A weir is a plate used to control flow from a clarifier.  Specifically, it makes sure the flow is evenly applied across the whole tank. Clarifier weirs often have v-shape notches.  Proper leveling and cleanliness are crucial to prevent flow short-circuiting. Weirs can be placed at the end, around the perimeter, or slightly inboard. Those positioned along the outer wall may experience higher solids loss due to upward currents.