The residues from special waste incineration (slag, filter dust, and the reaction products from off-gas purification—the salts) must be stored at special dumps since they are highly polluted with heavy metals. The extraction rate of heavy metals out of unfixed filter dusts and salts is very high. Therefore, salts can only be stored underground. On the other hand, getting licenses to build such special dumps is very difficult. A fixation of boiler ash (a special filter dust) or salts with water glass and aluminum phosphate (AlPO⁴) hardener reduces the leachability of heavy metal so well that these fixed residues can be dumped together with municipal or commercial and industrial refuse rather than in an underground repository.