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Separating waste, and then...?

What happens to the different waste streams?

There have been recycling bins in all of the University buildings since 2018. Waste bins were removed from staff offices at the same time. Waste such as paper, batteries and printer cartridges is now collected in separate streams. But the big question is: what happens to the different waste streams? We take a look behind the scenes.

Transporting and processing waste generates CO2 emissions. And incineration is a waste of raw materials. The University’s prime aim is therefore to produce as little waste as possible. 

The University has a three-pronged approach to reducing waste

  1. The main objective is to prevent waste. This is the most effective way of reducing the impact on the environment.
  2. Any waste that remains is processed sustainably so that raw materials can be recovered.
  3. The residual waste is incinerated in waste-to-energy plants.

This is how the University contributes to the circular economy in which waste is returned to the chain as a raw material.

Staff, students and visitors to Leiden University jointly produce around 900 tonnes of waste per year. 


Not everything can be recycled

The University collects around 40% of all the waste produced in separate streams, such as paper, organic waste, wood, construction waste, frying oil, glass, high-quality plastic, computers and other equipment, batteries and printer cartridges. Old furniture is sometimes reused within the University, but as yet there are no rules on this. A quarter of the waste collected is paper and cardboard.

Research has shown that we are better at recycling our ‘personal’ waste if we don’t have our own waste bin. This is why the waste bins were removed from the offices in the University buildings. Shared recycling and waste bins were placed throughout the University instead.

Two-thirds of the waste collected at the University is general waste. This is waste that cannot be recycled. It is burnt to produce energy.

How is each stream processed?


Plastic bottles and packaging, Metal and Drink cartons (PMD)

PMD is a combined stream of packaging materials. It includes plastic bottles, cans and milk cartons. According to the guidelines of the Ministry of Infrastructure and Water Management, the colour orange is used to denote PMD.

A dustbin lorry collects the University’s PMD and brings this to the waste and recycling centre in Zoeterwoude, where it is unloaded, quality checked and compressed. It is then transported in an optimised shipment to an intermediate station, where it undergoes further quality checks.

The compressed PMD is then transported to a sorting facility, where it is shredded and distributed over a conveyor belt. This mixed stream is then separated into nine mono-streams: not only plastic, cans and drinks cartons, but also different types of plastic, such as HDPE (e.g. shampoo bottles), PP (margarine tubs) and PET (drinks bottles).

The plastic is washed in a tank and quality checked. The quality of the plastic determines what happens to it. High-quality plastic is granulated, which means ground to a raw material in the form of granules. It goes into in an extruder, which is a special device for melting plastic. The melted plastic is pushed through a sieve to remove any last impurities, and any properties are added, such as a colour or scratch resistance. At the end of the extruder, the plastic is pushed through a second sieve. Then a machine cuts it into vermicelli-like pieces. This ‘regranulate’ is of comparable quality to new plastic, which is also known as virgin material. It can be used for the same purposes as new plastic.

Low-quality plastic, such as the foil from cured meats, is made into an agglomerate, which are granules of compressed plastic. This is used as a raw material in the production of garden benches, pallets or playground equipment, for instance.

After sorting, cans and aluminium go to certified smelting companies for further processing. Once they have been smelted, they are used for new tin and aluminium products. Drinks cartons go to the paper-processing industry, which makes new paper from the paper fibres from the cartons.

The sorting facility where the University PMD is processed uses the latest technology, so all the steps of the processing take place in a single installation. This not only saves transport and logistics costs but, at 80 to 90%, the recycling yield is higher than other facilities whose yield is around 50%. The facility also turns the low-quality plastic into an agglomerate that can be used as a new raw material.

The part that can’t be recycled (10 to 20%) is generally made up of water and high-calorific waste. The water is treated at the sorting facility. High-calorific waste is the remainder that can’t be recycled. This is turned into what is known as Solid Recovered Fuel (SRF). SRF is used as a fuel in the cement industry. The small remainder that is left is burnt in a waste incinerator.


Organic waste

Organic waste includes food waste. Swill is the specific term for cooked food waste. According to the guidelines of the Ministry of Infrastructure and Water Management, the colour green is used to denote organic waste.

Organic waste is used to generate green electricity. Once it has been collected, it is sent to a digester, where anaerobic digestion, a microbiological fermentation process, is set in motion. This is the same process used to ferment beer and prove bread, but here it is used to generate biogas. What remains is turned into high-quality compost. Around 94% of the organic waste can be reused as a raw material.



