Nutrients removal mechanisms in high rate algal pond treating rural domestic sewage in East China

This study focused on the evaluation of performance and mechanisms of a high-rate algal pond system (HRAP System) in nitrogen and phosphorus removal for rural domestic sewage treatment. A pilot scale HRAP System was located at Yangzhu Village, Jiangsu Province, east China, with treatment processes including a septic tank, two stages of HRAPs and an aquatic pond. Results showed that the HRAP System had a good performance in nutrient removal, especially in NH⁺₄‐N removal. Total removal efficiencies of COD, TN, NH⁺₄‐N and TP were 80, 51.8, 90.2 and 52.1%, respectively. About 61.6% of NH⁺₄‐N in the 1st-stage HRAP and 70.9% in the 2nd-stage HRAP were transformed into NO⁻₃‐N and NO₂⁻‐N through nitrification, and the nitrogen losses via ammonia volatilization was only 2.7% (for 1st-stage HRAP) and 8.8% (for 2nd-stage HRAP). The other 35.7% and 20.3% of NH⁺₄‐N removal were achieved by algae assimilation respectively. About 98.0% (for 1st-stage HRAP) and 84.8% (for 2nd-stage HRAP) removal of TN were attributed to algae sedimentation, while those through ammonia volatilization were only 2.0% (for 1st-stage HRAP) and 15.4% (for 2nd-stage HRAP), respectively. Therefore, the main mechanisms of NH⁺₄‐N removal in HRAP were nitrification and algae assimilation, and TN was mainly removed by algae sedimentation. About 50.7% (for 1st-stage HRAP) and 53.1% (for 2nd-stage HRAP) of phosphorus in the deposit were organic phosphates respectively, only 20.7% and 27.7% were calcium-bound and magnesium-bound phosphates. The removal mechanism of TP in HRAP could be mainly attributed to algae assimilation in forms of organic phosphate, and chemical precipitation in forms of calcium-bound and magnesium-bound.