Methods for Calculation of Stormwater Treatment Required for Meeting Receiving Waters Quality: Application in a Case Study

There is a need for tools relating stormwater effluent quality data to the ambient water quality guidelines listed, e.g., in the EU Water Framework Directive. We conducted a literature study and identified four methods to calculate the annual required treatment loads (kg/year) to satisfy the guidelines: (1) StormTac Web, (2) the Vollenweider OECD load model, (3) the model developed by the Swedish Water Authorities and SMHI, and (4) the method using effluent concentration criteria. In this paper, we compile and derive new equations based on these methods to enable calculation of annual acceptable load (kg/year) and annual required treatment load (kg/year) for different types of receiving waters and substances, i.e., not only nutrients but also priority chemicals listed in the Water Framework Directive. Obligatory input data consist of calculated or measured annual flow (runoff and baseflow), actual concentrations and criteria concentrations in the receiving water, or criteria concentration for discharge to the receiving waters. The four methods were applied to address total phosphorus in the urban watercourse of Bällstaån, Sweden. Results showed agreement of the calculated required treatment removals with measured phosphorus concentrations in the receiving water when Methods 1–2 were used but more varied results for Methods 3–4, which neglect the receiving waters concentrations and criteria. Based on these results, we recommend Methods 1–2 for use and further evaluation in projects related to improvements in the quality of receiving waters, as they are based on water quality criteria in the receiving waters themselves. Further investigations regarding input data, complementary measurements, and an evaluation of the validity for substances other than nutrients are recommended to improve the accuracy of calculations.