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 dry wash methods require we evaporate or distill the methanol into a flammable and toxic gas as part of the purification process.

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. Alternatively, you can take up to a week using less aggressive 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 aggressive wash method, moving on to more aggressive methods with later wash cycles. I like to start with an overnight Static Wash, then move on to bubble washing. With each wash, I put one part water for 5 parts biodiesel. If I have a 40-gallon batch, I replace the water with 8 gallons of fresh water for each wash. The first two bubble washes 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 Washing

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 contaminants. Most homebrewers let the static wash continue overnight before draining and starting a different technique. It is particularly effective as the first wash on very soapy biodiesel made from high FFA oils.


Mist Washing Biodiesel

Mist Washing

Mist Washing or Spray Washing are the process of spraying or misting water on top of biodiesel. As it sinks down through the biodiesel it absorbs impurities, cleaning the biodiesel. This method takes the least hands on time.



bubble washing biodiesel

Bubble Washing

Bubble Washing uses air bubbles to mix water and biodiesel. Water is on the bottom of the washtank with biodiesel on top. We make air bubbles in the water, which rise to the surface, taking some water with them into the biodiesel layer. This is the cheapest way to water wash.



Pump Washing Biodiesel

Pump Washing

Stir washing and pump washing are very aggressive techniques for washing. It is accomplished by placing water and biodiesel in the same tank and aggressively mixing to form an emulsion. The emulsion then separates over time. This is the fastest way to water wash.





Sooner or later everyone makes an emulsion that doesn't break down on its own. Emulsions are a mixture of water and biodiesel and other stuff like soap, and sometimes glycerin. It usually looks like mayonnaise. It can have other appearances as well. It is a problem for the beginner. Emulsions can be avoided and broken with a little care.

Avoiding Emulsions when washing biodiesel

Avoiding Emulsions

It is best to avoid emlusions when possible. To avoid emulsions we need to understand what makes them. The key factors are wash temperature, water hardness, soap content, and conversion completeness.




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 have washed enough. There are three ways to tell when to quit washing.

The Shakem'up Test

Many people recommend the Shakem’up Test 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 is 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

It takes some experience to read this test properly. If you wash with soft water, the water on the bottom is always 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 a useful quick test, once you have sufficient experience with it.


The Biodiesel Washwater Separation Test

The Separation Test

This test is used to determine when to stop washing your biodiesel. It works on the basic principle that we have removed enough impurities when the suspended water drops quickly to the bottom without forming an emulsion.




Biodiesel Soap Titration Test

Titrating for Soap

A quantitative test to determine how much soap is in biodiesel. Most shoot for between 50ppm to 200ppm. It gives specific numbers but is a complicated test to perform.




It ain't over till the paperwork is done.


Drying Biodiesel

Drying Biodiesel

After washing, biodiesel will have water left in the biodiesel. Water in the fuel can damage a diesel engine. So we need to dry it. Drying biodiesel can be as simple as letting the water settle to the bottom of your wash tank over a few days or it can involve complex equipment for drying it in a few minutes.  The method I prefer is bubble drying.



Wash Water Disposal

Wash Water Treatment and Disposal

We can pour our wash water on the ground and kill the grass and pollute our streams or we can pour it down the drain and overload our sewage treatment plants. There is an alternative; we can treat our spent wash water and make it cleaner with a little effort.



Biodiesel Wash Tank Plans


Steel Drum Biodiesel Washtank

Steel Drum Wash Tank Plans

A Steel 55 gallon drum converted into a biodisel wash tank is a studier than a converted a plastic drum. These plans let you build a steel washing tank wihtout having to crawl inside the drum.



Plastic Drum Washtank Plans

Plastic Drum Wash Tank Plans

Nothing beats a polyethelyene drum for easy to build. They can be built with basic woodworking tools in a few hours. This article shows you the tricks to building your own biodiesel washtank.



1000 liter biodiesel washtank

1000 Liter Polyethylene Wash Tank Plans

Water Washing is the bottleneck in producing biodiesel. This 1000 liter washtank lets you wash multiple batches together in the same tank. Modify a single drain tote into a standpipe tote with two drains at different levels.




Related Links

Biopowered wiki on water washing

University of Idaho Tech Note 25 - The advantages of water washing 

Water Washing Biodiesel 101 - Utah Biodiesel Supply

Biodiesel Emulsions 101 – Preventing And Curing Them - Utah Biodiesel Supply

Luc's Water Washing Page