Comparison of dry and wet digestion for solid waste

To reduce the amount of MSW going to landfills a number of technologies have been developed. Two main types of anaerobic digestion processes are distinguished for MSW organic waste, which are generally referred to as "wet" (10-15% DM) and "dry" (24-40% DM) anaerobic digestion processes. The input is conditioned to the appropriate DM content by adding process water as required. This article compares a full-scale dry process, Valorga in La Coruña, Spain and a wet process, Vagron in Groningen, The Netherlands. A crucial difference is that the Vagron facility applies a washing step prior to the AD process to remove inert materials for re-use and to prevent damage of the installation. In the Valorga facility the organic fraction of the mechanical separation process is fed directly to the AD process. Both processes can be considered proven technology. Specific gas production is practically identical. Waste water production is higher in the wet process, as is to be expected, but this is compensated by a smaller amount of digestate to be disposed of and the separation of inert materials suitable for recycling. The organic loading rate for the Vagron process appears to be higher and the required reactor volume smaller in comparison to the dry Valorga process. The applicability of AD processes is strongly determined by the environmental standards set for the products from digestion. The German standards for digestate result in a lower potential for recycling of inert material separated from organic MSW.