Study on the impact of hydraulic loading rate (HLR) on removal of nitrogen under low C/N condition by modular moving bed constructed wetland (MMB-CW) system

Traditional constructed wetlands often exhibit low nitrogen removal efficiency when treating low C/N wastewater, prompting the need for targeted technological innovation. Modular moving bed constructed wetlands (MMB-CWs) have demonstrated superior performance, achieving over 15% higher TN removal compared to traditional subsurface flow wetlands, even without the addition of carbon, further research is required to fully understand the operational effects of different hydraulic loads and the underlying nitrogen removal mechanisms. To address this gap, the contrast performance of MMB-CWs with three different substrate addition ratios 90% (unit A), 60% (unit B), 30% (unit C) was studied under low C/N conditions and different hydraulic loading rates (HLRs). Results show that there are no significant differences in TN removal among the three systems at 250 mm/d and 500 mm/d. NH4+-N and NO3--N removal rates don't significantly decrease (p>0.05). High HLR leads to significantly lower effluent NO2--N concentrations. HLR minimally affects CODCr removal, with unit C exhibiting better resistance. Functional gene analysis reveals a minor role of nitrification, emphasizing the crucial role of anaerobic ammonium oxidation. The copy number of the Anammox gene is 1.2–4.0 times that of the NIR gene, with a larger ratio in the secondary unit, indicating that denitrification is mainly driven by Anammox. The dominant genera in the first unit are Azospira, Thauera, and Denitratisoma, while Defluviicoccus, Mycobacterium, and Nocardioides dominate the ceramsite and biochar of the second unit. The addition of biochar enhances the composition and abundance of microbial communities in the system, and as biochar addition increases, the removal rates of NH4+-N and TN gradually increase, ensuring effective denitrification under varying loads.