomplished in the splash
zones.
• The aeration action is similar to a
flowing stream.
• Splash areas are created by placing
blocks across the incline.
• Cascade aerators used to oxidize iron and
to partially reduce dissolved gases.
• the oldest and most common type of
aerators.
Cone Aerators
• are used primarily to oxidize iron and
manganese prior to filtration.
• the water pumped to the top of the cones
and then allowed to cascade down
through the aerator.

WATER INTO AIR
Slat ( ) and Coke Aerators
• similar to the cascade and cone types.
• They usually consist of three-to-five stacked trays, which have spaced
wooden slats in them.
• The trays are filled with fist-sized ( pieces of coke, rock, ceramic
balls, limestone, or other materials.
• The primary purpose of the materials is provide additional surface contact
area between the air and water.
Spray Aerators
spray aeration is successful in
oxidizing iron and manganese and
is successful in increasing the
dissolved oxygen of the water.

WATER INTO AIR
Draft Aerators: the air is
induced by a blower.
Types:
• external blowers mounted at
the bottom of the tower to
induce air from the bottom of
the tower.
• Water is pumped to the top
and allowed to cascade down
through the rising air.
• The other, an induced-draft
aerator, has a top-mounted
blower forcing air from
bottom vents up through the
unit to the top.
• Both types are effective in
oxidizing iron and manganese
before filtration.

AIR INTO WATER
• These are not common types used in water treatment.
• The air is injected into the water through a series of nozzles submerged in
the water.
• It is more commonly used in wastewater treatment for the aeration of
activated sludge.
Air-into-water
• Diffuser
• Draft tube

AIR INTO WATER
Pressure Aerators
• Uses a pressure vessel.
• The water to be treated is sprayed into the
high-pressure air, allowing the water to
quickly pick up dissolved oxygen.
• A pressure aerator commonly used in pressure
filtration is a porous stone installed in a
pipeline before filtration.
• The air is injected into the stone and allowed
to stream into the water as a fine
bubble, causing the iron to be readily oxidized.
• The higher the pressure, the more readily the
transfer of the oxygen to the water.
• more O2 is available, more readily the
oxidation of the Fe or Mn.

AIR STRIPPING
• can be quite effective in removing volatile
organic chemicals (VOCs) from water.
• A major concern is that VOCs may be
carcinogens.
• Air stripping capable of removing up to 90
percent of the most highly volatile VOCs.
• water flow over cascade aerators or in
specially designed air-stripping towers.
• water is allowed to flow down over a
support medium or packing contained in the
tower, while air is being pumped into the
bottom of the tower.

VOC removable by air stripping

AIR STRIPPING
Diffused aeration air stripper
Low profile sieve tray air stripper

COMMON OPERATING PROBLEMS
• Aeration raises the dissolved oxygen content of the water.
• If too much oxygen is injected into the water, the water becomes supersaturated
• Supersaturation may cause corrosion whenever water and oxygen come into
contact with metallic surfaces
• False Clogging Of Filters-Air Binding (the spaces between the filter media
particles begin to fill with bubbles).
• Air binding causes the filter to plugged and in need of backwashing.
• Slow removal of the hydrogen sulfide from the towers, algae
production, clogged filters, and overuse of energy.
• Hydrogen sulfide is most efficiently removed, not by oxidation, but by the
physical scrubbing action of aeration.
• This removal is dependent on the pH of the water. At a pH of 6 or less, the
hydrogen sulfide is easily removed.
• high pH, the hydrogen sulfide will ionize, preventing removal by aeration.

OPERATIONAL TESTING
Three basic control tests are involved in the operation of the aeration process:
• Dissolved oxygen (to estimate whether the process is over or under aerated)
• pH (give an indication of the amount of carbon dioxide removal. pH increases
as the carbon dioxide is removed. pH can also be used to monitor the effective
range for hydrogen sulfide, iron, and manganese removal)
• Temperature (is important as the saturation point of oxygen increases as the
temperature decreases.
• As water temperature drops, the operator must adjust the aeration process to
maintain the correct DO level.

Adsorption
• Adsorption: is the adhesion of atoms, ions, biomolecules or molecules of
gas, liquid, or dissolved solids to surface
• This process creates a film of the adsorbate (the molecules or atoms being
accumulated) on the surface of the adsorbent.
• It differs from absorption, in which a fluid permeates or is dissolved by a liquid
or solid.
• The term sorption encompasses both processes, while desorption is the reverse
of adsorption. It is a surface phenomenon.

Types of Adsorption
• Types of Adsorption: Depending on the nature of the forces
between adsorbate & adsorbent, Adsorption is of two types.
• 1. Physical Adsorption or physisorption: Forces of attraction
between adsorbate and adsorbent are Van der Waal‟s forces,
they are weak.
• 2. Chemical Adsorption or chemisorption: Forces of
attraction between adsorbate and adsorbent are as strong as
chemical bonds, they are strong. This type of adsorption
cannot be easily reversed.

Adsorption
• Adsorption, is sorption processes in which certain adsorbates are selectively
transferred from the fluid phase to the surface of insoluble, rigid particles
suspended in a vessel or packed in a column.
• Adsorption systems treat water by adding a substance, such as activated carbon
or alumina, to the water supply.
• Adsorbents attract contaminants by chemical and physical processes that cause
them to „stick‟ to their surfaces for later disposal.

Adsorption
• the most commonly-used adsorbent is activated carbon
• Powdered activated carbon is often used when temporary quality
problems arise; it can simply be added to the water and discarded with
waste sludge.
• Granular activated carbon is often arranged in a bed through which
source water is slowly passed or percolated.
• Activated alumina treatment is used to attract and remove
contaminants, like arsenic and fluoride, which have negatively charged
ions.

Physical adsorption (Physisorption)
• In physical adsorption, the forces of attraction between the molecules of the
adsorbate and the adsorbent are of the weak van der Waals' type.
• van der Waals forces is the sum of the attractive or repulsive forces
between molecules (or between parts of the same molecule) other than those
due to covalent bonds or to the electrostatic interaction of ions with one another
or with neutral molecules
• A van der Waals bonding already illustrates the difference in bonding between
two molecules and one molecule with a solid.
• A van der Waals bonding between two molecules can be described as the
interaction between two point dipoles.
• Since the forces of attraction are weak, the process of physisorption can be
easily reversed by heating or decreasing the pressure of the adsorbate (as in the
case of gases). The attraction is not fixed to a specific site and the adsorbate is
relatively free to move on the surface

Chemical adsorption (Chemisorption)
• In chemisorption, the forces of attraction between the adsorbate and the
adsorbent are very strong;
• the molecules of adsorbate form chemical bonds with the molecules of the
adsorbent present in the surface.
• Adsorption is generally accompanied by release of energy, that is, most
adsorption processes are exothermic in nature.
• Enthalpy is a measure of the total energy of a thermodynamic system.
• It includes the internal energy, which is the energy required to create a
system, and the amount of energy required to make room for it by displacing
its environment and establishing its volume and pressure.
• Enthalpy of Physisorption, which is the enthalpy change for the adsorption of
one mole of an adsorbate on an adsorbent surface, is in the range of 20 kJ/mole
to 40kJ/mole
• Enthalpy values for chemisorption, are an order of magnitude high, that is, 200
kJ/mole to 400 kJ/mole

Comparison between Physisorption and
Chemisorption
Physisorption Chemisorption
Forces of attraction are vander Waals‟ forces
Forces of attraction are chemical
bond forces
Low enthalpy of adsorption (20 - 40
k.J/mole)
High enthalpy of adsorption (200 -
400 k.J/mole)
This process is observed under conditions of
low temperature
This process takes place at high
temperatures
It is not specific It is highly specific
Multi-molecular layers may be formed
Generally, monomolecular layer is
formed
This process is reversible This process is irreversible

Adsorption Equillibria
• Adsorption equilibrium is a dynamic concept achieved when the
rate at which molecules adsorb onto a surface is equal to the rate at
which they desorb.
• Most of the adsorption theories have been developed for gas solid
systems because the gaseous state is better understood than the
liquid.
• Till now the statistical theories developed for gas – solid systems
were applied for liquid solid systems with little confidence for
designing of the equipment.
• The most commonly used equilibrium models to understand the
adsorption systems was Freundlich and Langmuir isotherm
equation

ADSORPTION EQUILIBRIA / Isotherm
• Adsorption is usually described through isotherms
• If the adsorbent and adsorbate are contacted long
enough an equilibrium will be established between
the amount of adsorbate adsorbed and the amount of
adsorbate in solution. The equilibrium relationship
is described by isotherms.
• isotherms is the amount of adsorbate on the
adsorbent as a function of its pressure (if gas) or
concentration (if liquid) at constant temperature.
• The quantity adsorbed is nearly always normalized
by the mass of the adsorbent to allow comparison of
different materials.

Some general isotherms are shown in the figure below

Isotherm models
• The figures below show that there are four common models for isotherms.

Freundlich: K Freundlich isotherm coefficient and n
• The amount of adsorbate adsorbed is a function of the liquid-phase
concentration and called adsorption equilibrium isotherm.
• Different functions can be used to describe the adsorption
equilibrium.
• Freundlich isotherm data can be used.
• This adsorption isotherm empirical equation is.
• in which:
• qmax = loading capacity (g/kg)
• cs = equilibrium concentration (g/m3)
• x = adsorbed amount of compound (g)
• m = mass of activated carbon (kg)
• K = Freundlich constant ((g/kg).(m3/g)n)
• n = Freundlich constant (-)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Concentration
MassSorption
n = 0.5
n = 0.2
n = 2
n = 1
n = 5
n = 0.1

• The Freundlich constants K and n are influenced by water temperature, pH,
type of carbon and the concentration of other organic compounds.
• Using laboratory experiments the Freundlich constants can be determined
for a single substance with a specific type of activated carbon.
• Taking logs and rearranging:
log q = log K + n log Cs.
• When these graphs are plotted on a logarithmic scale the Freundlich
constant K can be determined from the intersection of the graph with the y-
axis.
• The slope of the line is equal to the Freundlich constant n.
• The higher the K-value, the better the adsorption.

Langmuir
• In 1916, Irving Langmuir published a new model isotherm for gases
adsorbed to solids. It is a semi-empirical isotherm derived from a proposed
kinetic mechanism.
It is based on four assumptions:
– The surface of the adsorbent is uniform, that is, all the adsorption sites are
equivalent.
– Adsorbed molecules do not interact.
– All adsorption occurs through the same mechanism.
– At the maximum adsorption, only a monolayer is formed: molecules of
adsorbate do not deposit on other, already adsorbed, molecules of
adsorbate, only on the free surface of the adsorbent.
• These four assumptions are seldom all true

Langmuir
• Langmuir suggested that adsorption takes place through this mechanism:
where Ag is a gas molecule and S is an adsorption site.
• Letting xo represent the total concentration of available sites on a given amount
of fresh solid substrate, we can define a fractional coverage (θ) as
(1)
where x is the concentration of occupied sites.
• The rate of adsorption va will be proportional to the concentration of gas or
liquid (c) above the surface and the fraction of the surface that is not covered (1-
θ), yielding a rate equation
(2)
where ka is the rate constant for adsorption.

Langmuir
• The rate of desorption is simply proportional to the fraction of the surface that is
already occupied, so the rate equation is
• (3)
• and kd is the rate constant for desorption. Setting equations (2) and (3) equal
yields an equilibrium statement that can be written as
(4)
•
The ratio of rate constants in equation (4) is equal to an equilibrium constant
(K = ka/kd).
• Upon substitution and further rearrangement, the fractional coverage is given by
(5)

Langmuir
• Equation (5) is plotted for an arbitrary value of K, (is often called a Langmuir
absorption isotherm)
• The surface sites becomes saturated as the concentration rises.
• The magnitude of K quantifies the relative affinity that a given solute has for
surface adsorption.
• Like all equilibrium constants, K is temperature dependent.
fractional
coverage

BET Isotherm
• Often molecules do form multilayers, that is, some are adsorbed on already
adsorbed molecules and the Langmuir isotherm is not valid.
• In 1938 Stephen Brunauer, Paul Emmett, and Edward Teller developed a
model isotherm that takes that possibility into account. Their theory is
called BET theory, after the initials in their last names.
• They modified Langmuir's mechanism as follows:
• A(g) + S ⇌ AS
• A(g) + AS ⇌ A2S
• A(g) + A2S ⇌ A3S and so on

Characteristics and general requirements
• Adsorbents are used in the form of spherical pellets, rods, moldings ), or
monoliths ( ) with hydrodynamic diameters between 0.5 and 10 mm.
• They must have high abrasion resistance ), high thermal stability and
small pore diameters, which results in higher exposed surface area and hence
high surface capacity for adsorption.
• The adsorbents must also have a distinct pore structure which enables fast
transport of the gaseous vapors.
Most industrial adsorbents fall into one of three classes:
• Oxygen-containing compounds – Are typically hydrophilic and polar, including
materials such as silica gel and zeolites.
• Carbon-based compounds – Are typically hydrophobic and non-polar, including
materials such as activated carbon and graphite.
• Polymer-based compounds - Are polar or non-polar functional groups in a
porous polymer matrix.

Silica gel
• a chemically inert, nontoxic, polar and dimensionally stable (< 400 C)
amorphous form of SiO2.
• Prepared by the reaction between sodium silicate and acetic acid, which is
followed by a series of after-treatment processes such as aging, pickling.
• After treatment methods results in various pore size distributions.
• Silica is used for drying of process air (e.g. oxygen, natural gas) and adsorption
of heavy (polar) hydrocarbons from natural gas
• silica gel is a solid
• it has an average pore size of 2.4 nanometers and has a strong affinity for water
molecules .

