The impact of the controlled emptying of in-sewer storage on wastewater treatment plant performance.

The use of in-sewer storage is generally considered to be an effective means of minimising the effects of intermittent discharges into receiving watercourses during combined flows. Despite this, very little information is available about the consequential effects these flows may have on recipient wastewater treatment plant performance. Typical problems may include biomass washout (hydraulic), and reduced biological reactor performance due to dilute loading (biological). A study is described where detailed analysis was carried out to ascertain the consequential effects of prolonged dilute loading on an activated sludge wastewater treatment plant in Perth, Scotland. Consideration was given to likely storage volumes which may have been utilised in the catchment to resolve local problems. A comprehensive analysis of resulting treatment plant performance was carried out for variations in flow and various wet weather loadings. It is concluded that storage may cause little or no benefit with respect to ammonia total emissions due to reduced treatment of dry weather flows subsequent to the prolonged combined loading period. This was exacerbated by the long regeneration times of nitrifying bacteria. However, an overall benefit with respect to BOD total emissions would always be expected, as appropriately sized storage would retain the first foul flush at the CSOs, thereby compensating the increased emissions from the downstream wastewater treatment plant.