Biogas process parameters—energetics and kinetics of secondary fermentations in methanogenic biomass degradation

Abstract

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 H 2 and CO 2 was energetically feasible whereas syntrophic acetate oxidation appeared to be possible in two biogas reactors, one operating at enhanced ammonia content (4.5 g NH 4 + -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.

Process Disturbances in Agricultural Biogas Production—Causes, Mechanisms and Effects on the Biogas Microbiome: A Review

“…Overloads often are related to acid accumulation (Figure 3), indicating an imbalance of microbial acid producers and consumers [40,63,68,69]. The underlying mechanism is based on different ways of obtaining energy and consequently on different growth rates: a thermodynamically more favorable energy production results in faster growth rates of the microorganisms [70][71][72][73].…”

“…While the energy balance of most hydrolytic and acidogenic reactions is negative -meaning a direct energy gain for the microorganisms-the reactions of acid-degrading microorganisms are thermodynamically unfavorable and require more energy than they yield [70,71,74]. Acid-degrading microorganisms often live in syntrophy with hydrogen scavengers, in the biogas process ideally hydrogenotrophic archaea, due to the fact that the conversion of most fatty acids is only possible via the formation of hydrogen (H 2 ).…”

Formate and Hydrogen as Electron Shuttles in Terminal Fermentations in an Oligotrophic Freshwater Lake Sediment

“…The supernatant was used directly without acidification for analysis of soluble compounds. Short-chain fatty acids were measured using a Shimadzu Prominence high-performance liquid chromatograph equipped with an Aminex HPX87H column, a photo diode array detector, and a trace sensitive method for organic acids (26). pH measurement was done with a potentiometric electrode in the supernatant of the centrifugation step.…”

“…Sulfate reducers compete for available hydrogen or formate as well, but only if sufficient sulfate is available (24,25). Beyond experiments with isolated cultures, several studies have tried to assess the energetic situation of secondary fermentations and methane formation in the past by measuring pool sizes of fermentation intermediates, including hydrogen partial pressures in situ, both in natural sediments and in technical settings, such as sewage sludge digesters or biogas reactors (26)(27)(28)(29)(30)(31)(32).…”

“…Only recently have techniques been developed that allow measurement of formate in the micromolar range in environmental samples, even in the presence of large amounts of complex organic matter such as, e.g., humic compounds (26,27). In the present study, the importance of formate as an alternative electron shuttle in methanogenic biomass degradation was investigated in sediments of the oligotrophic Lake Constance in southern Germany, focusing on the energetic limitations of the transformation processes involved.…”