Low temperature impact and HAP-enhanced one-stage partial nitritation/anammox process: Long-term stability, high-rate nitrogen removal, and operational strategies at 15°C

• High nitrogen removal rate of 0.97 kg N/m 3 /d was first achieved at 15 °C. • HAP-PNA granular sludge demonstrated advantages for efficient sludge management. • Direct and indirect impacts of low temperatures disrupted stable nitrogen removal. • AnAOB were able to self-regulate at low temperature, unlike AOB. • Selective sludge discharge was effective in maintaining stable operation at 15 °C. The one-stage partial nitritation and anammox (PNA) process have the serious problem of suboptimal performance at low temperatures due to the extremely low anammox activity and nitrite accumulation. To tackle the challenges of nitrite accumulation and performance deterioration at low temperatures, a long-term experiment of one-stage hydroxyapatite (HAP)-based PNA reactor was conducted. The study aimed to identify the major challenges and critical factors for maintaining high capacity under low-temperature of 15 °C by investigating nitrogen and phosphorus removal efficiency and its impact on bacterial activity and communities, thermodynamics, and granular sludge characteristics. After the operation at 15 °C for over 300 d, a high nitrogen removal rate of 0.97 kg/m 3 /d was first achieved. The anammox community demonstrated reduced sensitivity to temperature decreases, whereas ammonia-oxidizing bacteria did not. Considering the direct and indirect impacts of low temperatures on bacterial communities and granular sludge, which led to a decreasing AnAOB proportion, effective sludge management—encompassing sludge content control and selective discharge of non-HAP granular sludge—was particularly critical at lower temperatures. This approach helped prevent excess biomass accumulation, maintain balanced PNA reactions, and ensure optimal operational performance. Notably, the ability of HAP-PNA granular sludge to maintain good settleability at low temperatures further supported the effectiveness of sludge management practices.