Methods of Water Washing Biodiesel

After transesterification the biodiesel still contains contaminants that need to be washed out. Washing biodiesel removes these contaminants (soap, catalyst, methanol and free glycerin).


How Does Water Washing Work?

Excess methanol is suspended in the unwashed biodiesel. The soaps, catalyst, and glycerin are suspended as well and ‘hold on to’ the methanol. While washing, water is passed through the biodiesel. The methanol is attracted to the water and the biodiesel repels it. The methanol clings to the water molecules as they pass through, taking with it the other contaminants that were ‘holding on’ to it. All the contaminants eventually end up in the wastewater after several washes. Washing biodiesel will sometimes cause an emulsion. Learn how to deal with emulsions here.


Water Washing Methods:

There are three main methods of water washing biodiesel: bubble washing, mist washing, and stir washing.

Often, before doing these wash methods, a prewash is done. A prewash will decrease the chance of and emulsion, however not everyone finds it necessary.

Bubble washing – the equipment for this method is fairly cheap, such as an aquarium stone and timer. This method works by bubbling air up through the biodiesel. The water carries water with it, collecting contaminants along the way. The bubble bursts when it reaches the top, and the water droplet collects more contaminants on its way back down.

Stir washing biodiesel – this method should only be used with fully reacted fuel that is low in soap (you can check this by a simple test mentioned on the 'quality tests' page). Fuel that is not fully reacted or contains excess soap will not hesitate to emulsify! This method uses a paint stirrer attached to an electric drill to mix the water and biodiesel together rapidly. The stir-method can knock days off the bubble wash method.

Mist washing - Mist washing is done by misting water over the surface of the biodiesel. As the mist drops through the biodiesel, it takes contaminants with it which end up in the wastewater layer beneath the biodiesel. Mist washing decreases the chances of emulsification because it does not agitate the mixture as much as bubble washing or stir-washing.

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