Hydrothermal processes as treatment paths for biogenic residues in Germany: A review of the technology, sustainability and legal aspects

A considerable part of especially wet and sludgy biogenic residues is currently not in material or energetic usage in Germany. Therefore, a key issue for current research is to identify which technologies are most suitable at mobilizing these wet and sludgy materials. Hydrothermal Processes (HTP) appear to be promising treatment options for moist substrates because they require a high water content of 70%-90% for optimal processing. This review provides information on the state of the art and knowledge on HTP, and attempts to determine how suitable these processes are for mobilizing biogenic residues in Germany. We identified technological, economic, environmental and legal potentials and barriers of HTP using a modified content-analysis. About 120 relevant references were identified and analyzed using a structured sampling scheme. The results show considerable advantages of HTP for utilizing wet and sludgy biogenic residues in contrast to comparable biomass treatment processes. Especially, their high process energy-efficiency and low Global Warming Potential from a life cycle perspective. Nevertheless, technological, economic, environmental and legal barriers (e.g. missing data and knowledge on process kinetics; missing legal standards) must be taken into consideration. Finally, research needs are illustrated that must be fulfilled through structured and target-oriented research.