The liquid at the overflow level is at or near the 23% salt solution. The brine solution flows into the holding tank and is tested for the correct concentration using a hydrometer or salimeter.
If too dilute, the solution is recirculated back through the mixing tank
Or, if too concentrated, additional water is added.
Once the concentration is correct at 23.3%, the brine can be used to refill anti-icing and prewetting tanks, or offloaded to storage tanks, read to use when needed.