The influence of start-up strategies on the performance of an anaerobic baffled reactor

The start-up and performance of two 10 litre anaerobic baffled reactors (ABR), both of which contained eight compartments and were operated under mesophilic conditions (35 degrees C), were investigated. Two start-up strategies were evaluated: one with a constant hydraulic retention time (HRT) and a stepwise doubling of the organic feed strength (ABR 1), the other with a constant organic strength (4 g l(-1) chemical oxygen demand GOD, as sucrose) coupled to a systematic halving of HRT (ABR 2). The final desired baseline conditions were a feed strength of 4 g COD-1 and 20 hours HRT. The best stability and COD removal was demonstrated by the reactor started at a long retention time (ABR 2). The biomass (volatile suspended solids, VSS) lost from each reactor was greater in ABR 1 than in ABR 2 (200 g compared to 100 g after 140 days), and it was hypothesised that in ABR 1 low retention time and channelling controlled biomass washout, whereas in ABR 2 washout appeared to be caused by gas mixing. Microbial flee sizes were measured and found to decrease initially in most cases, but increased over time and after 270 days the average flee sizes were 152 and 185 mu m in ABX 1 and 2 respectively. The largest flocs were found in compartments 4 to 6 for both reactors, and it is proposed that this was due to a combination of low gas production and low but significant amounts of GOD. SER? and TEM photographs suggest that the population of methanogenic bacteria altered from primarily Methanosarcina-like organisms at the front where high substrate levels and low pH were present, to Methanosaeta-like bacteria at the rear of the reactors where there were low substrate levels and neutral pH conditions.