Water Resources Engineering (CVE 728)--Slide 4.pptx

Water Resources Engineering (CVE 728)
07/02/2025 1
Prepared by Ado Mohammed

Water Treatment
Water Treatment :-
1.It is the removal of impurities from the water,
which are harmful to the human health and
Environment.
2.It is the process for enhancing the quality of
water so that it meets water quality criteria for its
fitness for particular purpose.
3.Water treatment is the processes that makes
water more acceptable for an end-use, which may
be used for drinking, industry, or medicine.
07/02/2025 Prepared by Ado Mohammed 2

Water Treatment
Objectives of Water Treatment :-
1)To make water fit for various industrial & domestic uses such as
drinking, cooking, washing, etc.
2) To remove the unpleasant & objectionable color, odor & taste
from the water.
3) To remove obnoxious gases from the water.
4) To kill all the pathogenic germs, which are harmful to human
health.
5) To eliminate the tuberculating & corrosive properties of water
which affects the conduits and pipes.
6) To achieve technical feasibility & economy.
7) To provide required quantity and quality of water to consumer.
8) To maintain the water quality as per requirements.

Water Treatment
Water Treatments :-

Water treatment is a combination of physical, chemical & biological processes.

These are three types of unit operation and unit processes.
1)
Physical Unit Operation: in which physical forces are predominant for
removal of impurities. (screening, equalization, mixing, flocculation,
sedimentation, floatation, etc.)
2) Chemical Unit Processes: in which removal of contamination are brought
about by chemical activity. (chemical precipitation, disinfection, gas transfer,
adsorption, etc.)
3) Biological Unit Processes: : in which removal of contamination are brought
about by biological activity. (Aerobic, Anaerobic & Aerobic-anaerobic
Biological Unit processes)

Water Resources Engineering Cont.
Stages in water treatment:
Screening
Aeration
Plain Sedimentation or Primary Sedimentation
Coagulation & Flocculation
 Secondary sedimentation
Filtration
Disinfection
Storage
Distribution

Water Resources Engineering Cont.

Sedimentation
Sedimentation is a natural process by which solids with higher density
than the fluid in which they are suspended settles under the action of
gravity.
The setting velocity of a particle in a fluid is a function of its density, size
and shape as well as the density and viscosity of the fluid.
The process has been used to remove :
i) Grit in grit chamber
ii) Particulate matter in-primary settling and secondary settling basins.
iii) Destabilised floc in clariflocculators
iv) Biological floc in activated sludge, and
v) Solids concentration in the secondary settling tanks.

Sedimentation cont.
Sedimentation is classified into two categories :
a) Plain sedimentation, and
b) Sedimentation with coagulation

Sedimentation Basin (a.k.a Settling tank, Settling basin)
Types of Sedimentation Tank
A.Depending upon shape:
1)Circular
2) Rectangular
3) Square
B. Depending on process of operation:
2)Continuous flow Tank:
2) Intermittent Tank: ( Batch or Fill & Draw Type)

ZONES OF RECTANGULAR SEDIMENTATION TANK
1) Inlet Zone: The inlet or influent zone should distribute flow
uniformly across the inlet to the tank. The normal design includes
baffles that gently spread the flow across the total inlet of the tank and
prevent short circuiting in the tank.
2) Settling Zone: The settling zone is the largest portion of the
sedimentation basin. This zone provides the calm area necessary for
the suspended particles to settle.
3) Sludge Zone: The sludge zone, located at the bottom of the tank,
provides a storage area for the sludge before it is removed for
additional treatment or disposal. Sludge is removed for further
treatment from the sludge zone by scraper or vacuum devices which
move along the bottom.
4) Outlet Zone : The basin outlet zone (or launder) should provide a
smooth transition from the sedimentation zone to the outlet from the
tank. This area of the tank also controls the depth of water in the basin.

ZONES OF RECTANGULAR SEDIMENTATION TANK

SETTLEMENT TYPES
TYPE I Settlement: this is the settlement of discrete particles
(here force of gravity is always greater than the buoyancy
force)
UNITS
i. SEDIMENTATION TANKS
ii. FILTRATION UNITS
iii. COAGULATION & FLOCULATION

SETTLEMENT TYPES Cont.
TYPE II Settlement: this is the settlement of non-discrete particles
(here buoyancy force is greater than force of gravity)
UNITS
i. RE-SEDIMENTATION TANKS
ii. RE-FILTRATION UNITS
iii. DISINFECTION
iv. STORAGE

DESIGN OF RECTANGULAR SEDIMENTATION TANK
DESIGN CAPACITY (Q)
Q = P x D
Where;
Q is the Design Capacity in
m3
/day
P is the Population in thousand
D is per capita demand L/day
Example 1:
A town with a future population of
10,000 inhabitants, requiring an
average of 200 litres/day per
person. Assuming a maximum daily
demand of 1.2 times the average
demand, determine the design
capacity for twin tanks.

Example 1. Solution
Design Capacity (Q) = Population x Demand
= 10000 x (200/1000)
= 2000m3
/day
Maximum Design Capacity (Q) = 2000 x 1.2
= 2400m3
/day
Thus, design two(2) tanks to provide 1200m3
/day each

DESIGN Cont.
SURFACE LOADING (surface overflow rate
(SOR) )
SOR= Q/(BL)
Where;
SOR is the Surface Loading in
m/h
Q is the Design Capacity in
m3
/h
BL (width x Length) is the area
in m2
Example 2:
If surface loading (surface overflow
rate) of 1.25m/h and a depth(H) of
2m were adopted, compute the
sizing (Length(L), Width(B), Area(A)
and Volume(V)) of the tank for a
design capacity of 1200 m3
/day.

