Thermal Hydrolysis to Enhance Anaerobic Digestion Performance of Wastewater Sludge

Abstract

The objective of this review is to provide a closer check on the recent development in the thermal hydrolysis (TH) of wastewater sludge for anaerobic digestion performance enhancement with the focuses on the solubilization of sludge and macromolecules, organic micropollutants removal, TH-AD integration, and TH process (THP) energy balances. The recent research has developed improved understanding of the thermal hydrolysis mechanisms of proteins, carbohydrates, and lipid components of sludge and the characteristics of soluble COD in TH-treated sludge. Studies showed that TH treatment can partially remove micropollutants, but it has limited impact on the bioavailability of those compounds. For the THP and AD integration, the post TH and intermediate TH treatment have been introduced as two alternatives of the TH pretreatment, which have demonstrated the ability to achieve higher methane yields and VSS reduction. TH is a well-established technology for anaerobic digestion performance enhancement due to its ability to improve digestion loading rate, sludge biodegradability, and sludge dewaterability. The recent studies elucidated the mechanisms of sludge TH, characteristics, and biodegradability of hydrolysis products, fate of nutrient and micropollutants in sludge TH, and the effect of AD-TH integration on methane production and VS reduction. Future work in sludge TH is still needed to reveal the mechanisms of production of refractory organics during TH treatment, optimize the design and operation of the ITHP-AD and AD-PsTHP processes, and improve the energy efficiency of TH-AD processes through energy reduction and recovery.