Hydrothermal Processing of Organic Waste

Hydrothermal processing is an advanced thermochemical way of converting liquid to liquid and energy-rich fuels without the use of energy-intensive drying to transform high-moisture organic wastes. This chapter presents a thorough technical analysis of hydrothermal processing pathways, including hydrothermal carbonization, liquefaction, gasification, and supercritical water processing, together with a special focus on reaction mechanisms, operating conditions, and technological maturity. Hydrolysis, depolymerization, and reforming of complex organic matter can be effectively achieved by the unique physicochemical properties of water in the subcritical and supercritical states, which include a reduced dielectric constant and an increased ionic product. A wide range of feedstocks are considered, including agricultural residues, municipal organic waste, sewage sludge, food waste, and algal biomass, based on the content of moisture, elemental composition, and ash behavior. The effects of the key process parameters, including temperature, pressure, residence time, catalyst utilization, and the reactor configuration, on conversion efficiency and product distribution are analyzed. The key bioenergy products, such as hydrochar, bio-crude oil, and fuel gases, are defined by the energy density and the ability to be upgraded, along with the possibility of nutrient and organic recovery of aqueous by-products. The energy balances and life-cycle assessment indicators are used to measure process performance and environmental sustainability, demonstrating the decrease in the waste volume and greenhouse gas emissions. The technical viability and ability to integrate the system can be seen in the selected case studies, including the hydrothermal carbonization of sewage sludge in Europe and the pilot-scale hydrothermal liquefaction of food waste and microalgae in Asia. The chapter ends with a discussion of scale-up challenges and the latest technological developments that have made hydrothermal processing one of the essential elements in circular bioenergy systems.