Example 2. Solution

DESIGN Cont.
Detention Time (T)
T= V/Q
Where;
T= detention Time in h
V is volume of thank in m3
Q is the design capacity in m3
/h
Example 3:
Using the dimensions
adopted in example 2,
determine the detention
period (T) of the rectangular
sedimentation tank in hours.

Example 3. Solution

DESIGN Cont.
SETTLING VELOCITY (Vs)
Vs = H/T
where;
Vs is the settling Velocity in m/h
H is the depth of the tank in m
T is the detention time in h
Example 4:
Using the dimensions
adopted in example 2
& 3, determine the
settling velocity (Vs) of
the rectangular
sedimentation tank in
m/h.

Example 4. Solution

DESIGN Cont.
If settling velocity is greater than or
equal to the overflow rate, 100 percent
of discrete particles will remain in the
settling tank.
Check SOR and Vs

Coagulation and Flocculation
i. Coagulants
Coagulants are chosen depending upon
the pH of water. Alum or Aluminium
sulphate is normally used in all
treatment plants because of the low cost
and ease of storage as solid crystals over
long periods.

Coagulants
Types of Conventional coagulant:-
1) Alum, Al2(SO4)3.18H2O, Aluminium Sulphate
2) Copperas FeSO47H2O, Ferrous Sulphate
3) Chlorinated Copperas, Fe(SO4 ) 3 + FeCl3
4) Sodium Aluminate, Na2Al2O4
5) Magnesium Carbonate, MgCO3

Coagulants
Factors Affecting Coagulation:-
1)Dose of coagulant
2) Type of coagulant
3) Form of coagulant
4) Type of mixing
5) Type of feeding
6) pH value
7) Temperature
8) Impurities
9) Time

COAGULATION DOSAGE & Jar Test Apparatus
Jar test assembly.
➢ Used to determine optimum
coagulant dose.
Sample (1L) has to be placed into
➢
number of beaker.
Various amounts of dose in each
➢
beaker is added.
Paddle speed = 100 rpm for 1 minute
➢
then make it slow for next 10 to 25
minutes with 40 rpm.
After fast & slow mixing allow it to
➢
settle down for 30 minutes.
Smallest dose that produces a more
➢
floc is optimum coagulant dose.
This is trial and error method.
➢
One factor is varying and others are
➢
constant.

FILTRATION
SLOW SAND FILTER
Slow sand filters are best suited for the filtration of
water for small towns. The sand used for the filtration
is specified by the effective size and uniformity
coefficient . The effective size, D10, which is the sieve
in millimeters that permits 10% sand by weight to
pass. The uniformity coefficient is calculated by the
ratio of D60 and D10.
RAPID SAND FILTER
Rapid sand filter are replacing the slow sand filters
because of high rate of filtration ranging from 100 to
150m3/m2/day and small area of filter required.

DISINFECTION
The process of killing the infective bacteria from the water and
making it safe to the user is called disinfection.
REQUIREMENTS OF GOOD DISINFECTANTS
1. They should destroy all the harmful pathogens and make it
safe for use,.
2. They should not take more time in killing bacteria
3. They should be economical and easily available
4. They should not require high skill for their application
5. After treatment the water should not become toxic and
objectionable to the user.
6. The concentration should be determined by simply and
quickly.

METHODS OF DISINFECTION
A. PHYSICAL METHODS
1. BOILING : Boil the water for 15 to 20 minutes
and kills the disease causing bacteria. This
process is applicable for individual homes.
2. ULTRA-VIOLET RAYS: Water is allowed to pass
about 10cm thick ultraviolet rays. This process is
very costly and not used at water works.
Suitable for institutions.
3. ULTRASONIC RAYS: Suitable for institutions.

METHODS OF DISINFECTION Cont.
B. CHEMICAL METHODS
1. CHLORINATION : Using chlorine gas or chlorine compounds.
2. BROMINE AND IODINE : It is expensive and leaves taste and
odour.
3. POTASSIUM PERMANGANATE: This method is used for
disinfection of dug well water, pond water or private source
of water.
4. OZONE : Very expensive process, leaves no taste, odour or
residual.
5. EXCESS LIME TREATMENT: Needs long detension time for
time interval and large lime sludge to be treated.

METHODS OF DISINFECTION Cont.
POINTS OF CHLORINATION
Chlorine applied at various stages of treatment and distribution accordingly
are known as pre, post and Re-chlorination.
a) PRE-CHLORINATION
Chlorine applied prior to the sedimentation and filtration process is known as
Prechlorination. This is practiced when the water is heavily polluted and to
remove taste, odour, colour and growth of algae on treatment units. Pre-
chlorination improves coagulation and post chlorination dosage may be
reduced.
b) POST CHLORINATION
When the chlorine is added in the water after all the treatment is known as
Postchlorination.
c) RE-CHLORINATION
In long distribution systems, chlorine residual may fall tendering the water
unsafe. Application of excess chlorine to compensate for this may lead to
unpleasant smell to consumers at the points nearer to treatment point in such
cases chlorine is applied again that is rechlorinated at intermediate points
generally at service reservoirs and booster pumping stations.

Water Resources Engineering (CVE 728)--Slide 4.pptx

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