Food Waste Fermentation Liquid as a Supplementary Carbon Source for Enhanced Biological Nitrogen Removal from Rural Wastewater

Large variations in water quality and quantity are the main characteristics of rural wastewater in China, and the biggest impact caused by this is the lack of carbon sources. In this study, an anoxic–oxic (A/O) biological contact oxidation (BCO) reactor was used to explore the feasibility of using food waste fermentation liquid as the supplementary carbon source for enhanced nitrogen removal from rural wastewater. After using the carbon source supplements, the removal performance of the A/O BCO system was improved, with the removal efficiencies of COD, NH4+–N, and TN at 92.4%, 97.8%, and 67.6%, respectively. Mechanism studies showed that the activities of key denitrifying enzymes (NAR, NIR, NOR, and NOS) for nitrogen removal were improved, with NIR activity increasing by 36.9%. Microbial community analysis revealed that food waste fermentation liquid increased the diversity of denitrifying microbial populations. Notably, insights from metagenomics showed that the relative abundances of two key genes (nirS and nirK), which are vital indicators of the denitrification process, were significantly improved with the addition of food waste fermentation liquid as a supplemental carbon source, resulting in the enhancement of nitrogen removal from rural wastewater.