SLUDGE PROCESSING

Sudipta SarkarPradeep Kumar

Typical Process flow Diagram– Different Treatment Blocks

Bar Screens Grit RemovalPrimary Clarifier

O2

Aeration tank

Secondary Clarifier

Nutrient Removal

DISPOSAL

Dewatered Sludge to landfill

Anaerobic Digester

Gravity Sludge thickener

Filter Press

Screenings Grit

PRELIMINARY PRIMARY SECONDARY TERTIARY

Advanced Treatments

SLUDGE PROCESSING

Sludge Disposal

Thickening

Digestion

Mechanical dewatering

Disposal

aerobic, anaerobic

Vacuum filtration, centrifugation, filter press

Gravity, accelerated gravity

Land Application, burial, incineration

BIOLOGICAL SLUDGE DIGESTION

Lost heat EnergyEnergy Embedded in end products (such as CH4)Energy lost in respirationEnergy in cell synthesisUnused energy

Lost heat Energy

Energy lost in respiration

Energy in cell synthesis

ANAEROBIC DIGESTION AEROBIC DIGESTION

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Anaerobic digestion converts bulky, odorous, and putrescible raw sludge to a well-digested material that can be rapidly dewatered without the emission of obnoxious odors. Apart from stabilization and gasification of organic sludge, the volume of residue is significantly reduced by withdrawal of supernatant from digesters to thicken the sludge.

Aerobic digestion is only used when treating excess sludge from plants without primary clarifiers. Stabilization of sludge is accomplished by the extended period of aeration so that the organic material including the synthesized cells gets destroyed as the digestion goes in to the endogenous decay stage; however, thickening and dewatering of sludge is troublesome due to bulky nature of over-aerated sludge.

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Different Steps in Anaerobic Sludge Digestion Process

HYDROLYSIS

ACIDOGENESIS

METHANOGENESIS

pH tolerant and fast process

pH sensitive and slow process

Activity of methane bacteria gets affected by excess concentration of oxidized compounds, volatile acids, soluble salts and metal cations.

Limited temperature and pH range (Temperature: 30 -35 deg C and pH 6.7-7.4)

causes build up of organic acids

further reduction of the metabolism of acid-splitting methane formers

Process comes to an eventual halt. This is called souring of reactor.

TYPES OF ANAEROBIC REACTOR

Single-Stage Floating Cover DigestersRaw sludge is pumped into the reactor near the actively digesting zone at the center or at the gas dome. Alternatively, pumping sludge into the gas dome helps break the scum developed on the surface. Weight of the cover is supported by sludge; liquid forced up between the tank wall and the side of the cover works as a gas-seal. Gas collects at the gas dome.

The cover can float on the surface of the sludge between the landing brackets and the height of the overflow pipe.

Functions: 1) Digestion of volatile solids; 2) gravity thickening and 3) Storage of digested sludge and gas produced

The rate of digestion is dependent on the mixing. But mixing cannot be done as a good mixing would prevent supernatant formation. Thus, the efficiency is compromised to allow both digestion and thickening to occur in the same tank.

As a result of the stratification and the lack of mixing, not more than 50% of the volume of a standard-rate single-stage digester is used. Because of these limitations, the standard-rate process is used in small installations.

Detention time is typically 30-90 days.

Floating cover gas dome

Landing bracket

Stabilized thickened sludge

Sludge heater

Active digestion zone

Scum layer

Supernatant

Inlet

Outlet

overflow

High-Rate Completely Mixed Digesters

Actively digesting sludge

Fixed cover

Raw sludge inlet Stabilized

thickened sludge

Supernatant

Gas

Floating Gas dome

Digested Sludge draw-off

Supernatant draw-off

Stage-I (completely mixed) Stage-II (thickening and storage)

Completely mixed systems, with fixed or a floating cover dome, are very efficient in digesting the sludge because of uncompromised anaerobic biological process promoted by complete mixing. The tanks are often heated for increasing the rate of digestion. The feeding of sludge is continuous. The required detention time for high-rate digestion is typically 15 days or less .

The homogeneous nature of the digesting sludge does not allow for formation of supernatant. Thickening has to be done in a second reactor put in a series. It accomplishes thickening and storage of the digested sludge.

The volume capacity required for a single-stage floating-cover digester can be determined by the formula

sadaf TVTVV

V **2

V= Total Digester capacityVf= Volume of average daily raw-sludge feed, m3/dayVa= Volume of daily sludge accumulation in tank, m3/dayTd= Period required for digestion, daysTs= Period required for digested sludge storage, days

[approximately 20-30 days at 25-35 deg C)[normally 30-120 days)

For high rate digesters, volatile solids loadings rate is usually between 3-5 kg/m3.d for adequate volatile solid destruction and gas production and corresponding detention time is 10 day at the minimum.

