Anaerobic digestion of waste activated sludge—comparison of thermal pretreatments with thermal inter‐stage treatments

BACKGROUND: Treatment methods for improved anaerobic digestion (AD) of waste activated sludge were evaluated. Pretreatments at moderate thermal (water bath at 80 °C), high thermal (loop autoclave at 130–170 °C) and thermo‐chemical (170 °C/pH 10) conditions prior to AD in batch vials (40 days/37 °C) were compared with inter‐stage treatments under the same conditions carried out between two separate steps of AD (19–21 days/37 °C). Combined treatment at 80 °C with CO2/ NH3‐stripping was also evaluated. RESULTS: Pretreatment at 80 °C had no effect on methane yield while inter‐stage treatment gave a 20% increase, compared with controls. Combining inter‐stage treatment with CO2/ NH3‐stripping gave an increase in pH (7.1 to 9.3), a drop in ammonia‐N concentration (910 mg‐N to 510 mg‐N) and a methane yield improvement of 31%. Pretreatment at 130 °C, 170 °C and 170 °C/pH 10 considerably increased the methane production within the first 4 days but the improvement following 40 days of digestion was only 13%, 9% and 2%, respectively. In comparison, inter‐stage treatment led to improvements of 9% (130 °C), 29% (170 °C) and 28% (170 °C/pH 10). All treatment processes increased sludge solubilization. CONCLUSION: Thermal treatment of waste activated sludge for improved anaerobic digestion seems more effective when applied as an inter‐stage treatment rather than a pretreatment.