Zeolites
• Natural or synthetic crystalline aluminosilicates.
• Havin a repeating pore network and release water at high temperature.
• Zeolites are polar in nature. They are manufactured by hydrothermal synthesis of sodium
aluminosilicate or another silica source in an autoclave followed by ion exchange with
certain cations (Na+, Li+, Ca2+, K+, NH4+).
• The channel diameter of zeolite cages usually ranges from 2 to 9 Å.
• The ion exchange process is followed by drying of the crystals, which can be pelletized
with a binder to form macroporous pellets.
• Zeolites are applied in drying of process air, CO2 removal from natural gas, CO removal
from reforming gas, air separation…
• Non-polar (siliceous) zeolites are synthesized from aluminum-free silica sources or by
dealumination of aluminum-containing zeolites.
• The dealumination process is done by treating the zeolite with steam at elevated
temperatures, typically greater than 500 C.
• This high temperature heat treatment breaks the aluminum-oxygen bonds and the aluminum
atom is expelled from the zeolite framework.

Activated carbon
• non-polar and cheap, highly porous, amorphous solid consisting of
microcrystallites with a graphite lattice
• Usually prepared in small pellets or a powder.
• Its main drawbacks (it is reacts with oxygen at moderate temp. (over 300 C).
• manufactured from carbonaceous material, including coal, peat, wood, or
nutshells (e.g., coconut).
• The manufacturing process consists of two phases, carbonization and
activation.
• The carbonization process includes drying and then heating to separate by-
products, including tars and other hydrocarbons from the raw material, as well
as to drive off any gases generated.
• The process is completed by heating the material over 400 C in an oxygen-free
atmosphere that cannot support combustion.

Activated carbon
• The carbonized particles are then "activated" by exposing them to an
oxidizing agent, usually steam or carbon dioxide at high temperature.
• This agent burns off the pore blocking structures created during the
carbonization phase and so, they develop a porous, three-dimensional graphite
lattice structure.
• The size of the pores developed during activation is a function of the time that
they spend in this stage.
• Longer exposure times result in larger pore sizes.
• The most popular aqueous phase carbons are bituminous based
because of their hardness, abrasion resistance, pore size distribution, low cost
• Activated carbon is used for adsorption of organic substances and non-polar
adsorbates and it is also usually used for waste gas (and waste water) treatment.
• It is the most widely used adsorbent??? most of its chemical (eg. surface
groups) and physical properties (eg. pore size distribution and surface area) can
be tuned according to what is needed.

Activated carbon
Activated Carbon Granular Activated Carbon Powder

Characteristics of Different
Adsorbents
Table. Characteristics of Different Adsorbents
DisadvantagesUseCharacteristicsType
Difficult to
regenerate
Removal of organic
pollutants
Hydrophobic, favors
organics over water
Activated
Carbon
Low total
capacity
Air separation,
dehydration
Hydrophillic, polar,
regular channels
Zeolites
Trace removal
not effective
Drying gas streamsHigh capacity,
hydrophilic
Silica gel
Trace removal
not effective
Drying gas streamsHigh capacity,
hydrophilic
Activated
alumina

Factors Affecting the Rate of Adsorption
1. Surface area of the adsorbent:
• The rate of adsorption increases with increase in surface area of the adsorbent
• Rate of adsorption ά 1/ Diameter of adsorbent for powdered activated carbon
2. Nature of solute (adsorbate):
• Solubility of solute: Adsorption ά (1/Solubility of solute in solvent)
• Chain length of molecules: Adsorption ά chain length of molecule
• Molecular size of solute: Increase in molecular size of solute favors adsorption
• Geometry of the molecule: Branched- lesser tendency to get removed, Coiled-
more easily removed because easily entrapped in the interstices.
• Degree of ionization: Adsorption ά (1/ Dissociation constant) this is the reason
why salts are not much removed by activated carbon
3. Surface tension of the solvent:
• Those substances which lower the surface tension of solvent in which they are
dissolved become concentrated in surface layer (eg: organic substances), while
those substances which raise the surface tension are less concentrated in the
surface than in the bulk of solution. (eg.inorganic ions)

Ion Exchange
• The ion exchange process percolates water through bead-like spherical resin
materials (ion-exchange resins).
• Ions in the water are exchanged for other ions fixed to the beads.
• The two most common ion-exchange methods are softening and deionization.
• Ion exchange materials are insoluble substances containing loosely held ions
which are able to be exchanged with other ions in solutions which come in
contact with them.
• Exchanges take place without any physical alteration to the ion exchange
material.
• Ion exchangers are insoluble acids or bases which have salts which are also
insoluble, and this enables them to exchange either positively charged ions
(cation exchangers) or negatively charged ones (anion exchangers).
• Many natural substances such as proteins, cellulose, living cells and soil
particles exhibit ion exchange properties which play an important role in the
way the function in nature.

Ion Exchange
• Ion exchange uses a resin that removes charged inorganic contaminants like
arsenic, chromium, nitrate, radium, uranium, and fluoride.
• It works best with particle-free water and can be scaled to fit any size treatment
facility.
• Ion exchange is most often used to remove hardness (cation resin) or nitrate
(anion resin).
• In both instances, it can be regenerated with salt water.
• The use of ion exchange to remove radionuclides (an atom with an
unstable nucleus) is complicated by the fact that these materials accumulate in
the resin and occur at high levels in the regenerant, greatly complicating
operations.
• Activated carbon is generally preferred for removing organic
contaminants, whereas ion exchanges often best for removing inorganic
soluble molecules

Types of Ion Exchange Resin / Acid Cation
Resins
Strong Acid Cation Resins
• Behaving like strong acid
• highly ionized in both the acid (R-SO3H) and salt (R-SO3Na) form.
• Metal salt can be converted to the corresponding acid.
• They can be used for entire range of pH and are utilized for water softening
(calcium and magnesium removal).
• The regeneration takes place by contact with a strong acid solution
(hydrochloric or sulphuric)
Weak Acid Cation Resins
• Carboxylic acid (COOH) acts as the ionizable group in weak acid cation resins.
• They show more affinity for hydrogen ions. This results in the regeneration of
the hydrogen form with less acid than is required for strong acid resins.

Base AnionTypes of Ion Exchange Resin /
Resins
Strong Base Anion Resins
• They are suitable for entire pH range.
• They deionize water in hydroxide (OH) form.
• Acidic nature of the water can be removed and pure water can be obtained.
• The reaction can be put forward as:
• for which sodium hydroxide is used as the regenerant
Weak Base Anion Resins
• In weak base resins, intensity of ionization is affected by pH.
• They are incapable to split salts but can absorb acids

How ion exchange resins work
• The resins are prepared as spherical beads 0.5 to 1.0 mm in diameter.
• These appear solid even under the microscope, but on a molecular scale the
structure is quite open.
• This means that a solution passed down a resin bed can flow through the cross-
linked polymer, bringing it into intimate contact with the exchange sites.
• The affinity of sulphonic acid resins for cations varies with the ionic size and
charge of the cation.
• Generally the affinity is greatest for large ions with high valency.
• For dilute Solutions the order of affinity for some common cations is
approximately:

The general chemical reaction of ion exchange
Cation exchange: nRA + Bn+ = RnBn+ + nA+
Anion exchange: nRA + Bn- = RnBn- + nA-
Where:
R : ionized resin molecule with an attached functional group;
A+ / A- : exchangeable ion;
Bn+ / Bn- : cation/anion dissolved in the liquid;
n+ /n- : electrical charge of the ion B.

The general chemical reaction of ion exchange
• Suppose a resin has greater affinity for ion B than for ion A. If the resin contains
ion A and ion B is dissolved in the water passing through it, then the following
exchange takes place, the reaction proceeding to the right:
• When the resin exchange capacity nears exhaustion, it will be in the BR form.
• A mass action relationship applies :
• Q is the equilibrium quotient, and is a constant specific for the pair of ions and
type of resin.
• This expression indicates that if a concentrated solution containing ion A is now
passed through the exhausted bed, the resin will regenerate into the AR form
ready for re-use, whilst ion B will be eluted into the water.
• All large scale applications for ion exchange resins involved such exhaustion
and regeneration cycles.

Uses
• To remove unwanted ions from a solution passed through it (heavy metals from
metal wastes - salts from fruit juices)
• To accumulate a valuable mineral from the water which can later be recovered
from the resin.
• Strong cation resins in the hydrogen form are used for the hydrolysis of starch
and sucrose.
• Used in the laboratory to remove interfering ions during analysis or to
accumulate trace quantities of ions from dilute solutions.
• A cation resin in the hydrogen form can be used to determine the total
concentration of ions in a mixture of salts. The sample passing through a
column is converted to the equivalent quantity of acid and the amount readily
found by titration.
• Earliest applications of ion exchange was the separation of rare earth elements
(Promethium (element 61) and five new elements in the actinide series).

WATER TREATMENT
The two major types of treatment applied to water
are:
• Water softening - the replacement of ‟hard‟ ions
such as Ca2+ and Mg2+ by Na+
• Demineralisation - the complete removal of
dissolved minerals.

Water softening
• Softening is used primarily as a pretreatment method to reduce water hardness
prior to reverse osmosis (RO) processing.
• The softeners contain beads that exchange two sodium ions for every calcium
or magnesium ion removed from the "softened" water.
• In water softening a cation resin in the sodium form is used to remove hard
metal ions (calcium and magnesium) from the water along with troublesome
traces of iron and manganese
• These ions are replaced by an equivalent quantity of sodium, so that the total
dissolved solids content of the water remains unchanged as does the pH and
anionic content.
• At regular time intervals the resin is cleaned. This involves passing influent
water back up through the resin. In water softening the regenerant is a strong
solution of sodium chloride.

Example of Water softening
A) Sodium cation exchange:
• Ca+2 + 2Na.R = Ca.R + 2Na+
• Mg+2 + 2Na.R = Mg.R + 2Na+
Regeneration: using strong brine (NaCl)
• Mg. R + 2NaCl = 2Na.R + MgCl2
• Ca. R + 2NaCl = 2Na.R + CaCl2

Demineralisation = deionization
• Complete deionization can be achieved by using two resins.
• The water is first passed through a bed of cation exchange resin in hydrogen
ion form
• During service, cations in the water are taken up by the resin while hydrogen
ions are released.
• Thus the effluent consists of a very weak mixture of acids.
• Then water passes through second anion exchange resin in the hydroxide form.
• Here the anions are exchanged for hydroxide ions, which react with the
hydrogen ions to form water.
• twin bed units will reduce the total solids content to approximately 1-2 mg L-1.
• it is usual to pass water leaving the cation unit through a degassing tower.
• degassing tower removes the carbonic acid produced from carbon dioxide and
bicarbonate in the feed water and reduces the load on the anion unit.
• Without degassing the carbonic acid would be taken up by the anion bed after
conversion to carbonate.

Mixed resin
• Mixed resin produces water with much lower levels of dissolved material than
can be achieved by distillation.
• In laboratories, mixed resin is often used in disposable cartridges. These are
only used once, but larger mixed resin units can be regenerated.
• After exhaustion the bed is subjected to an up flow of water.
• Anionic resin beads are less dense than the cationic ones and they rise to the top
so that the bed is separated into two layers of resin.
• Each is regenerated in situ with the appropriate regenerant then rinsed with
clean water.

Cation and anion exchange and regeneration
i) Hydrogen cation exchange:
M+a + aH.R M.Ra + aH+
Examples:
Ca+2 + 2H.R Ca.R2 + 2H+
Na+ + H.R Na.R + H+
Regeneration: using strong acid
Ca. R + H2SO4 2H.R + CaSO4
2Na. R + H2SO4 2H.R + Na2SO4
ii) Hydroxyl anion exchange:
A-b + bR.OH Rb.A + bOH
Examples:
NO3
-+ R.OH R.NO3
- + OH
CO3
-2 + 2 R.OH R2.CO3
-2 + 2OH
Regeneration: using strong base (caustic soda)
R.NO3
- + NaOH R.OH + NaNO3
R2.CO3
-2 + 2NaOH 2R.OH + Na2CO3

Typical ion exchange installation

The ion exchange system in water treatment
a) Configuration of the ion exchanger
• Cylindrical steel tank (Diameter 1-2 m - Height 3-4 m )
• ion exchange bed occupies 1-3 meters of the tank height
• The water inters from the top (downflow) at the rate of 0.5 to 7 L/s.m2 .
• When breakthrough is reached the tank is taken off-line and backwashed by
applying water form the bottom upwards to remove any suspended solids.
• After backwashing the regeneration solution is pumped from the bottom up
wards at the rate of 0.7 to 1.5 L/s.m2.
• The same influent distributor is used to drain the upflow backwash water and
the regeneration solution ( brine, acid or base).
• At the end of the regeneration the bed is washed with clean water to remove the
residual of the regeneration solution
• An under drain piping system is installed at the bottom to collect the treated
water, and used to pump the upflow backwash water and the regeneration
solution.

The ion exchange system in water treatment
b) Pretreatment:
• The influent to the ion exchanger should be filtered to remove turbidity.
• Dissolved Organic matter should be removed by GAC before the IE because the
organic may coat the resin and reduce its exchange capacity.
• The IE is efficient for TDS less than 1000 mg/L.
c) Sizing ion exchanger : depends on the following factors:
• i) Contact time
• ii) Hydraulic loading rate
• iii) resin depth
• iv) number of columns.
d) Multiple tanks Operation:
• Ion exchange tanks can be operated in parallel or in series.
• Multiple units permit one or more units to remain in operation while one unit is
taken out of service for backwashing and generation or maintenance.

Exchange capacity of ion exchange resins
• The total solid phase concentration “q0” is termed ion exchange capacity.
• For cation exchange resins, “q0” is in the range of 200 to 500 meq/100 mg of
resin.
• During the exchange, the resin should be electrically neutral thus the all the
exchange sites should be occupied either by the original ion (such as Na+) or by
the replacing ions ( such as Ca+2 and Mg+2) and the ion exchange occupancy
should be equal to “q0” at any time.

Exchange capacity of ion exchange resins
• the Thomas kinetic equation is used for ion exchange columns.
• C = effluent concentrat ion of the ions, mg/l or meq/l
• Co = influent concentrat ion of the ions, mg/l or meq/l
• k1= rate constant, L/d . eq
• q0 maximum solid phase concentrat ion of exchange solute, eq/kg of resin
• M = mass of resin, kg
• V throughput volume, L ( )
• Q flow rate, L/d
• To apply this equation it is necessary to perform a laboratory column test or
pilot scale column to obtain the breakthrough curve.

