Operator terms · Water and wastewater
What is a weir in water treatment?
A weir is a wall or plate that water flows over at a controlled level, used in clarifiers and channels to spread the flow evenly and to measure or control it. In a clarifier the clear water flows over the weir at the top while solids settle below. Even weir flow keeps settling working well.
Last reviewed 2026-08-03
In a clarifier
Clarified water leaves a settling tank by flowing over weirs at the surface. Keeping the flow even across the weir stops short-circuiting and helps solids settle out.
Measuring flow
Because the depth of water over a weir relates to the flow, weirs are also used to measure how much water is passing through a channel.
Common questions
- What does a weir do in a clarifier?
- It lets the clear water flow off the top evenly while solids settle below.
- Can a weir measure flow?
- Yes, the depth of water over it relates to the flow rate.
- Why does even weir flow matter?
- Uneven flow short-circuits the tank and hurts settling.