| The recycling of waste streams produced by Drinking Water Treatment Plants(DWTPs) was widely considered as an effective way to solve the crisis of water resources. Researchers of developed or developing countries are devoted to studying the recycling of urban sewage and waste streams from DWTPs and improving the comprehensive utilization of water resources for the purpose of water resources sustainable development strategy. The recycling of waste streams can save water resources and reducing polluting emission. However, there are also problems should be concerned. Waste streams contained elevated concentrations of total solid(TS), organic matters, heavy metals and genetic toxic substances. Therefore, recycling waste streams may accumulate these pollutants in the treatment processes and increase the risk of finished water failing to conform to Chinese Standards for Drinking Water Quality. Other countries have made a lot of achievements in the field of recycling the waste streams of DWTP and promulgated standards, such as Filter Backwash Recycling Rule issued by USEPA. In China, waste water recycle has been widely adopted by DWTPs in the recent years. However, there is no criterion for the recycle of waste water streams in DWTPs until now, and it is still unclear whether the recycle procedure would worsen the drinking water quality. Generally, most of previous studies were bench- or pilot-scale experiments, focus on a certain recycle process(Filter Backwash Water(FBWW) or Combined Filter Backwash Water(CFBWW)) and regular water quality parameters, lacking of the study of organic parameters and comprehensive water quality parameters accumulation. Therefore, it is urgent to assess the risks of full-scale recycle practices to study the impacts of recycling waste streams on extensive water quality and comprehensive water quality parameters.The water intake of FB-DWTP is located near Beijiang River in Guangdong. B-DWTP have achieved "zero" emissions of waste streams and all kind of waste streams recycled in the treatment line except dewatered sludge residual. The recycle treatment procedure of FB-DWTP included waste water basins, waste sludge basins, sludge thicken basin and dewatering facility. FBWW or CFBWW(FBWW mixed with sludge thicken overflow) was recycled directly in the head of plant, mixed with raw water in the pipe before flocculation process. This kind of recycle procedure has been widely adopted by new built DWTPs in the recent years. In order to assess the impacts of recycle FBWW and CFBWW on regular water quality parameters, metals parameters, disinfection by-productions parameters and specific parameters(PAM). This research combined short-term period monitoring, long-term period monitoring with whole year monitoring results. Besides, in order to assess the impacts of recycle FBWW and CFBWW on genes, DNA, chromosome and reproductive cells, this research introduced a series of biological testing methods, such as Ames test, SOS/umu test, micronucleus test and mouse sperm malformation test. The main research results and conclusions are as following:Untreated, nonhomogeneous FBWW and CFBWW contained elevated concentrations of TS, SS, heavy metals and organic matters. The storage time and dissolved oxygen concentration of FBWW and CFBWW could affect the sludge in FBWW and CFBWW releasing of Mn into supernatant. Mn concentration of supernatant increased over storage time and low dissolved oxygen concentration. Besides, at the beginning of recycling, recycle water had high concentrations of TS and SS. Then it would come to a stable concentration over recycle time. Therefore, FBWW or CFBWW should be kept agitating in the waste water basins in order to decrease Mn concentration of finished water and the pollutants shock load of recycle water.It turned out that recycle untreated FBWW and CFBWW into raw water pipe would increase turbidity, TS, NH3-N, COD, DOC, Al, Mn and Cd concentrations of plant influent. However, it had no significant impact on the quality of settled water and filtered water except for TS, NH3-N and COD, which increased slightly(TS:settled water increased from 159mg/L to 181mg/L,filtered water increased from 137mg/L to 166mg/L; NH3-N: settled water increased from 0.11mg/L to 0.16mg/L,filtered water increased from 0.06mg/L to 0.08mg/L; CODMn: settled water increased from 2.0mg/L to 2.4mg/L,filtered water increased from 1.6mg/L to 1.8mg/L). The finished water treated by conventional processes with recycling FBWW or CFBWW conformed to Chinese Standards for Drinking Water Quality.Recycle FBWW and CFBWW would decrease genetic toxicity effect of plant effluent. Results of Ames test, mouse marrow cell(PCE) micronucleus test and mouse sperm abnormality test showed recycling FBWW and CFBWW had good removal effect of genetic toxicity of substances in plant effluent. However, SOS/umu test and CHO-K1 micronucleus test showed that recycling FBWW and CFBWW had no effect on plant effluent. All genetic tests showed that certain dose(equal to normal drinking water volume of an adult) samples were all negative(such as Ames test in a dose of 3L/vessel, PCE micronucleus test and mouse sperm abnormality in a dose of 10 times of daily adult drinking water volume) and it was safe to drink for long period.It is safe to recycle waste streams based on the results of different water quality indexes and genetic toxicity tests. |
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