Flue gas cleaning residues are made up of filter dusts. These pollutants are produced by the incineration of waste. Since the pollutants are concentrated in this residue, it requires special monitoring. Filter dust is used to fill hollow spaces in old mines.
 
Old salt domes are commonly used, because their special properties allow the formation of a firm geological barrier. The cavities located at great depths associated with shut-down mines also represent a potential danger for any towns situated above.  This is why backfilling is obligatory by law.

Cavities are filled up with filter dust as well as with power plant residues such as boiler ash and other industrial residues. This conserves natural resources. Adequate capacities for many years are available at present. Pollutants are withdrawn from the environmental cycle permanently in these absolutely reaction-neutral surroundings.

Backfilling Conducted By Certified Specialist Companies 
Certified recycling companies have become specialised in safe backfilling. A tough mixture is obtained by a specific combination of dusts, boiler ash and brine. This is carefully pumped into the cavities. Once the mixture has hardened, it is as strong as concrete and the cavity is adequately supported.
 
As opposed to other mines, salt domes do not tend to form cracks. The salt walls enclose the reaction-free material so safely that no contact to the outside world can take place any more. The water cycle within the mine is closed. That is why there is no danger of liquids having come into contact with the dusts reaching the environment.
 
Alternatively, the dusts are put in big-packs and used to fill hollow spaces underground.