The effect of shock loads on the performance of an anaerobic baffled reactor (ABR) .1. Step changes in feed concentration at constant retention time

A 10-litre anaerobic baffled reactor (ABR), with eight compartments, was used to examine the effect of organic shock loads, in the form of a step change in the feed chemical oxygen demand (COD) at constant hydraulic retention time (HRT), on reactor performance in terms of COD removal, and to obtain a greater insight into microbial responses and interactions during these shocks. In order to minimise feed variations, and have a totally biodegradable substrate, a synthetic carbohydrate (sucrose)-protein (meat extract) substrate was used. The reactor was operated at 20-h HRT, 4 g/litre COD (4.8 kg-COD/m(3) d), and 35 degrees C for 1 month as a base-line condition, and this resulted in 98% COD removal. It was found that a step change in the feed to 8 g/litre COD (9.6 kg-COD/m(3) d) at 20-h HRT for 20 days did not affect the substrate removal efficiency at all; however, when the concentration was increased to 15 g/litre COD (18 kg-COD/m(3) d) from 4 g/litre for 20 days, the removal efficiency decreased to 90%. It was found that the compartmentalised ABR consisted of three general zones (acidification, methanation, and a buffer zone where little acidification and methanogenesis occurs), and the function of high solids concentrations in the reactor was to enhance stability rather than improve COD removal. Hence, the structure of the ABR prevents most of the biomass being exposed to low pHs during shock loads, and enhances reactor stability. In addition, due to low pHs and high substrate concentrations in the first compartment, a microbial population seems to be selected which produces primarily acetate and butyrate rather than formate and propionate, and this also enhances the stability of the reactor during shock loads. In contrast to past results, formate did not seem to be an important interspecies electron carrier except under high mixing conditions and shock loads, and this was postulated to be due to differences in the structure of the microbial flocs. Based on these observations, the ABR holds some promise as a reactor design for anaerobic industrial wastewater treatment. (C) 1997 Elsevier Science Ltd.