Relationship between the removal of organic matter and the production of methane in subsurface flow constructed wetlands designed for wastewater treatment

The aim of this study was to evaluate the relationship between the organic matter removal and methane production in wetlands with a horizontal subsurface flow (HSSF) treating wastewater using Phragmites australis (Phr) and Schoenoplectus californicus (Sch). Four HSSF systems with a superficial area of 4.5 m2 a water table depth of 0.4 m that were planted with Phr and Sch were evaluated. For the evaluation, each HSSF was divided into three transversal zones (A, B, and C). The operation was controlled for the hydraulic loading (19.8–27.8 mm d−1), hydraulic retention time (3–7 d) and organic loading rates (3.3–4.2 g biological oxygen demand—BOD5 m−2 d−1). The removal efficiencies for the organic matter (BOD5 and chemical oxygen demand (COD)) and solids were evaluated in each HSSF during the 420 days of operation. To evaluate the methane produced in the HSSFs, maximum methanogenic activity assays from the biomass of three zones (A, B and C) along the HSSF were performed. The results show that the HSSF planted with Phr and Sch presented removal efficiencies of 70–80% BOD5, more than 60% for the COD and 70–95% of solids during the monitoring period. However, the methane biomass activity shows similar average methane production for HSSF-Phr and HSSF-Sch, although a mild difference was noted between Zone A (700 mL CH4 g volatile suspended solid—VSS−1), Zone B (1035 mL CH4 g VSS−1) and Zone C (368 mL CH4 g VSS−1). Moreover, mass balance determined that HSSF-Phr and HSSF-Sch are able to degrade 13.74 g COD d−1. Finally, the average methane production for HSSF-Phr and HSSF-Sch was 1455 ± 482 mg CH4 m−2 d−1 and 1305 ± 27 mg CH4 m−2 d−1, respectively.