All types of paper are welcome in the paper bins: letters, envelopes (window and all), newspapers, magazines and cardboard. It is important that the paper and cardboard is clean. Dirty paper such as used paper napkins or cake boxes should be thrown in the general waste bins. According to the guidelines of the Ministry of Infrastructure and Water Management, the colour blue is used to denote paper.

Around 75% of paper and cardboard in the Netherlands is recycled. Used paper and cardboard are the main raw material used to produce new paper and cardboard. Once the used paper and cardboard has been collected, this is cleaned and sorted and put into bales. At the paper factory, it is then immersed in water to separate the fibres. Sieving techniques remove pollutants such as staples and paperclips from the pulp.

Using flotation, a chemical process in which a mixture of substances are separated in water on the basis of hydrophobia, the ink is washed from the paper fibres. To make the clean pulp ready for paper production, it is bleached with hydrogen peroxide or ozone. What remains is used as a raw material for construction materials.


Coffee cups

The cardboard cups that the University cafés and vending machines serve coffee in are made of 95% cardboard. The remaining 5% is the plastic PE coating on the cup’s inside. Each coffee cup that ends up in the paper cups stream is fully recycled. The colour black is used to denote paper cups.

Specialised techniques are used to separate the paper fibres of the cardboard from the plastic coating. The paper fibres are used as a raw material in the production of toilet paper, whereas the PE coating enters its own recycling process.



Used batteries can be handed in at all University locations. They contain many valuable substances such as zinc, cadmium, iron, nickel, manganese, mercury, lithium and cobalt, so if they are recycled manufacturers can reuse these substances. Around 70% of batteries are recycled. The colour light blue is used to denote the batteries stream.

The used batteries are brought to a sorting centre, where they are sorted by shape in a sieve. A machine then sorts them electromagnetically according to their contents. Leaking, swollen or rusting batteries are sorted by hand. Once they have been sorted, the batteries are sent to different smelting plants to reclaim the valuable substances.


Ink cartridges and phones

Empty ink cartridges can be handed in at central points. This stream has the colour purple. The reusable parts, such as the loading roller, case and magnetic roller are dismantled and cleaned at recycling stations. Then the quality of each component is assessed: each component must meet the standard that applies for new ones. Those components that are approved are reused in new cartridges.

Old phones can also be handed in at the purple collections points at the University. In practice, these are sent for recycling by the staff from Real Estate. The chips in phones contain small quantities of different materials such as gold, silver and palladium. Recovering these materials through recycling reduces the need to mine for these metals, where people often work in harsh conditions.

Other waste streams

Alongside the waste streams above, the University has other specialised streams:

  • Waste wood is categorised as ‘A’ or ‘B’ wood: ‘A’ wood is untreated and unpainted wood; ‘B’ wood is wood that has been treated or painted. Both types of wood are suitable as a raw material for secondary materials in the wood fibre and chipboard industry or as fuel in bio-energy stations.
  • Waste that is produced during construction, renovation and demolition work has to be disposed of in containers. Fine aggregate from construction waste is processed into a sand and grit replacement for concrete. A secondary fuel is made from the remainder. Building companies are more likely to take responsibility for construction waste themselves than they were in the past. Hazardous or chemical waste is the collective term for waste that can harm people or the environment. These substances have to be removed, transported and destroyed according to strict government regulations.
  • Glass can be recycled without compromising its quality, which makes it the perfect raw material for producing new glass. A different procedure is used for lab glass: this is collected separately and processed in a specialised process.
  • Confidential documents can be deposited in the special blue paper containers with a slot. Strict regulations govern how they are destroyed and businesses have to be certified to process this paper stream.

Sustainable demolition of labs on the Science Campus

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General waste

General waste is the term for waste that can’t be recycled, such as bread sleeves made of paper and plastic or crisp packets. Dirty paper such as used paper napkins and tissues should also be disposed of in the general waste, as should blister packs for pills or chewing gum.

General waste is processed in the least environmentally friendly way, so the University aims to produce as little of this waste as possible. As well as recycling as much as possible, the University is working to prevent general waste. According to the guidelines of the Ministry of Infrastructure and Water Management, general waste is denoted by the colour grey.

General waste is burnt in waste-to-energy plants. This entails the loss of raw materials. The electricity and heat generated from burning general waste is reuse, however. The electricity is fed into the energy grid, and the heat is used to heat factories, houses and greenhouses, which does save fossil fuels.

Sustainable Development Goals

Our waste policy contributes to these Sustainable Development Goals of the United Nations:

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