The processing of mixed wastes and of source-separated wastes to recover materials involves a series of unit processes. The number of unit processes depends upon the degree of source separation of the wastes, as well as the types of materials to be recovered. In this chapter, waste processing facilities are discussed that process mixed municipal solid waste (MSW), sourceseparated materials, or both.
The nature and design of each unit process accommodate the physical and chemical characteristics of the particular materials or types of materials for which each is intended. Because most discarded materials have certain characteristics in common, basic processing principles (e.g., those pertaining to size reduction, air classification, and screening) usually are non-specific with respect to the materials in a category. However, the details of equipment design, size, degree of complexity, and cost of individual unit processes are strongly influenced by the nature and utility of the material to be recovered, and the extent and degree of source separation. The gradation also depends upon the function being served, e.g., recovery from the waste stream, readying the recovered material for reuse.
The basic types and principles of unit processes are described and discussed in the first part of this chapter. The discussion of unit processes is in the general sense. Therefore, the discussion focuses on mixed waste of which source-separated materials are a subset. Subsequently, the design of processing systems is discussed, i.e., the arrangement and sequence of unit processes to attain particular goals. This discussion includes the design of systems to process source-separated materials.
In this chapter, unit processes include both manual and mechanical processes. Mechanical processes include electromagnetic, fluid dynamic, pneumatic, and others. Of the categories of unit processes applicable to waste processing, those related to physical separation and removal from the waste stream of a particular type of material or mixtures of types of materials are the cornerstones of process design. The unit processes involving separation differ from the other unit processes in that they can be either manual or mechanical, or a combination of the two.
Resource recovery (primarily through scavenging) plays an important role in the economies of developing nations and in the economies of thousands of families. However, in most developing countries, the working conditions of scavengers are substandard. The following sections are presented in the hope that the concepts will be used to design systems in which the working conditions of the scavengers are substantially improved.
Some of the processing concepts and system designs described in this chapter have been instituted in industrialised countries and are predicated on the delivery of relatively clean mixtures of source-separated recyclable materials (e.g., containers or yard debris) to the processing location. The collection of source-separated materials and the mechanical and manual processing of them are popular and pervasive in many areas of North America. However, this method of collection and processing of wastes may not be readily instituted as a new system in developing countries, nor readily incorporated into the existing waste management systems. Economical institution of these recycling systems in North America has been a challenge as systems based historically on mixed waste collection and disposal are converted to an equal emphasis or greater emphasis placed on recycling. The problem of instituting similar recycling
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programs in developing countries in an economical manner is several-fold worse due to lack of, and competition for, financial resources.
The feasibility of processing source-separated materials has a partial grounding in the need for separation of materials by the generator and the storage of the materials on the generator’s premises. To fulfil this need, not only must the generator be motivated and trained in the separation of materials, but he or she must also have a separate container and space to store it. In North America, in many cases, the containers are provided at no cost to residential generators, as part of the motivational process. The supply of containers to householders in developing countries, the additional storage space requirement, and behaviour modification are among the reasons that collection and processing (the subject of this chapter) will have limited application in the short term for many developing nations.
Also confounding the feasibility and application of the processing of source-separated materials in developing nations is the circumstance that some of the highly mechanised designs presented in this chapter may be too costly and complex for many communities. However, despite the current limitations of applicability of highly mechanised designs, the concepts and designs are presented in this chapter for completeness and because the design process is relevant to less complex source separated programs, e.g., waste separated into wet (i.e., putrescible) and dry (i.e., non-putrescible) fractions. Communities in some developing countries are testing or have instituted forms of wet/dry collection and processing of wastes, e.g., Guatemala and the Philippines