Evaluating bioelectrochemically-assisted constructed wetland (METland®) for treating wastewater: Analysis of materials, performance and electroactive communities

METland® technology consists of a bioengineering strategy for treating wastewater by integrating microbial electrochemical concepts into constructed wetland systems to enhance pollutants removal. In this context, we have constructed planted (Iris sibirica) biofilters to assess the impact of different electrically conductive bed materials (electroconductive coke, electroconductive biochar, non-electroconductive biochar and gravel) by analyzing the (i) wastewater treatment efficiency (COD and nitrogen removal), (ii) bioelectrochemical response, and (iii) diversity of microbial communities. Electrically conductive materials outperformed non-conductive ones allowing removal rates as high as 175–180 gCOD/bed*m3 day capable to support footprint as low 0.4 m2/pe. In contrast, the highest nitrogen removal rates were achieved with non-conductive biochar in presence of plants (80 %) regardless the anoxic conditions of the assay. This was confirmed by the presence of annamox bacteria like Planctomycetes. Furthermore, the presence of a marked electric potential profile along the bed height in electroconductive materials together with redox pairs (cyclic voltammetry analysis) demonstrated an effective electron flow from bottom to uppermost layers of the bed (geoconductor mechanism). In electroconductive biochar, such effective conductivity-based model co-exists with a geobattery mechanism due to presence of electroactive phenolic and carbonyl/quinone groups and/or microporosity. Microbial biodiversity analysis revealed the impact of plants just at the upper layers of the biofilters where roots and Rhizobium predominate. Bacteria from genus Clostridium were dominant in gravel inert material; in contrast, bacteria from genus Geobacter (12%) and Trichococcus (30%) outcompete the rest of communities for an effective colonization of carbonaceous beds, suggesting their main role as part of the electrosyntrophies mechanism after METland®.