Weir loading rate is the calculation of how much water is being pushed into the launder weirs.
Launder weirs are installed at the basin or tank outlet to provide a uniform collection and flow-on of settled water. If the flow through the weir becomes too high then floc may be carried over in the settled water.
Weirs are usually designed for a maximum flow rate (per metre of length) based on optimum sedimentation conditions. Comparing actual weir loading rates to the design rates gives a quick idea of whether the weirs are overloaded and contributing to any water quality problems.