treatment and disposal of sludges

8/2/2019 Treatment and Disposal of Sludges

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A

PRESENTATION

ON

TREATMENT AND DISPOSAL OFSLUDGES


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There are two end products obtained from

various wastewater treatment plant:

(1)Effluent:The treated effluent is directly

discharged either in the receiving water or

on the land


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(2)Sludge:The sludges are to be first

processed before their final disposal

Objectives of processing sludge are to extract

water from solids and dispose the dewatered

residue through a combination of physical

chemical and biological operations


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SLUDGE TREATMENT

PROCESS

It includes following unit operation

1.Thickening or Concentration2. Digestion3. Conditioning

4. Dewatering5. Drying

6. Incineration


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1.Thickening

The purpose of thickening is to reduce

moisture content of the sludge and

consequently to increase the solids

concentration

Three types of thickening are as follows

I. Gravity thickeningII. Air floatation

III. Centrifugation


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Gravity thickening

It is the most common practice forconcentration of sludges.

It is the simplest and least expensive.

It is useful for primary sludge orcombined primary and activated sludge

but is not useful for activated sludgeindependently.

It is not effective when the activated

sludge exceeds 40% of the total sludge


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Centrifugal thickening

It is used both to thicken and to dewatersludges.

It involves the settling of sludge particles underthe influence of centrifugal forces.

The basic type of centrifuge used for sludgethickening is the solid-bowl centrifuge.


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Solid-bowl centrifugeIt consists of long bowl mountedhorizontally and tapered at one end.

Sludge introduce into the unit continuously,and the solids concentrate on the periphery,an internal helical scroll, spinning at aslightly different speed and accumulatedsludge towards the tapered end where

additional solids concentration occurs andthe thickened sludge is discharged.


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Different

ial speed

gear box

Cover Feed ports Rotating bowl

Main

drive

sheave

Feed

solids

Thickened

solids dicharge

port

Thickened solids

Rotating conveyer

Centrate

Centrate

dischargeport

(Schematic diagram of a centrifuge used for sludge thickening)


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2.Digestion

Sludge digestion is a biochemical

phenomenon involving organisms,

enzymes, food and environment.

1. Anaerobic digestion

2. Aerobic digestion


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1.Anaerobic digestion

It is the oldest process used for

stabilization of solids and biosolids.

It involves the decomposition oforganic matter and inorganic matter in

the absence of molecular oxygen.


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I.Single stage high rate

digestion

Heating, auxiliary mixing, uniform feeding

and thickening of the feed streamcharacterise the single stage high ratedigestion process.The sludge is mixed by gas recirrculation,

pumping and sludge is heated to achieveoptimum digestion rate.


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In this system a high rate digester is

coupled in series with a second tank.

The first tank is used for digestion and isheated and equipped with mixing facilities.

The second tank is usually unheated andused principally for storage. Tanks mayhave fixed roofs or floating covers.

This type of digestion is seldom usedbecause of expanse of building a large tankthat is not fully utilized and because the

second tank was of negligible benefit


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2.Aerobic digestion

It is used to treat

1. Waste activated sludge

2. Mixtures of waste activated sludge andprimary sludge

3. Waste sludge from extented aerationplants.


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Advantages of aerobicdigestion as compared toanaerobic digestion1. Lower BOD Concentrations insupernatant liquor.

2. Production of an odorless, biologicallystable end products.3. Recovery of more of the basic fertilizervalues in the sludge.

4. Operation is relatively easy.5. Lower capital cost.6. Suitability for digesting nutrient richbiosolids.


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Conditioning

It improves the drainability of digested

sludge.It is more amenable to dewatering. It

can be achieved by various method such as

elutriation, chemical conditioning, heat

treatment,freezing etc.


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4. DewateringIt is a physical unit operation used toreduced the moisture content of sludge and

biosolids.Various methods used in dewatering devicefor removing moisture are1. Mechanical dewatering devices

2. Physical devices


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Mechanical dewatering devices are used todewater the sludge more quickly

The physical device include filtration

squeezing, capillary action, centrifugalseparation and compaction.

Because of the improved design andperformance of solid-bowl centrifuges,imperforate basket centrifuge are seldomused in new dewatering installation.


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The selection of dewatering deviceis determined by the type of sludgeto be dewatered, characteristic of

the dewatered product and thespace available.The dewateringprocess that are commonly used

include centrifuges , belt filterpresses, drying beds and lagoons.


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5. Heat dryingIt involves the application of heat to

evaporate water and to reduce the moisturecontent of biosolids below that achievable

by conventional dewatering methods

advantage of this method is to reduceproduct transportation costs, improvedstorage capability, and marketabilty.

Heat drying methods1. Convection2. Conduction3. Radiation


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Radiation

In radiation drying systems, infrared lamps,electric resistance elements supply radiantenergy that transfers to the wet sludge andevaporates moistrues.Radiation heat transfers is expressed asfollows

qrad=CsA (Tr4-Ts

4)

Cs=radiation heat-transfer rateA=Emmisity of drying surface=Stefan-Boltzmann constantTr=absolute temp. of radiant source

Ts=Absolute temp. of the sludge drying surface


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Rotary sludge dryer

Rotary dryer

furnace Blender

cycloneHot gasses

Air

Fuel

Feed

sludge

Dried biosolids

Recycled product

Particulates

Dried

biosolids to

reuse

Exhaustgas to

odor

control/of

f gas

destructio

n system

Schematic diagram


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6. Incineration

It involves the combustion of the sludge ina reactor under high temperature along withauxiliary fuels. The purpose of incinerationis to destroy the organic material, theresidual ash being generally useful as landfill. During the process all the gassesreleased from the sludge are burnt off an all

the organism are destroyed. Cyclone ormultiple hearth and flash type furnaces areused with proper heating arrangement withtemp. control and drying mechanism.


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Various methods for incineration

1. Multiple hearth incineration

2.Fluidized bed incineration

3.Coincineration with multiple solid base


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Air Pollution control

Incineration method for waste water sludgehave the potential to be siginificant

contributors to air polltion. Aircontemenents associated with incinerationcan be divided into to categories:

1. Odors

2. Combustion emissions


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THANKYOU

treatment and disposal of sludges

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