Inhibition of free ammonia to the formation of aerobic granules

Abstract

This study investigated the inhibition of free ammonia to the development of aerobic granules for simultaneous organic carbon removal and nitrification in sequencing batch reactors. Five column sequencing batch reactors (SBRs) were operated at different N/COD ratios in the range of 5/100 to 30/100. Results showed that aerobic granules formed only when the free ammonia concentration were less than 23.5 mg l −1, and nitrification was completely inhibited at a free ammonia concentration greater than 10 mg l −1. It was found that the specific oxygen utilization rates of heterotrophic and nitrifying bacteria were reduced by a factor 5 and 2.5 as the free ammonia concentration increased from 2.5 to 39.6 mg N l −1. Free ammonia resulted in a significant decrease of cell hydrophobicity, and also repressed the production of cell polysaccharides. These would be responsible for the failure of aerobic granulation at high free ammonia concentrations. This study probably for the first time demonstrated that free ammonia could hinder the formation of aerobic granules through its inhibition to energy metabolism of microorganisms.