Collecting food fat from households and restaurants is being done today, partly to recycle the fat, partly to prevent it from being poured into the sewage and resulting in a plugged sewage pipes with extensively costs as a consequence. Existing systems for collection of used cooking oil from households, includes a lot manual handling for units responsible for collection, a safety risk for households and results in that the collected fat goes to incineration instead of recycling because of the uncertainty of the quality of the collected fat. The presented invention concept is a collection system and method that solves the above problem through a docking between one, dedicated to the purpose, container (2) and a receiving unit (1). The container (2) and the receiving unit (1) are both equipped with valves (5, 6) which interact at docking and open each other. This docking procedure means that the receiving unit (1) can only be filled by using the intended container (2).