If the RAS is decreased while the WAS remains unchanged, what change will occur in the RAS concentration?

When the return activated sludge (RAS) flow is decreased while the waste activated sludge (WAS) flow remains unchanged, the concentration of RAS will increase. This happens because the same amount of biomass (activated sludge) is present in a reduced volume of liquid.

To visualize this, consider the scenario where a fixed quantity of solids is cycling through the system but in a smaller volume due to the reduced RAS flow rate. Concentration is defined as the amount of solids per unit volume; therefore, if the volume decreases while the mass of solids remains constant, the concentration must increase.

Fundamentally, the relationship between flow rate and concentration illustrates how manipulating one aspect of the system directly affects the other. The constant WAS removal means that the mass being removed from the system does not change, but the less RAS means that the remaining solids become more concentrated.