Ion Exchange process analysis in the Fixed bed
a) Mass transfer inside the Ion exchange bed:
• When the polluted water is pumped on the ion exchange bed, the pollutant ions
replace the exchangeable ions in the resin.
• The area of the ion exchange bed in which the exchange occurs is called the
mass transfer zone (MTZ)
• No further adsorption occurs below the MTZ and the water leaving the MTZ
zone contains the minimum concentration value of the pollutant that the bed can
produce.
• With time a zone of saturation is created above the MTZ in which the resin has
reached its maximum exchange capacity and no further replacement occurs.
• The equilibrium concentration Ce of the pollutant in water in this zone is the
same as C0.
• The Zone below the MTZ essentially clean zone and no adsorbed material on it.
• With time the saturation zone depth increases and the MTZ is pushed down
until we reach to a point where the clean zone disappears and breakthrough
occurs.

Breakthrough curve
• Breakthrough is said to have occurred when the effluent concentration reaches to 5% of
the influent concentration (i.e, Cb = 0.05C0).
• After additional time the MTZ start to decrease until it disappears and the bed is called
exhausted.
• Exhaustion of the bed is assumed to have occurred when the effluent Concentration is
equal to 95% of the influent concentration (i.e, C = 0.95C0)
• The area above the breakthrough curve is equal to the mass of the pollutant adsorbed in the
column

Calculation of the length of the MTZ ***

Detection of resin exhaustion
• A resin is considered to be exhausted when the ions in the resin have
mostly been replaced by the ions that are being removed from the solution.
• Exhaustion of demineraliser is usually detected by an electrical
conductivity cell installed at the outlet.
• When the conductivity rises to indicate ionic breakthrough, a regeneration
cycle can be initiated automatically.
• With small units it is possible to incorporate a pH indicator on the anion
resin of a mixed bed cartridge.
• Exhaustion can be followed down the side of a transparent cartridge as the
alkaline anion resin is converted to the neutralised salt form.

Advantages and disadvantages in the use of
Ion-Exchange Resins
The advantages of ion exchange processes are:
• very low running costs.
• Very little energy is required,
• The regenerant chemicals are cheap and if well maintained resin
beds can last for many years before replacement is needed.

Disadvantages in the use of Ion-Exchange Resins
Calcium sulphate fouling
• Using Sulphuric acid as cation resin regenerant will react with calcium in water
forming calcium sulphate precipitates.
• This fouls the resin and blocks drain pipes with a build up of scale (hydrochloric
acid must be substituted).
Iron fouling
• Aeration allows oxidation of Fe2+ to Fe3+ and consequent precipitation of ferric
hydroxide which clogs resin beads and prevents ion exchange. Iron fouling is
the commonest cause of softener failure.
Adsorption of organic matter
• the presence of dissolved organic material can become irreversibly adsorbed
within the anion beads, reducing their exchange capacity.
• Removal of organics prior to demineralisation is achieved by flocculation with
alum or ferric salts followed by filtration.

Disadvantages in the use of Ion-Exchange Resins
Organic contamination from the resin
• The resins themselves can be a source of non-ionized organic contamination.
New commercial grade resin often contains organics remaining after manufacture.
when removal is needed, the demineralised water can be passed through an ultra
filtration membrane.
Bacterial contamination
• Resin beds do not act as filters for the removal of bacteria.
• Resin beds can generate a culture media for continued growth.
• Resins beds can be decontaminated with disinfectants such as formaldehyde
• heat or oxidising disinfectants as chlorine must not be used as these damage resins.
Chlorine contamination
• chlorine damages resins. It is customary to treat such feeds by passing them
through activated carbon which removes chlorine very efficiently.
ENVIRONMENTAL IMPLICATIONS
• The waste water for disposal after regeneration contains all the minerals removed
from the water plus salt from the spent regenerants.
• volume of it is equivalent to 1-5% of the treated water throughput.

Coagulation and Flocculation
Coagulation refers to all the reactions and mechanisms that result in particle
aggregation in the water being treated, including in situ coagulant formation
(where applicable), particle destabilization, and physical interparticle
contacts
• Coagulant formation, particle destabilization, typically occur during and
immediately after chemical dispersal in rapid mixing;
• inter particle collisions that cause aggregate (floc) formation begin during
rapid mixing but usually occur predominantly in the flocculation process.
• The physical process of producing inter-particle contacts is termed
flocculation.
• Flocculation defined as the uses gentle stirring to bring suspended particles
together so they will form larger more settleable clumps (groups) called floc.

A common classification of particles
• Molecules sizes smaller than 1 nm
• Colloids generally with
dimensions between 1 nm - 1 μm
• Suspended matter having sizes
larger than 1 μm.
• Colloids: humic
acids, proteins, colloidal
clay, silica and viruses.
• Suspended matter:
Bacteria, algae, silt, sand and
organic debris.
• Suspended matter-when it is larger
than 5-10 μm can be removed
quite easily by filtration or
sedimentation and filtration.
• The removal of colloids is possible

A common classification of particles

STABILITY OF PARTICLE SUSPENSIONS
• Coagulation process is used to increase the rate or kinetics of particle
aggregation and floc formation
• The objective is to transform a stable suspension [i.e., one that is resistant to
aggregation (or attachment to a filter grain)] into an unstable one.
• there are forces that tend to pull the interacting surfaces together
• The most important attractive force is called the London–van der Waals force.
• It arises from spontaneous electrical and magnetic polarizations that create a
fluctuating electromagnetic field within the particles and in the space between
them
• The most well-known repulsive force is caused by the interaction of the
electrical double layers of the surfaces (“electrostatic” stabilization).
• As particles approach one another on a collision course, the fluid between them
must move out of the way.
• The repulsive force caused by this displacement of fluid is called hydrodynamic
retardation.

Electrostatic Stabilization
• Origins of Surface Charge. Most particles in water, mineral and organic, have
electrically charged surfaces, and the sign of the charge is usually negative.
Three important processes for producing this charge are considered in the following
discussion.
• First, surface groups on the solid may react with water and accept or donate
protons.
• Second, surface groups can react in water with solutes other than protons.
• Third, a surface charge may arise because of imperfections within the
structure of the particle; this is called isomorphic replacement, or substitution

The Electrical Double Layer (EDL)
• In a colloidal suspension, there can be no net imbalance in the overall electrical
charge
• the primary charge on the particle must be counterbalanced in the system.
• If the particle is negatively charged, excess ions of opposite charge (positive)
accumulate in this interfacial region
• Ions of opposite charge accumulating in the interfacial region, together with the
primary charge, form an electrical double layer.
The diffuse layer results from:
– electrostatic attraction of ions of opposite charge to the particle
– electrostatic repulsion of ions of the same charge as the particle, and
– thermal or molecular diffusion that acts against the concentration gradients
produced by the electrostatic effects
• Because of the primary charge, an electrostatic potential (voltage) exists
between the surface of the particle and the bulk of the solution
• This electric potential can be pictured as the electric pressure that must be
applied to bring a unit charge having the same sign as the primary charge from
the bulk of the solution to a given distance from the particle surface

The zeta potential
• It is a scientific term for electrokinetic potential in colloidal systems (ζ-potential)
• It represents the net charge between the primary charge and the counter charge
in the EDL located between the surface and the shear plane
• zeta potential is electric potential in the interfacial double layer at the location of
the slipping plane versus a point in the bulk fluid away from the interface
• zeta potential is the potential difference between the dispersion medium and the
stationary layer of fluid attached to the dispersed particle.
• A value of 25 mV (positive or negative) can be taken as the arbitrary value that
separates low-charged surfaces from highly-charged surfaces.
• The zeta potential indicates the degree of repulsion between
adjacent, similarly charged particles in a dispersion.

The zeta potential
Zeta potential [mV] Stability behavior of the colloid
from 0 to ±5, Rapid coagulation or flocculation
from ±10 to ±30 Incipient instability
from ±30 to ±40 Moderate stability
from ±40 to ±60 Good stability
more than ±61 Excellent stability
• For molecules and particles that are small enough, a high zeta potential will
confer stability, i.e., the solution or dispersion will resist aggregation.
• When the potential is low, attraction exceeds repulsion and the dispersion will
break and flocculate.
• Colloids with high zeta potential (negative or positive) are electrically stabilized
while colloids with low zeta potentials tend to coagulate or flocculate

a net electrostatic repulsion/attraction developed
between colloids as a result of EDL
The net resultant force is a result of:
• Attractive potential energy (mostly
van der Waals forces).
• Repulsion potential energy
(electrostatic forces)

Relative strength of forces
Dipole-dipole interactions: electrostatic interactions of permanent dipoles in
molecules.
These interactions tend to align the molecules to increase the attraction
(reducing potential energy).
An example of dipole-dipole interactions can be seen in hydrogen chloride (HCl)
Hydrogen bond : is the attractive interaction between a hydrogen atom and
an electronegative atom, such as nitrogen, oxygen, or fluorine.
The hydrogen bond is a strong electrostatic dipole-dipole interaction.
It is stronger than a van der Waals interaction, produces interatomic distances shorter
than sum of van der Waals radii
Relative strength of forces
Bond type Dissociation energy (kcal)
Covalent 400
Hydrogen bonds 12-16
Dipole-dipole 0.5 - 2
London (van der Waals) Forces <1

Calculation of the zeta potential***
The magnitude of these forces is measured by the zeta potential, which is:
where:
• Z is the zeta potential,
• q is the charge per unit area,
• d is the thickness of the effective charge layer, and
• D is the dielectric constant of the liquid.
The greater the zeta potential, the greater are the repulsion forces between the
colloids and, therefore, the more stable is the colloidal suspension

Brief
There are two major forces acting on colloids:
1) electrostatic repulsion
(simply, negative colloids repel other negatively charged colloids)
2) intermolecular, or van der Waals, attraction
• Coagulants can be used to reduce the electrostatic repulsive forces. The
electrostatic repulsion reduced by the addition of countercharged ions such as
[Al3+]

Process Description
Purpose to aid in the removal of nonsettleable solids from water.
Coagulation is defined as:
• the destabilization of colloidal solids;
• the water treatment process which causes very small suspended solids to attract
one another and form larger particles.
Suspended particles in water resist settling for two primary reasons:
1. Particle size; and,
2. Natural forces between particles.
• Suspended particles in water normally have a negative (-) charge.
• Since these particles all have the same charge, they repel each other, keeping
each other from settling.
• This natural repelling force is called the zeta potential.
• Coagulation neutralizes the forces (zeta potential), which cause suspended solids
in water to repel each other and resist settling.
• Once the repulsive forces have been neutralized these particles can stick together
(agglomerate) when they collide.
• The force which holds the floc together is called the van der Waals force.

Flocculation
• After coagulation the destabilized particles can collide, aggregate so flocs can
be formed. This step is called flocculation.
• Flocculation: The process of agglomeration of the destabilized particles to
such a size that separation by sedimentation and filtration is possible.
• In flocculation one can make a distinction between peri-kinetic and ortho-
kinetic flocculation.
• Brownian motion is the driving force in the agglomeration of destabilized
particles up to 1μm-level peri-kinetic flocculation).
• Above ~ 1 μm the influence of Brownian motion on the collision rate of the
particles can be neglected, then artificial mixing is necessary to get an efficient
flocculation. That part of the flocculation process is called ortho-kinetic
flocculation.
• Flocculation uses gentle stirring to cause the particles to collide so that they
can stick together, for a particle (floc) large enough and heavy enough to settle

Flocculation Practice in Water Treatment***
• Coagulation and Flocculation Process Design
Where,
• G = velocity gradient, s-1
• P = power input, W
• V = volume of water in mixing tank, m3
• μ = dynamic viscosity, Pa.s
• Viscosity is a measure of the resistance of a fluid which is being deformed. The
SI physical unit of dynamic viscosity is the pascal-second (Pa·s), which is
identical to N·m−2·s
• Velocity gradient velocity gradient is the change in relative velocity between
parallel planes with respect to the change in perpendicular distance throughout
the depth of the material. Velocity gradient has the dimensions as rate of
shear, which is reciprocal seconds.

Types of Flocculation Tanks
Mechanical Flocculators
Paddle wheel Type (vertical and Horizontal Types)
Foil Type Mixing Blade

Types of Flocculation Tanks
• Hydraulic Flocculators
• The axial flow flocculators are typically used because they impart a nearly
constant gradient in each compartment.
• Flocculators are designed to have a minimum of three compartments to provide
for tapered (to make smaller gradually) mixing.
• The velocity gradient, G is tapered so that it is larger in the first compartment
and less is the other compartments as the floc grows.

Chemicals are used to neutralize the zeta potential
• These chemicals are coagulants, sometimes called primary coagulants, and
coagulant aids.
• Since most suspended particles in water carry a negative (-) charge, coagulants
consist of chemicals that provide positively (+) charged ions.
Common coagulants are:
• 1. Metal Salts
• a. Aluminum Salts (Alum (aluminum sulfate) - PACs (polyaluminum
chlorohydrate, and other variations)
• b. Iron Salts (Ferric Chloride - Ferric Sulfate - Ferrous Sulfate)
• 2. Polymers (polyelectrolytes)

Common coagulants / Polymers
• Polymers (polyelectrolytes) are extremely large molecules which produce
thousands of charged ions when dissolved in water.
1. Cationic Polyelectrolytes - Have a positive (+) charge. Used as either a primary
coagulant or as a coagulant aid. Cationic polymers:
• allow reduced coagulant dose;
• improve floc settling;
• are less sensitive to pH;
• improve flocculation of organisms such as bacteria and algae.
2. Anionic Polyelectrolytes- Have a negative (-) charge. Used primarily as a
coagulant aid. Anionic polymers are used to:
• increase floc size;
• improve settling;
• produce a stronger floc;
• They are not materially affected by pH, alkalinity, hardness or turbidity.
3. Nonionic Polyelectrolytes- Balanced or neutral charge.
• Used as a primary coagulant or coagulant aid.

Coagulant aids
Coagulant aids are chemicals which are added to water during coagulation to
improve coagulation by:
• building a stronger, more settleable floc;
• overcoming slow floc formation in cold water;
• reducing the amount of coagulant required;
• reducing the amount of sludge produced.
• The key reason coagulant aids are used is to reduce the amount of alum
used, which, in turn, decreases the amount of alum sludge produced.
• Alum sludge is difficult to dewater and to dispose of.

