Filter substrate comparison and biodegradable flocculant dosing for a compact constructed wetland treating combined sewer overflow

Combined sewer overflows (CSO) are a significant source of urban water pollution, with sediments originating from sewers contributing substantial loads of organic matter, nutrients, and particulate-associated contaminants to receiving waters. In Germany, constructed wetlands for CSO treatment (CSO-CWs) are well-established, yet their large surface area limits their implementation in urban areas. The present study investigates the potential of compact CSO-CWs, operated at doubled flow rate, with tailored filter substrates and the addition of biopolymers to enhance treatment performance. A series of flocculation experiments was first conducted to identify a suitable biopolymer for CSO treatment. Subsequently, small-scale pilot column tests were performed to evaluate compact CSO-CW configurations. Here, different filter substrates were investigated, including sand, sand-gravel, gravel, shells, sand-activated carbon, and sand-zeolite, with and without the selected biopolymer introduced into the inflow to assess its effect on treatment performance. Results show that compact CSO-CWs with suitable substrates and increased flow rates achieved removal efficiencies comparable to the reference CSO-CW in standard operation, exhibiting only minor deviations. Sorptive and fine-grained substrates (activated carbon, zeolite) demonstrated high performance for particulates, COD, DOC, and NH 4 -N, respectively. Biopolymer dosing significantly enhanced particle and COD retention in coarse filter substrates (gravel, shells), thereby reducing performance differences across configurations. In contrast, phosphorus removal remained limited across all configurations, independent of biopolymer dosing. These findings demonstrate that compact CSO-CWs can provide a robust treatment option for CSOs and urban runoff under increased hydraulic loading, particularly in densely populated areas.