A novel strategy for a stable low-ammonia nitrification system: low-temperature sludge storage recovery

Stable partial nitritation is difficult to sustain in low-ammonia wastewater, as nitrite-oxidizing bacteria (NOB) outcompete ammonia-oxidizing bacteria (AOB) for limited substrate and thereby reduce nitrite accumulation. This study adopted intermittent aeration combined with hydrazine dosing and compared two sludge storage approaches to rapidly start partial nitritation at an influent ammonium nitrogen (NH₄ ⁺-N) concentration of 40 mg L^-1. A single UASB reactor operated with dissolved oxygen (DO) < 0.7 mg L^-1, intermittent aeration, and hydrazine addition achieved stable performance in 24 days, with 60% ammonium conversion and 77% nitrite accumulation rate (NAR). Further reducing DO to 0.6 mg L^-1 cut aeration energy, stabilizing ammonium conversion at 50%, while NAR approached 100%. The measured AOB and NOB activities were 0.42 ± 0.01 and 0.1 ± 0.01 mg N (g VSS h)^-1, respectively. Nitrifying sludge was preserved for 5 months at 4 °C or at ambient winter temperatures (-25 to 12 °C), then seeded into reactors R1 and R2. R1 with refrigerated sludge stabilized within 15 days, maintaining 57% ± 4% ammonium conversion and 80% NAR, with far better performance than R2. PICRUSt2 functional prediction confirmed that 4 °C sludge storage boosted AOB enrichment and upregulated ammonia monooxygenase (AMO) expression. The integrated hydrazine regulation and low-temperature sludge storage strategy enables fast, stable partial nitritation under low ammonium loading, providing technical support for mainstream PN/A applications.