Effects of oxygenation and upflow liquid velocity on a coupled anaerobic/aerobic reactor system

By supplying a small amount of oxygen to a UASB (upflow anaerobic sludge be d) reactor and thereby supporting aerobic metabolism in an otherwise anaero bic environment, a coupled reactor system was evaluated at a laboratory sca le. In order to determine the optimal delivery of oxygen to the coupled rea ctor systems, two principal operating variables were investigated: the oxyg enation rate and the liquid recycle rate. Two UASB and four coupled reactor systems, each 1 litre and operated continuously at a one day hydraulic res idence time (HRT) at 35 degrees C, we .e employed in order to meet these ob jectives. The oxygenation rate increased the dissolved oxygen concentration to a maxi mum of 2.9 mg/L and the redox potential up to -9 mV (E-h). Methane was form ed even under these conditions, indicating that the aerobic and facultative microorganisms limited the O-2 penetration and maintained a reduced microe nvironment which was necessary for the methanogens to function. Oxygenation substantially decreased the methane yield and the ratio of methane:carbon diol;ide in the biogas. Oxygenation also led to a decrease in the mean part icle size of the biomass and lower volatile suspended solid (VSS) levels re tained in the reactors. Liquid recirculation aided in maintaining a low red ox potential. In spite of the effects of the increased upflow velocity remo ving the smaller diameter biomass particles from the reactors, more biomass was retained in the reactors at these higher liquid flow rates. No significant impact of oxygenation could be observed on the specific anae robic activities of the granules, up to the oxygenation mid-range. High oxy genation levels, obtained by combining both high aeration (highest flow of pure O-2) and higher liquid recycling rates are necessary to substantially impair the methanogenic activity. Crown copyright (C) 1999 Published by Els evier Science Ltd. All rights reserved.