A new breakthrough at Delft University of Technology(TU Delft) means that bottom ash, the toxic remains of waste incineration, can be purified to the point where it can become a valuable and pure raw material for the building of roads. The discovery is especially important as 1 million tonnes of bottom ash results from waste incineration in the Netherlands each year, and the material is in fact chemical waste full of heavy and toxic metals.  

A new breakthrough at Delft University of Technology(TU Delft) means that bottom ash, the toxic remains of waste incineration, can be purified to the point where it can become a valuable and pure raw material for the building of roads. The discovery is especially important as 1 million tonnes of bottom ash results from waste incineration in the Netherlands each year, and the material is in fact chemical waste full of heavy and toxic metals.   
 
In the Netherlands, almost all waste which cannot be used in some way goes into the incinerator. This is a good thing considering the energy that is thereby created and the reduction of about 90% in waste volume. Still, the amount of ash that comes out of incinerators is massive - a million tonnes each year. Bottom ash contains materials which make it unsuitable for many types of applications. Particles of metal contained in the ash, such as iron, lead, aluminium and copper, are not harmless. Rainwater can slowly but surely wash away toxic metals, creating a hazard for the environment.

Talus

Until now, bottom ash has been safely housed in talus used under Dutch motorways. Talus is made from a watertight layer of clay, followed by a layer of bottom ash packed in foil on top, and then a clean layer of soil for protective as well as aesthetic purposes.

Having to house bottom ash in talus is not an ideal situation. It is very expensive, the foils can leak, and monitoring is always necessary. A solution was needed that would enable the last particles of metal to be taken out of incinerator ash. Peter Rem, engineer at TU Delft, explains why it's such a complicated process:

"When the ash finally comes out of the incinerator, it needs to be cooled with water. And that makes it a damp mass that is sticky and in which all the particles you've lost track of, such as metals, are hidden. They remain untraceable in the moist damp ash, which is also lumpy, and the fine particles of metal are somewhere deep inside."

Because bottom ash has a sticky consistency, normal methods of metal recycling do not work. Filtering is impossible, and magnets are not strong enough to pull the metal particles from the sticky mass.

The new method

The new recycling method that has been developed in Delft involves giving it a hard shake, and in one go the particles shoot out through the bottom ash with great force in a kind of long collection room. The small particles come loose from the larger ones. The smaller ones fall at the front end of the collection room, the larger ones, which contain more kinetic energy, at the far end of the room.

Standing at the front end of the collection room, Walter de Vries, a young researcher hoping to do his PhD on the subject, explains why this makes the difference.

"What you now can see is that the sticky stuff lies here at the front. If I press it together, it sticks together like clay. But if I walk to the back... here, we see more material, but it is much coarser and completely loose. It is perfect for filtering and that's what we want. By filtering it, we can take out the metals."

The process of commercialising of this type of bottom ash recycling method is underway. The process is cost-effective, and the costly metals that are recovered help to pay for it, which makes this method of treatment cheaper than wrapping the bottom ash in foil and forever having to monitor it. The company that is to commercially launch this method is called Inashco, meaning Incineration Ash Company.

* RNW translation (sr)