Repurposing pre-pasteurization as an in situ thermal hydrolysis pretreatment process for enhancing anaerobic digestion of municipal sludge: a horizontal comparison between temperature-phased and standalone thermophilic or mesophilic anaerobic digestion
This study evaluated the effect of pre-pasteurization (70 °C for 45 min) on the performance of temperature-phased anaerobic digestion (TPAD) in horizontal comparison with standalone mesophilic and thermophilic anaerobic digestion (AD). The results suggested that existing pre-pasteurization system installed in utilities for the purpose of pathogen reduction can function in a similar way as the thermal hydrolysis pretreatment being implemented across the world for AD process intensification. For this reason, pre-pasteurization substantially increased soluble organic matter concentrations in terms of soluble chemical oxygen demand, protein, and carbohydrate by 22, 59, and 43%, respectively, and thus improved methane production, solids reduction, sludge dewaterability, and biosolids odor reduction. However, also because of the sludge biodegradability improvement by pre-pasteurization, high-rate AD such as the thermophilic phase of TPAD was subjected to extensive souring problem during startup which can be remedied by slow pace of SRT reduction and buffer addition. In spite of this challenge, upon arriving at steady state, the TPAD system fed with pre-pasteurized sludge outperformed all other AD processes, leading to at least 21% higher normalized methane production rate and 10% more volatile solids reduction.