Enhanced Nitrogen Removal of Low C/N Ratio Wastewater by Coupling Fe(Ii)-Driven Autotrophic and Heterotrophic Denitrification Using Conductive Carrier

The denitrification of low C/N ratio wastewater usually faces the issues of slow rate and low efficiency. Nitrogen removal rate can be greatly improved through heterotrophic denitrification by adding sufficient carbon source, but would greatly add the treatment cost. In this study, we report enhanced nitrogen removal of low C/N ratio wastewater by coupling Fe(II)-driven autotrophic and heterotrophic bioelectrochemical denitrification using conductive biofilm carrier. The introduction of conductive graphite carrier is proved to be able to enhance the Fe(II)-driven autotrophic denitrification. The bioreactor using graphite plate as biofilm carrier has a denitrification rate constant (kDN) of 0.016 h-1, which is 1.77 and 5 times as that of with non-conductive polypropylene and without carrier, respectively. The presence of Fe(II) and conductive graphite carrier promotes the denitrification of low C/N ratio wastewater. The kDN of the bioreactor for treatment wastewater with low C/N ratio of 0.76 with Fe(II) is 0.095 h-1, which is 5 times as that of without Fe(II). The reasons for the enhancement of the denitrification of low C/N ratio wastewater are probably that, the conductive carrier enhances the direct electron transfer and facilitates the growth of electroactive microorganisms, while the Fe(II) not only serves as electron donor but also plays the role of mediator and facilitate the mediated electron transfer, thus enhances the both Fe(II)-driven autotrophic and heterotrophic bioelectrochemical denitrification. Microbial community analysis show that the addition of Fe(II) changes the biofilm community composition and greatly enhances the enrichment of electroactive bacterium like Comamonas, and denitrifers such as Chryseobacterium and Castellaniella, in the conductive biofilm carrier under heterotrophic conditions. While the autotrophic conditions are beneficial to the enrichment of other microorganisms without denitrification function, such as soil Aquamicrobium Hydrotalea. This study provided a new strategy for enhancement of the denitrification low C/N ratio wastewater