Principles and potential of the anaerobic digestion of waste-activated sludge

Among different conversion processes for biomass, biological anaerobic digestion is one of the most economic ways to produce biogas from various biomass substrates In addition to hydrolysis of polymeric substances, the activity and performance of the methanogenic bacteria is of paramount importance during methanogenesis The aim of this paper is primarily to review the recent literature about the occurrence of both acetotrophic and hydrogenotrophic methanogens during anaerobic conversion of particulate biomass to methane (not wastewater treatment), while this review does not cover the activity of the acetate oxidizing bacteria Both acetotrophic and hydrogenotrophic methanogens are essential for the last step of methanogenesis, but the reports about their roles during this phase of the process are very limited Despite, some conclusions can still be drawn At low concentrations of acetate, normally filamentous Methanosaeta species dominate, eg, often observed in sewage sludge Apparently, high concentrations of toxic ionic agents, like ammonia, hydrogen sulfide (H2S) and volatile fatty acids (VFA), inhibit preferably Methanosaetaceae and especially allow the growth of Methanosarcina species consisting of irregular cell clumps, eg, in cattle manure Thermophilic conditions can favour rod like or coccoid hydrogenotrophic methanogens Thermophilic Methanosarcina species were also observed, but not thermophilic Methanosaetae Other environmental factors could favour hydrogentrophic bacteria, eg, short or low retention times in a biomass reactor However, no general rules regarding process parameters could be derivated at the moment, which favours hydrogenotrophic methanogens Presumably, it depends only on the hydrogen concentration, which is generally not mentioned in the literature

The roles of acetotrophic and hydrogenotrophic methanogens during anaerobic conversion of biomass to methane: a review

Interspecies electron transfer mechanisms between Bacteria and Archaea play a pivotal role during methanogenic degradation of organic matter in natural and engineered anaerobic ecosystems. Growing evidence suggests that in syntrophic communities electron transfer does not rely exclusively on the exchange of diffusible molecules and energy carriers such as hydrogen or formate, rather microorganisms have the capability to exchange metabolic electrons in a more direct manner. Here, we show that supplementation of micrometer-size magnetite (Fe3O4) particles to a methanogenic sludge enhanced (up to 33%) the methane production rate from propionate, a key intermediate in the anaerobic digestion of organic matter and a model substrate to study energy-limited syntrophic communities. The stimulatory effect most probably resulted from the establishment of a direct interspecies electron transfer (DIET), based on magnetite particles serving as electron conduits between propionate-oxidizing acetogens and carbon dioxide-reducing methanogens. Theoretical calculations revealed that DIET allows electrons to be transferred among syntrophic partners at rates which are substantially higher than those attainable via interspecies H2 transfer. Besides the remarkable potential for improving anaerobic digestion, which is a proven biological strategy for renewable energy production, the herein described conduction-based DIET could also have a role in natural methane emissions from magnetite-rich soils and sediments.

Magnetite Particles Triggering a Faster and More Robust Syntrophic Pathway of Methanogenic Propionate Degradation

Mesophilic and thermophilic anaerobic digestion of primary and secondary sludge. Effect of pre-treatment at elevated temperature

Anaerobic ammonium oxidation (anammox) is a recently discovered microbial pathway and a cost-effective way to remove ammonium from wastewater. Anammox bacteria have been described as obligate chemolithoautotrophs. However, many chemolithoautotrophs (i.e., nitrifiers) can use organic compounds as a supplementary carbon source. In this study, the effect of organic compounds on anammox bacteria was investigated. It was shown that alcohols inhibited anammox bacteria, while organic acids were converted by them. Methanol was the most potent inhibitor, leading to complete and irreversible loss of activity at concentrations as low as 0.5 mM. Of the organic acids acetate and propionate, propionate was consumed at a higher rate (0.8 nmol min(-1) mg of protein(-1)) by Percoll-purified anammox cells. Glucose, formate, and alanine had no effect on the anammox process. It was shown that propionate was oxidized mainly to CO(2), with nitrate and/or nitrite as the electron acceptor. The anammox bacteria carried out propionate oxidation simultaneously with anaerobic ammonium oxidation. In an anammox enrichment culture fed with propionate for 150 days, the relative amounts of anammox cells and denitrifiers did not change significantly over time, indicating that anammox bacteria could compete successfully with heterotrophic denitrifiers for propionate. In conclusion, this study shows that anammox bacteria have a more versatile metabolism than previously assumed.

Propionate Oxidation by and Methanol Inhibition of Anaerobic Ammonium-Oxidizing Bacteria

A mathematical model is developed to describe the dynamic behavior of mesophilic (35 +/- 5 degrees C) and thermophilic digestion (55 +/- 5 degrees C) Special emphasis is given to acetotrophic methanogenesis and propionate degradation, as the steps that determine the stability of anaerobic digestion, as well as to hydrolysis rate, which determines the degradation efficiency of particulate degradable organic carbon Within the range of 6-20 (mesophilic) and 2-8 d (thermophilic) hydraulic retention time (HRT), the observed maximum growth rates for acetotrophic methanogens are 033 and 13 d(-1), respectively, with a 15% decay rate Temperature and pH dependence as well as ammonia inhibition of acetate and propionate conversion are determined and included in the model, which allows us to simulate the effect of protein- and nitrogen-rich waste addition and the consequences of temporarily increased free ammonia at high pH No inhibition of hydrogen conversion was observed in the same free ammonia range The pH optimum is between 66 and 73 Acetotrophic methanogenesis is strongly inhibited below pH 62, whereas above pH 74 it can be inhibited by free ammonia For digesters fed with ordinary municipal sewage sludge, free ammonia inhibition of acetate conversion leads to an increase in acetate at about 35 and 140 mg of N/L for mesophilic (HRT = 20 d) and thermophilic (HRT = 6 d) conditions, respectively The hydrolysis rate constant is 025 and 04 d(-1) respectively for these two conditions The model is validated with load variation experiments in laboratory and full-scale digesters for step and shock loads

Dea Vogt

Papers

Mathematical model for meso- and thermophilic anaerobic sewage sludge digestion.