Sludge thickening and stabilization processes

Submitted by
NATTHU SHRIRAME
Under the guidance of
Prof. S.V. Ambekar

Introduction
 Sludge is composed of the pollutants that have been
removed by sedimentation and potential pollutants
produced when organic matter degrades.
 Sludge treatment is needed -
to reduce the water and organic content of the sludge
To remove odour
To reduce volume and weight
Improves hygiene by removing pathogen organism
and render the solids suitable for reuse or final
disposal.

Some important sludge treatment processes are-
 Preliminary operations
 Thickening
 Alkaline stabilization
 Anaerobic digestion
 Aerobic digestion
 Composting
 Conditioning
 Dewatering
 Heat drying
 Incineration

Sludge thickening
 Purpose – to increase solid content of sludge by
removing a portion of the liquid fraction
 Thickening is generally accomplished by physical
means, including co-settling, gravity settling,
flotation, centrifugation, gravity belt and rotary drum.
Applications- it is beneficial to subsequent treatment
process, such as digestion, dewatering, drying and
combustion from the following stand points

 Capacity of tanks and equipment required
 Quantity of chemical required for sludge conditioning
 Amount of heat required by digesters and amount of
auxiliary fuel required for heat drying or incineration,
or both
 Volume reduction is very desirable when liquid sludge
is transported by tank trucks for direct application to
land as soil conditioner.

DESCRIPTION AND DESIGN OF THICKNERS
 In designing thickness facilities, it is important to
Provide adequate capacity to meet peak demands
 and Prevent septicity, with its attendant odor
problems, during the thickening process.
 The six methods of thickening discussed in this
section are co-settling thickening, gravity, dissolved
air flotation, centrifugal, gravity belt, and rotary drum

TYPES OF THICKENINGS
 Co-settling thickening
 Gravity thickening
 Floatation thickening
 Centrifugal thickening
 Gravity belt thickening
 Rotary drum thickening

 Successful thickening of solids in primary clarifiers has
been achieved by a combination of the following :
1. Using one clarifiers in a bank of clarifiers for co-
settling thickening; dilute solids underflow from the
other clarifiers is discharged to the thickening
clarifier;
2. Maintaining the solids inventory for about 6 to 12 hr
3. Providing for the addition of coagulating chemicals
such as polymer & ferric chloride to condition the
solids to enhance settling.
4. The need for chemical addition depends upon the
clarifier overflow rates.

 Tank similar in design to a conventional sedimentation
tank. Normally a circular tank is used .
 Gravity thickening is most effective on primary sludge.
 Gravity thickeners are designed on the basis of solids
loading and thickener overflow rate. Recommended
maximum hydraulic overflow rates range from 15.5 to 31
m3/ m2d.

 Flotation thickening is used most efficiently for waste sludges
from suspended growth biological treatment processes such as
the activated sludge process or the suspended growth
nitrification process.
 Other sludge such as primary sludge , trickling filter humus,
aerobically digested sludge, and sludges containing metal salts
from chemical treatment have been flotation thickened.

 Centrifuges are used both to thicken and to dewater
sludges.
 Their application in thickening is limited normally to
waste-activated sludge.
 Thickening by centrifugation involves the settling of
sludge particles under the influence of centrifugal
forces.
 The basic type of centrifuge used for sludge
thickening is the solid-bowl centrifuge.
 Under normal conditions, thickening can be
accomplished by centrifugal thickening without
polymer addition.

 Maintenance & power costs for the centrifugal
thickening process, however can be substantial.
 Therefore the process is usually attractive only at
facilities larger than 0.2 m3/s (5 Mgal/d), where space
is limited and skilled operators are available or for
sludges that are difficult to thicken by more
conventional means.

 The equipment developed for thickening consists of
gravity belt that moves over rollers driven by a
variable- speed drive unit.
 The sludge is conditioned with polymer and fed into a
fed/distribution box at one end, where the sludge is
distributed evenly across the width of the moving belt.
 The water drains through the belt as the concentrating
sludge is carried towards the discharge end of the
thickener.

 The gravity belt thickener has been used for
thickening waste-activated sludge, anaerobically and
aerobically digested sludge and some industrial
sludges.
 Polymer addition is required.
 Testing is recommended to verify that the solids can
be thickened at typical polymer dosages.

