Return activated sludge flow rate may be adjusted by which of the following?

The adjustment of the return activated sludge flow rate is most effectively done through the SVI approach, which stands for Sludge Volume Index. The SVI is a measurement of the settling properties of the activated sludge and is calculated by dividing the settled volume of sludge by the concentration of solids in the sludge sample. By monitoring the SVI, operators can assess the performance of the activated sludge process and make informed decisions regarding the flow rate of returned activated sludge.

When the SVI is within an optimal range, the settling characteristics of the sludge are good, which means that the solids can effectively be returned to the aeration basin without causing issues like bulking or foaming. If the SVI indicates poor settling, adjustments to the return flow rate can be necessary to ensure that enough healthy biomass is maintained in the system, ultimately improving treatment efficiency.

Regulating the return flow rate based on SVI helps control the concentration of microorganisms in the aeration basin, thus maintaining the appropriate food-to-microorganism ratio, effectively stabilizing the process. Other factors like Mean Cell Residence Time (MCRT) and sludge age are important for understanding and managing the biological system, but they are not as directly tied to adjusting the flow rate as SVI metrics are.