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Home  > Question of the Week

Question of the Week

Permalink How do I wash and dry biodiesel?

Answer: Washing biodiesel is easy to do and requires only water and time. Washing removes impurities, including unfiltered particulates, methanol, and glycerin.

Unwashed biodiesel will not meet ASTM, formerly known as the American Society of Testing and Materials, standards. Equipment and engine manufacturers only warranty their equipment and engines for their material and manufacturer defects. Fuel manufacturers assume responsibility for any damage caused by the fuel.

There are several common techniques for washing biodiesel, including agitation washing, mist washing, and bubble washing. The process of washing biodiesel involves mixing it with water. Water is heavier than biodiesel and absorbs the excess alcohol, catalyst, and soap suspended in the fuel. After washing and settling, the water and the impurities in the water can be drained from the bottom of the container. Several wash cycles are generally needed. The first water drained off the bottom of the biodiesel will be milky, and the final wash water drained off will be clear. Excess catalyst in the biodiesel will form soap when mixed with water, and it takes awhile for the soap to settle out.

Depending on the method used, it takes roughly one gallon of water per one gallon of biodiesel for a wash cycle. The mixing should be thorough and the water should be dispersed throughout the biodiesel. Agitation washing means stirring water into the biodiesel, letting it settle and draining it off. Mist washing is spraying a fine mist of water over the surface of the biodiesel. Tiny droplets of water fall through the biodiesel and pick up impurities on the way down. Bubble washing is done by putting a bubbler in a layer of water beneath the biodiesel. The rising bubbles are coated with water, which picks up impurities as the water travels up and then back down through the biodiesel.

After the biodiesel is washed, it should be dried until it is crystal clear. This can be done by letting the biodiesel sit uncovered in a sunny location for a few days, or it may be heated to about 120 degrees Fahrenheit for a few hours. Another popular technique is recirculating the biodiesel from the bottom of the drying tank through a shower head or sprayer suspended above the top of the open tank. This increased contact with air will dry biodiesel in about an hour, depending on humidity. Reacted, washed, and dried biodiesel may be used in any diesel engine. It should have a pH of close to 7, or chemically neutral, and it should have no methanol left in it. Although professional testing of fuel may be prohibitively expensive, simple home fuel test kits can be purchased for a reasonable price. One such kit, The Biodiesel pHLip Test, is available at

For more information on producing biofuel, see ATTRA publication Biodiesel: Do-It-Yourself Production Basics at This publication is an introduction to home biodiesel production. It includes lists of equipment and materials needed to make small batches of biodiesel. It describes biodiesel and includes cautionary notes and procedures for making test batches and five-gallon batches. An extensive resource list is also provided.



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