Humic substances affecting the limitation of the activated sludge process for removal of micropollutants

In our previous investigation with the Bacillus subtilis rec-assay, a strong DNA-damaging potential was found in effluents of municipal wastewater treatment plants and nightsoil treatment plants using the activated sludge process. We hypothesized that the presence of non-biodegradable humic substances in the influent and the association of micropollutants inducing DNA-damaging chemicals to them may cause the comparatively lower removal rate of micropollutants than that of primary organic substrates. In this research, a batch type activated sludge process was set up in the laboratory to examine the fate of these toxic micropollutants and to evaluate the effect of humic substances. As neutral and polar micropollutants, pyrene and 1-aminopyrene was respectively selected, and their speciation (i.e., free, sorbed onto dissolved organic matters including humic acid, and sorbed onto activated sludge) was investigated. Almost all pyrene was sorbed onto activated sludge without any biodegradation. It was observed that the effect of humic substances made both pyrene and 1-aminopyrene leave the process in forms sorbed onto humic acid and free in the effluent. This complexity of the fate of micropollutants and the significant effect of humic acid leads to the necessity of further investigation not only in the activated sludge process but also into the other various natural environments.