Construction materials-based hybrid electroactive constructed wetlands for the treatment of hardly degradable wastewater: Circuit connections

This study employed two hybrid electroactive constructed wetland systems to treat recalcitrant landfill leachate. Each hybrid system included a media-based vertical flow (VF) stage, followed by a horizontal flow (HF) stage, and a water-column-based floating treatment (FT) unit as a post-polishing stage. The VF and HF stages between the two systems differed in the media used (stone dust vs. brick chips). The electroactive systems operated with (closed-circuit operation) and without (open-circuit operation) external resistor loads. Mean removals of 57%-89%, 42%-67%, 70%-87%, and 90%-93% for organic matter, nitrogen, phosphorus, and coliforms, respectively, were achieved by the two hybrid systems. The brick chips-based hybrid system achieved better pollutant removal than the stone dust-based system. Electrochemically unoxidized organic matter altered the redox gradient, which negatively influenced organic, nitrogen, and coliform removal of the upflow-based VF units. The HF and FT stages improved the removal performances of both systems. Filler material-induced adsorption controlled phosphorus removal. The closed-circuit operation enabled more intensive electrochemical oxidation of organic matter. Supplementary electron production via this electrochemical route supported NO 3 -N reduction, as the leachate wastewater was largely nonbiodegradable. Nutrient removal via plant uptake ranged from 0.1% to 2%, indicating their minor impact on overall removal. Power density production (across the two systems) ranged from 40 mW/m 3 to 1294 mW/m 3 during the closed-circuit operation. Internal resistance reduced bioelectricity production in the stone dust media-based system. This study identifies the pollutant removal mechanisms and the interacting factors (i.e., media, biodegradation ratio, redox potential, and external resistor load) in hybrid electroactive constructed wetland systems dosed with hardly degradable landfill leachate.