Algae, especially when growing on the surface of a pond. / Worthless, insignificant person or people.

A blowdown valve mounted at the water level of a boiler, used to blow down lighter oily or foamy deposits that float on the water.

plural of scum valve

The buildup of insoluble calcium or magnesium fatty acids from the use of soap with hard water, which leads to a dulling of surfaces and acts as a matrix for trapping soils.