Water Quality Simulation And Risk Management In A Reclaimed Wastewater Treatment System

Wastewater reclaimation and reuse is an important method to solve the problem of water shortage and water pollution. Guarantee of reclaimed water safety is a key issue of wastewater reclaimation. Many types of chemical pollutants and pathogens exist in secondary effluent of wastewater treatment plants and reclaimed water, which may cause water quality risk. The variation of operating conditions of wastewater reclaimation plants is also a source of effluent water quality risk. Moreover, increasingly stringent reclaimed water standards also challenge reclaimed water quality. Therefore, an urgent problem is put forward, i.e. how to establish effective strategies of risk management based on scientificly evaluating water quality risk of wastewater reclaimation plants.Aiming at solving this problem, a methodology was developed for assessing and managing water quality risk in a wastewater reclaimation plant based on a wastewater reclaimation plant model. With this methodology, the influence of influent water quality and operating conditions on water quality risk in a wastewater reclaimation plant could be simulated and evaluated, the critical control indexes of water quality risk in the wastewater reclaimation plant could be identified, and effective management strategies for reclaimed water safety could be formulated.The conceptual wastewater reclaimation plant model was developed. The model could simulate the variations of chemical oxygen demand, ammonia-nitrogen, fecal coliform, residual chlorine, genotoxicity and endocrine disrupting effect through ultrafiltration, ozonation and chlorination. The model was calibrated and validated against field monitoring data. Results indicated that the model had good simulation performance and a high proportion of sensitive parameters, and the values of model parameters coincided well with the literature values. Therefore, the model exhibited high reliability.The wastewater reclaimation plant model was applied for assessing and managing water quality risk in Qinghe wastewater reclaimation plant. The water quality risk assessment standards for conventional water quality indexes were determined according to related water quality standards for reclaimed water. The limiting values of effluent toxicity of the wastewater reclaimation plant, namely the water quality risk assessment standards for toxicity, were determined according to predicted no-effect concentrations of toxic equivalent concentrations. The probability distributions of effluent quality were simulated by applying the wastewater reclaimation plant model. Combing the simulation results with the water quality risk assessment standards, the effluent quality risk of Qinghe wastewater reclaimation plant was evaluated. Results indicated that water quality risk might be high in terms of ammonia-nitrogen, genotoxicity and endocrine disrupting effect. The critical control indexes that determine water quality risk were identified through regional sensitivity analysis. On this basis, effective management strategies such as setting water quality goals for secondary effluent and optimization of wastewater reclaimation plant with respect to operation were proposed.