Rapid Waste Activated Sludge Reduction and Stabilization via a Biofilm-Based Acidic Aerobic Digestion Process

Aerobic digestion is commonly used for waste activated sludge stabilization in small-to-medium wastewater treatment plants (WWTPs), but the long retention time (15–30 days) required for stability severely limits process intensification. Acidic conditions maintained by nitrification can enhance digestion efficiency, but at short hydraulic retention time (HRT), nitrifiers are washed out. Here, we utilized a biofilm-based solution to retain nitrifiers and produce in situ free nitrous acid (HNO 2 ) to expedite WAS stabilization and reduce pH. Two laboratory-scale sludge digesters were operated for over one year to evaluate this approach. This process reduced pathogen levels to well below the Class A biosolids standard within an extraordinarily short HRT of 1.75 days. Furthermore, it achieved a high volatile solids (VS) reduction rate of 0.87 ± 0.13 kg/m 3 /d. Model-based analysis indicated the biofilm-based digester achieved both a higher hydrolysis rate ( k ) (0.58 ± 0.07 d –1 vs 0.51 ± 0.13 d –1 ) and a greater extent of VS degradability ( f d ) (31 ± 2% vs 21 ± 2%) compared to the suspended sludge digester. Microbial community analysis revealed that biofilms enriched ammonia oxidizers and acid-tolerant heterotrophs, underpinning enhanced HNO 2 production and sludge degradation. Overall, this process provides a promising sludge management strategy for small-scale WWTPs.