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Microbiology. Microbiology of biogas Anaerobic digestion utilizes a consortium of microbes in four distinct phases Products of one phase are feedstocks

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  • Microbiology

  • Microbiology of biogasAnaerobic digestion utilizes a consortium of microbes in four distinct phasesProducts of one phase are feedstocks for next phase (serial process)For optimal performance, each rate keeps up with previousMicrobial populations are ever-evolving

  • What is anaerobic digestion?(Biochemistry perspective)Aerobic organisms use oxygen as their electron acceptorAnaerobic organisms utilize alternative electron acceptorsFacultative organisms are aerobic or anaerobic

  • What is anaerobic digestion?(Biochemistry perspective)Anaerobic electron acceptors (oxidizers)NO3-, Fe3+, Mn3+, SO43+, CO2Significantly different energy output30 ATP (aerobic) vs. 2 ATP (anaerobic)Anaerobic metabolism occurs throughout nature (hydric soils, ruminants, etc.)

  • Phases of digestionHydrolysis




  • Digestion ProcessHydrolysisAcidogenesisAcetogenesisMethanogenesis

  • HydrolysisLarge organic compounds are broken down into simpler compoundsPerformed by many organismsBacteria, fungi, protistsFacilitated by extracellular enzymesH2 and CO2 also produced


  • AcidogenesisProducts of hydrolysis are fermented into volatile fatty acidsAlso termed fermentative bacteriaTrace oxygen consumed by facultative bacteria Also produces acetate, H2, and CO2


  • AcetogenesisLong-chain volatile fatty acids are converted to acetate, H2, and CO2Metabolizes intermediates (proprionate, butyrate)Two groups of acetogensObligate hydrogen-producing acetogens (OHPA) more dominantHomoacetogens less dominant


  • Acetogenesis (OHPA)Produce acetate, CO2 and H2 from fatty acidsMetabolize in low-H2 environmentsMutualistic relationship with methanogens (Syntropy)Methanogens consume H2, OHPA produce acetate and consume fatty acidsPositive feedback cycle if balance disturbedHomoacetogens produce acetate from CO2 and H2 Also assist in reducing H2

  • MethanogenesisFinal metabolic phaseMethanogens produce methane gas from acetate, H2 and CO22/3 of methane produced is derived from acetateOften the rate limiting step in anaerobic digestionCan lead to accumulation of acids

  • MethanogensAll are ArchaebacteriaObligate anaerobesOptimum performance at neutral pH (7)Many genera convert H2 and CO2 to methane (Hydrogenotrophic)MethanococcusMethanospirillum

  • MethanogensTwo known genera convert acetate to methane (Acetoclastic)Methanosarcina Favor high concentrationMethanosaeta Favor low concentration


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