Bioelectrochemically assisted constructed wetlands for municipal wastewater treatment and the suitability of different biochar materials as system media

Microbial electrochemical technology constructed wetlands (METlands) use electroconductive (EC) materials in place of inert substrates such as gravel to enhance pollutant removal. This study evaluated whether EC biochar produced from waste biomass (olive pit, wood pellet, and woodchip) could function as an alternative METland media, an area with limited prior research. Biochars were tested in up-flow columns and compared with coke, a commonly used high-EC medium, and gravel, the typical inert substrate in conventional wetlands. Pollutant removal performance, material properties, biofilm-associated bacterial communities, and gas fluxes were assessed. Biochar and coke generally achieve higher removal of chemical oxygen demand (COD), nitrogen, phosphorus, and selected micropollutants than gravel, indicating that EC biochar can serve as a suitable METland medium. Electroactive bacteria were present at low relative abundance but were highest in biochar systems. Biochar columns also showed higher CO₂, N₂O, and CH₄ fluxes, though these patterns are only indicative due to limited GHG measurements, and further research is needed to clarify greenhouse gas emissions.