treatment and disposal of sludges
TRANSCRIPT
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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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