Effect of substrate size on partial denitrification-ANAMMOX in a subsurface vertical flow constructed wetland with step-feeding

Abstract

Subsurface vertical flow constructed wetlands (VFCWs) with step-feeding were used for the advanced treatment of anaerobically-digested swine wastewater taking rural domestic sewage as the external carbon source, the volume ratio between rural domestic sewage and anaerobically-digested swine wastewater after secondary treatment was defined as shunt ratio. This study explored the effect of substrate size (20–25, 7–10, and 3–5 mm) on partial denitrification-ANAMMOX (PD-A) in VFCWs at a shunt ratio of 1:3. Substrate size significantly affected the redox potential gradient and biofilm biomass content in the substrate layer, the diversity and abundance of the nitrogen-transforming microorganisms varied accordingly, leading to fluctuations in the intensity of nitrogen conversion processes (especially PD-A) in the VFCWs. When the substrate size was 7–10 mm, the abundance and activities of both ANAMMOX bacteria and denitrifying bacteria were significantly enhanced in the installation area of the shunt pipe, and there was an increase in the nitrifying-denitrifying performance in the upper substrate layer. Correspondingly, the total nitrogen, ammonia‑nitrogen, and nitrate‑nitrogen removal rates of the system reached (72.07 ± 2.61)%, (77.51 ± 1.98)%, and (64.46 ± 3.85)%, respectively. According to the contributions of functional gene groups associated with nitrogen transformations, it was concluded that appropriate substrate size enhanced PD-A and improved nitrogen removal through co-promotion of PD-A and simultaneous nitrification and denitrification in the VFCWs.