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. The materials are collected. 2. Materials are processed and manufactured into new products. 3. Consumers buy goods made from reprocessed materials.
Materials are source separated 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.
Before we go any further, however, we need to consider what the typical average analyzes of UK household rubbish might contain. Detailed lists are available on the web for the contents of these bins and wheelies, but in summary, the components can be classified as Putrescibles, Paper, Glass, Plastics, Metals, Textiles, Unsorted Fines, and Unsorted Material.
The largest amounts are of 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 contributor to waste is waste from the Household Recycling Center or Civic Amenities Site. Utility waste contains large and variable proportions of wood and yard waste, construction debris, furniture, and various large objects.
source separation
Source separation recycling schemes are the lowest cost, most sustainable and preferred. They are likely to concentrate on the easily recognizable metal, glass and plastic fractions to provide clean raw materials for recycling. Taken together, for household waste, it can be assumed that they include about a quarter of the wet weight and a similar proportion of the dry weight of the waste.
The paper fraction includes mainly newsprint, which is easily separated but difficult to economically recycle, as there tends to be more paper available from 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 part of the waste and 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 difficult. The older floors, for example, have only one garbage chute.
This means that in most areas, if recycling is to be much higher than 15-20%, additional waste separation will be necessary. This is called mechanical sorting and is carried out at MRFs (Materials Recycling Facilities) and may also be called MBT (Mechanical Biological Treatment) Plants when they include a method for biologically treating putrescible (organic) content after mechanical sorting.
Mechanical sorting of household waste
This is usually 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 targets of much higher and improved recycling rates.
Mechanical sorting can also be done to recover additional recyclable materials that have not yet been separated at source, or simply to provide a better feedstock for incineration or waste-derived fuel production.
Dry pulverization and screening are most common to provide a crude separation into a large combustible “paper and plastic” fraction and a smaller “putrescible and glass” fraction for anaerobic digestion or conventional composting. Wet spraying will direct more paper to the “putrescible and glassy” fraction.
Density separations and air classification techniques can further separate and concentrate heavy glass and light plastics to provide better material recovery and a broader 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 rapidly increasing demand for the expansion of the waste industry, and even if the public does its best to recycle, we will have to carry out increasingly sophisticated waste separation as target rates increase. This will be achieved through separation at source and through 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 Biological Mechanical Treatment (MBT).
