Screening, which removes ovesized solids, is distinguished from comminution and grinding, which reduce the size of solids.

Bar screens are the first unit treatment process in a wastewater treatment plant. They remove solids and trash that could otherwise damage or interfere with downstream operations such as pumps, valves, mechanical aerators, and biological filters. Coarse sreens consist of vertical or inclined steel bars spaced at equal intervals across a channel through which wastewater flows and are either manually or mechanically cleaned.