AbstractThe biological treatment process in high-altitude areas faces the problems of low biological activities, high aeration energy consumption, and low treatment efficiency due to the low atmosphere pressure and consequently low oxygen content. As a new type of simultaneous nitrogen and phosphorus removal process, simultaneous nitrification, denitrification, and phosphorus removal (SNDPR) has the advantages of high efficiency, low operation cost, and low aeration energy. Compared with the traditional sewage biological treatment process, SNDPR needs less dissolved oxygen (DO) and can save carbon source, so it may be suitable for solving the problems in high-altitude areas caused by low pressure and low oxygen. In this study, SNDPR system reactors were operated under different atmosphere pressures, one at 100 kPa and three at 72 kPa, with different anaerobic/aeration time. The chemical oxygen demand (COD) and total phosphate (TP) removal performance remained the same efficiency, while the nitrogen-removal rate increased slightly at 72 kPa. The low atmosphere pressure enhanced the phosphorus-accumulating organisms but inhibited ammonia-oxidizing bacteria based on the cycle study results. Studies on the activities of enzymes related to nitrogen removal showed that the processes of NH4+→NH2OH→NO2− and NO3−→NO2− were enhanced, while the processes of NO2−→NO3− and NO2−→NO were restrained as the atmosphere pressure declined. Under the condition of 72 kPa, the SNDPR systems were optimized by prolonging the anaerobic time and reducing the aeration time. The residual ammonium concentration increased predictably (<5 mg/L) as the aeration time decreased, while the nitrogen-removal efficiency improved. The aeration energy consumption at 72 kPa by a shorter aeration time was 89% of that at 100 kPa. The further study on the microbial community analysis showed that the abundance of Dechloromonas [identified as denitrifying phosphorus-accumulating organisms (DPAOs)] increased from 0.126% at 100 kPa to 5.499% at 72 kPa, which explained the nitrogen removal and phosphate uptake under the low atmosphere pressure.