The Impact of Sewage Treatment Plant Discharges on the Water Quality of Receiving Rivers

With increasing human populations, the need for sustainable management of wastewater becomes an ever‐increasing issue, yet studies that consider the impact of the final effluent on the receiving rivers, in a rigorous statistical manner, have been lacking. Furthermore, studies have not considered the impact of different treatment processes used at each works on water quality. In this study, we used water quality data for nitrate, phosphate, stream temperature, specific conductivity, pH, biochemical oxygen demand (BOD), chemical oxygen demand (COD) and suspended solids. The study considered the discharges from up to 317 sewage treatment works and 232 control river reaches across 22 years, and found that (i) sewage treatment works had a statistically significant impact on the receiving river for nitrate, phosphate along with stream temperature, BOD, pH and specific conductance; (ii) the impact of sewage treatment for BOD, phosphate, pH and specific conductance was getting worse over time. Conversely, for nitrate, the impact of sewage treatment was lessening; (iii) for phosphate, the increase in the receiving river due to sewage discharge was 84% and for nitrate, it was 19%—larger percentage impacts than for any other determinand considered; (iv) tertiary activated sludge treatment was significantly beneficial for both nitrate and phosphate, but secondary treatment was ineffective; and (v) the impact of a sewage treatment works on both phosphate and nitrate increased with the size of the works and works that have a significant impact for one nutrient tended to have a significant impact for the other nutrients.