Employing conductive carrier for establishing spontaneous microbial galvanic cell and accelerating denitrification

It is well-known that metal corrosion is accelerated by formation of galvanic cell. In this study, we reported the acceleration of denitrification by using conductive carrier through formation of microbial galvanic cell (MGC). Electrically conductive graphite plate (GP) was used as biofilm carrier and compared with the non-conductive polypropylene (PP) plate carrier. Cyclic voltametric analyses showed that biofilms with bidirectional electron transfer functions of bioelectrochemical denitrification (BEDN) and acetate oxidation could be enriched spontaneously onto the GP carrier, hinting the establishment of MGC. Further analysis using differential pulse voltammetry revealed that the redox mediator related to extracellular electron transfer was detected in both media of the GP and PP carrier. Microbial community analysis showed that the biofilms in both GP and PP carrier had identical microbial composition but varied in abundance. The genus of Comamonas, Pseudomonas, Paracoccus and Thauera were the dominance of electroactive denitrifiers responsible for BEDN in both the GP and PP carrier. The GP carrier had a 75.9% higher abundant enrichment of electroactive denitrifiers than the PP carrier. Denitrification performance analyses showed that the GP carrier had a denitrification rate constant (k_DN) of 1.25 and 2.66 h^-1 at 15 °C and 30 °C, respectively, which was nearly 76.1% and 92.7% higher than the non-conductive PP carrier with corresponding values of about 0.71 and 1.38 h^-1. Further, the result of conductive carrier accelerating denitrification was confirmed in scaled-up denitrification bioreactors with volume of 104 L using brush-like biofilm carriers. The acceleration of denitrification was attributed to the spontaneously established MGC, which promoted the direct and mediated electron transfer of the electroactive denitrifiers grown onto the conductive carriers and speeded up the BEDN. The result of this study demonstrated that the BEDN could be integrated to traditional biological denitrification system to accelerate denitrification in the form of MGC by simply employment of conductive carrier.