Sewage Pump Station throttling mechanism

[EngWade]

Hello everyone,

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, Thanks!

 

[SteveWag]

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.
Steve

 

[EngWade]

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).

 

[bimr]

With constant speed pumps, your pumps will start/stop over time. If you furnish the pumps with VFD motor controllers, you should be able to vary the flow from your lift station with the VFD's.

There are valves available for this application, but the VFD controls are the recommended approach.

 

[EngWade]

Thanks Bimr.

I've begun looking into the VFD option. However, just for my own general understanding, what are some examples of the valves that could also be used in an application like this?

Thanks a bunch.

 

[bimr]

GA Industries manufactures a surge valve for sewage applicatons:

http://www.gaindustries.com/EngProducts1.asp?PTID=3b&PID=20&MENUID=8&MENUID2=3

SURGE SENTINEL© FOR SEWAGE Figure SE5001E Long Radius Elbow Body Figure SE5001W Wye-body

 

[SteveWag]

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.
Steve

 

[gibfrog]

Flow Equalization.

A. You provide about 6,000 gallons net of flow equalization tank storage and pump your effluent out at a constant rate for 18 hours - from 6 am unitl 12 midnight. You will have septic conditions and will need to consider odor issues.

or

B. Convince the WWTP that they need a 20,000 gallon net flow equalization tank and you contribute 5/16 of the cost. You will build a standard small package lift station with grinder pumps.
-------------------------

As I understand the issues, if the small WWTP needs to upgrade, you will be on the hook for your fair share. Therefore, you need to keep the WWTP in compliance with its operationing permit and look towards the future. Go for option B.


Clifford H Laubstein
FL PE 58662

 

[Ron]

A variety of valves could be used. Butterfly with variable closure control, plug valves, etc. Use something that requires little maintenance and control (butterfly).

 

[bimr]

Treatment plants operate best when the flow is somewhat constant rather than stopping and starting.

The dirunal flow from your apartments will not be much different than from the other flows entering the treatment plant.

Your best option is to install VFD's. Use the level in the wet well as the control set point. Vary the pump capacity in order to maintain the the water level in the wetwell. With VFD operation, your pumps will always be operating )except at night) and this will eliminate the flow surges from starting and stopping of pumps. You will also need to maintain the minimum flow in the force main of greater than 2 ft/sec to prevent solids from settling out.

The other options that have been presented such as flow equalization and valves may work but will not be practical.

 

[gibfrog]

I do not think that this is an appropiate application for a VFD.
Using the guidelines from 10 States Standards - Recommended Standards for Wastewater Facilities, look at page 10-6.

http://www.broward.org/waterservices/states_standards_wastewater.pdf

Here are some preliminary numbers, assuming a standard duplex package list station -

Peak Factor
25,000 GPD = 250 people eq 4.11
80,000 GPD = 800 people eq 3.86
(Assuming 100 GPCD as per RSWF)

Hourly Peak Flow Est Pump Rate
gpm gpm
25,000 GPD = 71.4 80.0
80,000 GPD = 214.5 240.0

2 Minute Drawdwon Volume Min Cycle time
Gallons Minutes
25,000 GPD = 160 80.0
80,000 GPD = 480 240.0
Minimum cycle time when Qin = Qout/2,

Cycle Time @ Peak Flow Pump Run in Cycle
Minutes Minutes
25,000 GPD = 20.8 18.5
80,000 GPD = 21.0 18.8

First - Look carfully at RSWF page 10-6, HOURLY peak flows are determined and used for design in wastewater facilities not instantaneous peak flows.

Second - In my preliminary evaluation, at design peak inflow, the 25,000 gpd lift station pumps 80 gpm for 18.5 minutes, rest 2.3 minutes and then cycles again. If you were to throttle this flow, then you would pump 71.4 gpm for a constant 20.8 minutes. I do not see the benefit of a VFD.

Third - Flow equalization would pump the seasonal peak average daily flow over 18 hours. Let us assume that peak average daily flow = 1.6 X average daily flow of 25,000gpd and we want to pump this over 18 hours => 37.1 gpm. A significant difference from 80.0 gpm.

Four - With a flow of 5/16 of the WWTP capacity, you will be a significant stake holder and if the WWTP goes out of complaince, will be likely be assessed your fair share to correct any issues. Therefore, you might want to take a closer look at the WWTP operation and verify that there is an influent surge issue and not some other operational or design issue prior to designing a non-standard package duplex lift station.


