Reduced sludge production in a two-stage membrane-assisted bioreactor

The performance of different two-stage systems was compared for the treatment of synthetic wastewater. The first stage was a completely mixed reactor without sludge retention for the stimulation of dispersed bacterial growth. The second stage was an activated sludge system in which growth of protozoa and metazoa was stimulated. Solid-liquid separation was achieved either by sedimentation (conventional activated sludge (CAS) system) or submerged membrane filtration (membrane-assisted bioreactor (MBR) system). Some 80% of the chemical oxygen demand (COD) was removed in the first stage at volumetric loading rates up to 1.6 g COD l−1 d−1. In case the subsequent stage was conventional activated sludge, a further COD decrease was noticed. In case of a subsequent MBR system, however, the dissolved COD concentration tended to increase due to the retention of organics by the membrane. In the first stage of both systems, most of the nitrogen and phosphate was used for biomass incorporation. Increased mineralization by protozoa and metazoa in the second stage resulted in a partial release of nitrogen and phosphate to the effluent. The MBR system yielded a 20–30% lower sludge production than the CAS system under similar conditions of solids retention time and organic loading rate. This was attributed to the higher amount of predators in the second stage of the MBR configuration. However, the increased grazing of predators on nitrifying bacteria can result in overgrazing of the latter population. Overall, the two-phase system based on conventional activated sludge had as major point of weakness the wash-out of suspended solids, while the one based on the MBR was hampered by too intensive grazing on the nitrifiers, increased N and P concentrations and wash-out of soluble but humified COD.