To meet the safety requirements in an existing apparatus the used dusts have to be investigated on their explosive properties. Therefore the explosion indices maximum pressure rise, maximum explosion pressure and the minimum ignition energy are determined with the help of the MIKE 3-apparatus and the 20-l-apparatus. The used dust is a mixture of sewage sludge ash and a carbon carrier. To find out the safest alternative for this mixture, various carbon carriers are tested. In addition the ratio of carbon in the mixture alter to find the best mixture in terms of safety. Furthermore an impact of volatilities on explosibility can be seen. This parameter should also be regarded at the search for the best mixture between sewage sludge ash and carbon carrier.