Waste Collection and Sorting

The way that wastes are collected and sorted influences which waste management options can most effectively be used. The collection method significantly shapes the recovery of materials, compost or energy; this in turn determines whether markets can be found. Collection is also the point of contact between generators (e.g. households and commercial establishments) and the waste management system. Collection is rarely independent of subsequent sorting, since the type of

13 Oko-Institut eV, Waste prevention and minimisation: final report. Commissioned by European

Commission DG XI from Oko-Institut eV, Darmstadt, Germany, 1999.

collection affects the sorting needs, and some collection methods themselves involve a level of sorting.

Householder sorting. From the householder's viewpoint, commingled collection of all waste is probably the most convenient method, although this limits subsequent treatment options. Most treatment methods need some form of separation of the waste into different fractions in the home, prior to collection. At its simplest this might involve removing recyclable materials.

Household collection systems are often divided into 'bring' and 'kerbside' collection schemes. Bring systems are those where householders are required to take recyclable materials to communal collection points. Kerbside collection schemes require the householder to place recyclables in a container which they set, on a specified day, outside their property for collection. The extreme bring system is the central collection or Civic Amenity site to which householders transport materials such as bulky items and garden waste. Such sites often also have collection containers for recyclables. Other bring systems comprise materials banks at low density, often situated at supermarkets.

Kerbside collection is more narrowly defined, but collection can also be of separated fractions or of commingled waste.

Collecting household waste. Household waste has traditionally been collected mixed, but where household sorting has occurred the different waste streams are collected separately, whether by the same or different collection vehicles. The categories collected separately vary by geography: in Germany, the Duales System Deutschland (DSD) collects packaging material as a separate stream, whereas in Japan householders separate out combustible material for separate collection. In Europe and North America, separate collections are most commonly used for dry recyclables (paper, metal, glass, plastic), biowaste (kitchen and garden waste, with or without paper) and, in some countries, household hazardous waste (batteries, paint, etc.). A collection for remaining residual waste (known as restwaste) is also needed. Garden waste and bulky waste may be handled as separate streams, or alternatively included within the biowaste and restwaste streams, respectively.

Collecting dry recyclables. The greatest range of collection methods, from central or low density materials banks, to kerbside collection of recyclable materials in specially designed trucks. Single (mono) material banks ('drop-off') that collect a single material per container represent one of the best known forms of materials collection, mainly due to the success of 'bottle banks' for glass.

A range of kerbside collection methods have been used to collect recyclables, varying in the degree of sorting involved, and including boxes, bags and wheeled bins. In its simplest form, recyclables are separated by the household and stored together in a bag, box or wheeled bin ready for collection. As with the collection of mixed recyclables from street containers, collection can use existing collection vehicles, in some cases even with compaction. Commingled collection of recyclables, whether from communal kerbside containers or household bags or bins requires extensive subsequent sorting at a Materials Recovery Facility

(MRF). McDougall et a/.14 quote Schweiger (1992) which gives a comparison in recovery rates for kerbside and bring systems in Germany (Table 4) which shows that the former can achieve the highest levels of recovery, although great success can accompany the use of high density bring facilities.

Central sorting. Materials recovery facilities (MRFs) are facilities that process solid wastes for the purpose of recovering commodity-grade materials for sale, or recovering a mixed material fraction for subsequent processing or conversion (for combustion or mechanical-biological treatment). Depending upon the intent and design of the MRF, the wastes may be delivered to the facility as mixed solid wastes, as source-separated wastes (or alternatively, source-separated materials), or in both forms. However, facilities tend to process either mixed wastes or source-separated materials, not both.

The distinction between mixed solid wastes and source-separated materials is not a clear one. Mixed solid waste is heterogeneous, while source-separated materials are composed of specified components targeted for recycling. As more components are added to source-separated collection programmes in an effort to recycle more materials, the source-separated mixtures begin to approach the characteristics of mixed solid waste.

Both general categories of MRFs (i.e. those that process mixed wastes or those that process source-separated wastes) can utilise mechanical, manual, or both methods of processing materials. To achieve high levels of diversion of wastes from landfill, both source-separated and mixed waste MRFs may be required for a given community or region. For this discussion, high levels of diversion are those beyond the 40-50% level. The structuring of systems that would use both types of generic MRFs is an exercise in integration of the facilities in terms of compatibility and optimum efficiency and economics.

