Largus T. Angenent and Norman R. Scott

Abstract

Anaerobic digestion is a proven technology for bioconversion of agricultural wastes that are high in organic material. This engineered process with an undefined mixed microbial culture is based on the carboxylate platform (i. e., intermediates are channeled through short-chain fatty acids). The advantage of a mixed culture is that the waste material can be complex and variable in composition over the operating period, while sterilization is not necessary. The anaerobic food web is very efficient in transforming complex organic compounds into methane for stable digesters, because almost all intermediate products in the food chain are converted into methane with very low concentrations of carboxylates in the digester effluent and hydrogen in the off gas. The reason for such a high conversion is that the final products (methane and carbon dioxide) freely bubble out of the solution, which results in the cir­cumvention of product inhibition and separation costs. However, methane formation is sensi­tive to instabilities. Thus, practical studies are still being performed to answer the following questions: How do mixed substrates, such as agricultural residues and manures, affect methane fermentation during co-digestion? How can performance and stability be improved? What are the limitations of methane fermentation? What are the economic and environmental benefits of conversion of agricultural residues to methane? What are the future directions for improved methane fermentation? In this chapter we will address these questions for anaerobic digestion of slurries and solid wastes.