Resource recovery from organic solid waste using hydrothermal processing: Opportunities and challenges

Large amounts of organic solid waste are generated daily throughout the world. Hydrothermal processing has been shown to be a promising solution for dealing with this type of waste. This paper reviews the types of hydrothermal processing currently available, their advantages and disadvantages, and their suitability for converting organic solid wastes to value-added products. This technology can also be used to produce fuels such as bio-char, bio-oil, and biogas, which are more energy-dense than fuels produced after biochemical conversion of waste. Furthermore, hydrothermally produced fuels have similar energy densities to fossil fuels. Hydrothermal technology provides opportunities for resource recovery such as converting waste into value-added products (e.g., organic acids and fertilisers), nutrient recovery and metal removal from organic waste. The challenges associated with developing hydrothermal processing at an industrial scale are examined, and research perspectives and future developments are discussed in this article. Since the hydrothermal process is capable of eliminating and transforming organic waste efficiently, its customer readiness level (CRL) and societal readiness level (SRL) are moderate. However, this technology has a low technology readiness level (TRL) because it operates at high temperatures and pressures. Lack of fundamental thermodynamic data at extreme conditions, limited understanding of reaction kinetics and mass transfer effects on the process are the main challenges of hydrothermal processing. Other important factors of hydrothermal processing are its economic and safety considerations which must be considered before installing a hydrothermal processing unit. A commercial case study on hydrothermal technology (Terax™) revealed that the challenge of sewage sludge management can be addressed successfully using a combination of biological and hydrothermal processes.