During already more than 25 years, bottom ash produced in The Netherlands has been applied as construction material. Roughly 25 million tons of bottom ash has found its way to construction works, including highways (6 million tons), as ‘ICM’ building material (ICM = Isolation, Control, Maintenance). The long-term monitoring and maintenance responsibilities and liabilities, however, are not attractive for the owner of the construction, nor sustainable. Anticipating the need for a more sustainable use of bottom ash, as well as the agreement between the incinerator branch and the Dutch government in the “Green Deal” (which is to be realised fully by 2020), made Tauw invest in a process to upgrade bottom ash into a long term environmentally safe and stable product. This process transforms bottom ash into a 100% freely applicable construction material, and is called “EquiAsh”.
EquiAsh is a patented process that treats bottom ash in such a way that it is stable in the long run (i.e. in equilibrium with the atmosphere) and that it fulfills the Dutch leaching criteria for approved freely applicable construction (“NVG”) materials.
The first step is carbonation of the bottom ash. In about three weeks the bottom ash is actively aerated with air that is enriched with carbon dioxide (CO2). The bottom ash is neutralized from a pH of 11 to 8 (accelerated aging), organic matter is degraded and some metals (copper, lead, aluminum, barium en molybdenum) are fixated.
In the second step of the EquiAsh process, soluble salts (bromide, chloride and sulphate) and other soluble components are removed in a washing step.
In the third and last step, mineralization (storage) leads to further fixation of metals, especially antimony.
With the EquiAsh process, it is possible to transform bottom ash into a material that is stable under atmospheric, aerobic conditions. Advantage is that leachable contaminants have been washed out after aging, so there is no risk of an increase in leaching in the future. There is enough buffer capacity for maintaining a near neutral pH in the long term. The entire bottom ash is treated and no landfilling of residual fractions (fines) is necessary. Emissions to air and water are minimized. It is a sustainable process with CO2-fixation. It is an extensive process, wherein time and space are important.
We have developed the process from laboratory scale to practical scale by testing batches of 1,200 tons (maximum 3,000 tons for the first step). After treatment, all critical parameters fulfil the Dutch criteria for freely applicable construction materials.
In our presentation, we will show results of large scale experiments with bottom ash from AEB Amsterdam. We will discuss:
Practical problems and solutions related to the upscaling of the process like preferential flow of gas or water, precipitation of components during washing and corrosion
Process optimization
Results of different experiments
Assessment of the long term stability by pH controlled leaching and availability tests
Costs of the treatment
Lessons learnt from practical projects , Durability and ageing , New products, applications and machinery
