Optimizing nutrient removal of algal-bacterial symbiosis system for treating low C/N farmland drainage

The removal of nitrogen from farmland drainage is challenging due to the typically low carbon-to-nitrogen (C/N) ratio. In this study, an algal-bacterial symbiosis system was developed to treat low C/N farmland drainage. The investigation focused on the nutrient removal rates, microbial growth characteristics, extracellular polymeric substances (EPS) content, and microbial community species composition under varied disturbance frequencies and light conditions (intensity and duration). Results demonstrated that the optimal operating conditions were three disturbances per 24 h, a light intensity of 20,000 lux, and a 16-hour lighting duration. Under these conditions, the average removal rates of soluble chemical oxygen demand, total nitrogen, total phosphorus, nitrate nitrogen, and ammonia nitrogen reached 45.1 %, 73.3 %, 98.1 %, 63.1 %, and 97.3 %, respectively. Compared to continuous disturbance, intermittent disturbance reduced energy consumption by over 90 % and promoted higher biomass accumulation, with an average dry weight of 508.7 mg L⁻¹ and chlorophyll-a concentration of 521.0 μg L⁻¹. Meanwhile, a robust microbial community and a balanced bacterial-to-algal gene copy ratio (exceeding 25:1) were critical for nutrient removal. The optimized system facilitated symbiote secretion of bound polysaccharides (45.2 μg L⁻¹, double that of other reactors), promoting the formation of robust biofilms and enhancing nutrient removal. This work provides a technical reference for improving nutrient removal in low C/N wastewater treatment processes.