Types of Coagulant Aids
Activated Silica
• increases the coagulation rate;
• reduces the amount of coagulant needed;
• widens the pH range for effective coagulation;
• strengthens floc
Weighting agents (Bentonite Clay, Powdered Limestone; Powdered Silica)
provide additional particles that can enhance floc formation.
They are used to treat water that is:
• high in color; or,
• low in turbidity; or,
• low in mineral content.

Factors Which Affect How Well a Coagulant Work
Factors Which Affect How Well a Coagulant Work
(1) Mixing Conditions
(2) pH
(3) Alkalinity
(4) Water Temperature
(5) Turbidity
• If the alkalinity concentration in the water is not high enough, and effective floc
will not form when either alum or ferric sulfate is used. Metal salts (alum, ferric
sulfate, ferric chloride) consume natural alkalinity.
• Each mg/L of alum will consume 0.5 mg/l total alkalinity (as CaCO3).
• Each mg/L ferric sulfate will consume 0.75 mg/L total alkalinity (as as CaCO3).
• Each mg/L ferric chloride will consume 0.92 mg/L total alkalinity (as CaCO3).
• It may be necessary to add alkalinity to the water (lime, soda ash, caustic soda)
to the water in order for the metal salts to work properly. The doses should be
confirmed with jar testing.

Coagulation/Flocculation Facilities
Coagulation/Flocculation Facilities
• Flash Mix - purpose is to distribute the coagulant rapidly and evenly throughout
the water.
• Water should be stirred violently for a brief time to encourage the greatest
number of collisions between particles as possible.
• Types of Mixers: Mechanical - Pumps and Conduits
• Detention time should be 30 seconds or less (Design Criteria).
• Flocculation - provides for gentle mixing to encourage floc formation.
• Detention time of at least 30 minutes, with a detention time of 45 minutes
preferred.

Process Control
A. Chemical Selection
B. Chemical Application / Solution Preparation
C. Monitoring Process Effectiveness

Process Control / Chemical Selection
• A. Chemical Selection - These raw water characteristics should be monitored in
order to do a thorough job of chemical selection.
1. Temperature
• Low water temperatures slow chemical reactions, causing decreased efficiency
and slow floc formation.
• Higher coagulant doses may be required to maintain acceptable results.
2. pH
• Extremes can interfere with the coagulation/flocculation process.
• The optimum pH depends on the specific coagulant.
3. Alkalinity
• Low alkalinity causes poor coagulation.
• May be necessary to add alkalinity (lime, caustic soda, soda ash).
4. Turbidity
• Difficult to form floc with low turbidity water, may need to add weighting agents.
5. Color - Indicates presence of organic chemicals which can react with the
coagulant, and with chlorine to form disinfection byproducts.

Process Control / Chemical Application
B. Chemical Application:
Solution Preparation
• For Example when preparing potassium permanganate solutions, a three
percent solution is best. Potassium permanganate has a limited solubility of
about five percent at normal temperatures.
In order to prepare the solution needed the following information is required:
• Chemical required
• Volume of water required
• Specific gravity
• Weight of solution
• Concentration

Process Control / Monitoring Process
Effectiveness
C- Monitoring Process Effectiveness
• (1) Jar Test
• (2) pH
• (3) Turbidity
• (4) Temperature
• (5) Alkalinity

Process Evaluation and Troubleshooting
• Problems with coagulation and flocculation normally show up in subsequent
processes (sedimentation and filtration). Actual plant performance should be
monitored for:
Poor or inadequate flash mixing
• Gentle flocculation
• Adequate flocculation time
• Settled- and filtered water quality.
Indication of poor or inadequate flash mixing:
1. Very small floc (pin floc)
2. High turbidity in settled water should be less than 1.0 NTU when the raw water
turbidity is less than 1 NTU 95 percent of the time; less than 2.0 NTU when the
raw water turbidity is more than 1 NTU 95 percent of the time.
3. Too frequent filter backwashing
If any of these symptoms occur, check:
(1) detention time and mixing energy in the flash mix (detention time should be less
than 30 seconds);
(2) stirring speed in the flocculator – should have a peripheral speed of between 0.5
and 2.0 feet per second;
(3) flocculation detention time – should be at least 30 minutes, with 45 minutes
being preferred.

Jar Test
• Coagulation/flocculation is the process of binding small particles in the water
together into larger, heavier clumps which settle out relatively quickly.
• The larger particles are known as floc.
• changing water characteristics require the operator to adjust coagulant dosages
at intervals to achieve optimal coagulation.
• Different dosages of coagulants are tested using a jar test, which mimics the
conditions found in the treatment plant.
• The first step of the jar test involves adding coagulant to the source water and
mixing the water rapidly (as it would be mixed in the flash mix chamber) to
completely dissolve the coagulant in the water.
• Then the water is mixed more slowly for a longer time period (as flocculation
basin conditions and allowing the forming floc particles to cluster together).
• Finally, the mixer is stopped and the floc is allowed to settle out, as it would in
the sedimentation basin.

Jar Test
• A major goal of water treatment is turbidity removal.
• The jar test is a simulation of the treatment processes that have been developed
to accomplish turbidity removal
• Alum, ferrous sulfate, and ferric chloride are three common coagulants
• The best dose will also be a function of pH. The optimum pH for alum
coagulation is usually between 5.5 and 6.5.
• There is no way to “calculate” the best dose. It must be determined by trial and
error; hence, the jar test.
• The reaction chemistry varies according to the pH and alkalinity of the test
sample.
Alum coagulation proceeds according to the following equation
• if there is enough alkalinity in the water to react with the amount of alum
dosed:
Al2(SO4)3 • 14H2O + 6HCO3
- ↔ 2Al(OH)3(s) + 6CO2 + 14H2O + 3SO4
-2
• If there is insufficient alkalinity, the reaction will proceed according to the
equation:
• Al2(SO4)3 • 14H2O ↔ 2Al(OH)3 + 3H2SO4 + 8H2O
• An alkalinity test is usually performed before initiating a jar test to determine
whether alkalinity supplements might be required.

JAR TEST PROCEDURE
1. measure the initial pH, alkalinity, and turbidity of the sample to be tested.
Make any pH adjustment necessary.
calculate for the maximum alum dose you plan to use, what alkalinity
concentration is required to prevent significant pH reduction.
Compare this required amount to the amount measured in step 1, and
supplement the sample with alkalinity if necessary.
Al2(SO4)3 • 14H2O + 6HCO3
- ↔ 2Al(OH)3(s) + 6CO2 + 14H2O + 3SO4
-2
Decide on six dosages of the chemical(s) include coagulants, coagulant
aids, and lime
2. Prepare a stock solution of the chemical(s).
3. Collect a two gallon sample of the water to be tested. This should be the raw
water.
4. Measure 1,000 mL of raw water and place in a beaker. Repeat for the remaining
beakers.
5. Place beakers in the stirring machine.
6. With a measuring pipet, add the correct dosage of lime and then of coagulant
solution to each beaker as rapidly as possible.

JAR TEST PROCEDURE
7. With the stirring paddles lowered into the beakers, start the stirring machine and
operate it for one minute at a speed of 80 RPM.
8. Reduce the stirring speed to 20 RPM and continue stirring for 30 minutes
9. Stop the stirring apparatus and allow the samples in the beakers to settle for 30
minutes.
10. Determine which coagulant dosage has the best flocculation time and the most
floc settled out.
11. Test the turbidity of the water in each beaker using a turbidometer
12. If lime or a coagulant aid is fed at your plant in addition to the primary
coagulant, you should repeat the jar test to determine the optimum dosage of
lime or coagulant aid. Use the concentration of coagulant chosen in steps 10 and
11 and alter the dosage of lime or coagulant aid.
13. Using the procedure outlined in step 11, measure the turbidity of water at three
locations in the treatment plant - influent, top of filter, and filter effluent.
14. Prepare a graph of alum dose vs. remaining turbidity in order to identify the
dosage that produced optimum turbidity removal

Jar Test Example
Low alkalinity High alkalinity

Explanation of Jar Test Example
• Water A had low alkalinity and required less coagulant to achieve good
coagulation and flocculation than the higher alkalinity of Water B
• Plots of turbidity versus coagulant dose showed a continual decrease in turbidity
with an increase in coagulant dose.
• Water A, with FeCl3, showed a decrease followed by an increase (at 40 mg/L) in
turbidity.
• This dictates that adsorption and charge neutralization is taking place due to the
colloids restabilizing and not coagulating.
• Addition of coagulant to the low alkalinity waters lead to a drop in the pH of
Water A, which enhanced adsorption and charge neutralization.
• higher coagulant doses are needed with high alkalinity waters where pH remains
fairly constant.
• Although slightly less alum than FeCl3 was needed to reach an optimum
level, the residual turbidity when using the alum coagulant did not fall below 1
NTU.
• This means that even though alum may require a slightly smaller dose, it still
may not be able to meet the desired effluent regulations without the additional
help of a filter or polymer

Filtration
• After separating most floc, the water is filtered as the final step to remove
remaining suspended particles and unsettled floc
1. Rapid sand filters
• use relatively coarse sand and other granular media to remove particles and
impurities that have been trapped in a floc through the use
of flocculation chemicals-typically salts of aluminium or iron.
• Water and flocs flows through the filter medium under gravity or under pumped
pressure
• Water moves vertically through sand which often has a layer of activated
carbon or anthracite coal (a hard, compact variety of mineral coal).
• The top layer removes organic compounds
• Most particles pass through surface layers but are trapped in pore spaces or
adhere to sand particles
• To clean the filter, water is passed quickly upward through the filter, opposite
the normal direction (called backflushing or backwashing)
• compressed air may be blown up through the bottom of the filter to break up the
compacted filter media to aid the backwashing process

Rapid sand filters / Advantages and disadvantages
Advantages
• Much higher flow rate than a slow sand filter;
• Requires relatively small land area
• Less sensitive to changes in raw water quality, e.g. turbidity
• requires less quantity of sand
• Disadvantages
• Requires greater maintenance than a slow sand filter. For this reason, it is not
usually classed as an "appropriate technology,".
• Generally ineffective against taste and odour problems.
• Produces large volumes of sludge for disposal.
• Requires on-going investment in costly flocculation reagents.
• treatment of raw water with chemicals is essential
• skilled supervision is essential
• cost of maintenance is more
• it cannot remove bacteria

Slow sand filters
• Slow "artificial" filtration (a variation of bank filtration) to the ground, Water
purification plant
• The filters are carefully constructed using graded layers of sand with the coarsest
sand, along with some gravel, at the bottom and finest sand at the top.
• Drains at the base convey treated water away for disinfection
• effective slow sand filter may remain in service for many weeks or even months
• produces water with a very low available nutrient level and low disinfectant
levels
• Slow sand filters are not backwashed; they are maintained by having the top
layer of sand scraped off
• A 'large-scale' form of slow sand filter is the process of bank filtration in a
riverbank.

Slow sand filters / Advantages and disadvantages
• Advantages
• require little or no mechanical power, chemicals or replaceable parts,
• require minimal operator training and only periodic maintenance,
• often an appropriate technology for poor and isolated areas.
• simple design
• disadvantages
• Due to the low filtration rate, slow sand filters require extensive land area
for a large municipal system.
• Many municipal systems in grown cities installed rapid sand filters, due to
increased demand for drinking water.

Membrane filtration
• is a treatment process based on the physical separation of compounds from the
water phase with the use of a semi-permeable membrane.
• Most of the membranes used are synthetic membranes made of organic
polymers.
Can be divided into two catego-ries based on the pore sizes of the membrane:
• micro- and ultrafiltration (MF and UF) remove colloidal substances and
microorganisms
• nanofiltration and reverse osmosis (NF and RO) remove colloidal substances
and microor-ganisms but also dissolved substances like micropollutants and ions
• Micro- and ultrafiltration remove substances from the water phase by a sieve
mechanism
• Microfiltration removes bacteria and the larger viruses (to a size of 0.05 μm).
• Ultrafiltration also removes bacteria, but because of the smaller pore size all the
larger viruses are removed
• The removal of suspended solids of MF and UF is at least 99%.

Membrane filtration
• The removal of microorganisms is referred to in log units.
• A removal of one log unit corresponds to a 90% removal. The removal of 4 log
units cor-responds to a 99.99% removal.
• molecular weight cut-off (MWCO) can also be used as an indication of the
ability of membranes to reject compounds
• MWCO is the molecular weight of spherical molecules which are 90% rejected
by the membrane‟s pores.
• The unit of MWCO is Dalton (1 Dalton is the mass of one hydrogen atom =
1.66x10-27kg)
• The MWCO for MF/UF is in the range of 10,000 to 500,000 Dalton (10 to 500
kD).

Different filtration processes and size of
compounds removed

MF/UF for drinking water
• In drinking water treatment, UF can be used in different stages of the process:
– as a pre-treatment of surface water to remove suspended solids, heavy
metals, bacteria and viruses in order to prevent pollution of the dunes, or to
prevent clogging of the NF/RO membranes
– as treatment of backwash water from rapid sand filtration
– treatment of surface water as the first step in drinking water production.
In some cases, the MF/UF installation is preceded by a conventional
coagulation/flocculation/floc removal treatment process in order to reduce
the risk of membrane fouling

Log-removal capacity of MF and UF for
different microorganisms

Principle
• The membrane is the barrier responsible for the separation of compounds out of
the water phase. The membrane is semi-permeable.
• The pore size determines the removal of different compounds.
• The removed compounds remain at the raw waterside of the membrane and
accumulate on the membrane.
• Three water streams can be distinguished:
– the dirty water or raw water is called feed water
– the water passing the membrane is called the permeate or product water.
– the water with the rejected particles is called concentrate or retentate.

Possibilities for the use of MF and UF for
drinking water production

Dead-end filtration mode
• In dead-end filtration, all the feed water is through the membrane.
• The suspended solids remain on the feed side of the membrane.
• As a consequence, the resistance of permeation will increase in time.
• The water flux decreases if the pressure is constant, or the pressure increases if
the flux is constant.
• Periodically the membrane has to be cleaned in order to reduce the resistance of
permeation.
• The period of permeation is called filtration time. A filtration run is the filtration
time together with the cleaning time (also called filtration cycle).

