Refinery Processing Standards
A renewable diesel plant cleans fats and oils, then uses hydrogen to remove oxygen. The result is a hydrocarbon fuel that can share tanks and pipelines with petroleum diesel.
Biodiesel is a different process. It uses alcohol and leaves a fuel that is not identical to petroleum diesel. Both fuels can start from used cooking oil. The plant, the specification, and the finished product are not the same.
The kitchen never sees the hydrogen step. It sees the container and the pickup. Keeping water and food scraps out of the used-oil container is the kitchen's share of a clean feedstock. The plant's process, and the fuel it makes, belong to the refinery.
What pretreatment is protecting
A hydrotreater reacts oil with hydrogen over a catalyst at high temperature and pressure. The catalyst is expensive and it is poisoned by small amounts of the wrong material. Phosphorus from food, metals from equipment or from a contaminated drum, chlorine from a cleaner, and polyethylene from a melted spatula or a plastic cap all show up in refinery complaint logs. Pretreatment is the plant's filter, wash, and guard bed in front of that catalyst. It is also a cost, so a plant would rather receive oil that does not need the full treatment.
The kitchen never sees the reactor. The kitchen sees the practices that keep those poisons out. Do not store used cooking oil in a container that held a chlorinated cleaner or an unknown chemical. Do not let plastic utensils and fryer-filter bags accumulate in the caddy until they melt. Do not pump interceptor waste, with its detergents and food solids, into the oil tank. Detergent is a refinery problem as well as a commodity-quality problem. It helps water stay mixed with the fat, and it carries other contaminants with it.
Water is removed early because it uses energy and because it interferes with later steps. A load that looks like oil and weighs like oil can still be a quarter water if someone rinsed into the tank. The plant will find it. The invoice will show it. Cooling the fryer and draining oil, not wash water, is the entire technique.
GREASE.ORG collects the oil and pumps traps. It does not operate the hydrotreater and does not guarantee that a given load will meet a named refinery's pretreatment spec. That spec is the buyer's. Meeting it starts with a single-purpose tank, a lid, and a driver who will not mix a trap load into a yellow-grease truck. The feedstock-specification page lists the tests. This page is why those tests exist: a catalyst downstream that fails closed when the oil was treated like trash.