THE IMPACT OF SEWAGE TREATMENT WORKS DISCHARGES UPON RIVERS

Abstract

Abstract
Sewage treatment works (STWs) are essential for urban sanitation but remain important sources of ecological pressure on river systems. Previous studies often lacked suitable controls, particularly for small datasets, or assessments of pollutant bioavailability and ecological risk. This study conducts a national-scale, multi-decadal assessment of STW impacts on river water quality across England, using long-term Environment Agency monitoring data and multiple statistical approaches.
Both metal (calcium (Ca), magnesium (Mg), cadmium (Cd), copper (Cu), nickel (Ni), iron (Fe), zinc (Zn), and manganese (Mn)) and non-metal (stream temperature, biochemical oxygen demand (BOD), chemical oxygen demand (COD), nitrate, phosphate, pH, suspended solids, and specific conductance) determinands were evaluated. Paired upstream–downstream and control site comparisons were used to assess local impacts and results were used to assess the impact of difference treatment technologies between sewage treatment works. The cumulative impact of sewage treatment discharges was assessed by examining the impact of discharge on chlorophyll-a concentration and the exceedance relative to water quality standards. The wider impact of the sewage treatment works discharges was assessed by comparison with river water quality data from all rivers across England using principal component analysis.
The results showed that:
• STW discharges had a significant impact for all determinands except COD and suspended solids, and while for BOD, pH, nitrate, specific conductance, and suspended solids the impact of STW discharge is significantly decreasing, the impact on phosphate concentrations was significantly increasing.
• STW discharges significantly increased the concentration of Ca, Mg, Cu, and Mn in the receiving river, but decreased the concentration of Fe and Zn. Equally, STW discharges significantly increased the bioavailable concentration of Zn, Mn, and Ni, but not the bioavailable concentration of Cu.
• The impact of STW discharges did significantly differ between types of secondary treatment, and tertiary treatment did significantly reduce the impact of STW discharges on the phosphate concentration.
• The combined impact of STW discharge did not change the probability of eutrophication incidents in the receiving rivers.
The principal component analysis has shown that for non-metal water quality determinands from STWs’ the discharge was a dominant control on water quality across English rivers, for metals, STW discharges were not the dominant control and mining, and industrial discharges were more important.