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The difficulty of the start-up and maintenance the stability of partial nitritation (PN) was considered as a key challenge for mainstream anammox. This study demonstrated a novel approach for achieving rapid start-up of mainstream PN by sludge treatment using high salinity based on the phenomenon which high salinity is far more biocidal to nitrite-oxidising bacteria (NOB) than to ammonia-oxidising bacteria (AOB) in short-term (within 24 h). In this study, one-third of the nitrifying sludge from the continuous stirred tank reactor (CSTR) fed by mainstream wastewater was suppressed at 70 g NaCl/L for 24 h every day. The treated nitrifying sludge was afterwards returned back to the CSTR. Mainstream PN with high nitrite accumulation ratio (NAR) was rapidly established (in 14 d) after optimizing the suppression conditions and maintained stable (NAR = 95.69 ± 2.65 %) more than 45 days in the mainstream reactor …