An apparatus for treating oil-based sludge includes a boiler, a heat exchanger having a circulatory steam path in fluid communication with the boiler and a sludge path receiving sludge, a plurality of serially-aligned mixers in fluid communication with the sludge path of the heat exchanger, a separation tank in fluid communication with the plurality of mixers, and wherein the separation tank includes a tank having a plurality of walls, a hydrocyclone mounted within the tank receiving sludge from the plurality of mixers, wherein the hydrocyclone includes a solids outlet directing a solids component towards a bottom portion of the tank and a fluid outlet directing a fluid component into the tank, a means for conveying separated solids along the bottom portion of the tank to a solids outlet, and a plurality of fluid outlets vertically located along a tank wall, wherein each outlet includes a valve providing selective fluid communication from the tank.