Enrichment and characterization of acid-tolerant nitrifying sludge

•An acid-tolerant nitrifying sludge was successfully enriched at pH 6.0.•It had a high volumetric ammonia removal rate of 1.13 kg-N m−3 day−1 at pH 6.0.•It had a high specific ammonia conversion rate (0.29 g-N g−1 VSS day−1) at pH 6.0.•It tolerates acid due to the dominant Nitrosospira and ammonia-oxidizing archaea.

Nitrification is an acidifying process that requires the addition of external alkalinity because of the alkaliphilic nature of the most ammonia-oxidizing bacteria. In this study, aerobic activated sludge was used as inoculum in an internal loop air-lift reactor, which resulted in successful enrichment of acid-tolerant nitrifying (ACIN) sludge at pH 6.0 by sequential addition of tea orchard soil suspension. The results showed that ACIN sludge had a remarkable acid tolerant capability with a volumetric ammonia conversion rate of 1.13 kg N m−3 day−1. ACIN sludge showed a higher maximum specific ammonia conversion rate (0.29 g N g−1 VSS day−1) than neutrophilic nitrifying sludge (0.14 g N g−1 VSS day−1) at pH 6.0 and had good resistance against pH fluctuations, with a maximum specific ammonia conversion rate (0.584 g N g−1 VSS day−1) at pH 7.5. Microbial community analysis indicated that the higher abundance of acid tolerant Nitrosospira and ammonia-oxidizing archaea laid a solid foundation for the remarkable acid-tolerant capability of ACIN sludge.