We own our own parts washer and use mineral spirits as our solvent. Mineral spirit is a non-hazardous compound except for flashpoint. If you use the regular type it has a flashpoint below 140 degrees which makes the used mineral spirits a hazardous waste due to the explosion hazard. However, one can buy mineral spirits formulated with a flashpoint greater than 140 degrees. This type is non-hazardous. We use this type and buy it by the 55 gallon drum. A drum will last me about 2 years. When I want to change my parts washer solvent I then simply mix the old parts washer solvent in with my waste oil and it is all recycled and I get paid for it. I can do this because this material is simply non-hazardous minerals spirits, oil, and grease all of this has BTU value so the oil transporter is happy to get it. This is the cheapest option I have on waste handling and is a perfectly legitimate way to handle the waste according to the Indiana Department of Environmental Management and Federal EPA rules.