Automatic sorting of waste for recycling

Sorting of garbage for recycling saves resources, reduces environmental pollution and slows the build-up of landfills. Infrared light is the key to a sorting machine for drink cartons and plastic bottles developed by SINTEF.

In Norway most waste sorting takes place in the home, but several European countries have chosen other solutions. In Germany packaging waste ranging from wrapping paper to cardboard boxes and no-refund bottles is sorted at approximately 200 sorting centres. The sorting plants are financed by producers of the packaging in accordance with the cost of recycling their products.

The solution developed by SINTEF is based on infrared spectroscopy, using ordinary halogen lamps as light source. In the selected range of infrared wavelengths, objects have different reflection properties than for ordinary visible light. This is advantageous as the computer can distinguish between plastic-coated cardboard, ordinary cardboard and different kinds of plastics by small variations in the 'colour' of the reflected infrared light. The human eye, however, can detect only the visible colour of the objects.

The reflection at the various infrared wavelengths is recorded continuously as the waste crosses a transverse sensing line of the conveyor belt. The data is sent to a computer which analyses the readings and constructs a two-dimensional image of the debris on the conveyor. Based on this analysis, the computer identifies the location of the different elements in the waste on the conveyor and directs jets of pressurised air to blow eg, the beverage cartons into a container, while the rest of the waste continues to another conveyor. SINTEF has designed the system, including programming and data analysis, and made prototypes. Industrialisation has been done by the customer.

The automated sorting machines have been so successful that Ferd (the owner of Elopak which initiated the development) established its own company - TiTech Visionsort (former Titech Autosort) - that are now producing the units in larger numbers. Systems have now been developed that can sort any combination of common domestic plastic waste and beverage cartons. The largest machines can currently handle 10 tonnes of waste per hour on a conveyor belt that is 2.8 m wide, with a speed of 2.5 m/s. When sorting bottles, the selected plastic fraction is 97% 'pure' and 95% of the desired packaging type is picked up from the conveyor.

To make recycled newsprint from de-inked pulp only newsprint, magazine paper and white office paper are allowed as constituents. All cardboard, paperboard and coloured paper objects have to be removed from the return paper stream. A sorter based on analysis of NIR and visible light has been developed and several systems have been installed in paper sorting plants.