Biogas Production: Mechanical and Thermal Pre-treatment Technologies

Abstract

Mixed organic wastes are heterogeneous substrates with high moisture content as well as high levels of different contaminations. Therefore, in order to carry out an ideal anaerobic digestion process using this kind of substrates, i.e., highest possible biogas production while avoiding operation complications and failures, pre-treatment is a crucial stage of the whole process. In the present chapter, an overview on the mechanical and thermal pre-treatment technologies, with a focus on plants running on heterogeneous solid wastes, is presented. In general, mechanical pre-treatments are used for size reduction of organic matters to enhance the active surface area of hard-solid substrates as well as for separation of impurities (e.g., plastics, glass, stone, metal, etc.) to lower the cost of operation and maintenance. The current industrially-available mechanical pre-treatment technologies used in AD plants, i.e., hammer mills, impact reactor combined with a reject separation, pulpers, pressing as well as crushing technologies have been discussed. Following mechanical pre-treatment, organic fraction of waste can be further treated at high temperature and/or pressure values in order to disintegrate the structure of hardly-degradable substrates to increase the biogas production rate and/or to pasteurize the organic waste. Different thermal pre-treatment methods have also been presented herein.