Scum-collection systems in clarifiers remove floating material from the surface and typically include a scum baffle, skimmer arm, and scum trough. Circular clarifiers use skimmer-arm squeegees and deepened baffles to improve scum removal, while rectangular clarifiers rely on return flights to skim scum into a trough. Flushing mechanisms help clear collected scum. What issues can scum cause if it is not removed?