New insights into deep nitrogen removal in WWTP effluent: Performance and mechanisms of autotrophic denitrification filters.

Wastewater treatment plants (WWTPs) face increasing demands for enhanced nitrogen removal from their effluents, yet current removal efficiencies remain suboptimal. The sulfur-siderite-magnetite autotrophic denitrification (SSMAD) system has been shown to achieve high nitrogen removal efficiency in previous studies. Nevertheless, the design and optimization of such reactors have garnered insufficient systematic research attention. This study designed four reaction devices with distinct configurations. All devices were packed with fillers of the SSMAD system. The results indicate that the Vertical Baffled Denitrification Filter (VBDF) exhibits denitrification performance comparable to that of the Conventional Denitrification Filter (CDF) in terms of total nitrogen removal. When the HRT is less than 1 h, its total nitrogen removal efficiency remains stable at 80%, and its effluent total nitrogen concentration is maintained at approximately 3.49 mg/L. At an HRT of 30 min, the denitrification loading rates of the VBDF and CDF reach 9.04 g N/m3·h and 9.34 g N/m3·h, respectively. The baffle design of the VBDF increases the pressure in the first compartment of the denitrification filter, resulting in a higher flow velocity of the influent when it enters the subsequent compartments and thereby achieving superior mixing efficiency of the influent within the denitrification filter. In addition to Thiobacillus, the VBDF system was additionally enriched with Ferritrophicum and Geothrix, which in turn led to significantly higher expression levels of denitrification-related functional genes in the VBDF system-consistent with its enhanced nitrogen removal capacity. These findings provide a theoretical basis and design guidance for developing compact autotrophic denitrification systems.