The present invention relates to a process and an apparatus for separating environmentally hazardous substances, such as heavy metals and oil, from a contaminated mass, such as sewage sludge or soil from industrial sites. According to the process, an aqueous metal hydroxide solution comprising Na, K or Ca is added to the contaminated material in form of high-pressure jets, in order to disintegrate the material and provide a homogeneous mixture. Thereafter, liquid containing the hazardous substances is separated from the washed material and subsequently filtrated. The essentially pure water resulting from the filtration step may be used for producing new metal hydroxide solution to be used in the process.