Clifford H Laubstein
FL PE 58662

 

[bimr]

I agree that lift stations are designed for the hourly flows. However, hourly flows are not used for the design capacity of treatment plants. Refer to 11.26: "The wastewater treatment facility design capacity is the design average flow at the design average BOD".

How is it that it is possible for someone to calculate pump cycle times when the size of the list station wetwell is unknown?

If you use a VFD on a lift station, the pump capacity should be designed to be the peak hour capacity. That does not mean that you will be pumping at the peak hour capacity.

Use of a VFD also does not mean that you will pump at a constant flow rate. The flow rate will vary from the maximum (peak hour flow) to the minimum flow (the flow rate that will maintain 2 ft/sec in the force main).

http://www.pump-zone.com/controls/i...almost-utopia-lift-station-level-control.html

http://www.pumped101.com/vfd pump selection level control.pdf

With a VFD, you will not pump the lift station down on each pump cycle with start/stop pumping scenario. With a VFD, you will speed up and slow down the pump in order to control to the preset water level in the wet well. Maximum flow will be the peak hour. Minimum flow will be the flow rate that will maintain 2 ft/sec in the force main.

I have never seen a treatment plant that was not capable of accepting the diurnal flow. However, I have seen many where the starting and stopping of pumps upsets unit operations.

VFD's may be installed for very little cost. An equalization tank will be significantly more for little benefit.

http://www.fwea.org/cmsitems/attachments/9/56203 - Submersible Sewage Pumps 2009 FWEA.pdf

 

[gibfrog]

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.



Clifford H Laubstein
FL PE 58662

 

[bimr]

It seems that you are making assumptions and it is taking you off topic.

1. Note that the treatment plant design capacity has actually never been stated so there is no reason to assume treatment plant design defects or "With a flow of 5/16 of the WWTP capacity". EngWade stated "The WWTP handles about 80,000 GPD, and the apartment building is estimated to produce an additional 25,000 GPD." 80,000 GPD must mean that the treatment plant is currently processing 80,000 GPD and will then handle 105,000 GPD. The treatment plant could be designed for 200,000 or whatever, but the design capacity has not been stated.

It is likely that the capacity is at least 20% greater than 105,000 GPD. Otherwise, the treatment plant operator would be looking for money from EngWade to increase the size of the treatment plant.

2. The lift station size has never been stated. "I gave estimated pump cycle times based upon reasonable parameters." What point is it to make assumptions about pump cycle times when you do not know the lift station dimensions?

3. You state "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". Why assume that the treatment plant has not been designed adequately?

4. You also seem to be assuming that the treatment plant was built inadequately. You state "why the WWTP operator feels that surging is an issue." EngWade actually stated "he told me there are shock loading problems at the plant, and so would like to see a throttling mechanism on my pump station".

 

[gibfrog]

BIMR - You are also making some inncorrect assumptions.

Yes, I misunderstood the plant size, it is at least 105,000 GPD. However, without more information about the reported "shock loading problems at the plant", the proper course of action is indeterminate. You understood the issue as a pump pulse issue, I understood this as a diurnal peak issue. However, In my opinion, a properly designed and operated WWTP should be able to run at 100% rated capacity without any issues. Otherwise, its' capacity needs to be downrated.

I did not intend to imply that the WWTP was improperly designed. If we assume hydraulic shocks are an issue, it could be an operational issue or it could be design issue or it could be an unanticipated loading issue (loading beyond the normal design assumptions for the anticipated service area).

In any event, before I designed a non-standard package lift station, that is not a formal connection/permit requirement, I would want the WWTP's Engineer of Record to put this requirement in writing. There may be other or even better solutions to the problem, that the WWTP operator does not currently see.

I intend no disrepect to the WWTP operator. However, if we were told that the WWTP, currently operating at 80,000 gpd and is willing to accept another 25,000 gpd. The WWTP should have a signed & sealed design/permit for 105,000 gpd or more. I feel that the EOR who designed the WWTP (or another PE) should now become involved if the WWTP is experiancing operational issues at <81% capacity.






Clifford H Laubstein
FL PE 58662

 

[bimr]

I only thing that I assumed was that the "shock loading problems at the plant" are due to the flow. That is because an apartment building is not going to generate shock organic loadings. And I have never heard anyone term diurnal loadings to be "shock loadings".

In this part of the country, when a treatment plant reaches 80% of capacity, the authorities start to review additional service connections into the treatment plant.