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 bed) reactor and thereby supporting aerobic metabolism in an otherwise anaerobic environment, a coupled reactor system was evaluated at a laboratory scale. In order to determine the optimal delivery of oxygen to the coupled reactor systems, two principal operating variables were investigated: the oxygenation rate and the liquid recycle rate. Two UASB and four coupled reactor systems, each 1 litre and operated continuously at a one day hydraulic residence time (HRT) at 35°C, were employed in order to meet these objectives. The oxygenation rate increased the dissolved oxygen concentration to a maximum of 2.9 mg/L and the redox potential up to −9 mV (Eh). Methane was formed even under these conditions, indicating that the aerobic and facultative microorganisms limited the O2 penetration and maintained a reduced microenvironment which was necessary for the methanogens to function. Oxygenation substantially decreased the methane yield and the ratio of methane:carbon dioxide in the biogas. Oxygenation also led to a decrease in the mean particle size of the biomass and lower volatile suspended solid (VSS) levels retained in the reactors. Liquid recirculation aided in maintaining a low redox potential. In spite of the effects of the increased upflow velocity removing 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 anaerobic activities of the granules, up to the oxygenation mid-range. High oxygenation levels, obtained by combining both high aeration (highest flow of pure O2) and higher liquid recycling rates are necessary to substantially impair the methanogenic activity.