Reducing sludge production in aerobic wastewater treatment through manipulation of the ecosystem

Abstract The possibility of minimizing sludge production in aerobic wastewater treatment through manipulation of the ecosystem so that most of the bacterial biomass produced is consumed by predating protozoa and metazoa was studied. The study was carried out on different pulp and paper industry wastewaters. In all of the experiments, the wastewater was first subjected to treatment in a completely mixed, aerobic reactor without biomass retention, favouring the growth of fast-growing dispersed bacteria consuming the readily biodegradable organic matter in the wastewater. After treatment at this stage, the wastewater was led to a reactor for growth of predators consuming the bacteria. Different designs of the predator stage, i.e. biofilm and suspended growth reactors, were used in different experiments. In all of the experiments, the total suspended solids (TSS) content of the wastewater increased considerably in the first reactor because of the production of bacterial biomass after which it was reduced significantly in the second reactor in which large amounts of different types of protozoa and metazoa developed. The apparent sludge yield in the processes varied between 0.01 and 0.23 kg TSS/kg COD removed, which is considerably lower than the yields generally obtained for treatment of similar wastewaters in conventional treatment processes.