Master of Engineering Science
Civil and Environmental Engineering
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.
Summary for Lay Audience
Industrialization, urbanization, and the continuous growth of the world’s population has transformed the food system in many areas resulting in changes in diets and increased demand for food supplies, and consequently food waste. Despite the increasing interest in food preservation, almost one third of edible food is wasted, mostly ending up in landfills. The continuous disposal of municipal solids wastes is causing the prompt exhaustion of landfills, with municipal solid wastes ending up producing landfill leachate and green house gases. In addition to the increased food demand and waste, industrialization is also linked to increased water consumption, consequently increasing wastewater production. Biological wastewater treatment is one the most economical approaches for the removal of the contaminants in the wastewater, however, during the treatment process semi-solid slurries termed sludges are produced depending on the treatment process, while the treatment of these sludges makes up more than 50% of the operational cost of treatment plant. Anaerobic digestion is among the oldest and most attractive methods for the treatment of organic wastes such as sludges and food waste for its lower energy requirements and the production of methane gas, a potential energy source. Even so, the application of anaerobic digestion, especially to biological sludges which are predominantly microorganisms, is often limited because of the longer retention times required for the reduction of these wastes to be rendered environmentally safe.
Various pretreatment methods such as thermal hydrolysis pretreatment have been developed to overcome the longer retention times of anaerobic digestion. Additionally, anaerobic co-digestion, which is the simultaneous digestion of two or more organic wastes in a homogenous mixture, represents a viable option to not only the diversion of food waste from landfills, but also improving the stability of the anaerobic digestion process and methane production.
This study investigates the impact of both thermal hydrolysis pretreatment and co-digestion of biological sludges and food waste at different mixing ratios and temperatures on methane production and production rates. Modelling was also used to evaluate the impact of thermal hydrolysis on the different components of biological sludges and its implications at full-scale plants.
Ismail, Amr, "Modelling and Investigating the Impact of Thermal Hydrolysis Pretreatment on the Anaerobic Co-Digestion of Sewage Sludges and Food Waste" (2021). Electronic Thesis and Dissertation Repository. 8021.
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