We are adding a treatment system (for taste-not safety) which will have pressure drop proportional to flow.  The filter documentation lists the following pressure drops:
7 psid @ 30 gpm
15 psid @ 45 gpm
25 psid @ 60 gpm.
In cases where demand is 60 gpm, I guess the community water pressure would decrease to about 50 psi.  If the demand increased above 60 gpm, the pressure would drop further.  I don't think this will happen often, but it should be designed for.
I'd like to limit the amount of pressure drop when demand is greater than 60 gpm by installing a bypass line around the filter and including a pressure reducing valve (PRV) set at 50 psi.  I figure that when demand is greater than 50 psi, the PRV will open and allow unfiltered water to be blended with the filter water and meet the demands of the community while keeping the pressure at 50 psi.
That would work but if it is not used very often, the PRV might be stuck shut when it is needed.  You could put a PRV on the mainline after the filter, and set it for 70 PSI, then set your PID control to 100 PSI.  It won't be as efficient at the lower flow rates but, you are already using more energy anytime you vary the pump speed.  Might also be able to put the pressure transducer after the filter and not use a valve at all.

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