Solid waste digestors: process performance and practice for municipal solid waste digestion

The most common types of anaerobic digesters for solid wastes have been compared based on biological and technical performance and reliability. Batch systems have the most simple designs and are the least expensive solid waste digesters. They have high potential for application in developing countries. Two-stage systems are the most complex and most expensive systems. Their greatest advantage lies in the equalisation of the organic loading rate in the first stage, allowing a more constant feeding rate of the methanogenic second stage. Two-stage systems with biomass accumulation devices in the second stage display a larger resistance toward toxicants and inhibiting substances such as ammonia. However, the large majority of industrial applications use one-stage systems and these are evenly split between "dry" systems (wastes are digested as received) and "wet" systems (wastes are slurried to about 12% total solids). Regarding biological performance, this study compares the different digester systems in terms of organic loading rates and biogas yields considering differences in input waste composition. As a whole, "dry" designs have proven reliable due to their higher biomass concentration, controlled feeding and spatial niches. Moreover, from a technical viewpoint the "dry" systems are more robust and flexible than "wet' systems.