Pilot Testing of Bioaugmentation Using Post-Aerobic Digester Biomass as a Source of Nitrifiers

Anaerobic-aerobic phased digestion, or post-aerobic digestion, is a promising advanced digestion process applying the sequential operation of anaerobic digestion followed by aerobic digestion. Post-aerobic digestion provides total nitrogen removal without the need for chemical addition in addition to enhanced volatile solids reduction. Since nitrification occurs in the post aerobic digester, digester biomass may also represent an external source of nitrifiers for bioaugmentation. The City of Meridian, ID pilot tested bioaugmentation using post-aerobic digestion. Two 106-L activated sludge reactors with settling for biomass retention and one 16.5 L post-aerobic digester were constructed at the Meridian wastewater treatment plant. The activated sludge reactors were operated at a solids retention time (SRT) resulting in unstable nitrification. One reactor was bioaugmented with post-aerobic digester biomass. Bioaugmentation resulted in stable nitrification for ten days of reactor operation. However, the slow recovery of stable nitrification after an upset of the bioaugmented reactor suggests further optimization of the system is required to ensure stable bioaugmentation is maintained. No detrimental effects due to recycling of digested biosolids were observed in the bioaugmented reactor and the results of the current study suggest that further investigation of this new wastewater treatment concept is warranted. The performance of post-aerobic digestion was also demonstrated here. The post-aerobic digester achieved 80% ammonia removal and complete denitrification without the need for chemical addition. Thirty percent volatile solids reduction was achieved at a digester SRT of 9 days. The observed volatile solids reduction is twice as high as previous studies have reported at the same SRT and represents a significant decrease in the mass of biosolids for disposal if implemented full scale.