Evaluating the impacts of contrasting sewage treatment methods on nutrient dynamics across the River Wensum catchment, UK

Sewage effluent discharged from wastewater treatment plants (WWTPs) is a major driver of nitrogen (N) and phosphorus (P) enrichment, but tertiary treatment methods such as P-stripping have previously been shown to mitigate eutrophication risk. The aim of this study was to evaluate the impacts of sewage effluent discharged from WWTPs with contrasting classifications of tertiary treatment on nutrient dynamics across the River Wensum catchment, UK. River water samples were collected from 20 locations across the catchment at monthly intervals between October 2010 and September 2013, with 677 samples collected in total and analysed for a suite of hydrochemical parameters. The 20 sampling locations were divided into four classifications based on the type of upstream WWTP: (1) no WWTP; (2) WWTPs without P-stripping; (3) WWTPs with and without P-stripping; (4) WWTPs with P-stripping. Results revealed substantial overlaps in riverine nutrient composition making differentiation between classifications difficult. The majority of N (>97%) and P (similar to 75%) was present in dissolved bio-available forms across all sites and there was no significant difference in total N speciation between classifications. Total P (TP) speciation did, however, reveal higher proportions of particulate P at sites with no WWTP, indicating a greater P contribution of agricultural origin. Ratios of total dissolved to particulate P (TDP: TPP) and chloride concentrations proved effective discriminators of agricultural and sewage P, respectively, but phosphate-boron ratios (PO4:B) were ineffective discriminators in this catchment. Most importantly, there was no evidence that P-stripping reduced overall TP concentrations downstream of WWTPs, despite evidence of a per capita reduction, nor reduced the proportion of dissolved P released. These findings were attributed to P-stripping facilities serving larger populations and thus releasing greater effluent P load, thereby demonstrating that the presence of tertiary P-stripping alone is insufficient to overcome population pressures and ensure that rivers achieve good hydrochemical status. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

This study evaluated the impacts of wastewater treatment plants with different classifications of tertiary treatment on nutrient dynamics in the River Wensum catchment, UK. The majority of nutrients were found in bio-available forms, with phosphorus mainly of agricultural origin. Phosphorus-stripping facilities did not reduce total P concentrations downstream of WWTPs, indicating their insufficiency in overcoming population pressures and ensuring river reaches good hydrochemical status.