Inside-out filtration
• In a configuration with inside-out filtration, feed water enters the inside of the
capillaries or tubular membranes.
• The water is pushed from the inside to the outside of the membrane.
• Permeate is collected outside the membrane and transported to the permeate
tube.

Membrane fouling
• During filtration the resistance increases as a result of fouling of the membrane
surface.
• The resistance increases because the pores in the membrane are blocked and
because caked suspended matter is built up on the membrane surface.
• This resistance increase is referred to as fouling.
• The definition of fouling is: the deposition of suspended or dissolved substances
on the membrane surface or in front of the pores or in the membrane pores.
• fouling can be subdivided into different mechanisms:
- membrane resistance
- pore blocking
- adsorption in the pores
- cake resistance
- high concentration of dissolved substances near the surface.

Membrane fouling
• Due to the accumulation of solids on and in the membrane during dead-end
filtration, the total resistance increases with time.
• The mass of a cake layer is difficult to measure. The thickness of the cake layer
depends on The trans membrane pressure TMP.
• The thickness of the cake layer is in the range of several micrometers, depending
on the rejected compounds.

Cleaning
• As a result of the dead-end mode, the membrane has to be cleaned often in order
to remove the rejected compounds.
• The cleaning intervals can be constant in time or can be determined by a
maximum pressure.
• If possible, cleaning of membranes should be avoided because during the
cleaning no permeate is produced.
• Also, permeate and energy are used for the cleaning. With specific cleanings
chemicals are also used.
• Different methods or a combination of methods can be used to clean a
membrane module:
- forward flush (FF)
- back flush (BF)
- air flush (AF)
- chemical enhanced flush (CEF) or enhanced back flush (EBF)
- cleaning in place (CIP) or chemical soaking

Forward flush (FF) and back flush (BF)
The forward flush is a turbulent cross-flow
along the feed side of the membrane surface
This is the opposite of the filtration mode
where the flow is through the membrane
(flow direction perpendicular to the
membrane surface)
In the back flush, filtration direction is reversed
After removing the particles from the pores and
the membrane surface, the particles and the cake
have to be transported out of the module.
A combined back flush and forward flush can be
used. First, a back flush is used to clean the pores
and to lift the cake. Then, a forward flush is used
to transport the dirt out of the module

Air/water flush
• An air/water flush can be used to clean the membrane wall from adhering
fouling.
• The air/water flush is actually a forward flush with a combination of air and
water.
• The air is used to create a turbulent flow in the membrane under process
conditions where no turbulence is attained with the water flow.
• The cleaning efficiency depends on the kind of two-phase flow obtained in the
membranes.
• If the water/air ratio is high, only small air bubbles are present in the water and
the turbulence is only slightly enhanced.
• When the water/air ratio is too small, the air flows through the middle of the
membrane and the cleaning effect is low.
• The best cleaning is obtained with bullet-like air bubbles.

Chemical cleaning
• If forward flush, back flush and air flush are not enough to clean the
membrane, a chemical cleaning (often called enhanced back flush or chemical
back flush) can also decrease the clean water resistance.
• This kind of cleaning means that the module is soaked with a solution of
hypochloric acid, hydrogen chloride, hydrogen peroxide.
• After the soaking, a backwash or a forward flush removes the dissolved dirt.
• The main drawback of chemical cleaning is that the membranes age because of
the chemicals, and the lifetime of the membranes, therefore will be shortened.
• Also, the chemicals are a cost factor and with a chemical cleaning a chemical
waste stream should be discharged.
• Besides the periodical chemical cleaning which is part of the automated process
control of the installation, a more intensive chemical cleaning might also be
necessary.
• The so-called “cleaning in place” (CIP) can last from a few hours to several days
and is typically not automated.
• If the CIP is not able to clean the membranes, they are replaced by new ones.

Practice / Module design
Tubular membranes
Capillary membranes
Plate membrane
Cushion membranes
Spiral Wound Flat Sheet Membrane

Membrane Processes / Electro-dialysis
• What is a Membrane?
• The membrane can be defined essentially as a barrier, which separates two
phases and restricts transport of various chemicals in a selective manner.
• Membrane Separation Technology
• A membrane separation system separates an influent stream into two effluent
streams known as the permeate and the concentrate.
• The permeate is the portion of the fluid that has passed through the semi-
permeable membrane.
• The concentrate stream contains the constituents that have been rejected by the
membrane.

Membrane Processes
The main membrane processes are
• Dialysis
• Electro-dialysis
• Reverse osmosis
Driving forces that cause mass transfer of solutes are:
• Difference in concentration (dialysis)
• Difference in electric potential (electro-dialysis)
• Difference in pressure (reverse osmosis)

Dialysis Theory
• Dialysis depends on separating solutes of different ionic or molecular size in a
solution by means of a selectively permeable membrane.
• The driving force for dialysis is the difference in the solute concentration across
the membrane
The mass transfer of solute through the membrane is given by:
• M = mass transferred per unit time (gram/hour)
• K = mass transfer coefficient [gram/(hr-cm2)(gram/cm3)]
• A = membrane area (cm2)
• ΔC = difference in concentration of solute passing through
the membrane (gram/cm3)
• In environmental engineering, Dialysis is not used to an appreciable extent.
• In industrial applications, Dialysis can be used to recover Sodium Hydroxide
from textile wastewater.
• Dialysis is limited to small flows due to small mass transfer coefficient (K)

Electrodialysis (ED)
• ED is used to transport salt ions from one solution through ion exchange
membranes to another solution under the influence of an applied electric
potential difference.
• The cell consists of a feed (diluate) compartment and a concentrate (brine)
compartment formed by an anion exchange membrane and a cation exchange
membrane placed between two electrodes
• multiple electrodialysis cells are arranged into a configuration called an
electrodialysis stack, with alternating anion and cation exchange membranes
forming the multiple electrodialysis cells
• dissolved species are moved away from the feed stream rather than the reverse

Method
In an electrodialysis stack, the diluate (D) feed stream, brine or concentrate (C)
stream, and electrode (E) stream are allowed to flow through the appropriate cell
compartments formed by the ion exchange membranes

Anode and Cathode Reactions
At the cathode 2e- + 2 H2O → H2 (g) + 2 OH-
At the anode H2O → 2 H+ + ½ O2 (g) + 2e- or 2 Cl- → Cl2 (g) + 2e-
• Small amounts of hydrogen gas are generated at the cathode and small amounts
of either oxygen or chlorine gas (depending on composition of the E stream and
end ion exchange membrane arrangement) at the anode.
• These gases are combined to maintain a neutral pH and discharged or
recirculated to a separate E tank.
• hydrogen gas may be collected for use in energy production.
• ED systems can be operated as continuous or batch production processes
• Continuous process, feed is passed through a sufficient number of stacks placed
in series to produce the final desired product quality.
• Batch processes, the diluate and/or concentrate streams are re-circulated through
the ED systems until the final product or concentrate quality is achieved

Electrodialysis Reversal
• An improvement to ED, referred to as Electrodialysis Reversal
(EDR)
• EDR utilizes the same concept of ED except that there is periodic
automatic reversal of polarity and cell function to reverse the flow
of ions across the membrane.
• This returns anions across the anionic membranes and helps break
up scale formed on the concentrating face of the membranes and
minimizes membrane fouling.

Application
Some applications of electrodialysis include:
• Large scale brackish and seawater desalination and salt production or Small and
medium scale drinking water production
• Water reuse (e.g., industrial laundry wastewater, produced water from oil/gas
production, cooling tower makeup & blowdown, metals industry fluids.
• Pre-demineralization (e.g., boiler makeup & pretreatment, ultrapure water
pretreatment, process water desalination, power
generation, semiconductor, chemical manufacturing, food and beverage)
• Food processing
• Agricultural water
• Glycol desalting (e.g., antifreeze / engine-
coolants, capacitor electrolyte fluids, oil and gas dehydration, conditioning and
processing solutions, industrial heat transfer fluids, secondary coolants from
heating, venting, and air conditioning.
• Glycerin Purification

Example Treatment Train
• The conventional EDR treatment train typically includes raw water
pumps, debris screens, rapid mix, slow mix flocculator, basin or
clarifier, gravity filters, EDR membranes, chlorine disinfection, and clearwell
storage.
• Microfiltration (MF) could be used in place of flocculation, sedimentation, and
filtration.

Benefits
• ED and EDR can operate with minimal fouling or scaling, or chemical addition.
• Low pressure requirements.
• ED and EDR facilities are quieter than RO.
• Long membrane life expectancy.
• Unaffected by non-ionic sealants such as silica1.
• Low chemical usage for pretreatment1.
• Ability to treat feed water with higher SDI, TOC and silica concentrations, and
more turbidity than RO.
• Can operate with up to 0.5 ppm of free chlorine in the feed water to control the
biological matter in the feed water.

Maintenance
• ED membranes are durable, can run under a wide range of pH conditions (pH 2
– 11), and endure high temperatures during cleaning.
• They can be removed from the unit and scrubbed if necessary.
• If operated properly, membranes have an average life of 12 to 15 years.
• Solids can be flushed out by turning the power off and letting water circulate
through the stack.
• The ED stack must be disassembled, mechanically cleaned, and reassembled at
regular intervals.
• They can also be cleaned using a 5% hydrochloric acid solution

Limitations
• Non-charged, higher molecular weight, and less mobile ionic species will not
typically be significantly removed.
• less economical when low salt concentrations in the product are required
• large membrane areas are required to satisfy capacity requirements for low
concentration feed solutions.
• ED systems require feed pretreatment to remove species coat, precipitate onto, or
foul the surface of the ion exchange membranes.
• Fouling decreases the efficiency of the ED system. Species of concern
include calcium and magnesium hardness, suspended solids, silica, and organic
compounds.
• Water softening can be used to remove hardness, and micrometre or
multimedia filtration can be used to remove suspended solids.
• EDR systems seek to minimize scaling by periodically reversing the flows of
diluate and concentrate and polarity of the electrodes.

Reverse Osmosis
Principles of Reverse Osmosis
A. Osmosis:
• Osmosis is the natural passage or diffusion of a solvent such as water through a
semi-permeable membrane from a week solution to a stronger solution. This
natural phenomena is explained in many ways as follows:
• The movement is due to the difference in the vapor pressure of the two solutions
separated by the membrane.
• The vapor pressure of the pure solvent is higher than that of the solution with
dissolved solids. Thus the solvent moves from the higher pressure to the lower
pressure side.
• Others say that the solvent moves from the less concentrated (higher-potential)
solution to the more concentrated (lower-potential) one to reduce the solution
concentration.

Osmosis
• The solvent continues to move and water rises in the concentrated solution side
to a level with a hydrostatic pressure ( Δπ ) equivalent to the difference in vapor
pressure of two solutions. At this level the system is said to be at equilibrium.
• ( Δπ ) is called the Osmotic Pressure ( is the driving force for osmosis to occur).
• The osmotic pressure of a solvent depends on many factors such as the
characteristics of the solvent, the dissolved solids concentration, and
temperature.
• The osmotic pressure of any solution can be approximated by the following
equation:
• c = summation of the molar concentration of the dissolved ions
• R = Universal gas constant (82.05746 cm3 atm K−1 mol−1 or 8.314 J/K.mol)
T = Temperature in degrees Kelvin.
• ci = concentration ion (g/m3)
• Mi = molecular weight ion (g/mol)
• zi = valence ion (-)

Osmosis
In osmosis, water moves across the
membrane from the dilute to the
concentrated solution

Reverse Osmosis (RO)
B. Reverse Osmosis (RO):
• Reverse Osmosis is the forced passage of a solvent (e.g. water) through a
membrane against the natural osmotic pressure to accomplish separation of the
solvent from a solution of dissolved solids.
• If a pressure equal to the Osmotic pressure ( Δπ ) is applied to the side of higher
salt content, the water flow from lower to higher salt concentration will stop.
• If an additional pressure is exerted the water flow will be reversed and to the
direction from high to low salt concentration producing fresh water.
• The membrane allow the passage of the solvent while blocking the passage of
salt ions. The salt ions or the dissolved matter are called Solutes.
• Some salts move with water since each membrane has a rejection efficiency that
is less than 100%.

Reverse Osmosis Applied
In reverse osmosis, pressure is applied
to the concentrated solution reversing
the natural direction of flow, forcing
water across the membrane from the
concentrated solution into the more
dilute solution

Principle Reverse Osmosis (RO)

Reverse Osmosis Treatment (RO)

Osmotic pressure Calculation
• Example 2: In water from surface water (18o), the following ions are present
at the given concentrations:
• [HCO3-] 135 g/m3 M = 61.0 g/mol
• [SO42-] 63 g/m3 M = 96.1 g/mol
• [Cl-] 95 g/m3 M = 35.5 g/mol
• [Na+] 52 g/m3 M = 23.0 g/mol
• [Ca2+] 60 g/m3 M = 40.1 g/mol
• [Mg2+] 11 g/m3 M = 24.3 g/mol
• R= 8.314 J/K.mol
• Calculate the osmotic pressure of the water.
Example 3
Why is the osmotic pressure in the concentrate higher than in the feed?

Solution
Notes: By comparison, the osmotic pressure of brackish groundwater (2000 mg/l
NaCl) is 1.7 x 105 Pa (= 1.7 bar), the osmotic pressure of sea water (35.000 mg/l
NaCl) is 30 x 105 Pa (= 30 bar).
Example 3: Answer
The feed is separated into permeate and concentrate flows. The concentrate flow
contains the same amount of salts as the feed flow, however, they are dissolved in
less water. A higher salt concentration means a higher osmotic pressure.
Example 2: Answer

Osmotic pressure Calculation
atm = 14.6959488 Psi (pound per square inch

Theory
Mass balance
The water mass balance for a membrane element is given by:
Qf = Qc + Qp
in which:
• Qf = feed flow (m3/h)
• Qc = concentrate flow (m3/h)
• Qp = permeate flow (m3/h)
Also, the dissolved material of mass balance can be derived by:
QfCf = QcCc + QpCp
in which:
• Cf = concentration of dissolved material in feed water (g/m3)
• Cc = concentration of dissolved material in con-centrate (g/m3)
• Cp = concentration of dissolved material in per-meate (g/m3)

Recovery
• The recovery indicates the overall production of the system. It is the relationship
between permeate and feed flow (water recovery rate):
• A recovery of 80% means that 80% of the feed flow is produced as permeate.
This also means that the concentration of salts in the concentrate is 5 times
higher than the concentration in the feed flow, assuming that all salts are
retained.
• The recovery of one element is between 1 and 10%, therefore more elements
should be placed in a series to obtain the desired recovery of 80%.
• For sea water desalination, the maximum achievable recovery is about 50%. This
recovery is limited by the possibility of scaling caused by high salt
concentrations.
• For groundwater, however, recoveries up to 95% can be obtained.