The volume capacity required for a high-rate digester can be determined by the following equations:

df TVV *1 And, sathafd TVTVV

V **22

V1 and V2 are volumes of first and second stage digester, respectively.

Vfd= Volume of digested sludge feed = volume of average daily raw sludge (Vf), m3/day

Tth= Thickening time, days

Mechanical Dewatering

Dewatering of Digested Sludge

Open Air Drying Beds

•Vacuum Filtration

•Pressure Filtration

•Centrifugation

V E H I C L E O P E N I N G S

SLOPE SLOPE

Sludge inlet

Suitable for small communitiesSimple to design and operatePoor drying in damp weatherPotential odor problemsLarge land area requirement

Area required : 0.1 to 0.25 m2/capita

Avg. drying time: 2-7 days up to 6 wks P L A N

L O N G . S E C T I O N

SLUDGE DEWATERING PROCESSES – BELT FILTER PRESS

Basic Principle is to compress the sludge between two endless porous belts tensioned over a series of pulleys to squeeze out the water. Often it is pre-conditioned with a polymer to aggregate the solids.Initial dewatering takes place in the gravity drainage zone, where there is almost free draining of water through a highly porous belt . About half of the water content is removed.

Rollers are perforated stainless steel or carbon steel cylinders with corrosion-resistant coating

Belts are made of woven cloths of polyester or polypropylene fiber with visible clear openings

The rollers have gradually diminishing diameter to increase the pressure on the filter cakes as it gets dewatered.

Gravity drainage zone

Vacuum Filtration

Filter Press

Courtesy: www.asmfab.com

Operational Details of Filter Press

Sludge Dewatering Centrifuge

Solids are subjected to the influence of centrifugal field of 100-600 times force of gravity

Two main components are: a) rotating solid bowl and, b)interior rotating screw conveyor

Feed slurry is fed at the center and is spun against the bowl wall. Settled solids are moved by the conveyor to one end of the bowl for discharge. The liquid is discharged at the opposite end.

SLUDGE DISPOSAL

Weight and Volume Relationships of SludgeSpecific gravity of solid matter in a sludge / slurry

v

v

f

f

s

s

SW

SW

SW

Ws=weight of dry solids

Wf=weight of fixed (inorganic) solids

Wv=weight of volatile solids

Ss=Specific gravity of dry solidsSf=Specific gravity of fixed (inorganic) solids

Sv=Specific gravity of volatile solids

ϒ= Unit weight of water

Specific gravity of organic matter (volatile solids) is 1.2-1.4; Specific gravity of inorganic/ fixed solids is 1.8-2.5.

Specific gravity of wet sludge/slurry is calculated from:

)/()0.1/( ssw

sw

SWWWWS

Ww=weight of water

S=Specific gravity of sludge

The volume of waste sludge for a given amount of dry matter and concentration of solids is found by,

SmWV s

)100/(

S=Specific gravity of sludge Ws=weight of dry solids

m= solids content, % ϒ= Unit weight of water

Example 1. Consider a waste biological sludge with 10% solids with a volatile fraction of 70%. Find the specific gravity of sludge considering that sp. Gr. of fixed solids and volatile solids as 2.5 and 1.0 respectively.

v

v

f

f

s

s

SW

SW

SW

82.00.170.0

5.230.00.1

sS

22.182.01

sS

)/()0.1/( ssw

sw

SWWWWS

02.1)22.1/10()0.1/90(

1090

S

Example 2. Find out the volume of the above sludge weighing 40,000 kg/day.

SmWV s

)100/( day

mkgV /m 392

02.1*1000*)100/10(

kg/day 000,40 3

3

Example 3. The water content of wastewater sludge is reduced from 98% to 95% using mechanical dewatering. The slurry contains 70% organic matter of specific gravity 1.0 and 30% inorganic solids of specific gravity 2.0. What is the reduction in sludge volume due to dewatering? What are the specific gravities of 98 and 95% slurry?

v

v

f

f

s

s

SW

SW

SW

85.00.170.0

0.230.00.1

sS

17.185.01

sS

)/()0.1/( ssw

sw

SWWWWS

003.1)17.1/2()0.1/98(

298

S

CASE-IICASE-I

SmWV s

)100/(

31 m 0498.0

003.1*1000*)100/2(kg 1

V

For 1kg of slurry,

008.1)18.1/5()0.1/95(

595

S

32 m 0198.0

008.1*1000*)100/5(kg 1

V

Reduction in volume due to dewatering= %60100*0498.0

0198.00498.0100*1

21

VVV