Understanding how microbial electrolysis cell assisted anaerobic digestion enhances triclocarban dechlorination in sludge

Microbial electrolysis cell assisted anaerobic digestion (MEC-AD) has recently been considered as an efficient method for degradations of refractory pollutants. To date, however, knowledge about whether and how MEC-AD enhances the degradations of refractory pollutants in sludge remains largely unknown. This study therefore aims to fill this knowledge gap through investigating the transformation of triclocarban (TCC), a widely used antimicrobial agent, in MEC-AD reactors. Experimental results showed that over 83.3 % of TCC was dechlorinated to less toxic dichlorocarbanilide, monochlorocarbanilide and carbanilide in MEC-AD reactors. However, the mass loss of TCC in AD reactor (the electrodeless control) was merely 0.53 %. The presence of electrodes promoted TCC dechlorination in MEC-AD reactors, while the applied voltages (0.6 and 0.8 V) promoted hydrogenotrophic methanogenesis. H2-utilizing Nitrospira and homoacetogenic Acetobacterium were recognized as potential TCC dechlorinators, with their abundances in the planktonic sludge of MEC-AD reactors being 5.0-16.5 times higher than those in AD reactor. The carbon brush electrodes in MEC-AD reactors caused the enrichment of acetoclastic Methanothrix and the complete removal of acetic acid, which thereby thermodynamically accelerated homoacetogenesis and H2-producing acetogenesis in the planktonic sludge. Moreover, the direct interspecies electron transfer using hydrogenase as terminal electron acceptor was enhanced in the planktonic sludge of MEC-AD reactors, which could also improve H2 production rate and stimulate the growth and activity of TCC dechlorinators.

Automated summary

This study investigates the transformation of triclocarban (TCC) in microbial electrolysis cell assisted anaerobic digestion (MEC-AD) reactors, and fills the knowledge gap about whether and how MEC-AD enhances the degradation of refractory pollutants in sludge. The results show that over 83.3% of TCC was dechlorinated in MEC-AD reactors, while the mass loss in the control AD reactor was only 0.53%. The presence of electrodes promoted TCC dechlorination in MEC-AD reactors, and applied voltages promoted hydrogenotrophic methanogenesis.