Factors affecting washout in intermittently feed upflow anaerobic reactors: Experimental tests and modelling

This paper reports the results of experimental investigations carried out on a full-scale ABR and on a pilot UASB to determine biomass washout for various intermittent feed conditions. Particularly, the tests aimed to determine the extent to which sludge washout is affected by such factors as: daily flow distribution; upflow velocity; concentration and sedimentation properties of the biomass. Moreover, in order to obtain a tool to estimate sludge washout in different influent flow conditions, a simulation model of the sludge transport phenomena in the upflow anaerobic reactors is proposed. The results showed that when the influent flow is interrupted for long periods (above 1 hour), once it is resumed considerable losses of biomass into the effluent were found, independent of the upflow velocity. In contrast, with shorter periods, regular sludge expansion took place even with very high upflow velocities (up to 4 m h-1), giving rise to limited sludge washout and high removal efficiencies.