Butterfly valves are often used for throttling down of water lines, but I wouldn't use one on a sewage force main.
1) A WWTP has both a Hydraulic Capacity, a BOD Capacity and a Solids Capacity (here in Florida, also a denitrification capacity).  For example the typical extended aeration WWTP has a 18-36 hour hydraulic retention time for the aeration basin, based upon the influent loading.  The BOD capacity is more complex and an additional variable of O2 loading is included.
The hydraulic design of a small package WWTP is based upon both upon the Average Daily Flow and the Peak Daily Flow. See RSWF 72.232 -  for the recommended "Surface Overflow Rate at Design Peak Hourly Flow" for the clarifier or final settling tank of an extended aeration WWTP.
2) I gave estimated pump cycle times based upon reasonable parameters.  The minimum reccomended 1hp-10hp submersible sewerage pump run time is usually 2 minutes and this determines the minimum pump drawdown volume in a small package lift station.  (Of course you could have longer drawdown times and this tends to run up the costs due to the larger wetwell volumes.)  The peak flow rate is the minimum design rate a single pump must be able to pump at the required head.  Of course, the actual pump and system curves need to be included, but one must start somewhere and this is good starting point.  At least, this is where I start, so I can calculate a system head and then chose a pump, reiterate and determine a design operating point.   
3) My experiance with small extended aeration (<100,000 gpd) package plants is just the opposite.  It is the peak seasonal diurnal surge (like a super bowl sunday half time surge or a major holiday) that may create a problem in the clairfier, especially at high return activated sludge/skimmer rates or too high a sludge blanket (too little waste sludge).  Leaking toilets or just the typical unmetered water consumer profligate usage just makes this worse.   That is why it is important understand what is currently occuring at the WWTP and why the WWTP operator feels that surging is an issue.
The extended aeration WWTP usually has multiple compartments that gravity one to another.  The one/off pump pulse is only sensed in the first aeration basin.  If there is 80,000 gallons of aeration capacity @ 8 feet deep (a typical depth), a pulse of 480 gallons (the main influent pump running for two minutes) = 0.048'.  Furthermore, there is likely an 8"-12"  short pipe section or orifice connecting to the next chamber or clarifier to further attenuate this flow.  Maybe in a contact stabilization or another type of WWTP, influent pump on/off surging is a issue. 

MORE NEWS