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 (HNO₂) 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/m³/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 HNO₂ production and sludge degradation. Overall, this process provides a promising sludge management strategy for small-scale WWTPs.