Rejection
• RO has a very high efficiency of inorganic chemicals.
• However, it also has a very high efficiency in removing dissolved organic
matter 90-99%, but it is preferred to remove these materials using other methods
such as carbon adsorption. This is due to the fouling nature of organic matter.
• RO is capable of removing more than 99% of microorganisms.
Rejection indicates the amount of material rejected by a membrane. Rejection is
calculated by:
in which:
• R = solute rejection rate
X 100

RO Contaminant Rejection efficiency

Estimate Quantity and Quality of Waste Stream From
Reverse Osmosis Facility
Example 4: Estimate Quantity and Quality of the Waste Stream, and total quantity
of water that must be processed from reverse osmosis facility that is produce
4000 m3/d of water to be used for industrial cooling operations. Assume that
both the recovery and rejection rates are equal to 90 % and that concentration of
feed stream is 400 g/m3
Answer:
1. Determine the flow rate of the concentrated waste stream and the total amount of
water that must be processed
• Qc = Qp (1-r)/r
• Qc = (4000 m3/d)(1-0.9)/0.9 = 444 m3/d
2. Total amount of water that must be processed to produce 4000 m3/d of RO water
• Qf = Qp + Qc = 4000 m3/d + 444 m3/d = 4444 m3/d
3. Determine the concentration of the permeate stream.
• Cp = Cf (1-R) = 400 g/m3 (1-0.9) = 40 g/m3
4. Determine the concentration of concentrated waste stream
• Cc = (QfCf – QpCp) / Qc
• Cc = {(4444 m3/d)(400 g/m3) – (4000 m3/d)(40g/m3)} / (444 m3/d)
• Cc = 3643 g/m3

Checking the need for pretreatment
• RO membrane may be fouled by many ways, such as colloidal mater, bacterial
activity, iron and manganese, chlorine, scale from calcium carbonate.
• Particulate matter will be retained and is an ideal nutrient for biomass, resulting
in biofouling.
• Both fouling processes (scaling and biofouling) should be avoided as much as
possible to efficiently operate reverse osmosis.
• Many parameters have been used to assess the need for pretreatment of feed
water before the RO units.
• The most common indexes are the silt density index (SDI), and the modified
fouling index (MFI).
• Fouling indexes are determined from simple membrane tests. The samples are
passed through a 0.45μm Millipore filter at a gage pressure of 210 kPa . The
time needed for the test varies between 15min-2hr depending on the fouling
nature of the water. The same equipment is used for the two indexes.

SDI Calculation
Where
Q = average flow, L/s
A = constant
MFI = modified field fouling index, s/L2
V = volume of water filtered in the test
The equation of the MFI

Example 4: Determine the silt density index for a proposed feed water from the
following test data. If a spiral wound RO membrane is to be used, will
pretreatment be required?
• Total run time = 30 min
• Initial 500 mL = 2 min
• Final 500 mL = 10 min
Answer:
• SDI = 100{(1 – (ti/tf)} / t
• SDI = 100{1 – (2/10)}/30 = 2.67
Compare the SDI to the acceptable criteria
Calculated SDI value 2.67 is less than 3; therefore no further pretreatment would be
needed normally. As a practical matter, because the SDI value is close to 3.0 it
may prudent to consider some of pretreatment to prolong the filtration cycle

Nanofiltration
• It is not always necessary to remove all dissolved ions. For example, when water
has to be softened nanofiltration will be sufficient.
• Nanofiltration removes divalent ions (like Ca2+ , Mg2+ and SO42-), while
monovalent ions are not rejected.
• Nanofiltration membranes have larger pores than reverse osmosis membranes,
resulting in a lower resistance for filtration and also lower operational pressures
(2 - 10 bar).
• The pores of nanofiltration are smaller than ultrafiltration pores.
• Nanofiltration membrane modules can be constructed as spiral-wound
membranes, and now as capillary membranes as well.

Softening
• Due to the long residence time in the subsoil, groundwater is in chemical
equilibrium (i.e., calcium carbonate equilibrium).
• Groundwater comes in contact with the atmosphere when it is pumped up or
discharged into surface water. When carbon dioxide disappears from the water, it
is not in calcium carbonate equilibrium anymore.
• Also, when water is heated the equilibrium is changing, the Ca2+ and HCO3
- -
ions will precipitate in the form of calcium carbonate (CaCO3) leads to calcium
carbonate scaling (e.g., deposits in water boilers).
• To prevent precipitation of calcium carbonate at the customers‟ taps, calcium
ions are partially removed from the water by drinking water com-panies. This is
called softening.

Water quality and softening
• The hardness of water is classified from very soft to
very hard.
• The produced water always needs to comply with the
standards.
• The most important water quality parameters
influenced by softening are
acidity, hardness, bicarbonate, sodium, and the
solubility potential for metals like copper and lead.

• Acidity (pH): guideline = 8.0 < pH < 8.3
• Directly after the softening process, acidity of the water is higher than the
above-mentioned guideline. By means of pH-correction (acid dosing), pH is
decreased to the desired value.
• Hardness: guideline = 1.5 < hardness < 2.5 mmol/l
• Hardness is defined as the sum of the concentration of dissolved calcium and
magnesium ions.
• Bicarbonate concentration: guideline > 2 mmol/l
• The bicarbonate concentration should be higher than 2.0 mmol/l, resulting in
water with sufficient buffering capacity (pH stability);
• Sodium concentration: guideline = as low as possible
• Because sodium influences blood pressure and therefore, indirectly, heart and
vascular diseases, the concentration should not be higher than 120 mg/l.

• Solubility potential
• Softening also works to reduce the solubility of metals from pipe material. For
drinking water the most important metals are lead (Pb) and copper
(Cu), because these metals have a health impact.
• guideline Cu2+ < 2 mg/l - guideline Pb2+ < 0.01 mg/l
• The values for copper and lead solubility are determined by a pipe test. The pipe
test is performed in stagnant water and takes 16 hours. Empirical relationships
have been derived to give a rapid indication about the release:
• Cumax = copper dissolving capacity (mg/l)
• TAC = Total An organic Carbon (mmol/l)
• Pbmax = lead dissolving capacity (mg/l)
• SO4
2- = sulfate concentration (mmol/l)
• T = temperature (oC)
• TAC concentration can be influenced by the softening process (mainly caused
by the decrease in HCO3
-).

Softening processes
• Groundwater extracted from calcareous subsoils can have a high degree of hardness.
• Water extracted from deep sand layers is, on the other hand, fairly soft (0.5 mmol/l).
• The hardness of surface water is normally from 2.0 - 3.0 mmol/l.
• The hardness of water can be decreased by means of different processes:
1. Dosing of a base: (NaOH, Ca(OH)2 or Na2CO3 ) - Because of a shift in the
calcium carbonic acid equilibrium, spontaneous crystallization occurs. By dosing the
base in a reactor with seeding grains (sand), crystallization will occur on the surface
of the seeding grains, forming limestone pellets (softening in a pellet reactor)
2. Ion exchange: Ca and/or Mg are exchanged with other ions (Na is used the most).
3. Membrane filtration:the hardness is removed partly (nanofiltration) or fully (RO).

Softening in a treatment plant
• Softening installation (including post treatment) into an existing groundwater
treatment process, the following possibilities are considered:
• Softening of raw water
• Softening of aerated water
• Softening after rapid filtration
Softening of raw water
• If iron and manganese are present in dissolved form in the water (anaerobic
water), these substances will be trapped in the CaCO3 grains.
• The advantage of this is that the loading on the sand filters is reduced.
Disadvantage:
• CaCO3 grains become less pure
• high dosage of base is needed due to the high concentration of carbon dioxide
in raw water. So before the softening reaction starts, the carbon dioxide needs
to be converted to HCO3
- and CO3
2-.

Softening in a treatment plant
• Softening after an aeration:
• lower chemical dose will be sufficient, because some of the carbon dioxide is
removed during aeration.
• An additional (possible) cost advantage of softening (aerated) raw water is
that, in many cases, existing filters that have been used for iron and manganese
removal up till this point, can also be applied as „carry-over‟ filters. When
softening after filtration is applied, the purest pellets are formed. Iron and
manganese are removed by the filters.
• A disadvantage is that after softening a new (expensive) rapid filtration step must
be added to the process to remove the „carry-over.‟

Theory / Equilibrium
• The calcium carbonic acid equilibrium determines whether calcium carbonate
precipitates:
• In groundwater abstracted a high concentration of calcium ions is present.
• Given that there is no CO3
2- in the water , calcium does not precipitate but
remains in dissolved form in the water, resulting in water with a high degree of
hardness.
• This reaction only happens if sufficient CO3
2- ions are present:
Ca2+ + CO3
2- = CaCO3
• To realize this the pH has to be increased.
• The equilibrium reactions of carbonic acid shift to the right and the CO3
2-
concentration increases (carbonic acid equilibrium):
CO2 + 2H2O = H3O+ + HCO3
-
H2O + HCO3
- = H3O+ + CO3
2-

Theory / Equilibrium
• The increase of pH is realized by adding a base solution that binds the H3O+ -
ions.
• Sodium hydroxide (NaOH caustic soda ) is used to achieve this.
• The more NaOH is added, the more the pH rises and the stronger the softening.
• The above-mentioned reactions are irreversible; in reality, equilibrium will be
set.
• Also for the bases Ca(OH)2 and Na2CO3, similar reaction equations can be
formulated.
• By dosing a base, the carbonic acid equilibrium shifts to the left, forming
calcium carbonate. The Saturation Index exceeds 1. At a similar
SI, crystallization of calcium carbonate occurs, forming a deposit on the seeding
grains present in the reactors.

Langelier Saturation Index (LSI)
• LSI is a calculated number used to predict the calcium carbonate stability of
water.
• It indicates whether the water will precipitate, dissolve, or be in equilibrium with
calcium carbonate.
• In 1936, Wilfred Langelier developed a method for predicting the pH at which
water is saturated in calcium carbonate (called pHs).
• The LSI is expressed as the difference between the actual system pH and the
saturation pH: LSI = pH (measured) - pHs
• For LSI > 0, water is super saturated and tends to precipitate a scale layer of
CaCO3.
• For LSI = 0, water is saturated (in equilibrium) with CaCO3. A scale layer of
CaCO3 is neither precipitated nor dissolved.
• For LSI < 0, water is under saturated and tends to dissolve solid CaCO3.
• If the actual pH of the water is below the calculated saturation pH, the LSI is
negative and the water has a very limited scaling potential.
• If the actual pH exceeds pHs, the LSI is positive, and being supersaturated with
CaCO3, the water has a tendency to form scale.
• At increasing positive index values, the scaling potential increases.

• In practice, water with an LSI between - 0.5 and + 0.5 will not display
enhanced mineral dissolving or scale forming properties.
• Water with an LSI below -0.5 tends to exhibit noticeably increased dissolving
abilities
• water with an LSI above +0.5 tends to exhibit noticeably increased scale
forming properties.
• LSI is temperature sensitive. The LSI becomes more positive as the water
temperature increases.
• This has particular implications in situations where well water is used. The
temperature of the water when it first exits the well is often significantly lower
than the temperature inside the building served by the well or at the laboratory
where the LSI measurement is made. This increase in temperature can cause
scaling, especially in cases such as hot water heaters.

• LSI is defined as: : LSI = pH (measured) - pHs
• Where: pH is the measured water pH
• pHs is the pH at saturation in calcite or calcium carbonate and is defined as:

• As an example, suppose the drinking water supplied to animals has the following analysis:
• pH = 7.5
• TDS = 320 mg/L
• Calcium = 150 mg L-1 (or ppm) as CaCO3
• Alkalinity = 34 mg L-1 (or ppm) as CaCO3
• The LSI index is calculated at two temperatures, i.e. 25oC (room temperature) and 82oC (cage wash
cycle). The colder incoming water will warm to room temperature in the manifolds. Residual water in the
rack manifold can be heated to 82oC when the rack is in the cage washer.
• LSI Formula: LSI = pH - pHs
• pHs = (9.3 + A + B) - (C + D) where:
• A = (Log10[TDS] - 1)/10 = 0.15
• B = -13.12 x Log10(oC + 273) + 34.55 = 2.09 at 25 C and 1.09 at 82 C
• C = Log10[Ca2+ as CaCO3] - 0.4 = 1.78
• D = Log10[alkalinity as CaCO3] = 1.53
• Calculation at 25oC:
• pHs = (9.3 + 0.15 + 2.09) - (1.78 + 1.53) = 8.2
• LSI = 7.5 - 8.2 = - 0.7
• Hence No Tendency to Scale
• Calculation at 82oC:
• pHs = (9.3 + 0.15 + 1.09) - (1.78 + 1.53) = 7.2
• LSI = 7.5 - 7.2 = + 0.3
• Hence Slight Tendency to Scale

Background: Current Methods of
Disinfection
• Large-Scale:
– Chlorination
– Ozone
– UV irradiation
• Small Scale:
– Boiling
– Iodine tablets
– Filters

Use of Disinfectants as Chemical Oxidants
Oxidation is a chemical reaction where electrons are transferred from one species
(the reducer) to another species (the oxidant)
Disinfectants are used for more than just disinfection in drinking water treatment.
While inactivation of pathogenic organisms is a primary
function, disinfectants are also used oxidants in drinking water treatment for
several other functions:
1. Minimization of Disinfection Byproducts formation : Several strong
oxidants, including potassium permanganate and ozone, may be used to control
DBP
2. Prevention of re-growth in the distribution system and maintenance of
biological stability;
– Removing nutrients from the water prior to distribution;
– Maintaining a disinfectant residual in the treated water; and

Continue: Use of Disinfectants as Chemical Oxidants
3. Removal of color: Free chlorine is used for color removal. A low pH is
favored. Color is caused by humic compounds, which have a high potential
for DBP formation
4. Improvement of coagulation and filtration efficiency;
a. Oxidation of organics into more polar forms;
b. Oxidation of metal ions to yield insoluble complexes such as ferric iron
complexes;
c. Change in the structure and size of suspended particles.
5. Oxidation is commonly used to remove taste and odor causing
compounds. Because many of these compounds are very resistant to
oxidation, advanced oxidation processes (ozone/hydrogen
peroxide, ozone/UV, etc.) and ozone by itself are often used to address taste
and odor problems. The effectiveness of various chemicals to control taste
and odors can be site-specific.

