Integrated approach for waste activated sludge stabilization under ultra-short solids retention time

Sludge stabilization is essential for safely disposing waste activated sludge (WAS). This study introduces an integrated approach to stabilize unthickened WAS under ultra-short solids retention time (SRT). Two aerobic digesters were operated: a control at 15-day SRT and an experimental digester with SRT reduced from 15 to 1.5 days. At 2.5-day SRT, pH decreased to 5.0 ± 0.3 due to ammonia oxidation by influent nitrifiers, but nitrite accumulation was insufficient to generate the required free nitrous acid (FNA). To address this, chemical acid was added to lower the pH to 4.3 ± 0.1, promoting FNA formation. This enabled effective stabilization, achieving Class A standards with a specific oxygen uptake rate of 0.2 ± 0.1 mg O₂/g VS/h and pathogen levels of 2.3 ± 0.1 log MPN/g total solids. Economic analysis showed this integrated approach is cost-effective, with microbial ammonia oxidation driving most of the acidification and minimizing chemical use.