May 22, 2024


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What is Recycling – Source Separation and Mechanical Sorting of Household Waste

3 min read

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 purchase the goods made with reprocessed materials.
Materials are either source-separated and collected, or collected without segregation. The latter is often called black-bag waste, due to the colour of the bags used in most countries.

Before we go any further though, we should consider what the average typical analyses of household refuse in the UK might contain. Detailed lists are available on the web for the contents of these bins and wheelies, but in short the components can be classified as putrescibles, paper, glass, plastics, metals, textiles, unsorted fines, and unclassified material

The largest quantities are of paper (and card), and putrescible fractions, and together these contribute most of the organic matter and moisture content of the waste. Plastics make up a large and increasing proportion of the volume.
Another contributor to waste is Household Recycling Centre or Civic Amenity Site waste. Civic amenity waste contains large and variable proportions of wood and garden wastes, building rubble, furniture and miscellaneous large objects.

Source Separation

Source separation recycling schemes are the lowest cost, and most sustainable and are preferred. They are likely to concentrate on the easily recognisable metal, glass and plastics fractions to provide clean feedstocks for recycling. Together for household waste these can be assumed to comprise about a quarter of the wet weight and a similar proportion of the dry weight of the refuse.

The paper fraction comprises mostly newsprint, which is easily separated but difficult to recycle economically as there tends to be more paper available from recycling than is used by industry. The glut which results depresses the value of the recycled material.

So, source separation will only be effective for a proportion of the wastes, and it will not be suitable everywhere. Some inner city areas find that certain groups of people are reluctant to participate in recycling, no matter what incentives are given, and some property types make recycling harder. Older flats for example have only single rubbish chute.

This means that in most areas if recycling is to be taken much above 15% to 2o%, additional separation of the waste will be needed. This is called mechanical sorting, and carried out in MRFs (Materials Recycling Facilities) and these may also be called MBT (Mechanical Biological Treatment) Plants when they include a method for biologically treating the putrescible (organic) content after mechanical sorting.

Mechanical Sorting of Household Refuse

This is usually done to increase the proportion of material which is separated, and very many of these sorting plants will be needed in the next few years to achieve EU targets for improved and much higher recycling rates.

Mechanical Sorting can also be undertaken to recover additional recyclable materials not already separated at source, or simply to provide a better feedstock for incineration or production of refuse-derived fuel.

Dry pulverising and screening is the most common to provide a crude separation into an oversize combustible “paper and plastics” fraction and an undersize “putrescible and glass” fraction for anaerobic digestion or conventional composting. Wet pulverising will direct more of the paper into the “putrescible and glass” fraction.

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


There is a rapidly increasing demand for expansion of the waste industry, and even if the public do their best to recycle, we will have to carry out more and more sophisticated waste separation as the target rates rise. This will be achieved by source separation and by mechanical separation techniques in facilities called MRFs 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 at Waste Technology and Mechanical Biological Treatment (MBT).

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