Sequential Sedimentation-Biofiltration System for the purification of a small urban river (the Sokolowka, Lodz) supplied by stormwater

•A multi-zone constructed wetland was studied for small urban river purification.•System provide reduction despite small system area compared to its watershed.•Average removal efficiencies 61.4% (TSS), 37.3% (TP), 46.1% (TN) and 44.8% (NO3−).•Sedimentation zone played key role in overall removal efficiency.•Investigated system reduces chloride pollution - 64%.

The study analyses the efficiency of a Sequentional Sedimentation-Biofiltration System (SSBS) built on the Sokolowka river in Lodz (Poland). It was constructed to purify a small urban river whose hydrological regime is dominated by stormwater and meltwater. The SSBS was constructed on a limited area as multi-zone constructed wetlands. The SSBS consists of three zones: sedimentation zone with structures added to improve sedimentation, a geochemical barrier made of limestone deposit and biofiltration zone. The purification processes of total suspended solids (TSS), total phosphorus (TP), total nitrogen (TP) and other nutrients: phosphates (PO43−), ammonium (NH4+) and nitrates (NO3−) of the SSBS were analyzed. Chloride (Cl−) reduction was investigated. Monitoring conducted in the first two hydrological years after construction indicated that the SSBS removed 61.4% of TSS, 37.3% of TP, 30.4% of PO43−, 46.1% of TN, 2.8% of NH4+, 44.8% of NO3− and 64.0% of Cl−. The sedimentation zone played a key role in removing TSS and nutrients. The geochemical barrier and biofiltration zone each significantly improved overall efficiency by 4-10% for TSS, PO43−, TN, NO3− and Cl−. Although the system reduced the concentration of chloride, further studies are needed to determine the circulation of Cl− in constructed wetlands (CWs), and to assess its impact on purification processes.