The performance of an anaerobic baffled reactor (ABR, 10 l and eight compartments) treating a dilute wastewater (500 mg COD/l) was studied. The reactor was started with a hydraulic retention time (HRT) of 80 h at 35 degrees C, and this was steadily reduced to 10 h eventually; all HRTs tested resulted in more than 80% COD removal. The temperature was then reduced to 20 degrees C, which gave a removal efficiency of 70%, and finally to 10 degrees C, which resulted in only 60% COD removal. Residence time distribution studies were carried out to monitor hydrodynamic flow characteristics and dead space after each decrease in temperature. Not much difference in mixing or dead space was observed under the different conditions tested. The average dead space was 25-30%, and the flow pattern within the reactors showed an intermediate behaviour between plug-flow and ideally mixed. Finally, anaerobic bioassays were used to evaluate the activity of the biomass. The biomass retained its initial specific acetogenic and methanogenic activity throughout the compartments of the reactor, and hardly any selection for psychrophilic bacteria had taken place. (C) 2000 Elsevier Science Ltd. All rights reserved.
