Bioaugmentation with Mixed Hydrogen-Producing Acetogen Cultures Enhances Methane Production in Molasses Wastewater Treatment

Hydrogen-producing acetogens (HPA) have a transitional role in anaerobic wastewater treatment. Thus, bioaugmentation with HPA cultures can enhance the chemical oxygen demand (COD) removal efficiency and CH4 yield of anaerobic wastewater treatment. Cultures with high degradation capacities for propionic acid and butyric acid were obtained through continuous subculture in enrichment medium and were designated as Z08 and Z12. Bioaugmentation with Z08 and Z12 increased CH4 production by glucose removal to 1.58. Bioaugmentation with Z08 and Z12 increased the COD removal rate in molasses wastewater from 71.60% to 85.84%. The specific H2 and CH4 yields from COD removal increased by factors of 1.54 and 1.63, respectively. Results show that bioaugmentation with HPA-dominated cultures can improve CH4 production from COD removal. Furthermore, hydrogen-producing acetogenesis was identified as the rate-limiting step in anaerobic wastewater treatment.

Syntrophic butyrate and propionate oxidation processes: from genomes to reaction mechanisms

In anoxic environments such as swamps, rice fields and sludge digestors, syntrophic microbial communities are important for decomposition of organic matter to CO2 and CH4 . The most difficult step is the fermentative degradation of short-chain fatty acids such as propionate and butyrate. Conversion of these metabolites to acetate, CO2 , formate and hydrogen is endergonic under standard conditions and occurs only if methanogens keep the concentrations of these intermediate products low. Butyrate and propionate degradation pathways include oxidation steps of comparably high redox potential, i.e. oxidation of butyryl-CoA to crotonyl-CoA and of succinate to fumarate, respectively, that require investment of energy to release the electrons as hydrogen or formate. Although investigated for several decades, the biochemistry of these reactions is still not completely understood. Genome analysis of the butyrate-oxidizing Syntrophomonas wolfei and Syntrophus aciditrophicus and of the propionate-oxidizing Syntrophobacter fumaroxidans and Pelotomaculum thermopropionicum reveals the presence of energy-transforming protein complexes. Recent studies indicated that S. wolfei uses electron-transferring flavoproteins coupled to a menaquinone loop to drive butyryl-CoA oxidation, and that S. fumaroxidans uses a periplasmic formate dehydrogenase, cytochrome b:quinone oxidoreductases, a menaquinone loop and a cytoplasmic fumarate reductase to drive energy-dependent succinate oxidation. Furthermore, we propose that homologues of the Thermotoga maritima bifurcating [FeFe]-hydrogenase are involved in NADH oxidation by S. wolfei and S. fumaroxidans to form hydrogen.