How to identify suitable ways for the hydrothermal treatment of wet bio-waste? A critical review and methods proposal

A considerable amount of wet biogenic residues and waste has no resource-efficient use in several European countries yet. Hydrothermal processes (HTP) seem to be promising for treating such biomass as they best work with substrates with 70% to 90% water content. However, thus far the suitability of HTP for this purpose has not been sufficiently evaluated, for which this work aims to identify suitable multi-criteria analysis (MCA) methods that can be used to identify promising ways for the hydrothermal treatment of wet bio-waste. A review on 31 recent MCA studies in bio-waste management was conducted with the aim of comparing them to methodological requirements for evaluating HTP. Furthermore, an MCA approach for HTP based on the review findings is proposed. Results show that no observed MCA method is directly transferable for assessing HTP, for which a customized approach combining the analytical hierarchy process and the technique for order preference by similarity to ideal solutions is proposed and preliminarily validated with literature data. These preliminary calculations indicate that hydrothermal gasification seems most promising under consideration of multiple criteria using the available average and exemplary data. However, needless to say there is still a long way to go to obtain the sufficient adequate data to validate and use the model appropriately, for which further studies are necessary to acquire more reliable data and to assess also future technology developments of HTP.