How to Filter Vegetable Oil, Biodiesel and Other Liquids

This information is aimed for filtering vegetable oil and biodiesel but is essentially universal for any liquid filtering. Filtering can be done using strainers, filter bags, settling techniques and in the poor man’s method aka: using old clothing (not recommended as it makes a mess and requires a LOT of old clothes.)

Discussion Topics:

Why Filter?

Why is filtering important? For vegetable oil and biodiesel, contaminants must be removed before entering an engine. Injectors, pumps and pistons/seals will be ruined if large particles or the wrong fluids enter them. Engines require lubricating fluids to ensure long operational life. Diesel fuel is a lot more lubricating than gasoline. This is one of the main factors why diesel engines tend to go 500k to a million miles with no problem while most gasoline engines are pretty much junk by 200k.

Coarse Filtering

It is extremely important to filter your vegetable oil with coarse filters before fine filtering. Why? Fine filters collect practically everything. Which means all that big stuff that would be easy to scrape out of a coarse filter is now embedding itself into the fine filter you use. Your fine filters will take a beating all the way, they will clog fast and it will cost you a lot of money to replace them often.

The first thing you should do when filtering vegetable oil is to start with about 400-600 micron. A micron is defined as 1x10^-6 meters. 100 micron is around the width of a human hair. You will not see any particles floating around in your veggie oil until about the 400-600 micron range. If you do not see anything in the oil, your best guess is that you need to start with 400 micron. 600 and 1000 micron are great to get out all the french fries and junk you can see easily.

If your oil comes in those 5 gallon jugs, it’s best to use strainers. The EZ-strainers in our store are meant for placing on top of a 5 gallon bucket or a 55 gallon drum. Simply place them over the container and pour. The best part about them is you can stick them inside of each other, and pour right through a 600-400-200-100 micron series all at once.When you filter in stages like this, each filter will catch it’s "share" of particles but allow the veggie to continue flowing through by not clogging up from too much of the bigger stuff that the previous strainer catches. You will be able to pour more oil at once and will not need to clean the strainers as often.

If your oil is already in a large container, it is best to use a pump and bag filters with an adapter head. Simply attach the bag filters to the adapter and attach the outlet of your pump’s hose to the adapter. You can now pump oil from your container, through the bags and into your next container. The adapter head should be attached to a rigid pipe so that it can hang over the container and the bag will hang from the adapter. It’s also best to filter with multiple bags on one adapter. (We use 3). This will help strengthen the bags against any pressure brought on by the pump and also get successive filtering done in one step.

With filter bag adapters, you are limited to a low pressure application because the bag is hanging freely and is not supported by anything to prevent a blow out. If too much pressure or too high of a flow rate is applied, the bag filter may developer leaks in the fabric or the bag filter can even blow right off of the adapter head. In short, the adapter head prevents overflow so you do not need to control the flow of the pump too much. Essentially with the filter bag adapters, you are still mostly using gravity and just a little boost from the pump.

The pump you use should be a slow enough flow so that they won"t blow the bags, or you will need to use a ball valve or bypass to prevent blowouts of the bag. We have a small diaphragm pump which pumps biodiesel quite well and can be used for filtering with the adapter heads with little worry of harming the bag filters. Also for biodiesel, we have the series fuel oil pumps which should be ok for 16" and 32" long filter bags. A ball valve may be needed to choke down the inlet to the pump if flow is too high though.

For vegetable oil and thicker oils, it is recommended that a gear pump be used. Because of the thickness of the oils, most small motor pumps can"t handle moving the oil and some, like the series fuel oil pumps will break from the high viscosity. The gear pumps are all metal and will push any thicker fluid through with no problem. The drawback to the filter bag adapter with these pumps is that they may pump too well, and the inlet should be reduced with a ball valve to reduce flow and to help prevent blowing out the bag filter.

If a high flow application is required, then our stainless steel filter bag housings should be used. These vessels can be fully pressurized and can handle much higher flow rates than the bag filters. They have a basket inside of them to support the bag filters and prevent blow out of the bags and are fully enclosed so fluid can be pumped in and out to wherever you want to put it, without having to hang it over a container.

Note: Hot vegetable oil filters a lot faster than cold oil. It’s important to raise your oil temp to 100+ degrees whenever possible. If it’s in jugs, let it sit out in the sun for a day. If in a metal container, you can connect a water heating element to a spliced wire and drop it right into the bottom of the drum. Oil/water does not conduct electricity so don’t not be afraid. As long as the white and black wires are connected to their own terminal, it will be fine. (Do not plug in until it’s submerged in the oil)


Before proceeding with fine filtering, you should first de-water your fuel. There are a few methods for doing this.

