Methane oxidation in vertical flow constructed wetlands and its effect on denitrification and COD removal

Most types of subsurface flow constructed wetlands include pretreatment units for separation of particulate matter. Due to long retention times of the settled sludge in pretreatment units of small constructed wetlands (CWs), anaerobic degradation processes occur and cause methane emissions into the environment. To minimize methane emissions, small-scale trials were carried out, in order to investigate whether vertical subsurface flow CWs (VF wetlands) are suitable for removal of methane and whether it can be utilized to improve the denitrification rate of VF wetlands. The results show that methane was completely removed for surface loads of up to 19 g CH4/(m(2).d). Based on mass balancing, 2% of the fed methane was oxidized with nitrate while 22% were oxidized with oxygen. The major part was attributed to biomass build-up. A correlation between effluent COD load and removed methane load was observed. Published literature indicates that COD accumulation is a result of production of extracellular polymeric substances. Low methane loads have only a slight impact on COD accumulation. It is expected that typical methane loads resulting from pre-sedimentation units may cause effluent COD concentrations to increase by up to 20% for municipal wastewater treatment with VF wetlands. Hence, combined wastewater treatment and methane oxidation was successfully demonstrated, but enhanced denitrification could not be achieved.