Gen Questions about Low Pressure Sewers 2

[cieg22]

I'm trying to gain a preliminary understanding of low pressure sewer systems, having never worked with them before.

I understand that generally, each residence has its own pump that delivers the flow into the low pressure main. Is there a central pump station maintains pressure in the main?

Is there a good source for guidelines on designing low pressure sewer systems?

I read that a cleansing velocity of 2 ft/s needs to be maintained. Would this make the low pressure sewer a bad choice if only a couple residences are being tied into it?

Thanks everyone for your help!

 

[RWF7437]

http://www.eone.com/sewer_systems/intro/engineers.html

 

[bimr]

"I understand that generally, each residence has its own pump that delivers the flow into the low pressure main. Is there a central pump station maintains pressure in the main?"

No. The semi-positive displacement grinder pump in the LPS pump station has a nearly vertical H-Q curve. This is the best type of pump for successful parallel operation of many pumps into a system of common low pressure mains. Since each pump will be located at a different point along common low pressure mains and at various elevations, each pump should operate in an efficient and predictable manner, whether one pump or numerous pumps are operating at a given moment; the pumps in such a system do not have a single fixed “operating point,” but must operate consistently over a wide range of heads that are continually, and often rapidly, changing.

The E/One grinder pump has the capability of operating above the LPS system design criteria of 80 psig. Based on the maximum daily number of pumps operating simultaneously versus the number of pumps connected to the system at the design pressure of 80 psig, the capability to operate significantly above the system’s design pressure is mandatory in order for the system to operate properly during the approximately bimonthly peaks when the “absolute maximum” numbers of pumps are operating. This feature also ensures that pumping will continue under those conditions when higher–than–normal pressure occurs in the pipeline.

"Is there a good source for guidelines on designing low pressure sewer systems?" Contact Orenco or E/one for design information.

"I read that a cleansing velocity of 2 ft/s needs to be maintained. Would this make the low pressure sewer a bad choice if only a couple residences are being tied into it?"

No. As a general rule, pipe sizes should be selected to minimize friction losses while keeping velocity near or above 2 feet per second. When the pump(s) are operating, you will have the 2 feet per second velocity.

 

[cieg22]

Thank you both for this information - it's a huge help.

I'm looking at another scenario as well, where the system would be installed initially with only one user - a commercial property that operates one or two days a week at best. The flow produced on those days should be 1, maybe 2 edu's at best. Could an LPS work in this situation?

Under such conditions, would it be possible for flow to end up sitting in the pipe and going septic, once the pump shuts down?

If this materials settled out during the down time, would the min 2 ft/s flow be able to scour the clogs out, once the pump started up again?

Thanks.

 

[bimr]

If you only have one user, the setup would amount to being a lift station.

Have you considered using an onsite treatment system? Something like a sand filter.

The material will go septic in the pipe. You should not be overly concerned with that. It will be more of an issue where the force main discharges. If it discharges into a larger sewer, then it probably is not a problem.

A flowing force main has a minimum velocity of 2 ft/sec to keep materials in suspension. If you have a force main that stops flowing, you need about 3.5 ft/sec to resuspend the material.

 

[cieg22]

I would love to keep it on septic, but this is for a funeral home with an embalming room.
Thanks everyone, for all of your help!

 

[RWF7437]

"....but this is for a funeral home with an embalming room...."

Will you be required to pre-treat this waste stream before discharging to a public sewer ?

What future land uses are anticipated or planned for ?

 

[cieg22]

Good questions. Luckily, pretreatment is being addressed by others. Once again, thank you, sir for all of your help!