Assessment Of Biological Toxicity And Ecological Safety For Urban Black-Odor River Remediation

The urban black-odor river is mainly the organic pollution caused by the direct discharge of domestic sewage and industrial wastewater without effective treatment.These pollutants are degraded by microorganisms,which consume oxygen to make the water anoxic or anaerobic,thus producing black-odor river.The urban black-odor river not only makes the water lose the function of water resources,aggravates the shortage of water resources,destroys the landscape environment,but also affects the natural environment and the surrounding residents.At present,the treatment of black-odor river in China mainly focuses on the following aspects: source control and pollution interception,endogenous treatment,water purification and ecological restoration.However,the difficulties in the remediation of urban black-odor river are not only by its complicated causes,but also lack of standards to evaluate the black-odor river.Most indexes for evaluating black-odor river are commonly sensory physical indexes and water quality chemical indexes.Although physical and chemical analysis can understand the existence of pollutants,it can not reflect the toxicity of pollutants to aquatic organisms.So it is urgent to identify the hazards of urban black-odor river and establish the evaluation indexes and methods for ecological safety after remediation,so as to evaluate the remediation effect of black-odor river,and further provide a scientific method for the treatment and management of the urban black-odor river.The Whole-Effluent-Toxicity test(WET)can directly test the comprehensive toxicity effect of drainage according to aquatic organisms without identifying the toxic source of the drainage.In this study,a set of combined biological toxicity tests were applied to evaluate the ecological effects of one urban black-odor river(Shentongbang)according to the concept of WET.The special growth rate of Chlorella vulgaris and mortality rate of Daphnia magna were used to assess acute toxicity.The Salmonella Typhimurium/Reverse Mutation Assay(Ames test)was applied to test the mutagenicity.The overlying water and sediment were studied.By teste results on the physical-chemical analysis and the toxicity change before and after the remediation of Shentongbang,the remediation effect of urban black-odor river was assessed.The results of this study are as following:(1)The physical-chemical analysis of Shentongbang before and after the remediation showed that,the overlying water was inferior to class V water body before the remediation.The main pollutants were COD,TN,TP,NH4+-N and DO.The pollutants of sediment mainly come from five kinds of heavy metals(Cr,Cu,Mn,Pb,Zn)and polycyclic aromatic hydrocarbons(PAHs).After remediation,the water quality of the overlying water at all points were improved.The total concentrations of TOC,heavy metals and PAHs in the sediment were significantly reduced.The results indicated that the remediation of Shentongbang effectively reduced the pollutants in the overlying water and sediment.However,TN and TP in the overlying water after remediation still exceed the class V standard,and Cr,Zn,PAHs still existed in the sediment,which may cause toxicity risks to aquatic organisms.(2)The biotoxicity results of the overlying water before and after the remediation of Shentongbang are:(1)The tests by C.vulgaris growth showed that there were no inhibition effects before and after remediation by overlying water,in contrast promoted the growth of C.vulgaris.(2)The tests by D.magna showed slight toxicity on site 3# before remediation and nontoxic after remediation.(3)The mutagenicity of organic extracts from overlying water at all sampling sites were positive before remediation which could cause frameshift and base-pair substitution mutations,but were eliminated after remediation except from 3 of 4sites on TA98 strain.The addition of the liver microsomal S9 induced the positive mutagenicity to TA100 strain on site 4# compared to S9 absence,indicating that the organic chemicals present in the overlying water will be changed by the metabolism of microorganism.(3)The results of biotoxicity of sediments before and after the remediation of Shentongbang are:(1)The tests by C.vulgaris growth showed that,the pore water at all sites before the remediation had inhibitory effects on the growth of C.vulgaris,and the biological toxicity varied from strong to weak from sites 3#,7#,5#,and 4#.After remediation,the toxicity of pore water at sites 5 # and 7 # decreased,but sites 3 # and 4 # increased,which might be caused by the re-release of sediment pollutants.The toxicity after remediation from strong to weak was 4 #,3 #,7 #,5 #,while site 5 # promoted the growth of C.vulgaris.(2)The tests by D.magna showed that,the pore water at point 4 # was nontoxic to D.magna before remediation,and the other sites were media toxic.After remediation,the toxicity of pore water decreased except site 4 # which is slight toxic to D.magna,while the toxicity of site 3 # still showed media toxic,sites 5 # and 7 # were reduced to slight toxic.(3)The mutagenicity of organic extracts from sediments(above 2 mg/Dish)at all sampling sites were positive before remediation,which could cause frameshift and base-pair substitution mutations.After remediation,the mutagenicity decreased,only a few of the sediments above5 mg/Dish remained weak mutagenicity,and the organic extracts of sediments at all sites were not mutagenic at 2 mg/dish.The results showed that the remediation of black-odor river reduced the biological toxicity and genotoxicity of the overlying water and sediments,but the organic pollutants present in the black-odor river still have potential ecological risks,and the toxicity risk of sediments to aquatic organisms was higher than that of overlying water.