Pilot-Scale Demonstration of Two-Phase Anaerobic Digestion of Activated Sludge

An innovative pilot-scale two-phase anaerobic digestion process was developed to stabilize concentrated (7-5%) activated sludge at a 12-day SRT and a loading rate of 5 kg VS/m3 d. The pilot system exhibited an unusually high VS reduction of 73%, an aggregated carbohydrate-protein-lipid reduction of 71%, and a methane yield of 0.3 m3/kg VS added. Optimum acidogenic fermentation producing 9500 mg/l of organic acids was achieved at an HRT of 3 days and a loading rate of 16 kg VS/m3.d or higher. Enhanced acidogenic hydrolysis and prehydrolysis of polymerics and nocardial residues eliminated digester foaming. Sulfate and nitrate reductions, and syntrophic methane fermentation occurred during acidogenic conversions. Acetogenesis and aceticlastic methane fermentation were predominant in the methane digester, which generated 93% of the system methane production. Contrary to literature reports, there was no inhibition of acetogens or methanogens at a high ammonia-N concentration of 2500 mg/l and pH 7.7. The acid and methane digesters could be started quickly. They were resilient to temperature drops and loading fluctuations. Acid fermentation of 8% of the plant's WAS and addition of the resulting fermentation products (enzymes, organic acids, etc.) to the full-scale high-rate digester increased VS reduction by 46% and eliminated severe foaming. Full-scale two-phase digestion of WAS is scheduled to start in 1990.