Anaerobic Anaerobic Digestion and Digestion and BiogasBiogasTerminology and designsTerminology and designs

Total Solids, Volatile Total Solids, Volatile SolidsSolids

Total Solids (TS)= Dry weight of Total Solids (TS)= Dry weight of substratesubstrate

Volatile Solids (VS)= Combustible Volatile Solids (VS)= Combustible proportion of TS, organic proportion of TS, organic mattermatter

Non-volatile Solids (Ash)Non-volatile Solids (Ash) = = Minerals etc. left over Minerals etc. left over from from combustioncombustion

Total Solids, Volatile Total Solids, Volatile SolidsSolids

Total Solids Ash

COD/BODCOD/BOD

Chemical/Biochemical Oxygen DemandChemical/Biochemical Oxygen Demand Measures amount of oxygen required to Measures amount of oxygen required to

degrade a substratedegrade a substrate Used widely in aerobic water treatmentUsed widely in aerobic water treatment Characterizes wastewaters and organic Characterizes wastewaters and organic

feedstocksfeedstocks Estimates energy content of substrateEstimates energy content of substrate

BMPBMP

Biochemical Methane Potential AssayBiochemical Methane Potential Assay Developed as correlative test of BOD in Developed as correlative test of BOD in

anaerobic systemsanaerobic systems Realistic measure of total methane potential Realistic measure of total methane potential

of a feestockof a feestock Can determine non-biodegradable materials Can determine non-biodegradable materials

in feedstockin feedstock

Organic loading rateOrganic loading rate

Rate at which feedstock is fed to the Rate at which feedstock is fed to the digester (i.e. g VS / L reactor)digester (i.e. g VS / L reactor)

Varies by feedstock, reactor type, Varies by feedstock, reactor type, temperature, etc.temperature, etc.

Too low= too large of reactorToo low= too large of reactor Too high= overload reactor, acidificationToo high= overload reactor, acidification

Hydraulic Retention TimeHydraulic Retention Time

How long substrate remains in active How long substrate remains in active volume (i.e. turnover rate)volume (i.e. turnover rate)

Optimized for higher biogas/ reactor Optimized for higher biogas/ reactor volume ratiovolume ratio

Too high= Unnecessarily large reactorToo high= Unnecessarily large reactor Too low= Reduced biogas output, Too low= Reduced biogas output,

washout microbeswashout microbes

TemperatureTemperature

Anaerobic metabolism much more Anaerobic metabolism much more sensitive to temperature than aerobicsensitive to temperature than aerobic

Methanogens more affected than Methanogens more affected than acidogensacidogens Leads to unbalanced pH at low temps.Leads to unbalanced pH at low temps. Loading rate reduced at low temps.Loading rate reduced at low temps.

Mesophilic vs. Mesophilic vs. ThermophilicThermophilic

Temperature classes for microbes in Temperature classes for microbes in digesterdigester

Mesophilic (30-40Mesophilic (30-40°C), Thermophilic (50-°C), Thermophilic (50-60°C)60°C)

Different microbial communities operate Different microbial communities operate at each temp. classat each temp. class

Most digesters operate at mesophilicMost digesters operate at mesophilic

pHpH

Very important parameterVery important parameter Optimum 7.0, acceptable 6.0-8.0Optimum 7.0, acceptable 6.0-8.0 Methanogens cannot function at low pH. Methanogens cannot function at low pH. Ammonia toxicity above 8.0Ammonia toxicity above 8.0 Slow recovery after sustained drop in pH.Slow recovery after sustained drop in pH.

Different digester typesDifferent digester types

Batch vs. ContinuousBatch vs. Continuous Two phaseTwo phase CSTRCSTR UASBUASB Fixed-filmFixed-film

Batch vs. continuously Batch vs. continuously fedfed

Batch – Digester loaded once, emptied Batch – Digester loaded once, emptied once fully degradedonce fully degraded

Continuously fed – Digester regularly Continuously fed – Digester regularly loaded (usually daily), effluent produced loaded (usually daily), effluent produced at each loadingat each loading

Two phase digestionTwo phase digestion

Single phase- all in one reactorSingle phase- all in one reactor Two phase- acidogenesis and Two phase- acidogenesis and

methanogenesis separatedmethanogenesis separated BenefitsBenefits

Increased over-all efficiency (short HRT of Increased over-all efficiency (short HRT of acidogenic reactor)acidogenic reactor)

More pH control of methanogenic reactorMore pH control of methanogenic reactor

Two phase digestionTwo phase digestion

CSTRCSTR

Continuously-Stirred Tank ReactorContinuously-Stirred Tank Reactor Simple but effective designSimple but effective design

UASBUASB

Up-flow Anaerobic Sludge BlanketUp-flow Anaerobic Sludge Blanket Frequently used Frequently used

for sewage for sewage wastewaterwastewater

Fixed Film ReactorFixed Film Reactor

Increased substrate-bacteria contactIncreased substrate-bacteria contact Utilizes biofilmsUtilizes biofilms Short HRTShort HRT Decreased sizeDecreased size Used as DRUUsed as DRU

Digesters in developing Digesters in developing nationsnations

“Bag digester”, Costa Rica

Small scale digester, rural India

Digesters in agricultureDigesters in agriculture

Covered Lagoon, Tulare, CA

Corn sillage digester, Neumunster, Germany

Digesters in industrial Digesters in industrial settingssettings

Lübek mechanical biological treatment plant, Germany

Mechanical biological treatment plant, Tel Aviv, Israel

Digesters as architectural Digesters as architectural features?features?

Reading Sewage Treatment Works, Reading, United Kingdom

Appleton Wastewater Treatment Plant, Appleton, Wisconsin

Questions?Questions?