A compact, self-contained apparatus for treating wastewater containing as impurities and pollutants various non-volatile (at water boiling points) fluids such as greases, oils and soaps, a vessel for collecting the wastewater, a heating chamber including electric heating elements and a heat transfer liquid such as mineral oil to heat the wastewater admixture to boil off the water and to reduce the volume of liquid for disposal, and a wastewater supply tank positioned under the apparatus. The apparatus may further include a disposable liner to isolate the wastewater from the vessel itself and facilitate rapid and clean removal and disposal of waste material after evaporation of the water, and thermally-activated sensors to maintain a desired temperature differential between the heating elements and the heat transfer liquid contained in the apparatus.