Some makers of biodiesel are satisfied to let their newly produced mixture simply sit untouched for about a week to allow the bulk of the soap residues settle to the bottom before running the product through a filter and into their fuel tank. But for a cleaner biodiesel that burns better, runs better, gets better mileage, and minimizes wear and tear on your engine, you may wish to consider taking this stage of the process a step further.
There are numerous ways to wash biodiesel, the simplest being to pour water with a dash of white vinegar mixed in into a translucent container made of PVC or some other similar material, or a 5-7 gallon bucket, if washing large batches, fitted with a valve at the bottom that can be opened or closed at will. Then pour your biodiesel on top of it and gently stir, avoiding agitating the soaps you're attempting to remove. Afterwards, let the mixture sit for 12-24 hours.
At the end of this time the oil (biodiesel fuel) and the water (cloudy with soap) will have separated, and you can decant the biodiesel out the top, and then drain the soapy water out the bottom.
You'll want to repeat this process 1 or 2 more times, using fresh water each time (though after the 1st wash, you need not add any vinegar to the water you use for subsequent batches.) In the interest of preserving water, by the way, the remaining water from your 3rd wash could be used for the 1st wash on a subsequent batch.
If, after 3 washings, there still seems to be residual soaps remaining, they can then be removed by slowly reheating the oil, decanting out the clean biodiesel and leaving the impurities behind.
There is, however, another way to wash your biodiesel that, while taking longer to complete, uses up far less water. Brought to us by the University of Idaho, this technique, called Bubble Washing, utilizes an aerator and an air stone, both of which you can buy for under $50 collectively at your local aquarium supply shop,.
If the aerator you purchase comes with a filter, remove it now before proceeding. Add into a tank or container filled halfway with biodiesel 30 milliliters of vinegar for every 100 liters of biodiesel and then fill it just about the rest of the way with plain water.
Drop into the mixture the air stone that's attached to the end of your aerator. It will immediately sink to the tank's bottom. Once you turn the aerator on, bubbles begin to emanate from the stone, rising up through the tank, washing the fuel as it does so.
When the bubble arrives at the surface of the biodiesel, it then pops, releasing a water droplet that then drops back through the mixture, washing it a second time.
Now if washing your biodiesel this way only succeeds in making it cloudier, then there's probably too much alkali in your mixture. This can easily be reduced by adding a small amount of lemon juice or white vinegar to the water you're washing with.
Let the bubble washing continue for 2 to 3 days. For faster results, you might choose to attach the pump (aerator) to the side of your tank or container. The vibrations assist in the separation process and help expedite its completion.
Periodically check the pH of the mixture as the process rolls along. Whether you wash your biodiesel "manually" (as in the first method outlined) or using the latter bubble washing method, you should only consider the process complete when the pH of your biodiesel reaches approximately 7-7.5 (neutral).
