Outlook and challenges for recovering energy and water from complex organic waste using hydrothermal liquefaction

Abstract

The amount of organic waste is rapidly increasing, and so do the environmental impacts and costs associated with its management. Hydrothermal liquefaction (HTL) is a promising technology that can convert organic waste into a biocrude oil which, once upgraded, has similar thermochemical properties to petroleum-derived transportation fuels. HTL also produces significant amounts of aqueous, solid, and gas products, which can be further processed to maximize energy production and water recovery, as well as to restore nutrients into the value chain. In this review, the main factors influencing the yields of the HTL products, and the potential alternatives for the valorization of these products, are discussed. Additionally, an extensive review of the different types of organic waste subjected to HTL is presented together with an assessment of the potential energy and water recovery for representative organic wastes (i.e., animal manure, wastewater sludge, food waste, and agricultural waste). Analyses show that, by coupling HTL with anaerobic digestion (AD), it is possible to recover between 54 and 99% of the energy contained in the wastes, and to reduce between 70 and 92% the organic matter content in the influent (on a COD basis). These results were applied in selected regions (i.e., the USA, the EU, and China) based on available waste disposals. An integrated HTL/AD approach could potentially recover ca. 11.3 exajoules per year in these countries combined, and recover over 3600 million cubic meters of water with only a fraction of the organic waste's COD.