A Biotoxicity-based Ecological Risk Assessment Of Streams Augmented By Reclaimed Wastewater

Due to water scarcity in China, reclaimed wastewater has been widely used as analternative source to augment streams and lakes. However, numerous toxic substancesmay exist in reclaimed wastewater and thus have long-term and cumulative adverseimpact on aquatic ecosystems. Since contaminants in reclaimed wastewater arecomplicated and not thoroughly known and possibly interact with each other, it becomesa scientific problem as well as a practical challenge for risk management of wastewaterreuse to assess the ecological risk of reclaimed wastewater for river and lakeaugmentation.This study focused on the comprehensive ecological risk assessment of streamsaugmented by reclaimed water and a method for ecological risk assessment based on abattery of bioassays was correspondingly established by applying modern bioanalyticaltechniques. Three bioassays, i.e. estrogenic effect, neurotoxicity and genetic toxicity,which could efficiently characterize the toxicity of various contaminants, were selectedas water quality indicators in this study. Effects assessment was conducted using speciessensitivity distribution (SSD) models based on the toxicity data of the equivalentsubstances of the three bioassays for a series of species. Exposure concentrations of theequivalent substances were acquired through a water quality monitoring program, andfurthermore a water quality model was developed to simulate the variation of theseequivalent substances in surface water. Ecological risk was finally characterized by theproportions of species affected in aquatic ecosystems supplemented by reclaimedwastewater. Xiaojiahe Reach of the Qing River, which was supplemented by the effluentfrom the Qinghe Wastewater Reclamation Plant, Beijing, was selected as a case study.SSD models were developed based on the available toxicity data of around430nativespecies in Beijing, and used to assess the ecological risk of the Xiaojiahe Reachassociated with reclaimed wastewater augmentation. The influence of upstream waterflow and quality as well as the treatment processes of wastewater reclamation on theecological risk was also evaluated, and suggestions for ecological risk amanagementwere finally proposed.Considering the uncertainty of SSD models, ecological risk caused by substances leading to estrogenic effect, neurotoxicity and genetic toxicity were0~0.064,0.403~0.693and0.259~0.304, respectively, and the joint ecological risk caused by allthese substances were0.598~0.780, where the reclaimed wastewater was augmented.Compared with the scenario of no augmentation, reclaimed wastewater did not bringadditional risk to the river significantly, while it may even mitigate the ecological riskwhen the upstream water quality was poor. With the goal of protecting95%of thespecies, the results of the Burr III distribution model revealed that, the length of riverthat failed to reach the goal would increase with the upstream water flow given thecurrent upstream water quality. Improving upstream water quality could significantlylower the ecological risk in the Xiaojiahe reach. Ultrafiltration and ozonation couldproduce an effluent that reduces the ecological risk, yet chlorine disinfection may do thereverse. Potentially feasible measures for ecological risk management were proposed,including delineating an adjustable mixing zone in response to upstream water flow,identifying the sources of these biotoxicity-leading pollutants and managing them at thesource, renovating disinfection processes, and installing other advanced treatmentprocesses.