The effect of shock loads on the performance of an anaerobic baffled reactor (ABR). 2. Step and transient hydraulic shocks at constant feed strength

Abstract

Two Anaerobic Baffled Reactors (ABR) were used to examine the effect of transient and step hydraulic shock loads on reactor performance in terms of chemical oxygen demand (COD) removal and microbial responses to hydraulic shocks in each compartment. The reactors were operated at 20-h hydraulic retention time (HRT), 4 g/litre COD and 35°C as a base-line condition. Hydraulic shocks with an HRT of 1 h (an increase of 20 times in the influent flowrate), 10 h and 5 h were applied to the reactors for 3 h, 2 weeks and 3.5 weeks, respectively, and a variety of key intermediates monitored over time in each compartment. The baseline conditions (4.8 kg-COD/m 3 d) resulted in 98% COD removal; however, when the HRT decreased to 10 h (9.6 kg-COD/m 3 d) removal dropped to 90%, and at 5-h HRT (19.2 kg-COD/m 3 d) removal was only 52%. It was found that the ABR was very stable to large transient shocks, and while biomass loss was substantial, it recovered back to its basline performance only 9 h after the shock ceased. Tracer studies found that the total dead space in the reactor at 24 g-VS/litre and 20-h HRT was about 18% by volume; however, when the HRT was reduced to 10 and 5 h, this increased to 39%. Comparison of this data with previous results for varying feed concentration and recycle ratio in the ABR revealed that removal efficiency could be tentatively explained in terms of HRT; organic loading rate; the concentration/activity and kinetics of the biomass; the dead space in the reactor; the severity and type of channelling in each compartment; the substrate concentration in each compartment driving mass transfer into the flocs; and floc size. Hence, it seems that this type of reactor configuration has potential in treating industrial wastes that vary in both flow and concentration, and still enable high removal rates to be achieved.