Identification of characteristic factors for water quality indicators and development of a wastewater source signature system for receiving rivers

Abstract

Wastewater treatment plant (WWTP) discharge has become a focal point in watershed management, and its aggregate impacts on receiving rivers have been preliminarily elucidated. However, the characteristic water quality patterns in receiving rivers under the influence of different WWTP discharges (domestic, mixed, and industrial) remain unclear. To address this gap, water quality indicators were analysed in samples collected upstream and downstream of the outfall during different water periods and characteristic factors were identified. A threshold system for identifying the characteristic water quality patterns was established based on indicator concentration ratios, and the threshold ranges for source-type water quality signature ratios were determined. The characteristic patterns were validated by selecting three characteristic section types (different regions, double outfalls, and long distances). The results showed that the concentrations of most indicators at the downstream of outfalls were 5 % − 70 % higher than those at the upstream, and the water quality index quantified downstream deterioration (0.46 − 0.69). Furthermore, anions and metallic elements were identified as the characteristic factors. Based on these analyses, threshold ranges for source-type water quality signature ratio were determined: domestic (< 6.22), mixed (6.22 − 9.86), and industrial (> 9.86). Validation across the other characteristic sections confirmed that the results were within the threshold ranges. The strength of the indicator interaction by industrial wastewater discharge exceeded that of other wastewaters, thereby elucidating the differential characteristics mechanisms. This study provides a novel methodological framework for watershed water quality characterization, and the established threshold system holds significant practical value for aquatic environment management.