Low-temperature thermal hydrolysis for anaerobic digestion facility in wastewater treatment plant with primary sludge fermentation

Abstract

Primary sludge (PS) fermentation prior to anaerobic digestion (AD) is becoming more common in wastewater treatment plants (WWTPs) for the production and utilization of volatile fatty acids-rich sludge liquor in biological nutrient removal processes. However, the thermal hydrolysis process (THP) for enhancing AD of sludge in WWTPs with PS fermentation has rarely been investigated. This study examined low-temperature THP (50–90°C, 30–90 min) for enhancing co-digestion of fermented primary sludge (FPS) and thickened waste activated sludge (TWAS). The experiments were conducted under two schemes: scheme-1 (THP of TWAS + FPS) and scheme-2 (THP of TWAS only). Scheme-2 was more effective in solubilizing COD (up to 20.4% increase in SCOD) and various macromolecular compounds (e.g., proteins and carbohydrates). In contrast, scheme-1 was more efficient in VSS solubilization (up to 26.12%). Scheme-1 also resulted in a greater enhancement in methane production over the control (scheme-1: 56.28% at 90°C, 90 min vs. scheme-2: 43.4% at 90°C, 60 min). Thus, these results suggested that low-temperature THP of TWAS alone (scheme-2) would result in solubilization of refractory macromolecular compounds, leading to a relatively lower positive impact on methane yields than scheme-1. The preliminary economic assessment considering THP operating cost, enhancement in energy recovery, and saving in biosolids handling costs indicated that THP at 90°C (90 min) under scheme-1 could provide the highest net saving of $79.55/dry tonne solids compared to the control (AD without THP). These results will provide technical guidance in adopting THP in WWTPs with PS fermentation.