In sport, a foul describes an inappropriate or unfair action by a player. “Foul” can be translated as “dirty” and so FOULING also stands for contamination. This is the formation of deposits on membranes, which can lead to a drop in the performance of the entire system. Even though FOULING is reminiscent of scaling, they are still two completely different phenomena: While scaling can be attributed to precipitation of calcium and magnesium, FOULING involves contamination by primarily organic substances.