What is recycling? Separation at source and mechanical classification of household waste

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What is recycling? It is the process by which materials are collected and used as “raw” materials for new products.

There are three steps in recycling: 1. Materials are collected. 2. Materials are processed and manufactured into new products. 3. Consumers buy products made from reprocessed materials.
Materials are sorted at source and collected, or collected without segregation. The latter is often referred to as black bag waste, due to the color of the bags used in most countries.

However, before proceeding further, we should consider what the typical average UK household waste analysis might contain. Detailed lists are available on the web for the contents of these containers and wheelies, but in summary, the components can be classified as putrescible, paper, glass, plastics, metals, textiles, unsorted fines, and unsorted material.

The largest amounts are paper (and cardboard) and putrescible fractions, and together they contribute most of the organic matter and moisture content of the waste. Plastics make up a large and growing proportion of the volume.
Another factor that contributes to waste is waste from the home recycling center or civic service site. Civic service waste contains large and variable proportions of wood and yard waste, construction debris, furniture, and miscellaneous large objects.


Source separation

The source separation recycling schemes are the cheapest, most sustainable and preferred. They are likely to focus on easily recognizable metal, glass and plastic fractions to provide clean raw materials for recycling. Together, for household waste, it can be assumed that they comprise about a quarter of the wet weight and a similar proportion of the dry weight of the waste.

The paper fraction mainly included newsprint, which is easily separated but difficult to recycle economically as there tends to be more paper available for recycling than is used by industry. The excess that results depresses the value of the recycled material.

Therefore, source separation will only be effective for a proportion of the waste and will not be adequate everywhere. Some downtown areas find that certain groups of people are reluctant to participate in recycling, no matter what incentives they are given, and some types of properties make recycling more difficult. Older floors, for example, only have one garbage drop.

This means that in most areas, if recycling is to be done well above 15% to 20%, additional waste separation will be necessary. This is called mechanical sorting and is carried out in MRF (Material Recycling Facilities) and can also be called MBT (Mechanical Biological Treatment) plants when they include a method of biologically treating putrescible (organic) content after mechanical sorting.

Mechanical sorting of household waste

This is generally done to increase the proportion of material that is separated, and many of these sorting plants will be needed in the coming years to achieve the EU’s goals of much higher and improved recycling rates.

Mechanical sorting can also be performed to recover additional recyclable materials that have not yet been separated at the source, or simply to provide better feedstock for incineration or waste-derived fuel production.

Spraying and dry screening are most common to provide a crude separation into a large “paper and plastic” combustible fraction and a smaller “glass and putrescible” fraction for anaerobic digestion or conventional composting. Wet spraying will direct more paper to the “putrescible and glass” fraction.

Density separations and air sorting techniques can further separate and concentrate heavy glass and light plastics to provide better material recovery and a wider range of recovered products, and there is a “trade off” between the quality of the product and yield of any selected fraction. .

Conclution

There is increasing demand for the expansion of the waste industry, and even if the public does everything possible to recycle, we will have to undertake increasingly sophisticated waste separation as target rates increase. This will be achieved through source separation and mechanical separation techniques in facilities called MRF and MBT plants. In fact, these plants will include a wide variety of processes of which we have only touched the tip of the iceberg in this article, and which are described in detail in Waste Technology and Mechanical Biological Treatment (MBT).

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