Water prewash

Prewashing biodiesel with 7% water

A 7% water prewash is a way to give your biodiesel a preliminary wash after the reaction is finished but BEFORE draining off the glycerol byproduct. Performing a 7% water prewash stops the transesterification reaction dead in its tracks by drawing much of the water-soluble methanol and catalyst out of the biodiesel and into the glycerol layer. This can help reduce soap, reduce the chance of emulsion formation, and make further washing easier.

Why you may want to use a 7% water prewash

Washing has traditionally been thought of as one of the more challenging, time-consuming aspects of making biodiesel. For people who use the 7% water prewash method, this is no longer the case. Most people who have used the 7% water prewash method report that their biodiesel is much easier to wash afterwards.

Although this technique may help you save wash water overall, be aware that it will add water to your glycerol byproduct and therefore may make it harder to recover the methanol from it. If you don't recover methanol from your glycerol, carry on.

How to do a 7% water prewash

Start by running the biodiesel reaction in your processor as normal. Process as usual until your batch is finished. But BEFORE turning off your pump or mixer, add in your 7% water prewash and continue mixing. Here are the details for the two most common base reactions:

Single stage base reaction Process as usual until your batch is finished but keep your pump or mixer running. With your pump or mixer still running, add 7% water (that's 70ml per litre of water for every litre of liquid in the processor---oil plus methoxide.) After you have added in the 7% water, continue pumping/mixing for an additional 15 to 20 minutes.

Two-stage 80/20 base reaction Process as usual until your batch is finished but keep your pump or mixer running. With your pump or mixer still running, re-introduce the glycerol you drew off at the end of Stage 1 back into your processor PLUS 7% water (that's 70ml per litre of water for every litre of liquid in the processor---oil plus methoxide.) After you have added in the Stage 1 glycerol PLUS the 7% water, continue pumping/mixing for an additional 15 to 20 minutes.

When you are finished mixing the prewash, settle the batch as usual in your processor or after pumping into your settling tank or wash tank.

After the batch has settled, drain off the glycerol byproduct. Now that it has 7% water mixed in with it, your glycerol byproduct will be much thinner than usual and there will be more of it.

Using your usual technique, wash your biodiesel until the wash water is clear. With the 7% water prewash you may find that you can use a faster, more vigorous washing technique with less risk of causing an emulsion. You may also find you need to wash your biodiesel just once or twice instead of three, four or five times it previously needed. This technique can save you a lot of time, water and aggravation!