Using a microbial fuel cell to balance the carbon-nitrogen mismatch in submerged fixed-bed reactors for the resilient treatment of mariculture wastewater

Two of the main challenges in treating mariculture wastewater are the variable carbon to nitrogen ratio as well as its high sulfur content. Here we developed and tested the ability of a microbial fuel cell (MFC) to balance the mismatch between carbon and nitrogen loads in a submerged fixed-bed bioreactor (SFBBR) treating mariculture wastewater. Three reactors were tested including an SFBBR without an MFC, an SFBBR coupled to an MFC in an open-circuit (OC) arrangement, and an SFBBR-MFC in a closed circuit (CC) arrangement. The latter allows for flow of electric current. Both MFC configuration reduced sulfide yield by 65.6-111.7 g m(-2) compared to the SFBBR alone. All reactors presented high nitrogen removal efficiency (> 95 %) when an external carbon source was applied. Results of highthroughput sequencing and the Picrust2 revealed that the mixotrophic denitrification, namely sulfur autotrophic denitrification and heterotrophic denitrification, contributed to the high nitrogen removal efficiency. When external carbon was withheld, however, the SFBBR-MFC-OC and SFBBR-MFC-CC showed a significantly higher nitrogen removal efficiency than the SFBBR, i.e., 79.9 +/- 6.3 % or 76.1 +/- 6.3 % vs. 56.0 +/- 6.0 %. This difference was most likely driven by the presence of reduced sulfur compounds (e.g., S-0, SO32-) generated at the first anodes of the MFC reactors which could support continued nitrogen reduction for up to 48 h without carbon addition. Thus the MFC configuration promoted the formation of a "sulfur-pool" that could balance the carbon-nitrogen mismatch in the SFBBR, allowing for resilient treatment under variable influent characteristics.