To make more biogenic waste materials useable as substrates in anaerobic digestion plants, various pre-treatment options are under discussion. Usually, the main problem is that readily degradable substances are surrounded by non- or hardly degradable ones. Therefore, they are not accessible for the various groups of microorganisms realizing the overall anaerobic digestion. To improve the accessibility of these substances, often a mechanical pre-treatment is carried out. But, there are also approaches to treat these organic waste materials thermally. Against this background, the aim of this paper is to summarize the large number of existing studies on such thermal pre-treatment techniques and to combine their key findings. To be more precise, hydrothermal, vapothermal, microwave and steam explosion pre-treatment are compared in terms of their effects on digestibility as well as the respective energy efficiency. This closes an existing gap because currently there are no studies providing such a comparison based on the latest measurement results. The state of knowledge on these pre-treatment options is presented using the example of three different types of biogas substrates (lignocellulosic biomass, sewage sludge, and food wastes). For this purpose, the main ingredients of the mentioned biomass types are considered. Based on this, the reactions taking place during thermal pre-treatment are identified. The advantages and challenges of the individual procedures are highlighted allowing to choose the most promising pre-treatment option for each individual case.
