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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