Effect of low temperatures on the performance of an anaerobic baffled reactor (ABR)

The effect of a decrease in operating temperature on the performance of two 10 dm(3) anaerobic baffled reactors (ABR) was examined in terms of steady state chemical oxygen demand (COD) removal efficiency. To minimise variations, and have a totally biodegradable feed, a synthetic carbohydrate (sucrose)protein (meat extract) substrate was used. The reactors were operated at 20 h hydraulic retention time (HRT), 4 g dm(-3) COD, and 35 degrees C as a base-line condition, Because of their different histories, the reactors responded differently to a decrease in operating temperature to 25 degrees C. Reactor 1 remained stable at 97% COD removal, whereas Reactor 2 decreased to 93% removal, but rose to 97% after adding an effluent recycle of 0.25. At 15 degrees C, the efficiency of Reactor 1 dropped to 75%, while the removal of Reactor 2 declined to 83%, and no improvement in efficiency occurred with an effluent recycle at 0.25. At 25 degrees C, the decreased rare of catabolism of the slow-growing syntrophs and methanogens resulted in a shift of the volatile fatty acids (VFA) peak to the second compartment. However, the biomass present in the reactor prevented VFAs breaking through in the effluent, Nevertheless, at 15 degrees C VFAs were present in the effluent, perhaps due to the lower rates of metabolism and an increase in the K-s for VFAs. Finally, at 15 degrees C part of the increase in the effluent COD was due to the enhanced production of soluble microbial products (SMP), or a decrease in their metabolism, with these compounds constituting some 10% of the inlet COD.