We're trying to filter WVO through 50 micron, 25 micron and 10 micron filter bags. The idea is to get MIU content to less than 2%. For some oil this works great, but some just doesn't want to get filtered. When I put a sample in a pharmaceutical centrifuge, I get about 5-10% powdery ash substance as residue on the bottom. Any idea of what this stuff is and how to get rid of it? If I try to burn this oil in my diesel truck, it clogs up the fuel filter very quickly. The filtration process happens after the oil has been heated to 180F and settled overnight. All water has been drained out, but when I empty the heating tank, there is a lot of black gooey residue on the bottom.