Characterization, DBPs formation, and mutagenicity of soluble microbial products (SMPs) in wastewater under simulated stressful conditions

Abstract

Soluble microbial products (SMPs) are an important group of components in activated sludge-treated wastewater effluents. Chlorination is the most widely used method for disinfecting wastewater effluents. During chlorination of wastewater effluents, SMPs may act as precursors to form disinfection by-products (DBPs), which may pose adverse impacts on the organisms in the receiving water body. In this study, SMPs were prepared by simulating activated sludge under different stressful conditions, including high ammonia content (HA), high salinity (HS), high levels of heavy metals (HM) and high temperature (HT), as well as a normal state (NS). The molecular weight (MW) distribution of SMPs was characterized using gel permeation chromatography (GPC). Chlorination of SMPs was conducted with sodium hypochlorite. Several species of DBPs were detected using gas chromatography–mass spectrometry (GC–MS). The mutagenicity of SMPs solutions and SMPs solutions treated with chlorination was also evaluated using the umu test. Compared with NS, stressful conditions were induced to produce more low MW components of SMPs. Stressful conditions, had no significant effect on the species of DBPs, but played a crucial role in the levels of DBPs produced in SMPs solutions during chlorination. Among the stressful conditions tested, HT and HS resulted in higher levels of both carbon-containing DBPs (C-DBPs) and nitrogen-containing DBPs (N-DBPs). Levels of C-DBPs and N-DBPs were lower under HM condition. HA stimulated the production of N-DBPs, but had no impact on the levels of C-DBPs. The mutagenicity of SMPs solutions was higher under the stressful conditions than that under NS condition in both before and after chlorination. For each SMPs solution, the mutagenicity of SMPs increased after the chlorination, except for SMPs solution under HM and NS conditions.