Biogas process parameters—energetics and kinetics of secondary fermentations in methanogenic biomass degradation
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Pool sizes of short-chain fatty acids (formate, acetate, propionate, and butyrate), hydrogen, and carbon monoxide were assayed in digesting sludge from four different methanogenic reactors degrading either sewage sludge or agricultural products and wastes at pH 8.0 and 40 or 47 °C. Free reaction energies were calculated for the respective degradation reactions involved, indicating that acetate, propionate, and butyrate degradation all supplied sufficient energy (−10 to −30 kJ per mol reaction) to sustain the microbial communities involved in the respective processes. Pools of formate and hydrogen were energetically equivalent as electron carriers. In the sewage sludge reactor, homoacetogenic acetate formation from H2 and CO2 was energetically feasible whereas syntrophic acetate oxidation appeared to be possible in two biogas reactors, one operating at enhanced ammonia content (4.5 g NH4 +-N per l) and the other one at enhanced temperature (47 °C). Maximum capacities for production of methanogenic substrates did not exceed the consumption capacities by hydrogenotrophic and aceticlastic methanogens. Nonetheless, the capacity for acetate degradation appeared to be a limiting factor especially in the reactor operating at enhanced ammonia concentration.
KeywordsMethanogenesis Energetics Pool sizes Fatty acids Syntrophy Secondary fermentations
The authors are grateful to Melanie Hecht, Thomas Dickhaus, and Sarah Refai for organizing the sampling campaigns at the biogas reactors at Troisdorf and to Erich Kronenthaler and Martin Kaspar for the supply of sewage sludge and operation parameters of the reactor at the wastewater treatment plant in Konstanz. The technical help with specific experiments in the lab by Antje Wiese, Ye Schmidt, and Stefan Bieletzki is highly appreciated.
Compliance with ethical standards
This study was funded by the German Federal Ministry for Education and Research, project BioPara, project number 03SF0421E.
Conflict of interest
The authors declare that they have no competing interests.
This article does not contain any studies with human participants or animals performed by any of the authors.
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