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

Area of Science:

• Environmental Engineering
• Microbiology
• Biotechnology

Background:

• Aerobic digestion is standard for sludge stabilization in small-to-medium wastewater treatment plants (WWTPs).
• Long retention times (15-30 days) limit process intensification.
• Nitrification can enhance digestion but nitrifiers are lost at short hydraulic retention times (HRT).

Purpose of the Study:

• To develop a biofilm-based solution for rapid sludge stabilization.
• To utilize in situ free nitrous acid (HNO₂) production to expedite waste activated sludge (WAS) stabilization.
• To retain nitrifying bacteria within the digester at short HRTs.

Main Methods:

• Operated two laboratory-scale sludge digesters for over one year.
• Employed a biofilm-based approach in one digester and a suspended sludge approach in the control.
• Analyzed pathogen levels, volatile solids (VS) reduction, hydrolysis rates, VS degradability, and microbial communities.

Main Results:

• Achieved Class A biosolids standards for pathogen reduction within 1.75 days HRT.
• Reached a high VS reduction rate of 0.87 ± 0.13 kg/m³/d.
• Biofilm digester showed higher hydrolysis rate (0.58 d^-1 vs 0.51 d^-1) and VS degradability (31% vs 21%) than suspended sludge digester.
• Biofilms enriched ammonia oxidizers and acid-tolerant heterotrophs.

Conclusions:

• The biofilm-based process significantly accelerates WAS stabilization.
• This method is effective in reducing pathogen levels and volatile solids at short HRTs.
• The process is a promising sludge management strategy for small-scale WWTPs.