Evaluation Of Water Quality To The Recipient River And Its Ecological Effects Of Reclaimed Water

As the "second water source" of water-deficient cities,reclaimed water is widely used for ecological recharge of urban river courses.However,after the reclaimed water is treated by sewage plants,the residual toxic and harmful substances in the reclaimed water will cause certain potential ecological effects of the replenished river.Therefore,it is particularly important to evaluate the ecological effect of reclaimed water.In this paper,the reclaimed water replenishment reach of Licun River in Qingdao is taken as the research object,12 water quality indexes of Licun River are monitored,and the change rule of water quality of Licun River is comprehensively analyzed by combining principal component analysis method and water quality identification index method.At the same time,based on the nutrient level of the bacteria-algae-fish food chain,luminescent bacteria,Chlorella and zebrafish embryos were used as test organisms to carry out ecological effect evaluation based on the species sensitivity distribution curve,in order to provide reference for the recharge of reclaimed water in river channels.The results of water quality analysis in Licun River showed that the effluent quality of reclaimed water was stable throughout the year.Replenishment of reclaimed water had a certain dilution effect on ammonia nitrogen,total phosphorus and chemical oxygen demand,but it would increase the concentration of total nitrogen in the river.Affected by surface runoff in wet season,the concentrations of ammonia nitrogen,total phosphorus and chemical oxygen demand in summer were higher than those in other seasons,and the variation range of total nitrogen concentration was small.In dry season,the ammonia nitrogen concentration was low,and the variation range along the river was small,while the total phosphorus and chemical oxygen demand showed a higher concentration in the upstream inflow and storage area,and a lower concentration in the ecological stream area.The results of principal component analysis showed that the overall water quality of Licun River was greatly affected by the recharge of reclaimed water and the nitrogen and phosphorus nutrients in the upstream inflow.The analysis results of water quality index method showed that the overall water quality level of the river was in good grade,and the comprehensive water quality status along the river in winter dry season changed little.During the summer wet season,the water quality grade will decline in the river storage area,resulting in the annual comprehensive water quality of each monitoring section from the upstream to the downstream presents a trend of water quality deterioration.Based on the ecological effects of different trophic levels of organisms,the results showed that the reclaimed water showed certain biological inhibition on the three tested organisms,and the inhibition sensitivity was in the order of zebrafish embryo >luminescent bacteria > Chlorella.The test and analysis results of ecological effects along the river channel showed that the inhibitory effect of the river channel on organisms increased at the recharge outlet of reclaimed water,but the inhibitory effect was reduced after upstream inflow dilution and ecological stream area wetland purification,with the maximum reduction rate of 46%.In addition,in the whole monitoring channel,there is artificial laying of hardened foundation,where the water depth is the shallowest and the biological inhibition effect is the least.The result of water quality safety classification method shows that the overall water quality safety grade of reclaimed water recharge channel is high,and the lowest safety grade is B.According to the experimental results of biological effects,the results showed that the reclaimed water and the channel branches had a high level of ecological impact on the high trophic level organisms.The highest level of impact of reclaimed water on consumers was 6.38%,and the ecological impact on the low trophic level decomposing water and producers was low.Both of them were below the ecological impact limit which was aimed at protecting 95% of species.Considering the influence of water quality and seasonal flow changes in the upper reaches of Licun River and Zhangcun River,the pollutant limit,which is aimed at protecting 95%species,can be extended to 90% species during the wet season.Therefore,the ecological impact of reclaimed water replenishment on the middle and lower reaches of Licun River is relatively low.