Treatment of hydrocarbon-contaminated materials

A method of treating of hydrocarbon-contaminated materials in order to recover the hydrocarbon-based contaminants from such materials which comprises mixing the hydrocarbon-contaminated material with water, and subjecting the mixture to treatment and thereafter subjecting the mixture to one or more separation treatments to separate hydrocarbon-based contaminant material from the contaminated material, characterized in that process water is added to a feed of hydrocarbon-contaminated material to adjust the viscosity of the mixture upstream of at least one hydrodynamic cavitation treatment and at least one hydraulic shock treatment, in any sequential order, the at least one hydraulic shock treatment comprising continuous, pulsed or periodic release of water pumped into the mixture at high pressure relative to the pressure flow of the mixture or continuous, pulsed or periodic release of gas into the mixture where the gas is compressed to a high pressure relative to the flow of the mixture, the use of the at least one hydrodynamic cavitation treatment and the at least one hydraulic shock treatment being such as to cause collisions between particles transported in suspension in the process water, with the consequent detachment of surface materials and adhering hydrocarbons.