High-efficiency nitrogen and phosphorus removal for low C/N rural wastewater using a full-scale multi-stage A2O biofilm reactor combined with horizontal-vertical flow constructed wetlands system

Rural wastewater treatment faces significant challenges in achieving stable effluent quality due to factors such as temperature fluctuations, variations in water quality and quantity, and low carbon-to-nitrogen (C/N) ratios. This study developed a full-scale, non-membrane, multi-stage anaerobic-anoxic-oxic (MSA2O) biofilm reactor integrated with horizontal-vertical flow constructed wetlands (HVCWs), which was operated continuously for approximately 320 days with an average flow of 11.9 m3/d in a rural area of northern China. Key parameters were optimized: hydraulic retention time (HRT) of 21-32 h, aeration rate of 4.0 m3/h, carbon source dosing at 1.25 L/h, PAC dosing at 0.55 L/h, and mixed liquor reflux ratio at 200 %. The system demonstrated high removal efficiencies for COD (74.2 %), NH4+-N (93.4 %), TN (90.6 %), and TP (86.3 %), consistently meeting the class 1A of GB18918-2002, China (COD <= 50 mg/L, NH4+-N <= 5 mg/L, TN <= 15 mg/L, TP <= 0.5 mg/L), even under challenging conditions such as low C/N (3.3) and rainy seasons. More than 70 % of nitrogen and phosphorus were removed in the MSA2O system. Microbial analysis revealed the enrichment of many functional bacteria. Proteobacteria play a key role in denitrification and phosphorus removal. Actinomycetes, Acidobacteria, and Firmicutes to nitrogen fixation and organic matter degradation. Nitrosomonas dominated ammonia oxidation, while Dechloromonas and Accumulibacter significantly contributed to phosphorus uptake. Seasonal variations in microbial diversity enabled consistent and highly efficient nutrient removal. The HVCWs system contributed 16.3 % of total phosphorus removal through selected plant species and phosphorus-absorbing modified ceramsite, ensuring effluent polishing and stability. With low operational costs ($0.12/m3), the integrated system provides an effective and scalable solution for rural wastewater treatment, delivering high-quality effluent with minimal energy consumption.