Continue: Use of Disinfectants as Chemical Oxidants
6. Removal of Iron and Manganese
7. Prevention of algal growth in sedimentation basins and filters:
Prechlorination will prevent slime formation on filters, pipes, and tanks, and
reduce potential taste and odor problems associated with such slimes.
Manganese (II)
(mg/mg Mn)
Iron (II)
(mg/mg Fe)
Oxidant
0.770.62Chlorine Cl2
2.451.21Chlorine Dioxide, ClO2
0.88*0.43Ozone, O3
0.29014Oxygen, O2
1.920.94Potassium Permanganate,
KMnO4
Source: Culp/wesner/Culp, Langlais et al., 1991
Optimum pH manganese oxidation using ozone is 8-8.5 source Reckhow et al.,

Factors affecting disinfection effectiveness
• Time
• pH
• Temperature
• Concentration of the disinfectant
• Concentration of organisms
• Nature of the disinfectant
• Nature of the organisms to be inactivated
• Nature of the suspending medium

CT Factor
• One of the most important factors for determining or predicting the germicidal
efficiency of any disinfectant is the CT factor, a version of the Chick-Watson
law (Chick, 1908; Watson, 1908).
• The CT factor is defined as disinfectant contact time, the mathematical
product of C x T, where C is the residual disinfectant concentration measured
in mg/L, and T is the corresponding contact time measured in minutes.
• CT values for chlorine disinfection are based on a free chlorine residual.
• Chlorine is less effective as pH increases from 6 to 9.
• In addition, for a given CT value, a low C and a high T is more effective than
the reverse (i.e., a high C and a low T).
• For all disinfectants, as temperature increases, effectiveness increases.

CT Values for Inactivation of Viruses
Various levels of deactivation can be achieved. This is often expressed as a log
reduction:
log reduction = 90% deactivation
log reduction = 99% deactivation
log reduction = 99,9% deactivation
log reduction = 99,99% deactivation

Chick's Law
• The death of microorganisms is first order with respect to time
• Thus, the remaining number of viable microorganisms in
water, N, decreases with time, t, according to:
where k is an empirical constant descriptive of the microorganism, pH and
disinfectant used.
• Integrating with respect to time, and replacing limits (N = No at t = 0)
yields:
• Integrating with respect to time, and replacing limits (N=No a t = 0) yields:

Chick-Watson Law
• Incorporates the Concentration-time Product concept into Chick's Law.
Disinfection can be modeled using the Chick-Watson Law:
where
• N = number of microorganisms at time t
• N0 = number of microorganisms at time = 0
• k = inactivation constant
• C = disinfectant concentration, mol/L
• n = constant of dilution, usually close to 1.0
• t = time, min
tkC
N
N n
0
ln

Chick Law
Example. The specific lethality of hypochlorous acid is
5L.min/mg for the Polio virus. Determine the time
required to obtain 99.99% inactivation using 1.0 mg/L of
HOCl.
Solution: N/No = 1 - Removal = 1 - .9999 = 0.0001

N/Not (sec)
0.076904
0.006338
0.0005012Solution:
1. Plot the -Ln (N/N0) against tine
2. Execute linear regression for
experimental points. This yields
the slope of the line (k = 0.634/s).
3. The time required for a 10,000 fold
reduction is:
The following table shows the disinfection of poliomyelitis virus using hypobromite as a
disinfectant. Determine Chick's constant and the time required to reduce the
concentration of viable polio virus to 1/10,000 of the original concentration
Example

Chlorine
Chlorine has many attractive features that contribute to its wide use in the industry.
Four of the key attributes of chlorine are that it:
• Effectively inactivates a wide range of pathogens commonly found in water;
• Leaves a residual in the water that is easily measured and controlled;
• Is economical; and
• Has an extensive track record of successful use in improving water treatment
operations
There are, however, some concerns regarding chlorine usage that may impact its
uses such as:
• Chlorine reacts with many naturally occurring organic and inorganic
compounds in water to produce undesirable DBPs;
• Hazards associated with using chlorine, specifically chlorine gas, require
special treatment and response programs; and
• High chlorine doses can cause taste and odor problems.

Chlorine purposes in water treatment
• Taste and odor control;
• Prevention of algal growths;
• Maintenance of clear filter media;
• Removal of iron and manganese;
• Destruction of hydrogen sulfide;
• Bleaching of certain organic colors;
• Maintenance of distribution system water quality by
controlling slime growth;
• Restoration and preservation of pipeline capacity;
• Restoration of well capacity, water main sterilization; and
• Improved coagulation by activated silica.

Chlorine Chemistry
• Chlorine gas hydrolyzes rapidly in water to form hypochlorous acid
• Hypochlorous acid is a weak acid (pKa of about 7.5), meaning it dissociates
slightly into hydrogen and hypochlorite ions
• Between a pH of 6.5 and 8.5 this dissociation is incomplete and both HOCl
and OCl- species are present to some extent (White, 1992). Below a pH of 6.5,
no dissociation of HOCl occurs, while above a pH of 8.5, complete
dissociation to OCl- occurs.
• As the germicidal effects of HOCl is much higher than that of OCl-,
chlorination at a lower pH is preferred.

Effect of pH on relative amount of hypochlorous acid
and hypochlorite ion at 20 C.

Commonly Used Chlorine Sources
Sodium hypochlorite and calcium hypochlorite are the most common sources of
chlorine used for disinfection of onsite water supplies.
• Sodium Hypochlorite (common household bleach)
• Sodium hypochlorite is produced when chlorine gas is dissolved in a sodium
hydroxide solution.
• Clear to slightly yellow colored liquid with a distinct chlorine odor.
• Common laundry bleach - 5.25 to 6.0 percent available chlorine, when bottled.
• Do not use bleach products that contain additives such as
surfactants, thickeners, stabilizers, and perfumes.
• Always check product labels to verify product content and use instructions.

Sodium Hypochlorite (common household bleach)
• Higher concentrations of chlorine in sodium hypochlorite solutions are
generally not available.
• Above 15 percent, the stability of hypochlorite solutions is poor, and
decomposition and the concurrent formation of chlorate is of concern
• Sodium hypochlorite solutions are of an unstable nature due to high rates of
available chlorine loss
• Over a period of one year or less, the amount of available chlorine in the
storage container may be reduced by 50 percent or more.
• Solutions more than 60 days old should not be counted upon to contain the full
amount of available chlorine originally in solution
• Swimming pool chlorine - 10.0 to 12.0 percent available chlorine.

Sodium Hypochlorite (common household bleach)
• The stability of hypochlorite solutions is greatly affected by
heat, light, pH, initial chlorine concentration, length of storage, and the
presence of heavy metal cations
• These solutions will deteriorate at various rates, depending upon the specific
factors:
1. The higher the concentration, the more rapidly the deterioration.
2. The higher the temperature, the faster the rate of deterioration.
3. The presence of iron, copper, nickel, or cobalt catalyzes the deterioration
of hypochlorite. Iron is the worst offender

Commonly Used Chlorine Sources
• Calcium hypochlorite is formed from the precipitate that results from
dissolving chlorine gas in a solution of calcium oxide (lime) and sodium
hydroxide.
Calcium Hypochlorite
• Dry white powder, granules, or tablets - 60 to 70 percent available chlorine
- 12-month shelf life if kept cool and dry - If stored wet, looses chlorine
rapidly and is corrosive.
• A chlorine test kit should be used to check the final chlorine residual in a
prepared chlorine solution to assure that you have the concentration
intended.

Which is Best, Sodium Hypochlorite or
Calcium Hypochlorite?
• Sodium hypochlorite is more effective
• This may be associated with the quality of the ground water in the well being treated
rather than with the source of the chlorine itself.
• If there is an abundance of calcium based materials in both bedrock wells. Calcium
hypochlorite already has a high concentration of calcium.
• At 180 ppm of hardness, water is saturated with calcium to the point that it
precipitates out of the solution, changing from the dissolved state to a solid state.
• Introducing a calcium hypochlorite solution into a calcium rich aquifer can cause the
formation of a calcium carbonate (hardness) precipitate that may partially plug off
the well intake.
• Sodium hypochlorite does not have the tendency to create the precipitate.
• If the calcium carbonate concentration in the ground water is above 100 ppm
(mg/l), the use of sodium hypochlorite is recommended instead of calcium
hypochlorite.

Typical Chlorine Dosages at Water Treatment Plants

Chlorine Added
Initial chlorine concentration added to water
Chlorine Demand
Reactions with organic
material, metals, other
compounds present in water
prior
to disinfection
Total Chlorine
Remaining chlorine
concentration after chlorine
demand of water
Free Chlorine
Concentration of chlorine
available for disinfection
Combined Chlorine
Concentration of chlorine
combined with nitrogen in the
water and unavailable for disinfection
Chlorine Addition Flow Chart

DISINFECTANT DEMAND REACTIONS
• Reactions with Ammonia
• In the presence of ammonium ion, free chlorine reacts in a stepwise manner to
form chloramines
• monochloramine (NH2Cl), dichloramine (NHCl2 ), and trichloramine
(NCl3), each contribute to the total (or combined) chlorine residual in a water.
• The terms total available chlorine and total oxidants refer, respectively, to the
sum of free chlorine compounds and reactive chloramines, or total oxidating
agents.
• Under normal conditions of water treatment, if any excess ammonia is
present, at equilibrium the amount of free chlorine will be much less than 1
percent of total residual chlorine.

Chlorine residual
• Chlorine persists in water as „residual‟ chlorine after dosing and this helps to minimize
the effects of re-contamination by inactivating microbes which may enter the water
supply after chlorination. It is important to take this into account when estimating
requirements for chlorination to ensure residual chlorine.
• The level of chlorine residual required varies with type of water supply and local
conditions.
• In water supplies which are chlorinated there should always be a minimum of 0.5mg/l
residual chlorine after 30 minutes contact time in water.
• Where there is a risk of cholera or an outbreak has occurred the following chlorine
residuals should be maintained:
– At all points in a piped supply 0.5mg/l
– At standposts and wells 1.0mg/l
– In tanker trucks, at filling 2.0mg/l
• In areas where there is little risk of a cholera outbreak, there should be a chlorine
residual of 0.2 to 0.5 mg/l at all points in the supply. This means that a chlorine residual
of about 1mg/l when water leaves the treatment plant is needed.

Combined Chlorine
What is it?
• Free chlorine that has combined with ammonia (NH3) or other nitrogen-
containing organic substances.
• Typically, chloramines are formed .
Where does NH3, etc come from?
• Present in some source waters (e.g., surface water).
• Contamination; oxidation of organic matter
• Some systems (about 25% of U.S. water supplies) actually Add ammonia.
Why would you want to Add ammonia?
• Chloramines still retain disinfect capability (~5 % of FAC, Free Available
Chlorine)
• Chloramines not powerful enough to form THMs.
• Last a lot longer in the mains than free chlorine,
– Free chlorine + Combined chlorine = Total Chlorine Residual
• Can measure “Total” Chlorine
• Can measure “Free” Chlorine
• Combined Chlorine can be determined by subtraction

pH Effect on Chlorine
• Chlorine is a more effective disinfectant at pH levels between 6.0 and 7.0, because
hypochlorous acid is maximized at these pH levels
• Any attempt to disinfect water with a pH greater than 9 to 10 or more will not be
very effective.
• The pH determines the biocidal effects of chlorine.
• Chlorine will raise the pH when added to water.
• By increasing the concentration of chlorine, and subsequently raising the pH, the
chlorine solution is actually less efficient as a biocide.
• Controlling the pH of the water in the aquifer is not practical. However buffering or
pH-altering agents may be used to control pH in the chlorine solution being placed
in the well.

Free Chlorine Distribution with pH

Temperature Effect on Chlorine
• As temperatures increase, the metabolism rate of microorganisms
increases.
• With the higher metabolic rate, the chlorine is taken into the microbial cell
faster, and its bactericidal effect is significantly increased.
• The higher the temperature the more likely the disinfection will produce
the desired results.
• Virus studies indicate that the contact time should be increased by two to
three times to achieve comparable inactivation levels when the water
temperature is lowered by 10°C (Clarke et al., 1962).
• Steam injection has been used to elevate temperatures in a well and the area
surrounding the Well bore

Contact time
• Time is required in order that any pathogens present in the water are inactivated.
• The time taken for different types of microbes to be killed varies widely.
• it is important to ensure that adequate contact time is available before water
enters a distribution system or is collected for use
• In general, amoebic cysts are very resistant and require most exposure.
Bacteria, including free-living Vibrio cholerae are rapidly inactivated by free
chlorine under normal conditions.
• For example, a chlorine residual of 1mg/l after 30 minutes will kill
schistosomiasis cercariae, while 2mg/l after 30 minutes may be required to
kill amoebic cysts.
• Contact time in piped supplies is normally assured by passing the water, after
addition of chlorine, into a tank from which it is then abstracted.
• In small community supplies this is often the storage reservoir (storage tank). In
larger systems purpose-built tanks with baffles may be used. These have the
advantage that they are less prone to "short circuiting" than simple tanks.

