At the end of the day, the truck is heavy with the combined grease of dozens of restaurants. Corn, canola, and other leftover oils have mixed together into a single fatty sludge. But the process has really only just begun. To turn the discarded grease into a usable product, the oils are first sent through a processing machine. The machine warms the grease and, in its porous state, filter the grease through screens. That removes any suspended matter, like wayward french fries. The next bit is up to gravity; for as long as two days, the solution sits until it naturally separates into grease, water, and fine solids, according to the EPA. Last, the emulsion goes through a centrifuge, which spins it until the solids and water are sorted out and the grease product can be skimmed off and carted away.