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

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 (PO₄^3-), 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 PO₄^3-, 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, PO₄^3-, 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.