A Novel Layered and Advanced Nitrogen Removal Filter with Gravel and Embedded Bio-Organic Carrier Based on Autotrophic and Heterotrophic Pathways

Given increasingly prominent environmental issues, there is a pressing need to satisfy more stringent emission standards for wastewater treatment plants (WWTP) while concurrently prioritizing energy conservation; a new up-flow layered nitrogen removal filter was constructed on a laboratory scale using gravel (for the bottom and top layers) and embedded bio-organic carriers (for the middle layer) containing microorganisms as fillers to treat the secondary effluent by introducing a portion of raw water. This study investigated the nitrogen removal effectiveness and transfer pathways of synthetic wastewater at varying mixing ratios, promoted the enrichment of Anammox Bacteria (AnAOB) by embedding microorganisms, and analyzed the microbial community structure using high-throughput sequencing techniques. The findings showed that the highest total nitrogen (TN) removal efficiency was achieved with chemical oxygen demand (COD), ammonia (NH4+-N), and nitrate (NO3−-N) contents in the mixture at 77, 10, and 8 mg·L−1, respectively, with an average efficiency of 89.42%. NO3−-N was mostly removed through denitrification (heterotrophic), while NH4+-N was eliminated by partial nitrification (PN) and anaerobic ammonium oxidation (Anammox, autotrophic). According to high-throughput sequencing results, denitrifying bacteria such as Thauera (1.30–6.96%), Flavobacterium (0.18–0.40%), and Parcubacteria (0.14–0.32%) were present in all the filter layers, and Anammox bacteria such as Candidatus_Kuenenia were predominant in the middle layer at a 0.88% abundance, with the aid of organic carriers.