Effects of Free Oxygen on Anaerobic Digestion
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Oxygen can have multifaceted effects in bio-gasification using anaerobic digestion (AD), related to various biochemical interactions involved in AD. The apparent impact of aeration on the methane yield of a digestion system hence depends on the eventuality of these diverse biochemical and physiochemical reactions involving oxygen. The rates of these different reaction pathways would also depend on the operating conditions of a digestion system, such as the nature and concentrations of substrates (soluble, particulate or other specific ingredients like S), hydraulic and operational configuration of the reaction system (CSTR, UASB, batch or continuous) and the characteristics of the inoculum used (biomass concentration, oxygen adaptation, aggregated biomass, etc.). In contrast to the conventional viewpoint of oxygen being toxic for anaerobic digestion systems, it is shown that digesters have a significant capacity to maintain stable performance under a broad range of oxygen input. Possibility of co-existing anaerobic and aerobic cultures in a single reactor environment is proposed and demonstrated in experiments and in model simulations. Oxygen shielding effect by facultative biomass, diffusion barriers due to aggregated biomass and some intrinsic oxygen tolerance of anaerobic organisms are the main factors contributing to this. Several experimental studies have been conducted on the oxygen effects in anaerobic digestion. A mechanistic model has also been proposed to describe the oxygen effects in AD. It is demonstrated that the biogas /methane generation and gas quality can either be affected positively or negatively by oxygenation based on the operating conditions of the reactor. It is suggested that oxygen could enhance the hydrolysis stage of AD leading to enhanced methane yields. Aeration would, however, lead to oxidation of some available substrate by aerobic respiration. Existence of an optimum oxygenation level corresponding to a maximum methane yield is suggested. Limited aeration can be a successful strategy for reducing the risk of intermediates acids accumulation in digesters, especially at the start up periods. This would lead to enhanced starting-up performance in AD and also more stable operation of digesters. Partial aeration assisted anaerobic digestion can be a successful mean of treating a variety of ecotoxic and rather recalcitrant industrial organic wastes, including monoethanolamine (MEA) wastes generated in post combustion CO2 capture. Post-anaerobic aeration can be used to achieve successful nitrification of anaerobic digestates containing significant amounts of ammonia N. This is a useful strategy for converting these anaerobic effluents into a stable high quality organic fertilizer product, attaining the total nutrient recycle and radically improving the environmental sustainability of the overall treatment strategy. This study recognizes partial aeration assisted anaerobic digestion as en enhanced waste treatment and a renewable energy generation scheme with multiple benefits. Further research on the topic, specially related to full scale plant demonstrations, can lead definitely to establish this scheme as a standard treatment option.