I'm designing a pump station to convey your typical waste from an apartment building.  The pump station is designed with two submersible centrifugal grinder pumps, and connects into an existing municipal low pressure sewer force main.  I've spoken with the Wastewater Treatment Plant operator, and he told me there are shock loading problems at the plant, and so would like to see a throttling mechanism on my pump station.  So does anybody have any recommendations for this?  I know butterfly valves are often used for throttling down of water lines, but I wouldn't use one on a sewage force main.  Any help or insight is greatly appreciated.

Must be a big apartment building or a very small treatment plant. A rubber pinch valve (Red Valve Co.) is very good for throttling sewage, especially following a grinder pump. They are best when operated by air, so an air source and control would be needed. If the flow leaving the facility is to be controlled, some storage volume will be required and this means perhaps an aerated (and smelly) tank. The apartment complex may not like that.

You are right on both accounts.  It is a big apartment building, and it is a small WWTP.  The WWTP handles about 80,000GPD, and the apartment building is estimated to produce an additional 25,000GPD.  The only alternative I can really think of is to slowly ramp up the pumps so as to alleviate water hammer, and to pump fairly small volumes at a time (1100 gallons per cycle).

The G-A surge valve is only used to dissipate water hammer and will not handle continuous flows. If the WWTP is willing to work with you, get a daily flow chart for the plant and study it Then develop a theoretical chart for the apartment complex. Is it an old folks home or a yuppy enclosure where everyone leaves for the day? Then combine the plant flow chart and the apartment chart and determine what flow the plant operator feels is excessive. That area of the chart that represents excessive flow needs to chopped off, stored and delivered to the plant during a low flow period.

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.

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