Coupled process of in-situ sludge fermentation and riboflavin-mediated nitrogen removal for low carbon wastewater treatment.

Volatile fatty acid recovery from waste activated sludge (WAS) was highly suggested to supplement carbon source for nitrogen removal. However, it was not easy to separate them from the metabolites under the ex-situ fermentation. In this study, in-situ WAS fermentation combined in the denitrification system was established to treat low carbon wastewater (COD/TN = 4), and riboflavin was employed as a redox mediator. This coupled process could simultaneously enhance the WAS fermentation and nitrogen removal, and riboflavin could significantly enrich the fermentative bacteria (Firmicutes phylum), denitrifying bacteria (Denitratisoma genus) and related functional genes (narGHJI, napABC, nirKS, nosZ, norBC), generating more available carbon sources for efficient nitrogen removal. This resulted in the effluent TN (<15 mg/L) satisfying the required discharge standard in China. This study provided new insights into the efficient nitrogen removal from low carbon wastewater, realizing the carbon-neutral operation of new concept wastewater treatment plant in China.