Impact of sewage treatment discharges on the water quality of receiving rivers

Most modern societies rely on rivers for both water supply and for disposal of waste. With increasing population there is increasing pressure on receiving rivers, and therefore, the aim of this study was to assess whether discharges from sewage treatments works (STWs) detrimentally impact the water quality of receiving rivers.

 

The approach of this study was to consider any sewage treatment works in England where there was a monitoring point above and below the STW discharge without any other input between these monitoring points. Any determinand could be expected to change downstream with or without the presence of a sewage works discharge, and therefore, the result from river reaches with a sewage discharge were compared to results from control river reaches where there was no sewage discharge present. Downstream changes were assessed relative to date factors; type of reach (control vs. sewage discharge reach); and individual river reach. In addition, for each river reach a series of covariates were also considered – distance between monitoring points; percentile river flow at time of sampling; and upstream altitude. Where significant, results for each sewage treatment works were compared to characteristics of the sewage treatment works to see whether particular treatment processes contributed to, or mitigated, any water quality impact on the receiving rivers. The determinands considered were stream temperature; nitrate; phosphate; biochemical oxygen demand (BOD); chemical oxygen demand (COD); pH; suspended solids.

 

The results show that:

• Discharge from sewage treatment work significantly altered the temperature, BOD, COD, suspended solids, pH, nitrate, phosphate and specific conductance of the receiving river.
• Comparing impact on water quality to the nature of the sewage treatment works showed that only stream temperature was significantly altered by the nature of the secondary treatment present at any works.
• The size of the sewage treatment works, as judged by population equivalence and dry weather flow, had a significant impact on the magnitude of effect for all except nitrate.
• principal component analysis showed that sewage treatment works grouped together according to their COD or according to their nutrient behaviours.

 

The study shows that the impact of sewage treatment works is widespread, independent of the range of technologies used. At the same time, that the results show that works were rarely a problem for both suspended solids or nutrients.