Achieving nitrification-denitrification in a low-SRT activated sludge system through bioaugmentation of sidestream nitrifying granules

This study investigated the ability of sidestream nitrifying granular activated sludge to increase the nitrification capacity in mainstream flocculent activated sludge systems through bioaugmentation and selective retention of granules. Nitrifying granules were grown in a sidestream reactor fed municipal anaerobic digestion centrate and added in an initial slug dose and subsequent smaller doses to a non-nitrifying mainstream system at 12°C and 2.5 day flocculent sludge aerobic solids retention time (SRT). Effluent NH3-N concentrations less than 1 mg/L were achieved with bioaugmentation, and nitrification was immediately lost when granules were removed 30 days after the initial bioaugmentation. Molecular microbial analyses also indicated that bioaugmented nitrifying organisms remained attached to granules and that nitrifying bacteria loss to flocculent sludge was scarce. Maximum specific nitrification activity of the granules decreased in mainstream treatment, but the decrease was proportionate to an increase in total granule mass, which indicated that some granule growth occurred. This study demonstrated that bioaugmentation with nitrifying granules can intensify nitrification capacity in low-SRT flocculent activated sludge systems to enable low effluent NH3-N concentrations and nitrogen removal.