The impact of treated sewage wastewater discharges on the phosphorus levels and hydrology of two second order rivers flowing into the Thames

Abstract

Rivers Bourne and Hogsmill, urban tributaries of the Thames, were sampled up and downstream of sewage treatment works (STWs) wastewaters outflows. River water was analysed for total phosphorus (TP) and soluble reactive phosphorus (SRP) and, in conjunction with river flow data, this study aimed to assess the impact of treated wastewaters on P concentrations in rivers where wastewater volumes frequently exceed those of the underlying river and to consider whether these rivers are likely to achieve "good ecological status" under the Water Framework Directive (WFD). P concentrations downstream of the STW discharge points were generally an order of magnitude greater than upstream, the flow weighted mean concentration in the River Bourne increased from 0.078 mg SRP l(-1) to 0.45 mg SRP l(-1) downstream of the input source. In the Hogsmill, the flow weighted mean concentration rose from 0.19 mg SRP l(-1) to 1.78 mg SRP l(-1) downstream of the input source prior to the introduction of P removal at the STW. Once P removal commenced, flow weighted mean concentrations downstream of the STW reduced to 0.56 mg SRP l(-1). Headwaters in the River Bourne, upstream of the STW outflow showed evidence of irregular, minor, diffuse inputs but overall mean figures indicate that the proposed UK Technical Advisory Group (UK TAG) for the WFD limit of 0.12 mg SRP l(-1) would be met. Headwaters in the Hogsmill are subject to small, continuous discharges of sewage effluent, sufficient to raise SRP levels above the UK TAG limit. Downstream, neither river meets the UKTAG recommendations in terms of P, despite the use of P-stripping processes at both STWs, an indication that current discharge consents for P concentrations in wastewaters are too high.