Modelling and Investigating the Impact of Thermal Hydrolysis Pretreatment on the Anaerobic Co-Digestion of Sewage Sludges and Food Waste
Abstract
In this study, the impact of thermal hydrolysis pretreatment (THP) on the mono- and co-digestion of thickened waste activated sludge (TWAS) and food waste (FW) was carried out at temperatures of 150, 170, 190, and 210 ˚C for a contact time of 30 min and mixing ratios of 90:10, 70:30, and 50:50 on a volumetric basis in batch anaerobic tests. Thermal hydrolysis influenced the solubilization of the particulate organic constituents of TWAS, reduction in volatile suspended solids (VSS) and particle size with the increase in temperature to 210 ˚C, achieving a reduction in VSS up to 62% and a reduction in the mean particle size up to 75% relative to the raw TWAS. Thermally pretreated TWAS showed an 18% improvement in methane yields and biodegradability, and a 39% increase in the methane production rate relative to the raw TWAS at 170 ˚C. Increasing the THP temperature beyond 170 ˚C detrimentally impacted digestion and was associated with the formation of refractory compounds. Thermal hydrolysis showed no significant influence on the solubilization of FW, VSS and particle size reduction with the increase in temperature, achieving solubilization of only 7% of the particulate material and an increase in the mean particle size up to 62% relative to the raw FW. Thermally pretreated FW showed a 3% increase in methane yields and biodegradability, and a 10% increase in the methane production rate relative to raw FW at 150 ˚C. The increase in the THP temperature beyond 150 ˚C was associated with the formation of refractory compounds which consequently decreased methane yields, biodegradability, and kinetics relative to raw FW.
Co-digestion improved methane yields, biodegradability, and kinetics as the volumetric contribution of FW increased, with the detrimental effects of THP on the individual feedstocks being reflected in their co-digestion. Co-digestion of thermally pretreated TWAS with FW showed the largest improvements of 27% in methane yields and biodegradability, and a 29% improvement in methane production rates relative to the raw 90:10 mixture at 170 ˚C, with no synergetic effects observed. Co-digestion of thermally pretreated FW with TWAS showed a 14% improvement in methane yields and biodegradability, and a 25% increase in methane production rates at 150 ˚C, with improvements up to 21% in experimental yields relative to theoretical yields due to synergism. Thermal hydrolysis of the TWAS and FW mixtures showed a 52% increase in the methane yield and biodegradability of the 90:10 at 150 ˚C, and a 92% increase in the methane production rate at 170 ˚C, with improvements becoming less pronounced with the increase in the volume of FW and temperature.
The efficacy of thermal hydrolysis at the full-scale is a function of the aerobic solids retention time (SRT) of the activated sludge (AS) system. The increase in the aerobic SRT is associated with an increase in the inactive fraction of the biological sludge, including endogenous decay products which are hardly biodegradable. A fraction of the endogenous products is converted with THP to particulate materials more favourable for microbial consumption. The solubilization of a mixture of primary sludge (PS) and TWAS is lower relative to TWAS only, since two of the main components of PS (i.e. starch and cellulose) do not degrade at common THP temperatures of 160–180 ˚C. Improvements in anaerobic biodegradability with THP stem from improvements associated with the TWAS not PS; primarily, it is the conversion of endogenous products which unlocks methane potential, while the improved kinetics allow for improved degradation at shorter anaerobic SRTs.