Nitrate removal without carbon source feeding by permeabilized Ochrobactrum anthropi SY509 using an electrochemical bioreactor

Biological denitrification is very attractive due to its economical and environmental advantages. However, an external carbon source is required to maintain biological activity in biological denitrification. Permeabilized cells have been used to achieve denitrification that does not require an external carbon source. In this study, permeabilized Ochrobactrum anthropi SY509, which contains the denitrifying enzymes, nitrate reductase, nitrite reductase, nitric oxide reductase and nitrous oxide reductase, was used. NADH participates in denitrifying enzyme reaction as an electron donor. However, in the bioelectrochemical method, a redox molecule was used for electron transfer to the enzyme, and the denitrification was performed bioelectrochemically. This redox molecule functions as an artificial electron transferring agent, and is thus called a mediator. Mediated bioelectrocatalysis is a useful system for many redox biocatalyst reactions as the aforementioned mediator mediates the biocatalytic reaction between electrode and biocatalyst. In this study, neutral red was selected as a mediator. For application to a real wastewater treatment system, immobilization of biocatalyst and mediator on the electrode is essential. Carbon nanopowder was used as a support material to immobilize neutral red. When both the permeabilized cells and the carbon nanopowder with the mediator were immobilized on a graphite felt electrode, mediator leakage was completely prevented and high denitrification efficiency was obtained.