ROTARY DRUM THICKENING
 Rotary-drum thickeners can be used as prethickening
step before belt-press dewatering and are typically
used in small to medium size plants for waste
activated sludge thickening.
 A rotary-drum thickening system consists of
conditioning system and rotating cylindrical screens.
 Polymer is mixed with dilute sludge in the mixing and
conditioning drum.

 The conditioned sludge is then passed to rotating
screen drums, which separate the flocculated solids
from the water.
 Thickened sludge rolls out the end of the drums, while
separated water decants through the screens.
 Some design also allow coupling of the rotary drum
unit to a belt filter press for combination thickening
and dewatering.

INTRODUCTION TO STABILIZATION
 More specifically, the purpose of sludge stabilization is
1. to reduce pathogens
2. eliminate offensive odours
3. inhibit, reduce or eliminate the potential for
putrefaction (which is done by biological reduction
of the volatile organic fraction or addition of
chemicals).

Alkaline stabilization
 To eliminate nuisance condition in sludge by using
alkaline material to render the sludge unsuitable for
survival of m/o.
 In lime stabilization, lime is added to untreated sludge so
that ph is 12 or more.
 high ph retards microbial reactions which are producing
odour and vector attraction.
 process inactive virus, bacteria, m/o present.

 CHEMICAL REACTION IN LIME STABILIZATION:
 Calcium : Ca2+ + 2HCO3
- + CaO 2CaCO3 + H2O
 Phosphorus :
2PO4
3- + 6H + + 3CaO Ca3 (PO4)2 + 3H2O
 Carbon dioxide :- CO2 + CaO CaCO3
 Reactions with organic contaminants :
 Acids : RCOOH + CaO RCOOCaOH
 Fats : Fat + Ca (OH)2 Glycerol + fatty acids

HEAT GENERATION
 In sludge, Quick lime (CaO)+ water = hydrated lime.
 Reaction is exothermic.
 Reaction between quick lime and carbon dioxide is
also exothermic.
 this rises the temp.

APPLICATION OF ALKALINE STABALIZATION
PROCESS:
 Three methods of alkaline stabilization are commonly used :
 Addition of lime to sludge prior to dewatering, termed “lime
pretreatment”,
 The addition of lime to sludge after dewatering or “lime post
treatment”.

LIME PRETREATEMENT
 Pretreatment of liquid sludge with lime has been used
for either -
1. The direct application of liquid sludge to land or
2. Combining benefits of sludge conditioning and
stabilization prior to dewatering .
 When pretreatment is used prior to dewatering,
dewatering has been accomplished by a pressure type
filter press. Lime pretreatment is seldom used with
centrifuges or belt filter presses because of abrasive
wear and scaling problem.

 Because lime stabilization dose not destroy the
organics necessary for bacterial growth, the sludge
must be treated with an excess of lime or disposed of
before the ph drops significantly.
 An excess dosage of lime may range up to 1.5 times
the amount needed to maintain the initial pH of 12.

 ADVANCED ALKALINE STABALIZATION
TECHNOLOGIES:
 Most of the technologies that rely on additives, such as cement
kiln dust, lime kiln dust, or fly ash, are modifications of
conventional dry lime stabilization.
 The most common modifications includes, the addition of other
chemicals, a higher chemical dose, and supplemental drying.
 These processes alter the characteristics of the feed material and
depending on the process may increase product stability,
decrease odor potential, and provide product enhancement. To
utilize these technologies, dewatered sludge is required.

Aerobic digestion
 Used to treat-
1. waste activated sludge only
2. Mixture of waste activated sludge or trickling filter
sludge and primary sludge.
3. Waste sludge from extended aeration plants
 Used primarily in plants of size < 0.2, m3/s but in
recent years it is employed in larger waste water
treatment plants with capacities up to 2 m3/s.

Process description-
 Similar to activated sludge process.
 As the food supply is depleted, micro-organisms begin
to consume their own protoplasm to obtain energy for
cell maintenance operation.
 when energy source is cell tissue then micro-
organisms are in endogenous phase.
 Cell tissue oxidised aerobically to CO2, H2O, ammonia
 75-80% of cell tissue oxidises, 20-25% is not
biodegradable.
 Ammonia oxidises to nitrate as digestion proceeds.
 Non biodegradable volatile suspended solids will
remain in final product.

