The recent upsurge of interest in composting the organic fractions of municipal solid waste (MSW) builds on largely successful efforts with composting yard trimmings, agricultural wastes, and sewage sludges. While experience with these other materials is helpful in considering an MSW composting program, there are a number of challenges unique to MSW which need to be addressed. MSW contains materials which vary widely in size, moisture, and nutrient content, and the organic fractions can be mixed with varying degrees of non-compostable wastes and possibly hazardous constituents. Manufacturing a marketable compost product from this material requires a range of physical processing technologies in addition to the biological process management common to other types of composting.
Four tasks are central to the design of a modern MSW composting system: collection, contaminant separation, sizing and mixing, and biological decomposition. This fact sheet reviews the various technologies and options currently available for preprocessing MSW and accomplishing the first three tasks. Fact Sheet 2 discusses biological processing and overall system evaluation.
The first of the preprocessing tasks, collection, largely determines the processing requirements of the remaining tasks because they must be tailored to the characteristics of the incoming waste. Separation processes at the composting facility generate recyclable and reject streams, usually at several places in the process. Size reduction increases the surface area of the organic wastes, enhancing opportunities for biological activity, while mixing ensures that nutrients, moisture and oxygen are adequate throughout the material. Options for accomplishing these first three tasks are described below.