Control valve F&A 2353 can be used on any waste water treatment plant with a stable continuous recirculation flow rate. Manually adjusting a ‘telescopic head’ requires many adjustments particularly when the flow of feed to the clarifier is variable. Other parameters such as sludge viscosity or a variation in concentration in the aerated tank may require to adjust the telescopic head settings. In addition to avoiding to bog down the clarifier and to remove the need to monitor recirculation, it provides for a more stable concentration of sludge to the sludge thickening system and to reduce the amount of flocculent.