 Biomass destruction:
C5 H7NO2 + 5O2 4CO2 + H2O + NH4HCO3
 Nitrification of released ammonia nitrogen
NH4
+ + 2O2 NO3 + 2H+ + H2O
 Overall equation with complete nitrification
C5H7NO2 + 7O2 5CO2+ 3HO2 + HNO3
 Using nitrate nitrogen as electron acceptor
C5H7NO2 + 4NO-
3 + H2O NH+
4 + 5HCO-
3 + 2NO2
 With compete nitrification/ denitrification
2C5H7NO2 + 11.5O2 10CO2 + 7H2O + 2N2

 Organic matter converted to nitrate increases hydrogen ion
conc. , decreases ph if sufficient buffering capacity is not
available in sludge.
 7 kg of (alkalinity) CaCO3 destroys per each kg of
ammonia oxidised.
 50% of alkalinity consumed by nitrification can be
recovered by denitrification.
 If DO is very low, nitrification will not occur.
 When buffering capacity is insufficient, ph is < 5.5,
alkalinity feed equipment is installed.

Advantages
1. In aerobic digestion, volatile solid reduction is
approx. equal to anaerobic digestion.
2. lower BOD conc. in supernatant liquor.
3. production of an odourless, biologically stable end
product.
4. recovery of more of the basic fertilizers values in the
sludge.
5. Operation is relatively easy.
6. Lower capital cost
7. Suitability for digesting nutrient rich biosolids.

Disadvantages
 high power cost for supplying required oxygen.
 Digested biosolids produced have poorer mechanical
dewatering characteristics.
 The process is affected significantly by temp., location,
tank geometry, conc. of feed solids, type of tank
material.

Anaerobic digestion
 Decomposition of organic or inorganic matter in absence of
molecular O2.
 Dominant process because of energy conservation, recovery of
beneficial waste water solids
 Many times it produces digester gas to meet energy requirement of
plant.
 3 stage process-
1. hydrolysis
2. Fermentation also called as acidogenesis (the formation of
soluble organic compounds and short chain organic acids)
3. Methenogenesis (the bacterial conversion of organic acids into
methane & carbon dioxide).

 Factors affecting anaerobic sludge digestion-
1. Solid and hydraulic retention time
2. Temperature
 Mesophilic temp. range
 Thermophilic temp. range
3. Alkalinity

Mesophilic anaerobic digestion process-
 In single stage high rate digestion-
Heating, auxiliary mixing, uniform feeding and
thickening of the feed stream characterize the single
stage high rate digestion.
Uniform feeding is important and sludge should be
pumped to digester cont. or on 30 min to 2 hr time
cycle to help maintain constant condition in reactor.

Separate sludge digestion-
Reasons-
 The excellent dewatering characteristics of digested
primary sludge are maintained.
 The digestion process is specially tailored to the sludge
being treated .
 Optimum process control conditions can be
maintained.

PROCESS DESIGN FOR MESOPHILIC ANAEROBIC
DIGESTION-
 The design process depends on-
 Solid retention time
 The use of volumetric loading factors
 Volatile solid destruction
 Observed volume reduction
 Loading factors based on population.

Thermophilic anaerobic digestion process-
 Occurs at temperature between 50 and 570 C ,condition suitable
for thermophilic bacteria.
 Because biochemical reaction rate increase with temperature,
doubling with every 100C rise in temperature until a limiting
temperature is reached, thermophilic digestion is much faster
than mesophilic digestion.
 Advantages include increased solid destruction capability,
improve dewatering, and increase bacterial destruction.
 Disadvantages are - higher energy requirement for heating,
poorer quality supernatant containing larger quantities of
dissolve solid, odors, and less process stability .

 Single stage thermophilic digesters have been used
only in limited application for municipal sludge
treatment; they have been mainly used as the first
stage of a temperature phased anaerobic digestion
process.
 Two phase anaerobic digestion
In search for improved anaerobic digestion
performance several option for phasing or staging the
digestion process in multiple rectors have been
investigated. Four basic phasing methods have been
evaluated (1) staged mesophilic digestion (2)
temperature phased digestion (3) acid/gas (A/G)
phased digestion and, (4) staged thermophilic
digestion.

CONCLUSION
 In thickening process, due to reduction in fraction of
water content, volume of reduced sludge helps in
subsequent treatment process of sludge.
 Due to stabilization, we can remove the pathogens
present in the sludge, make it in such a way that
putrefication is not possible.

Sludge thickening and stabilization processes

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Sludge thickening and stabilization processes