sludge handling and disposal

RAKESH KUMAR (2012UCE1293)DEEPAK SHERAWAT (2012UCE1280)RIJUL (2012UCE1438)

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Sludge refers to the residual, semi-solid material left from industrial wastewater, or sewage treatment processes.

Sustainable sludge handling may be defined as a socially acceptable, cost-effective method

that meets the requirement of efficient recycling of resources while ensuring that harmful substances are not transferred to humans or the environment.

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1. Primary operations

2. Thickening

3. Stabilization

4. Conditioning

5. Dewatering

6. Heat drying

7. Incineration

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This process includes◦ Grinding

-particle size reduction

◦ Screening

-removal of fibrous materials

◦ Degritting

-grit removal

◦ Blending

-homogenization

◦ Storage

-flow equalization

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Using higher dosage of chemical coagulant/flocculent to remove not only suspended solids but also the soluble organic matter

APT will also provide

persistent sulfide

odor absorption

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APT program requires addition of the Ferric Chloride(3 to 15ppm ferric iron)at a point just upstream of the primary clarifiers.

In general the higher the dosage of coagulant the more BOD will removed, but dosage will limited by amount of alkalinity available and by economy

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To increase solids content percentage of sludge by removing a portion of liquid fractionvolume reduction of approximately 30 – 80 % can be reached with sludge thickening2

METHODS :-gravity thickening flotation thickening Centrifugation Rotatary drum thickening

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Objectives ◦ Reduce pathogens

◦ Eliminate offensive odors

◦ Reduce potential for putrefaction

◦ Production of usable gas (methane).

Mechanismso Biological reduction of volatile content

oApplication of heat to disinfect sludge

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Alkaline stabilization :

lime is added to untreated sludge , to raise the PH to 12 or higher .

Retards microbial reaction .

Materials such as cement kiln dust ,fly ash are used instead of lime .

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A biological process that uses bacteria in an oxygen free environment.

These bacteria converts volatile solids into carbon dioxide, methane and ammonia .

single stage digester

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Organic part decomposes in presence of oxygen by micro organisms .

Mostly used in plants with average flow less than 5 MGD .

Bacteria converts organic matter into carbon dioxide .

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Advantages Disadvantages

Lower capital cost High power cost

End product odourless and stable

Digested solids have poorer dewatering characteristics

Lower BOD concentrations in supernatant liquor

Useful by product such as methane is not recovered

More basic fertilizer values recovered

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In this process sludge solids are treated with chemicals or other means to prepare sludge for dewatering.

It improves the drainability of digested sludge. Sludge becomes more amenable to dewatering.

It can be achieved by various methods such as elutriation, chemical conditioning, heat treatment, freezing etc.

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A physical unit operation used to reduce the moisture content of sludge

Difficulty due to fact that particles are very fine, colloidal in nature and possess a gel-like structure due to polymeric flocculation

Compared to thermal (evaporative processes) for water reduction, mechanical dewatering is often selected due to its low energy

requirement3

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1. Centrifugation – for separating liquids of different densities, thickening slurries .

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Uses principle of gravity drainage and mechanically applied pressure.

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It involves the application of heat to evaporate water and to reduce the moisture content of biosolids through conduction, convection or radiation .

Advantage of this method is to reduce product transportation costs, improve storage capability, and marketability.

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1. Direct drying- the wastewater solids come into contact with hot gases, which cause evaporation of moisture.

Dryers such as rotary dryer and fluidized bed dryer are used.

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Indirect drying- Moisture evaporates when the wastewater solids contact the metal surface heated by the hot medium. Dryers such as paddle dryers ,hollow flight and disc dryers are used.

paddle dryer

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0

5

10

15

20

25

30

35

40

45

50

1 10 20 30 40 50 60 70 80 90 100

dry matter [%]

ma

ss [

t] (

volu

me

[m

³])

.

Thickening Dewatering Drying

Dry matter

Water

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Involves total conversion of organic solids to oxidized end products

Sludge stabilization is not necessary before incineration .

Water, carbon-dioxide and ash are primarily end products .

The sludge generally has calorific values

ranging from11 to 17 MJ/kg

(reference :Werle and Wilk,2010)4

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1. multiple hearth incinerator

Dewatered sludge

to dry ash.

used in large plants.

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2. FLUIDIZED BED

INCINERATION

wastes are quickly

and uniformly

incinerated with

utilizing

thermal capacity

of hot fluidizing

sand

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Energy recovery potential 5

Pathogens would be reduced by a pasteurization process

Waste volume reduction

Destruction of combustible toxins

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Emission of mercury & NOx leading to air pollution

High capital and operational costs

Ash must be landfilled and may be hazardous

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Sewage sludge contains both compounds of agricultural value and pollutants.

Agriculture value- organic matter, nitrogen, phosphorus and potassium.

Pollutants- heavy metals, organic pollutants and pathogens

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1. Land fill

2. Agricultural use

3. Other methods

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A site for the disposal of waste materials by burial and is the oldest form of waste disposal.

Some landfills are also used for waste management purposes, such as the temporary storage, consolidation and transfer, or processing of waste material (sorting, treatment, or recycling).

Problem with this method is that many landfills are filling up, and towns are having trouble finding places to put new ones.

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The purpose of using sludge in agriculture is

partly to utilize nutrients such as phosphorus

and nitrogen and partly to utilize organic

substances for soil improvement.

All types of sludge can be spread on farmland if they fulfill the quality requirements (heavy metals, pathogens, pre-treatment)

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Utilization of nutrients contained in the sludge, i.e. phosphorus and nitrogen.

Utilization of organic substances contained

in the sludge for improvement of the humus layer of the soil (i.e. soil improvement).

The cheapest disposal route

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Major investments in storage facilities as sludge can only be spread on farmland a few times a year.

Dependency on the individual farmers and considerable administration of agreements

Lack of knowledge as to the content of organic micro-pollutants and pathogenic organisms in sludge and their impact on the food chains

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Ocean Disposal – Dumping or controlled release of sewage sludge from a barge or other vessel into marine water.

Distribution and Marketing – The give-away, transfer, or sale of sewage sludge or sewage sludge product in either bagged or bulk form.

Surface Disposal – A controlled area of land where only sewage sludge is placed for a period of one year or longer. Sludge placed in this area is not provided with a daily or final cover.

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1.http://www.californiawatertechnologies.com/pdf/APTBulletin.pdf

2. http://www.lenntech.com/library/sludge/thickening/sludgethickening.htm#ixzz3BV2qbCFu

3. Vaxelaire J, Bongiovanni JM, Puiggali JR (1999) Mechanical dewatering and thermal drying

of residual sludge. Environ Technol 20:29–36

4. Werle and Wilk,2010

5. U. S. EPA (1985), seminar publications composting of municipal wastewater, EPA/625/4-85/014

6. Wastewater Treatment By Metcalf & Eddy, TMH

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