Mixed waste MRFs, in industry jargon, are also called 'dirty' MRFs, whereas source-separated MRFs are also called 'clean' MRFs. The term 'dirty' refers to the high level of contamination and moisture usually found in mixed solid wastes, while 'clean' describes the relatively uncontaminated materials that are provided by source-separated collection programmes.

Mixed waste MRFs. Mixed waste MRFs are designed to process residential, commercial, institutional, or industrial mixed solid wastes, or any combination of these. Levels of contamination in the delivered wastes are expected to be high, and non-processible items will require special handling. Non-processible items include stringy items such as cable, textiles, and magnetic tape and over-size objects such as exhausts from vehicles, water heaters, and rolled carpet. The gross recovery rate for commodity-grade secondary materials can be expected to be low (e.g. 10-30% of the quantities processed) due to the high levels of contamination and to concomitant low yield of products of marketable quality.

An exception can be the processing of mixed solid wastes from a specific type of generator where higher diversion is possible due to large concentrations of non-contaminated material (e.g. paper, in the case of commercial businesses).

14 F. McDougall et ai, Integrated solid waste management, a life cycle inventory, 2nd Edition, ISBN

0 632 05889 7, 2001.

Table 4 Comparison of German kerbside and bring systems

Paper

Glass

Recovery rate kg/person/y (%) kg/person/y

__Mixed dry

Recovery rate Metals Plastics recyclables Biowaste

(%) kg/person/y kg/person/y kg/person/y kg/person/y

Bring systems

1000 persons/bank 10-25 17-42 10-20 26-51 0.5-2.5

500 persons/bank 15-50 25-50 15-25 38-64 0.5-2.5

15-50

Kerbside collection paper collected (in bundles):

Every week Every 2 weeks Every 4 weeks

20-35 15-25 10-20

Paper (in containers) 35-55 Glass (in containers) Multi-material (glass, metals, plastics) 30-50 Bag collection 5-25

Bio-bin

33-58 25-42 17-33

58-92

50-83 8-42

15-30

12-30 5-20

38-77

31-77 13-51

30-60 50-140

Rates of recovery for mixed waste MRFs processing residential mixed wastes can be expected to be, and usually are, lower than those of facilities processing commercial and industrial mixed wastes. Typically, mixed waste MRFs have processing rates in the range of 200-1500 tonnes per day.

Source-separated MRFs (SSMRFs). Source-separated MRFs are designed to process individual components (e.g. aluminium cans), mixtures of individual components (e.g. commingled tin, glass, and aluminium containers), or both. Consequently, source-separated MRFs (SSMRFs) can be further divided according to the degree of mixing or commingling of the components, and also the number of processing lines dedicated to processing the different source-separated mixtures. For all SSMRFs, the quality of the materials delivered to the facility is expected to be high and contamination low. Process residues from SSMRFs typically are in the range of 3-10% of the weight processed, although values as high as 15% have been experienced.

General considerations. The primary objective of a MRF is the processing of the raw feedstock into a marketable or usable end-product. The processing must be carried out such that the end-products meet a given set of specifications. Also, especially in times of tight markets, recovered materials of high quality are desirable to assure continuity of end-product sales.

A variety of equipment is used in a materials processing facility. Conveyors (predominantly belt conveyors) are essential pieces of equipment in a MRF. Shredders, crushers, magnetic separators, screens, and densifiers (e.g. balers) are also frequently used. Rolling equipment used at MRFs includes front-end loaders, forklifts, and trailers. The design of mixed waste MRFs must incorporate processes to open bags to liberate their contents for segregation and potential recovery. Source-separated MRFs that accept bagged materials also require similar processes.

Regardless of whether a mixed waste or source-separated MRF is employed for recovering recyclable materials, successful design and equipment selection must accommodate the particle size distribution and other pertinent characteristics of the materials to be processed, achieve the necessary level of purity and yield of end-product, and prepare the end-products to the other specifications of the marketplace, including, for example, bulk density.

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