Evaluation of the impact of treated wastewaters discharges on river water phosphorus and metal concentrations
It is standard practice for sewage treatment plants to discharge treated wastewaters to water-bodies and, in the UK, this has been the custom since the first sewerage system was completed in London around 1865. Before sewerage networks untreated sewage waste was discharged to rivers (wet carriage) or removed from domestic dwellings by "honey wagons" (dry carriage) and taken to rural areas to be used as fertiliser (Adams & Papa, 2000). Wastewater inputs to rivers are governed by discharge consents issued by the Environment Agency in accordance with existing legislation, taking into account the physical and ecological characteristics of the receiving river. However, it has become apparent in recent years that wastewaters may have a detrimental effect on receiving rivers, particularly in terms of nutrients as they frequently discharge phosphorus (P) in concentrations > 1.0 mg P 1[sup]-1. Metals are not specifically removed by the wastewater treatment process and, although their affinity for particulates results in some removal during processing, it is suspected that dissolved metals may be present in treated wastewaters. In 2000, the E.C. introduced the Water Framework Directive, which requires water bodies within member countries to attain "good ecological status" by 2015 in terms of biological and physico-chemical water quality. An umbrella Directive, it combines existing legislation on a range of contaminants including nutrients and metals with additional environmental standards to improve the quality of European waters. The purpose of this study was to consider whether the discharge of treated sewage wastewaters is likely to prevent rivers from achieving the standards required by this legislation. During an eighteen month period, water samples were collected from two second order rivers, the Bourne and Hogsmill, up and downstream of sewage treatment works. Samples were analysed for a range of P species and metals in filtered and unfiltered river water. Ultra-filtration was carried out on a selection of samples, as finer fractions are likely to be more bioavailable. River flow data was used to calculate potential contaminant loads downstream of the input source; this data was compared against published concentrations of agricultural derived P, historically considered the major contributor of P to UK riverine waters. Until the introduction of P removal processing at the Hogsmill, downstream concentrations of all P species in both rivers, were greater than upstream by an order of magnitude. P removal reduced P concentrations in the Hogsmill by more than 60% although they remain significantly greater than upstream. The majority of P occurred as soluble reactive P (SRP), the most bioavailable species, and the continuous nature of wastewaters discharge meant concentrations were high during the growth season. Current P-removal processes may not remove sufficient P to meet quality targets recommended by the U.K. Technical Advisory Group on the Water Framework Directive. Estimated P loads upstream of the wastewaters outflow are < 8 kg SRP day[sup]-1 for both rivers, the combination of increased P concentrations and greater river water volume downstream of the outflow result in estimated P loads of > 45 kg SRP day[sup]-1 in the Bourne and > 80 kg SRP day[sup]-l for the Hogsmill (after P-stripping). Comparison of P export figures from treated sewage wastewaters, calculated using daily load figures, with those from agriculture indicate that in all but the most rural catchments, sewage wastewaters are a greater source of P. Downstream, dissolved metals concentrations were not significantly different from upstream except for As in the Bourne and Pb in the Hogsmill; neither metal exceeded regulatory limits. River sediment from the Hogsmill and Bourne, and two additional rivers receiving sewage wastewaters, the Mole and Blackwater, were analysed for total metals and P. The ability of sediments to release (SRP) was investigated using kinetic release analysis. In the absence of U.K. standards, metals concentrations in sediments were assessed using the US EPA sediment quality guidelines (SQG). A range of metals in sediments from both the Hogsmill and Mole exceeded SQG to the extent that biota was likely to be affected. Although the greatest concentrations of total phosphorus in sediment were found in the Bourne, it is the Hogsmill which exhibits most potential for the release of SRP from sediment to the water column.