Reader Question: Straight Veggie Oil

Thanks to our many readers and those who write to us with questions--we'll periodically share questions (and our answers) here. We recently received this query from Ray, who wrote:

"I have asked several restaurants about collecting their WVO (waste vegetable oil) but one of them cautioned me regarding a substance they use to clean their fryers (sodium hydroxide) ... he said he did not know if this would be an issue with the internal health of the engine. I have a direct injected Listeroid engine that is supposed to run on straight veg oil. Do you know of any issues with the cleaning chemicals and does making it into biodiesel make any difference? Thanks in advance for your time."

Hi Ray,

The restaurant probably filters out most of the sodium hydroxide (NaOH) when they drain the oil from their fryers. They use this compound to neutralize the acid in heated vegetable oil, and that makes it last longer. It usually goes in as a powder and turns into a light paste-like substance when it comes out. If you are going to use the oil for an SVO (straight vegetable oil) system, you'll probably be fine. Any trace amount left in the oil will drop out in your settling drum, and any further suspended amounts will be caught when you filter it.

Using it for making homemade biodiesel is another matter. The biodiesel reaction is very sensitive, and the trace NaOH could disrupt your chemistry. Sodium hydroxide is similar to potassium hydroxide (KOH) that is normally used as the catalyst for the biodiesel reaction (transesterification). If you are going to try using this oil for BD, be sure to do a thorough titration or mini batch test to see if the NaOH residue will interfere.

Thanks for visiting our site,
Scott & Christine

bucket of oil - photo © Scott Gable