VFDs (Variable Frequency Drives) are great energy savers. But pump systems as a general rule see little to no energy savings by controlling with a VFD. The operative word in VFD is "variable". If the pump flows varies greatly and often, an energy savings may be realized by using a VFD. But most pump applications do not vary flow great enough (big curve) or often enough to justify the extra control cost. In that case control valves are used to vary or set flow. Also realize the extra cost associating with using a VFD is not just energy cost, especially when you are talking 100 pumps or more. That will get you in the range of generating too much harmonics if powered from one main, plus initial VFD cost, long term maintenance cost, etc.
Now I am not disking VFDs, it just happens you picked the one product they are least effective on (general speaking). If you want to control machine motors, elevators, conveyors, etc. The operative word in VFD is "variable", and over simplifying it. Another example in line with that theme, a conveyor. If the conveyors only function is start at the beginning of the week, and shut down on weekends, running only one non-critical speed, a VFD would not result in savings. (And that would be one more VFD in your plant adding to the harmonics.