Sizing ratios for stormwater treatment facilities

Wet weather effluents in both combined and separate sewer systems contain significant loads of pollutants that have negative acute and cumulative impacts on receiving waters depending on the observation time scale. Based on long data series and calibrated models, sizing ratios, with confidence intervals and/or associated with specific return periods, have been evaluated for 2 types of stormwater treatment facilities (storage-settling tanks and on-line settlers), for event, annual and inter-annual time scales. These ratios have been calculated for an interception efficiency E defined as the mass of pollutant intercepted by the facility divided by the mass of pollutant transported in the sewer system. Calculations have been carried out for two representative catchments (one separate, one combined) with 1362 rainfall events from a 17 years long rainfall data series. Results are highly variable for event time scale, but mean values for annual and inter-annual time scales are relatively stable. Storage-settling tanks with specific volumes of 100 m³/active ha intercept approximately 80 % of the annual TSS pollutant load, and on-line settlers designed for 8L/s/active ha can intercept 82 to 85 % of this load. Differences exist between combined and separate systems but are usually less than 10-15 %.