Toxicity Assessment Of Effluent Dissolved Organic Matter From Typical Sewage Treatment Processes Based On Vitro Cell Model

Effluent dissolved organic matter?EfOM?was discharged into the rivers and lakes with the effluent,which is an important source of DOM in the receiving waters.Previous studies have found that some trace components in EfOM,such as PPCPs,EDCs and DBPs,potentially have biological activity and toxicity.But so far,little known about the overall biological effects of EfOM.The sewage treatment plants in the main city of Kunming can not completely remove the DOC,and the effluent inflowed the Dianchi Lake through the river,which is related to the eutrophication of the Dianchi Lake.At present,the characteristics and composition information of the EfOM of each sewage treatment plant is very scarce.Therefore,it is particularly urgent to collect EfOM characteristics and composition related information and evaluate its toxicity,which is great significance to EfOM's environmental behavior and risk prediction,and also has certain guiding significance for river basin environmental management.This study selected four exemplary treatment processes in the Dianchi Lake Basin,including the first sewage treatment plant?Carrousel/Orbel oxidation ditch?,the third sewage treatment plant?ICEAS?,the fourth sewage treatment plant?MBR?,and the eighth sewage treatment?A²/O?.The DOM in the secondary effluent?SE?,advanced effluent?AE?and dewatered sludge?DS?of each sewage treatment plant was obtained by reverse osmosis enrichment method.Elemental and spectral characteristics were further analyzed.Human hepatocarcinoma?HepG2?cells and human normal liver?L02?cells were used as vitro cell models.The cytotoxic effects were evaluated by comparing cell proliferation,apoptosis and ROS generation after treatment with different DOM.The main findings are as follows:?1?Different sources and treatment processes resulted in different structural and molecular weights of EfOM.The aromaticity of AE_DOM in STP4 and STP8 was increased after ozone and ultraviolet oxidation,whereas the reduction of the aromaticity was observed in STP1 and STP3.Comparing the effluent of four different secondary treatment processes,we found that the oxidized ditch effluent contained the most aromatic protein substances;The molecular weight of AE_DOM in four sewage treatment plants decreased after advanced treatment.?2?HepG2 and L02 cells showed obvious toxic effects after exposed to higher concentration of DOM?>20 mgC/L?.The toxic effects of DOM from different sections in the same STP showed a consistent pattern:DS_DOM>SE_DOM>AE_DOM,the toxicity of secondary effluent was reduced after extensive treatment.We speculated that this may be related to advanced oxidation reducing the molecular weight of DOM.?3?Compared with SEDOM exposure,the total apoptosis of two cell line were decreased after treatment with AE_DOM in four STPs,but the total apoptosis were higher than HA and FA alone exposure,which may be related to the molecular weight of DOM,and then the total percentage of apoptosis in the oxidation ditch and ICEAS effluent DOM exposure was higher than the other two processes,and the early apoptosis was dominant,probably because the oxidation ditch and ICEAS effluent DOM contained more aromatic substances.?4?Exposure to DOM from different sources,HepG2 and L02 cells produced oxidative stress damage,indicating that DOM is genotoxic.The secondary effluent from the four STPs reduced intracellular ROS production after advanced oxidation,which is consistent with MTS and apoptosis results.We hypothesized that oxidative stress is an important mechanism for DOM-induced genotoxicity.