Performance of stormwater detention tanks in an experimental catchment of northern Italy

The performance of stormwater detention tanks with alternative design configurations and operating conditions have been evaluated according to an integrated approach. Various performance indices have been adopted to describe the mitigation of the pollution impact to the natural environment, the reduction of the management and maintenance charges for the urban drainage system, the preservation of the normal purification efficiency, and the limitation of the costs at the treatment plant. A conceptual model based on the instantaneous unit hydrograph of a single linear reservoir system has been used to simulate the rainfall-runoff process and the pollutant dynamics on an experimental urban catchment and combined sewer network for a continuous run of events and inter event periods of one year. Stormwater detention tank combined with flow regulator demonstrates good performance with respect to environmental pollution: satisfactory performance indicators can be obtained with fairly low flow rate of flow regulator (1 L/s per hectare of impervious area) and tank volume of about 35-50 m3 per impervious hectare. These solutions also ensure rather low number and duration of overflows. Intermittent emptying controls the volume sent for purification thus reducing the costs and the risks of impairment in the normal treatment efficiency of the plant. Dangerous anaerobic conditions can be avoided also for a considerable volume of the tank (i.e. 75 m3/haimp) while adopting rather high emptying flow rate of the tank (i.e. 1-2 L/s/haimp).