Bubble wash

Bubble wash is a method of final washing of home-made biodiesel through air agitation. Biodiesel floats above a quantity of water. Bubbles from an aquarium air pump and air stone are injected into the water causing the bubbles to rise. At the water/biodiesel interface, the air bubbles carry water up through the biodiesel by surface tension. Diffusion causes water soluble impurities in the biodiesel to be extracted into the water. As the bubble reaches the surface and breaks, the water is freed and percolates back down through the biodiesel again.

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Bubble washing is a cheap way to wash biodiesel, but it can degrade the product by polymerizing it somewhat. If unsaturated oils are used to make the biodiesel, those oils will have double-bonded carbons. If the bubbles are made with air (as is commonly done), the oxygen in those bubbles will attack the double bonds, breaking them, and causing them to polymerize with other carbon chains. The process is the same one that causes linseed oil to "dry" and turn into a tough varnish. The result is a plastic-like residue in the biodiesel which can varnish engine parts.