Germicidal Efficiency of Chlorine
• The major factors affecting the germicidal efficiency of the free chlorine
residual process are: chlorine residual concentration - contact time – pH -
water temperature.
• Increasing the chlorine residual, the contact time, or the water temperature
increases the germicidal efficiency. Increasing the pH above 7.5 drastically
decreases the germicidal efficiency of free chlorine.
• Chlorine dissolved in water, regardless of whether sodium hypochlorite or
calcium hypochlorite is used as the source of the chlorine, generally exists in
two forms, depending on the pH of the water:
- HOCl - hypochlorous acid (biocidal)
- OCl - hypochlorite ion (oxidative)
• Hypochlorous acid is the most effective of all the chlorine residual fractions
• Hypochlorous acid is 100 times more effective as a disinfectant than the
hypochlorite ion

Distribution Diagram for Chloramine Species
with pH****

Breakpoint Chlorination****
• The type of chlorine dosing
normally applied to piped water
supply systems is referred to as
breakpoint chlorination.
Sufficient chlorine is added to
satisfy all of the chlorine
demand and then sufficient
extra chlorine is added for the
purposes of disinfection.
• As the applied Cl2:N ratio
increases from 5:1 to
7.6:1, breakpoint reaction
occurs, reducing the residual
chlorine level to a minimum.
• Breakpoint chlorination results
in the formation of nitrogen
gas, nitrate, and nitrogen
chloride.
• At Cl2:N ratios above 7.6:1, free
chlorine and nitrogen
trichloride are present.
Distilled water and rainwater (no Cl2 demand) will not show a
breakpoint.
Breakpoint

Breakpoint- why should we care?****
The importance of break-point chlorination lies in
the control of:
The killing power of chlorine on the right side of
the break point is 25 times higher than that of the
left side
taste,
odor,
and increased germicidal efficiency.
Complaints of “chlorine” odor and “burning eyes”
from pools/ spas that people usually attribute to
over-chlorination is actually due to chloramines!
(i.e. UNDER-chlorination)

The “Breakpoint”…another look****
Chlorine is
reduced to
chlorides by
easily
oxidizable
stuff (H2S,
Fe2+, etc.)
Chloramines
broken down
& converted to
nitrogen gas
which leaves
the system
(Breakpoint).
Cl2 consumed
by reaction
with organic
matter. If
NH3 is
present,
chloramine
formation
begins.
At this
point,THM
formation
can occur

• Measure Free and Total chlorine
• Bump up chlorinator to increase chlorine dose a certain
known amount
• On the following day, re-test Free and Total chlorine.
• If Total increases but Free does not, you are NOT at
breakpoint.
• Repeat process until both Total and Free chlorine increase
similarly upon adjustment
Ensuring you are at Breakpoint

1. Chlorine is a health concern at certain levels of exposure.
2. Drinking water containing chlorine well in excess of drinking
water standards could cause irritating effects to eyes and nose.
3. Some people who drink water containing chlorine well in
excess of standards could experience stomach discomfort.
4. Drinking water standards for chlorine protect against the risk
of these adverse effects.
5. Little or no risk with drinking water that meets the drinking
water standard level and should be considered safe with
respect to chlorine.
Can you have too much chlorine?****

Advantages and Disadvantages of
Chloramine Use****
Advantages
• Chloramines are not as reactive with organics as free chlorine in forming
DBPs.
• The monochloramine residual is more stable and longer lasting than free
chlorine or chlorine dioxide, thereby providing better protection against
bacterial regrowth in systems with large storage tanks and dead end water
mains. However excess ammonia in the network may cause biofilming.
• Because chloramines do not tend to react with organic compounds, many
systems will experience less incidence of taste and odor complaints when
using chloramines.
• Chloramines are inexpensive.
• Chloramines are easy to make.

Advantages and Disadvantages of
Chloramine Use ****
Disadvantages
• The disinfecting properties of chloramines are not as strong as other
disinfectants, such as chlorine, ozone, and chlorine dioxide.
• Chloramines cannot oxidize iron, manganese, and sulfides.
• When using chloramine as the secondary disinfectant, it may be necessary to
periodically convert to free chlorine for biofilm control in the water
distribution system.
• Excess ammonia in the distribution system may lead to nitrification
problems, especially in dead ends and other locations with low disinfectant
residual.
• Monochloramines are less effective as disinfectants at high pH than at low
pH.
• Dichloramines have treatment and operation problems.
• Chloramines must be made on-site.

Chlorine Residual Testing****
The presence of chlorine residual in drinking water indicates
that:
• a sufficient amount of chlorine was added initially to the
water to inactivate the bacteria and some viruses that cause
diarrheal disease; and,
• the water is protected from recontamination during storage.
The presence of free residual chlorine in drinking water is
correlated with the absence of disease-causing
organisms, and thus is a measure of the potability of water.

Analyze samples for chlorine immediately after
collection.
Free chlorine is a strong oxidizing agent; unstable in
natural waters.
It reacts rapidly with various inorganic compounds and
more slowly oxidizes organic compounds.
Factors including reactant concentrations,sunlight, pH,
and temperature influence decomposition of free
chlorine in water.
Avoid plastic containers  may have a large chlorine
demand.
Chlorine Sampling Issues****

Pretreat glass sample containers to remove any chlorine demand
 Soak in a dilute bleach solution for at least 1 hour
 Dilute bleach solution =1 mL bleach to 1 liter of deionized
water.
 Rinse thoroughly with deionized or distilled water.
Common error in chlorine testing is obtaining an unrepresentative
sample.
 If sampling from a tap, let the water flow for at least 5
minutes to ensure a representative sample.
 Let the container overflow with the sample several times,
then cap the sample containers so there is no headspace (air)
above the sample.
Chlorine Sampling Issues****

Restricted Water Use During
Chlorination****
1. Do not drink the water and avoid all body contact.
2. Water use should be minimized to assure that chlorine remains in
the well during the minimum contact period.
3. If strong chlorine odors are detected, ventilate the effected area
immediately, and minimize exposure to the fumes.
4. Avoid doing laundry, filling fish tanks, watering plants and using
water for other purposes where the chlorine may have an adverse
effect.

Dechlorination ****
• Dechlorination (removing residual chlorine from disinfected wastewater
prior to discharge into the environment/sensitive aquatic waters or in a
treated water to be lowered prior to distribution
• the chlorinated water can be dosed with a substance that reacts with or
accelerates the rate of decomposition of the residual chlorine.
• Compounds that may perform this function include thiosulfate, hydrogen
peroxide, ammonia, sulfite/bisulfite/sulfur dioxide, and activated carbon;
• Hydrogen peroxide is not frequently used because it is dangerous to handle
• only the latter two materials have been widely used for this purpose in
water treatment (Snoeyink and Suidan, 1975).
• (1) SO3
-2 + HOCl = SO4
-2 + Cl- + H+
• (2) SO3
-2 + NH2Cl+ H20 = SO4
-2 + Cl- + NH4
+
• On a mass basis, 0.9 parts sulfur dioxide (or 1.46 parts NaHSO3 or 1.34
parts Na2S2O5) is required to dechlorinate 1.0 part residual chlorine.

Dechlorination / ADVANTAGES AND
DISADVANTAGES ****
Advantages
• Protects aquatic life from toxic effects of residual chlorine.
• Prevents formation of harmful chlorinated compounds in drinking water
through reaction of residual chlorine with water born organic materials.
Disadvantages
• Chemical dechlorination can be difficult to control when near zero levels
of residual chlorine are required.
• Significant overdosing of sulfite can lead to sulfate formation, suppressed
dissolved oxygen content, and lower pH of the finished effluent.

Ozone Generation
• 3O2 2O3
• Ozone Generator

Primary Uses of Ozone
Ozone is used in drinking water treatment for a variety of purposes including:
• Disinfection;
• Inorganic pollutant oxidation, including iron, manganese, and sulfide;
• Organic micropollutant oxidation, including taste and odor compounds,
phenolic pollutants, and some pesticides; and
• Organic macropollutant oxidation, including color removal, increasing the
biodegradability of organic compounds, DBP precursor control, and reduction of
chlorine demand.

Pathogen Inactivation and Disinfection Efficacy
• Ozone has a high germicidal effectiveness against a wide range of pathogenic
organisms including bacteria, protozoa, and viruses.
• Ozone cannot be used as a secondary disinfectant because the ozone residual
decays too rapidly.
• Ozone disinfection efficiency is not affected by pH although because of
hydroxyl free radicals and rapid decay, efficiency is the same but more ozone
should be applied at high pH to maintain “C”.
• Inactivation of bacteria by ozone is attributed to an oxidation reaction. The first
site to be attacked appears to be the bacterial membrane Also, ozone
disrupts enzymatic activity of bacteria
• The first site of action for virus inactivation, particularly its proteins and RNA
• aqueous ozone penetrates into the Giardia cysts wall and damages the
plasma membranes, additional penetration of ozone eventually affects the
nucleus, and ribosome

CT Values for Inactivation of Giardia
Cysts by Ozone (pH 6 to 9)

CT Values for Inactivation of Viruses by
Ozone (pH 6 to 9)

Advantages and Disadvantages of Ozone Use
Advantages
• Ozone is more effective than chlorine, chloramines, and chlorine dioxide for
inactivation of viruses, Cryptosporidium, and Giardia.
• Ozone oxidizes iron, manganese, and sulfides.
• Ozone can sometimes enhance the clarification process and turbidity removal.
• Ozone controls color, taste, and odors.
• One of the most efficient chemical disinfectants, ozone requires a very short
contact time.
• In the absence of bromide, halogen-substitutes DBPs are not formed.
• Upon decomposition, the only residual is dissolved oxygen.
• Biocidal activity is not influenced by pH.

Advantages and Disadvantages of Ozone Use
Disadvantages
• DBPs are formed, particularly by bromate and bromine-substituted
DBPs, in the presence of bromide, aldehydes, ketones.
• The initial cost of ozonation equipment is high.
• The generation of ozone requires high energy and should be generated on-site.
• Ozone is highly corrosive and toxic.
• Biologically activated filters are needed for removing assimilable organic
carbon and biodegradable DBPs.
• Ozone decays rapidly at high pH and warm temperatures.
• Ozone provides no residual.
• Ozone requires higher level of maintenance and operator skill

Fundamentals of UV
• History of UV.
• What is UV?
• Terms
• Concept of UV Dose
• Types of UV Reactors
• Inactivation mechanism
• Environmental Effects
• Advantages

Ultra Violet - Beyond the Violet
Cosmic
Rays
Gamma
Rays
X Rays
Ultra
Violet
Visible
Light
Infrared
Micro
Waves
Radio
Waves
Electromagnetic Spectrum
UV C UV B UV A Visible Light
Expanded Scale of Ultraviolet Radiation
Germicidal Wavelengths 245-285 nm

UV Dosage D = I · t
Where: D = UV Dose, mW s/cm2
I = Intensity, mW/cm2
t = Exposure time, s
Terms
• UV Output - Energy Delivered (W/lamp)
• UV Intensity - Rate of Energy Delivery (mW/cm
• UV Transmittance - Ability of water to transmit UV light
Terms and Dosage
Two main factors affect ultraviolet intensity:
WATER QUALITY - Water quality refers to the clarity of the water to be treated
and the degree to which it allows ultraviolet light to pass through it unobstructed
LAMP OUTPUT - Proper lamp output is easily maintained by regular cleaning of
the quartz sleeve that encases the lamp (generally every six months) and by lamp
replacement once per year.

Measuring UV Dosage
• Intensity (Irradiance)
– actinometry (instruments used to measure the heating power of radiation)
is a chemical system or physical device which determines the number of
photons in a beam integrally or per unit time.
– Radiometers or UV sensors
– Mathematical models
• Time: The time of exposure to ultraviolet light (retention time) is directly
related to the flow rate of water passing through the disinfection chamber. By
changing the retention time for a given ultraviolet intensity, the dosage can be
increased or decreased as needed.
• Bioassay Methods: (is a type of scientific experiment to measure the effects
of a substance on a living organism )

Lamp Construction
The lamps typically used in UV disinfection consist of a quartz
tube filled with an inert gas, such as argon, and small quantities of
mercury.
Tungsten Electrode Tungsten Electrode
Mercury Vapor and Argon Quartz Envelope

UV application
Path of UV Light in Water
• Lamp Encasement - Water Quality - Target Organism
Pretreatment
Raw
Water
Filtration UV
Reactor
To
Distribution
UV Treatment Application

UV Applications in Drinking Water
• Disinfection of Surface Water
• Disinfection of Ground Water
• Oxidation of Organic Chemicals
• Oxidation of NOM
• By Using UV methods: no natural physiochemical features of the
water are changed and no chemical agents are introduced into the
water.
• formation of THM or other DBPs with UV disinfection is
minimal

Inactivation Mechanism
• UV radiation is efficient at inactivating vegetative and sporous forms of
bacteria, viruses, and other pathogenic microorganisms.
• Electromagnetic radiation in the wavelengths ranging from 240 to 280nm
effectively inactivates microorganisms by irreparably damaging their nucleic
acid.
• The germicidal effects of UV light involve photochemical damage to RNA and
DNA within the microorganisms.
• DNA damage irreversible over time so UV contactors should be designed to
either shield the process stream or limit the exposure of the disinfected water to
sunlight immediately following disinfection.

Limitations of UV treatment
“Point” Disinfection
• UV units only kill bacteria at one point in a watering system and do not
provide any residual germicidal effect downstream. If just one bacterium
passes through unharmed (100% destruction of bacteria cannot be
guaranteed.
• Cells Not Removed
• Bacteria cells are not removed in a UV unit but are converted into pyrogens.
The killed microorganisms and any other contaminants in the water are a
food source for any bacteria that do survive downstream of the UV unit.
• Due to these limitations, the piping in a watering system treated by UV
disinfection will need to be periodically sanitized with a chemical
disinfectant.

UV Effectiveness
Crypto/Giardia - log
Bacteria - log
Virus - log
Dose Removal/Inactivation
(mJ/cm

• Effective for Crypto/virus
• No chemicals added to water
• Small footprint
• Pressurized system
• Cost Effective
• No DBPs
• Oxidize organic chemicals
Benefits of UV

Applicability of Alternative Disinfection
Techniques

Water Quality Control and Treatment Water Treatment

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