anaerobic digestor

7/28/2019 Anaerobic Digestor

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What is Anaerobic Digestion?

The use of microbes in the absence of oxygen

for the stabilization of organic material by

conversion to methane, carbon dioxide, new

biomass and inorganic products.

This process is used to treat waste water of high

COD concentration.


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Introduction

Anaerobic digestion consists of a series of

microbiological processes that convert organic

compounds to methane and carbon dioxide,

and reduce the volatile solids by 35 percent to

60 percent, depending on the operating

conditions

Anaerobic digestion has long been used forthe stabilization of wastewater sludges


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Stages involved:

STAGE1 STAGE2STAGE3


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Raw materials

Farm Waste

Sewage and sewage sludges

Breweries and distillery waste Vegetable and Energy Crops

Solid waste

meat and slaughter house waste Pure and mix substrates


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Reasons for using Anaerobic Treatment

Higher loadind rates

Lower power Requirement

Useful end products Organic matter stabilisation

Lowering of water Activity

Easier handling of Sludge Low levels of microbial growth


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Essential conditions for efficient anaerobic treatment

Enough nutrients (N & P) and trace metals especially, Fe, Co, Ni, etc.

Avoid excessive air/O2 exposure

No toxic/inhibitory compounds present in the influent

Maintain pH between 6.8 7.2

Sufficient alkalinity present (mainly bicarbonates)

Low volatile fatty acids (VFAs)

Temperature around mesophilic range (30-38 oC)


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PROCESS DESCRIPTION

1.Single-Stage Digestion

Anaerobic digesters are largefermentation tanks provided withmechanical mixing,

heating, gas collection, sludgeaddition and withdrawal, andsupernatant outlet. Sludge digestionand settling occur simultaneously. inthe tank. Sludge stratifies and formsseveral layers from the bottom to thetop of the tank: digested sludge,actively digesting sludge, supernatant,

a scum layer, and gas. Higher sludgeloading rates are achieved in the high-rate version where sludge iscontinuously mixed and heated


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2.Two-Stage Digestion

This process consists of two digester one tank iscontinuously mixed and heated for sludgestabilization and the other one for thickeningand storage prior to withdrawal and ultimatedisposal. Although conventional high-rate

anaerobic digestion and two-stage anaerobicdigestion achieve comparable methane yieldand COD stabilization efficiency, the latterprocess allows operation at much higher

loading rates and shorter hydraulic retentiontimes.


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Organisms involved

Complex process of anaerobic digestion involves

succession of microbial activity.

more than one type of organisms having special role

and metabolic activity.1.Hydrolytic Bacteria

2.Fermentative Acidogenic Bacteria

3.Acetogenic bacteria4. Methanogens


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Group 1: Hydrolytic Bacteria

Consortia of anaerobic bacteria break down

complex organic molecules (e.g., proteins,

cellulose, lignin, lipids)

Hydrolysis of the complex molecules is catalyzedby extracellular enzymes such as cellulases,

proteases, and lipases. However, the hydrolytic

phase is relatively slow and can be limiting inanaerobic digestion of wastes such as raw

cellulolytic wastes that contain lignin


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Group 2: Fermentative Acidogenic

Bacteria

Acidogenic (i.e., acid-forming) bacteria (e.g.,

Clostridium) convert sugars, amino acids, and

fatty acids to organic acids alcohols and

ketones Acetate is the main product of

carbohydrate fermentation. The products

formed vary with the bacterial type as well as

with culture conditions.


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Group 3: Acetogenic Bacteria

Acetogenic bacteria (acetate and H2-producingbacteria) such as Syntrobacter wolinii andSyntrophomonas wolfei convert fatty acids (e.g.,propionic acid, butyric acid) and alcohols into acetate,

hydrogen, and carbon dioxide, which are used by themethanogens. This group requires low hydrogentensions for fatty acid conversion, necessitating a closemonitoring of hydrogen concentration. Under relativelyhigh H2partial pressure, acetate formation is reduced

and the substrate is converted to propionic acid,butyric acid, and ethanol rather than methane. There isa symbiotic relationship between acetogenic bacteriaand methanogens.


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Group 4: Methanogens

Methanogenic microorganisms grow slowly

in wastewater and their generation timesrange from 3 days at 358C to as high as 50days at 108C.

Methanogens use a limited number ofsubstrates that include acetate, H2,CO2,formate, methanol, and methylamines.

All of these substrates are reduced tomethylCoM (CH3SCoM), which isconverted to CH4 by methylCoM reductase


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Hydrogenotrophic methanogens (i.e., hydrogen-using

chemolithotrophs) convert hydrogen and carbondioxide into methane

CO2+4H2 CH4+2H2O Acetotrophic methanogens, also called acetate-

splitting methanogens, convert acetate into methaneand CO2.

CH3COOH CH4+CO2

This group comprises two main genera:Methanosarcina,Methanothrix, and Methanosaeta.

About two-thirds of methane is derived from acetate

conversion by acetotrophic

methanogens. The other third is the result of carbondioxide reduction by hydrogen.


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ORGANISM ACTIVITY

Acetivibrio,Clostridium,Ruminicoccus Cellulose Degradation

Clostridium,Ruminicoccus,Cellulomonas,T

hermoanaerobacter

Hemicellulose Degradation

Bacteroids,Butyrivibrio, Clostridium Starch degradation

Clostridium,Eubacteria Pectin Degradation

Bacteroids,Butyrivibrio, Clostridium,Selenomonas Protein degradation


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Up-Flow Anaerobic Sludge Blanket

(UASB) Process


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Upflow Anaerobic Sludge Blanket (UASB)

The upflow anaerobic sludge blanket (UASB) usesimmobilized biomass to allow the retention of the sludge inthe treatment system.

The UASB-type digester consists of a bottom layer ofpacked sludge, a sludge blanket and an upper liquid layer.

Wastewater flows upward through a sludge bed, which iscovered with a floating blanket of active bacterial flocs.

Settler screens separate the sludge flocs from the treatedwater and gas is collected at the top of the reactor.

This process results in the formation of a compact granularsludge, which settles well and which withstands the shearforce caused by the upflow of wastewater.


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The sludge is immobilized by the formation ofhighly settleable microbial aggregates that growinto distinct granules.

Scanning electron microscopy and transmissionelectron microscopy (TEM) have shown that thegranules are three-layered structures.

The inner layer consists of Methanothrix-like

cells, which may act as nucleation centersnecessary for the initiation of granuledevelopment.

The middle layer consists of bacterial rods that

include both H2-producing acetogens and H2-consuming organisms.

The outermost layer consists of a mixture ofrods, cocci, and filamentous microorganisms.


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A:- exterior

heterogenous layer with rods, filaments, and cocci.

B:- second layer containing predominantly rodsand cocci.

C:- homogenous core containing a large number of cavities, surrounded by one

bacterial morphology..


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FACTORS CONTROLLING ANAEROBIC

DIGESTION

1.Temperature

Optimum temperature:-250C-500C

2. Retention Time

depends on wastewater characteristics and

environmental conditions

From 10-60 days3.pH

Optimum pH: 7-7.2


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Applications of Biogas

Biogas can be used readily in all applications designed fornatural gas such as direct combustion including absorptionheating and cooling, cooking, space and water heating,drying, and gas turbines.

It may also be used in fueling internal combustion enginesand fuel cells for production of mechanical work and/orelectricity.

If cleaned up to adequate standards is may be injected into

gas pipelines and provide illumination and steamproduction.

Finally, through a catalytic chemical oxidation methane canbe used in the production of methanol production


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References

anaerobic digestor

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