Anaerobic digestion highly reproducible at low substrate concentrations.
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Increase of sludge volume positively influences process stability and productivity.
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Intensive mixing increases productivity at lower substrate concentrations.
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Intensive mixing inhibits methanogenesis at higher substrate concentrations.
Abstract
Anaerobic digestion processes were compared in multiple reactors using glucose and starch as substrates. For reproducibility studies, five reactors were run in parallel as consecutive 7d-batches and supplied with increasing amounts of starch (2 to 13 g L−1). For concentrations < 10 g L−1, production of biogas and organic acids as well as the population structure changed but remained similar, while populations split and statistically significant differences in metabolic intermediate concentration and biogas composition/volume were observed at higher starch concentrations. Based on this reproducibility study, the influence of sludge volume and mixing intensity were investigated using reactors with working volumes of 0.75 L and 1.5 L, with and without mechanical mixing. Mixing positively influenced reaction rates at lower substrate concentrations (up to 4 g L−1), while a negative influence on the process stability was observed at higher concentrations. In general, the larger reactors showed better productivities and stabilities than the corresponding smaller ones.