Valorization of animal and human wastes through hydrothermal liquefaction for biocrude production and simultaneous recovery of nutrients

Abstract

Intense farming activities and the growth of the population produce increasing amounts of wastes, which represent an environmental concern and require an adequate disposal. Animal manure, fish sludge, and sewage sludge are all wet wastes consisting of organic, but also inorganic material. Hydrothermal liquefaction is proposed to treat these wastes as wet feedstocks can be processed without any drying. The organic fraction is valorized, being converted into biocrude oil, while the inorganics are recovered primarily in the solid products. The decomposition of these wastes is investigated under sub- (350 °C) and supercritical (400 °C) conditions, and with and without the addition of K2CO3 catalyst with focus on the biocrude yield and quality. High yields of biocrude are obtained from the liquefaction of all the feedstocks, especially from fish sludge (ca. 50% d.a.f.) and sewage sludge (ca. 45% d.a.f.). A reduction in biocrude production is observed at supercritical conditions for animal wastes, however, the quality of manure-derived biocrudes is improved when using supercritical conditions and by the addition of the catalyst. Carbon is primarily recovered in the biocrude: 50–60% for swine and cow manure, 55–80% for fish and sewage sludge. Considerable quantities of nitrogen and sulfur are transferred to the biocrude, respectively 26–60% and 33–66%. Most of the inorganics (e.g. Ca, Mg, P) are recovered in the solids (above 70%), except for potassium and sodium, which show a higher degree of solubility in the aqueous phase.