Microbial electrochemical systems outperform fixed-bed biofilters in cleaning up urban wastewater

In this work, we present for the first time the concept of integrating microbial electrochemical technologies (MET) with natural wastewater treatment biofilters used in constructed wetlands (CW) to form METlands. In order to validate this technology, three lab-scale horizontal subsurface flow (HSSF) biofilters, two hosting electroconductive materials and one gravel biofilter (control) were operated for 525 days to define the best design and operational conditions to maximize the removal of wastewater pollutants. Organic loading rates tested ranged from 2 to 24 g BOD5 m(-2) d(-1) at hydraulic retention times (HRT) from 4 days to as low as 0.5 day, respectively. The electroconductive biofilter showed the best COD and BOD removal rates per volume of bed, achieving mean values of 213 g COD m(-3) d(-1) and 119 g BOD m(-3) d(-1) at the lowest HRT (0.5 d). Ammonia and total nitrogen maximum removal efficiencies at 3.4 days of HRT were 97 and 69%, respectively, in the electroconductive biofilter. Bacterial communities were studied by 16S rDNA Illumina sequencing with the aim of understanding the role of the electrically conductive material in selecting microbial populations. Deltaproteobacteria (a known electroactive taxon) were enriched in the presence of an electrically conductive bed. Geobacter and Geothrix were the dominant genera in the deeper zone of the electrically conductive bed where oxidation of organic matter occurred. The results suggest that the enhancement in biodegradation rate will significantly reduce the area requirements of classical CW.