Combined biochar vertical flow and free-water surface constructed wetland system for dormitory sewage treatment and...

A two-stage treatment system that included vertical flow (VF) and free-water surface (FWS) constructed wetlands was investigated for the dual purposes of sewage treatment and reuse. The VF included four layers (biochar, sand, gravel, and sandy soil), and the FWS was installed after the VF and used as a polishing tank. Two types of local plants, namely Colocasia esculenta and Canna indica, were planted in the VF and FWS, respectively. The system operated for approximately six months, and the experimental period was categorized into four stages that corresponded to changes in the hydraulic loading rate (HLR) (0.02-0.12 m/d). The removal efficiencies for total suspended solids (TSS), chemical oxygen demand (COD), biological oxygen demand (BOD₅), ammonia (NH₄-N), and total coliform (Tcol) were 71 ± 11%, 73 ± 13%, 79 ± 11%, 91 ± 3%, and 70 ± 20%, respectively. At HLRs of 0.04-0.06 m/d, the COD and BOD₅ levels satisfied Vietnam's irrigation standards, with removable rates of 64% and 88%, respectively, and the TSS and Tcol levels satisfied Vietnam's standards for potable water. Furthermore, the NO₃-N levels satisfied the reuse limits, whereas the NH₄-N levels exceeded the reuse standards. At high HLRs (e.g., 0.12 m/d), all the effluent parameters, except Tcol and NO₃-N, exceeded the standards.