How Recycling Sorting Works and Common Mistakes

From the kerb to the sorting plant

Recycling is a chain of steps, and the quality of the material at the start affects every step after it. Understanding the chain helps explain why the rules on your bin lid are not arbitrary.

After collection, mixed recycling is usually taken to a materials recovery facility, often called an MRF. There, the load is tipped onto conveyors and sorted by a mixture of machines and people. Exactly how this works varies between facilities and countries, but typical stages include:

1. Pre-sorting. Workers remove obvious problems such as plastic bags, nappies, clothing, garden hoses and other items that can jam machinery.
2. Size and shape separation. Screens and rotating discs separate flat items such as paper and card from rounder, heavier items such as bottles and cans.
3. Magnets and eddy currents. Magnets pull out steel cans, while eddy-current separators use a changing magnetic field to push aluminium away from other material.
4. Optical sorting. Sensors identify different plastics by how they reflect light, and jets of air divert them into separate streams. Some sites can also distinguish colours.
5. Manual quality checks. People often check the final streams to remove items that machines miss.

The sorted materials are baled and sold to reprocessors, who clean, shred or melt them for use in new products. If the material is too dirty or mixed, buyers may reject it or pay less.

Some areas collect materials separately at the kerb, for example paper in one container and glass in another. Separate collections tend to produce cleaner material, but they need more vehicles and space, which is one reason why systems differ.

What contamination means

Recycling contamination is anything in the recycling stream that should not be there, or material that is too dirty to be processed. Examples include food waste, liquids, non-recyclable plastics, and items that are made of mixed materials. Contamination matters for several reasons:

• It can damage or jam machinery, and plastic bags are a well-known cause of stoppages.
• It lowers the quality of baled material, which can reduce its value or make it unsellable.
• Wet or food-soiled paper and cardboard lose fibre quality.
• If contamination is high, a load may be sent to disposal instead of being recycled. How often this happens depends on the local system and the market for each material.
• Some hazardous items, especially batteries, can start fires.

A small number of wrong items does not usually ruin a whole load, but the problem builds up over many households.

Common mistakes

“Wishcycling”. This means putting something in the bin hoping it will be recycled even when you do not know. The intention is good, but unrecyclable items add work and cost. When in doubt, check your council’s list or leave the item out.

Bagging recycling. Many schemes ask for loose items. A bag of recycling may be treated as rubbish because workers cannot see inside it, and the bag itself is a problem for machines. Check local instructions, since a few areas do ask for clear bags.

Dirty containers. Items do not need to be spotless, but a quick rinse or scrape removes most food and liquid. Greasy pizza boxes are a classic problem because grease soaks into card fibres.

Small items. Very small pieces, such as bottle caps in some areas, loose lids or shredded paper, can fall through screens and end up in the wrong stream. Local guidance varies, with some areas asking you to refit lids on bottles and others asking you to remove them, so follow your own scheme.

Black plastic. Some optical sorters struggle to detect the dark pigment in black plastic, so it may not be sorted correctly, though some newer systems cope better.

Mixed materials. Coffee cups, crisp packets, pouches and some cartons combine paper, plastic or foil layers that are hard to separate. Some areas collect certain cartons and not others.

Putting batteries, electricals or chemicals in the bin. These need separate routes, as with battery recycling and electrical items.

Compostable or “biodegradable” items. These are not the same as recyclable plastics, and they can contaminate plastic streams.

Assuming a recycling symbol means it is accepted locally. A triangle with a number identifies a plastic type, not whether your council collects it.

Why the rules differ

Local rules reflect the equipment at the nearest sorting plant, the markets for each material and how the collection is organised. That is why a pot or tray accepted in one area may be refused in another. The question of local rules also fits into the wider picture of waste management, in which collection and processing are planned at a regional level.

What you can do

• Look up your local list of accepted items and keep it somewhere visible.
• Empty, quickly rinse and, if told to, keep items loose.
• Keep out anything you are unsure about.
• Prioritise reduction and reuse, since recycling is a last step. The zero-waste ladder puts it below refusing, reducing and reusing for good reason.

Recycling does work, particularly for metals, glass, paper and common plastic bottles, but it is not a way to make waste disappear. Putting the right items in the right bin, and fewer items overall, is the most reliable way to help the system work as designed.