Optical characteristics and cytotoxicity of dissolved organic matter in the effluent and sludge from typical sewage treatment processes.

The environmental ecological risks of dissolved organic matter (DOM) extracted from diverse sewage treatment plants and processes have attracted urgent attention. The correlations between the toxicity of DOM and its compositions or properties deserved to be explored to evaluate the environmental risk. Human liver cancer (HepG2) and normal liver (L02) cell lines were used in in vitro experiments evaluating the environmental risks of dissolved organics discharged from secondary and advanced sewage treatment processes. Organics extracted from dewatered sludge were also tested. Elemental compositions were determined and optical characterization was performed. The results indicated that the organics in the effluent from anaerobic-anoxic-oxic processes contained more oxygen-containing groups and hydrophilic substances than those in other types of effluent. The sludge extracts showed the greatest cytotoxicity, followed by the effluent from secondary treatment and then the effluent from an advanced treatment process. The sludge extracts inhibited cell proliferation while the other effluents promoted it at a 5 mgC/L concentration. The organics discharged from secondary and advanced treatment processes induced relatively little production of reactive oxygen species. That stimulated cell self-repair and free radical scavenging and consequently resulted in cell proliferation with the cell lines tested. Oxygen-containing groups in the dissolved organics promoted cell proliferation and ROS removal. The atomic ratios and UV spectroscopy indices contributed mainly to the cell viability among the positive indicators. These results provide theoretical basis for managing the ecological risks posed by dissolved organics released from sewage treatment processes.