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, w as 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 hydr aulic retention time (HRT), on reactor performance in terms of COD rem oval, and to obtain a greater insight into microbial responses and int eractions during these shocks. In order to minimise feed variations, a nd have a totally biodegradable substrate, a synthetic carbohydrate (s ucrose)-protein (meat extract) substrate was used. The reactor was ope rated 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 re moval. 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 increa sed to 15 g/litre COD (18 kg-COD/m(3) d) from 4 g/litre for 20 days, t he removal efficiency decreased to 90%. It was found that the compartm entalised ABR consisted of three general zones (acidification, methana tion, 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 compartm ent, a microbial population seems to be selected which produces primar ily acetate and butyrate rather than formate and propionate, and this also enhances the stability of the reactor during shock loads. In cont rast to past results, formate did not seem to be an important interspe cies electron carrier except under high mixing conditions and shock lo ads, and this was postulated to be due to differences in the structure of the microbial flocs. Based on these observations, the ABR holds so me promise as a reactor design for anaerobic industrial wastewater tre atment. (C) 1997 Elsevier Science Ltd.