Water Washing Biodiesel
By Rick Da Tech
Washing Biodiesel with water is the oldest and most common method of cleaning biodiesel. About 3% of raw unwashed biodiesel is methanol. Methanol is a solvent, it captures soap and other impurities and holds them dissolved in the biodiesel. Water soaks up that methanol, releasing impurities to be washed away with water.
Keeping the methanol liquid and diluted in water makes water washing the safest way to clean biodiesel. Most drywash methods require we evaporate or distill the methanol into a flammable and toxic gas as part of the purification process. Spent wash water can be used for irrigation after minimal treatment. Other wash methods generate hazardous waste streams that are difficult to dispose of.
Water washing is the most flexible way to purify biodiesel. Under the right conditions, you can power wash in just a few hours with extremely aggressive wash methods. Or you can take up to a week using less aggressive wash methods. Some methods conserve water, letting you wash with 1 part water for every 5 parts biodiesel, while most use a 1:1 water to biodiesel ratio. Water washing can be automated, and you can mix and match different wash methods to fit your personal needs.
Water Wash Methods
It takes several washes to clean your biodiesel. Most people start with a less agressive wash method, moving on to more agressive methods as the biodiesel is cleaned. I like to start with an overnight Static Wash, then move on to bubble washing. With each wash I will put one part water for 5 parts biodiesel. If I have a 40 gallon batch, I'll replace the water with 8 gallons of fresh water for each wash. The first two bubble washes will usually last about 8 hours each, with the last water wash usually lasting 48 hours. I use a lamp and appliance timer to control the bubble wash so I only need to attend to it for a few minutes a day.
Static or gravity washing is the least aggressive and least likely to generate an emulsion. It is simply placing water and biodiesel in the same tank without any mixing. Impurities migrate from the biodiesel to the water through the boundary layer over time. This process takes anywhere from 4 hours to 24 hours to saturate the water with contaminates. Most homebrewers will let the static wash continue over night before draining and starting a different technique. It is particularly effective as a first wash for biodiesel from high FFA oils. Gravity washing is best used on very soapy water.
Sooner or later everyone makes an emulsion that doesn't break down on it's own. Emulsions are a mixture of water and biodiesel and other stuff like soap, and sometimes glycerin. It's usually a white mayonnaise looking stuf. It can have other apearances as well. It is a problem for the beginner. Emulsions can be avoided and broken with a little care.
Tests for When to Stop Washing
It usually takes three to four washes to clean biodiesel completely. We can use one of several tests to help us decide when we’ve washed enough. There are three tests that can tell us when to quit washing.
The Shakem'up Test
This test is commonly recommended as a test for soap. In the Shakem'up test we fill a bottle about a third full of biodiesel, then add an equal amount of water, and Shake like crazy. If it separates into water and biodiesel quickly with the water being not too white (soapy) then it's a pass.
The test needs a lot of experience to read properly. If you wash with soft water, the water on bottom will always be soapy. If you have hard water, it may be clear when there is still soap in the biodiesel. Different oils can also give different readings. Even though it is subjective, it can still be useful as a quick test, once you have sufficient experience with it.
Appearance of the Wash Water
Some will wash until the spent water is clear or nearly clear. This is another subjective test that requires much experience before it is useful. Soft water will never be completely clear.
It ain't over till the paperwork is done.
Biodiesel Wash Tank Plans
Biodiesel Washtank - Home-Made-Biodiesel.com explains water washing.
Water Washing Biodiesel 101 - Utah Biodiesel Supply
Biodiesel Emulsions 101 – Preventing And Curing Them - Utah Biodiesel Supply