One method would be to heat the oil to 212 degrees F. Once the temperature passes 212 degrees, all water should be boiled out. The water in the oil cannot raise higher than 212 degrees at sea level or it must vaporize. If you are higher than sea level it will evaporate before this temperature, so by 212 degrees it is safe to assume all water has been vaporized. Although easiest, this method is not the best. It uses a lot of energy,whether electrical or by some other means. And it also increases the risk of producing more free fatty acids which is what we are taking out when making biodiesel. They are what gels vegetable oil up when cold. Polymerization may also occur which is a "black goo" you may find at the bottom of your containers. This occurs rapidly when oil is hot and in contact with metal.

Another method used to remove water is to settle it. Simply put the oil into a container which has a drain installed at the bottom (or side bottom) and let it sit still for about a week or two. Water is about 15% more dense than oil so it and many of the solid particles will drop to the bottom during this time. They can then be drained out through the bottom or if not available, the oil can be siphoned off the top. It’s extremely important to remove all water whether running on straight vegetable oil or biodiesel so be sure not to siphon up oil anywhere near the bottom. (preferably stop 6 inches or higher from the bottom).

If you need to settle the water out faster, you can heat the oil to about 110 degrees F and maintain the heat for 4 hours and let it settle for an additional 4-8 hours. The water should settle more rapidly.

Another method for removing water is to use a water separator/filter. This is essentially the same as settling but it’s meant for only trace amounts of water. The idea is when the fluid flows through the filter, water will sink to the bottom and the fuel will draw from the top. It’s great for on-board vehicle applications as a safety precaution but terrible for initial filtering of vegetable oil. There are high chances of a lot of water being in vegetable oil especially when coming from a dumpster and a water separator will not catch it all.

Testing for water in your oil

One easy method for testing your oil is to use the crackle test. Heat a frying pan to approximately 275 degrees. The best way to figure out if it’s hot enough is to splash a few drops of water onto the pan. When the drops instantly vaporize with a loud crackle you have gotten it hot enough. Don’t let it get too hot as your oil can catch on fire if it’s too hot.

Next pour a small sample of your oil onto the pan. If it makes any crackle sounds from the water in it vaporizing, you have water. If no sounds occur, look for bubbles. A lot of small bubbles may be a sign of some water content. Only small trace amounts of bubbles are normal. If that’s all you get then you’ve passed the test for water-free oil. If not, de water it again by settling or heating and re-test later to be sure all water has been removed.

Fine Filtering

In the world of fuel, fine filtering starts at about 100 micron or less. You should use polyester filter bags to achieve these smaller ratings. Polyester is cheap and good at catching small debris. It can withstand temperatures up to 300 degrees and handle a good amount of pressure from a pump. It’s also very chemical resistant. Some web sites will claim that polypropylene is a better material to use and they are wrong. It is simply the same quality of filtering with lower temperature ratings and at a higher price. Polypropylene is meant for special applications involving filtering of certain acids and bases. We still offer polypropylene bags for those who are convinced it is better by these other sites.

To filter with your polyester filter bags, it is best to use a pump. Although you can still gravity filter using the provided hanging strap and by pouring the fluid into it little by little. This is not recommended as when the filter begins to clog, the oil will flow slower and slower and you will consume more and more time.Or you will spend more money on more filters. Instead, use a diaphragm pump with an adapter head to pressurize the filtering. It will force the oil through the bags quickly and efficiently. Best of all, you can run your filters until they are clogged completely.

IMPORTANT: Heat your oil to 100+ degrees. Higher is not necessarily better, but any lower risks the chance of your filters catching fats in the oil. They will clog FAST (within 20 gallons) if the oil is cold. And the oil will also flow very very slowly. With a pump it may not be so bad but the filter will still not last as long as it should. Let the fats melt and pass through the filters so you can either take them out when making biodiesel or just burn them in a straight vegetable oil set-up.

As usual, you should filter in succession. Start by filtering with a 50 micron filter,then through a 25,then a 10 etc... The more bags you use, the longer they last and the quicker the filtering. Bags can be stuffed inside of each other so the oil passes through all the stages in one process. About 3 bags can fit easily onto our adapter heads. It’s possible to fit more but they begin to choke down on the bags. If using an adapter, choose the 3-4 bags you need to filter with and go with it. Or run two filtering cycles doing 50-25-10 then 5 and 1 after.

If you are making biodiesel, you ideally only need to filter to about 25 micron before processing. At that micron range the particles left are small enough to not affect your process. You can do a final filtering of 5 or 1 micron after the biodiesel process is completed.

If filtering for straight vegetable oil usage, you should go to at least 5 micron. Most diesel filters are around the 5 micron range and you should filter under this value to preserve them. The more you filter now the less your vehicle needs to filter later. 1 micron is recommended as the final stage in the filtering process. It’s also best to filter again with the 1 micron when pumping from your storage tank to your fuel tank. Gas stations filter on every single transfer of the fuel. You should too.

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