Hybrid system for the purification of street stormwater runoff supplying urban recreation reservoirs

Abstract

A high percentage of urban areas are covered by impermeable surfaces which reduce infiltration and landscape retention of stormwater. Moreover, the pollution flushed from these areas, particularly after intensive rainfall, is often drained directly to rivers and reservoirs which, in many cases, also serve a recreational function in cities. Stormwater runoff contributes to degradation of aquatic ecosystems and their intensified eutrophication which, in growing seasons, results in toxic cyanobacterial blooms. The hybrid system (combined of engineering and biological measures) tested in this research was constructed in 2013 in Łódź city (POLNAD) to retain and purify stormwater runoff from a street that runed directly to a cascade of recreational reservoirs. The hybrid system consists of an underground separators system that is combined with a sequential sedimentation-biofiltration system (SSBS). In the first two years of the system’s operation, it effectively reduced pollution transported to the urban river system by reducing 86.0% of total suspended solids, 71.5% of total nitrogen (TN), 66.7% of total phosphorous (TP), and from 40.7% to 78.3% of PO43− and NO2−, respectively. In addition, the system was able to reduce the hydraulic stress induced by extreme discharges and mitigated discharges for precipitation amounts less than 9mm. The hybrid system is an example of a nature-based solution measure reducing the negative effects of nutrients transfer, eutrophication and flooding in urbanized areas, as part of the blue-green infrastructure.