What is the purpose of having vertical bar in the first stage of WWTP?

A bar screen is a mechanical filter used to remove large objects, such as rags and plastics, from wastewater.^[1] It is part of the primary filtration flow and typically is the first, or preliminary, level of filtration, being installed at the influent to a wastewater treatment plant. They typically consist of a series of vertical steel bars spaced between 1 and 3 inches apart.^[2]

Bar screens come in many designs. Some employ automatic cleaning mechanisms using electric motors and chains, some must be cleaned manually by means of a heavy rake. Items removed from the influent are collected in dumpsters and disposed of in landfills.^[1] As a bar screen collects objects, the water level will rise, and so they must be cleared regularly to prevent overflow