Dehydration of methanization digestates : economics analysis of 4 process protocols

Methanization is not, strictly speaking, a treatment solution, but the co-generation industry can nevertheless provide its input by dehydrating a fraction of the digestate ahead of export. We led an economics analysis on 4 process protocols coupling dehydration with a methanization unit. Scenarios were built around two digestate volume targets for dehydration (3000 and 10000 m3/yr) and three surplus levels (20, 50 and 80%). The results highlight that scenarios employing a drier-conveyor appear most profitable in most of the settings studied, except (1) for larger-scale methanization units working at 80% surplus, where the heat deficit makes it necessary to employ an expensive add-on boiler requiring heavy investment and fuel-cost outlays,making it economically more profitable to use multi-stage evaporator-concentrators; (2) for smaller-scale methanization units working at 80% surplus, where no scenario proved profitable. This paper underlines that technical factors (type of co-product obtained, equipment technology, etc.) also need to be integrated when selecting a process protocol.