Pre-processing is applied during MSW composting for one or more of the following reasons.
· To increase the proportion of compostable materials in the feedstock – Glenn 1991 – MSW contains a high proportion of non-compostable or poorly compostable material, hence composting the whole MSW stream is inefficient and leads to a very low grade of compost. See also the Critical Review Section, Feedstocks and composition - Physical characteristics.
· To improve feedstocks for other recovery options Removal of compostable materials (and materials removed in parallel such as glass) can improve the efficiency of down stream sorting processes to recover energy and dry recyclables in mechanised plant (Anon 1991, Barton 1983, 1984 and 1986, Barton and Poll 1983, Barton and Wheeler 1988 )
· To reduce levels of contamination by inerts and trace elements. MSW contains items which may be hazardous in a finished compost, for example glass which may be sharp or plastics which can injure grazing animals, and also give the compost an unsatisfactory appearance. It also contains levels of trace elements which could restrict, or eliminate, the usefulness of the finished compost. Pre-processing (often combined with post processing, or refining) is used to control the levels of hazardous items and substances in composts, for example exploiting known differences in contaminant distribution, such as the tendency of fines (<5-10 mm) to be enriched with trace elements. See also the Critical Review Sections: Feedstocks and composition - Physical characteristics, Refining and Product quality and environmental impacts
· To recover other recyclable or re-usable materials (such as ferrous metal). Many of the non-compostable fractions of refuse may potentially be recovered for other purposes, for example recycling or energy recovery. Poorly compostable materials such as paper and card may also be better recycled or combusted for energy recovery. As a rule MSW processing takes place in a plant that, as far as possible, integrates several processing routes to divert fractions of the waste to the most appropriate recovery approach. See the Critical Review Sections: Feedstocks and composition - Physical characteristics and Composting: Past and Present.
· To condition the feedstock to make it more easily compostable. Large items will only degrade slowly, and may also degrade anaerobically beneath their surface. Compostable materials may be embedded in non-compostable items such as plastic bags. Conditioning liberates the compostable material and controls the size of particles to support a more efficient biological processing step. See the Critical Review Section, Biology of Composting - Process Optimisation. Pre-processing with chemical amendments, for example to control pH or change CN ratios has been applied for MSW feedstocks, but is uncommon.
· To mix materials, ensuring even and thorough distribution of the moisture, nutrients and substrates.
· To reduce contents of pathogens and parasites, for example by the suggested use of autoclaving. Pre-processing using microwave irradiation or autoclaving to kill pathogens has been trialled, mainly at pilot scale, but is not in widespread use, nor considered necessary Some of these processes are described in the Environment Agency Waste Technology Data Centre at
http://www.environment-agency.gov.uk/wtd/.
Most of the pre-processing techniques that are applied are mechanical in nature. There are two broad categories: shredding and separation. Separation technologies exploit differences in properties between components of interest. For example MSW fractions that pass through a 50 mm screen tend to be enriched in putrescibles. Separation is achieved by exploiting one or more differences in size, shape, density or electro-magnetic properties. Shredding or pulverisation is typically achieved by attrition, usually in knife mills or hammermills. However, water based systems have been applied, most frequently the combined shredding and screening approach “wet pulverisation”, but various maceration techniques have also been applied, rarely at practical scales. For MSW streams “debagging” is also necessary, as wastes are often contained in one or more plastic or paper bags. This may be achieved by pulverisation or shredding, or using spikes in rotating trommel screens that pierce and rip sacks.
In many cases an MSW processing plant, such as an MBT facility, will include several of these processes arranged in various “circuits”. There are two broad families of approach that depend on the initial step taken to deal with the input MSW:
· size reduction by milling or shredding
· trommel screening to achieve separation into size ranges followed by subsequent separations.
This chapter discusses the following pre-processing methods in more detail:
· Separation technologies (handpicking, size and density based techniques, use of electric / magnetic fields)
· Size reduction approaches
· How size reduction and separation are combined
· Other conditioning approaches
· Materials handling issues.
This chapter is not intended as a comprehensive review of mechanical and other waste separation technologies, which have been discussed elsewhere. A review of MBT plants is the subject of another major SET project (see http://www.sitaenvtrust.co.uk/).