Plumbing Question: Submersible Sewage Simplex Pump
A client wants to act as a general contractor on her own small addition project. She asked me to help her understand the plumbing issues involved, as I’ve done with rainwater harvesting. Today I’m researching the installation requirements for a Submersible Sewage Simplex Pump, which will have to accommodate a new bathroom and possibly another small bathroom / kitchen if she converts a garage in the future.
The addition will be “slab on grade,” approximately 18 inches below existing plumbing fixtures. Therefore, a pump is required. The idea is to dig the submersible sewage simplex pump into the ground, deep enough for the two new gravity sewage lines (4”) to connect to. We can run the new discharge line (2”) under the house, in the crawl space. It will tie into the existing gravity sewage line at the cleanout (see diagram below).
First step is to look up what kind of pump I installed for my own use. It’s a Waste-Mate 270 series by Zoeller. Well, something like this will probably work, but I need to see if there’s a way to connect a second (future) line to it. I call a rep and instead of answers, she emails a pump-sizing questionnaire to fill out.
To size a submersible sewage simplex pump, I need to answer the following plumbing questions:
- What type pump is it? It is a residential sewage & grinder type. We only need one pump, which makes it a “Simplex” as opposed to a “Duplex” or “Triplex.”
- What size is the discharge pipe and material? It’s a 4” PVC pipe.
- Including the pit depth, how high is the pump ejecting? If the pit is four feet deep, then the total height is five feet.
- What is the total length of the discharge pipe from pump to gravity? Maximum 38 feet.
- How many 90 and 45-degree turns are there? It’s a straight shot.
- Is a high water alarm required? And if so, would it be indoors or outdoors? An alarm is always beneficial (though less common than it should be). Otherwise, the first indication that the pump has worn out will be when the plumbing backs up, but that takes about 10 years.
- What is the list of fixtures? Two bath groups consisting of lavatory, tub/shower, and W/C + kitchen sink with disposer; perhaps a dishwasher.
- Is the basin required? Yes. Fiberglass or Poly? While fiberglass, in general, is more rigid than polyethylene; however, either is more than adequate for the application.
- Lid style? Blank cover because we want to discharge through the sidewall of the basin. There’s a chance that the blank cover is only available with a fiberglass basin.
- What is the depth of “Invert” (Inlet) from finish grade? I am guessing two feet.
Next, I call a manufacturer’s rep for Liberty Pump to clarify and confirm.
Norris picks up the phone and provides all the technical support I need. It turns out, the requisite discharge velocity of a sewage ejection system is calculated on the potential volume of waste (expressed in “Drainage Fixture Units”). For two bathrooms, it should be twelve DFU. The minimum velocity of a forced sewage main in 2” pipe is 21 gallons per minute, which is suitable for up to 34 DFU, so we can comfortably assume that the 21 GPM will suffice. Sewage pumping at the minimum velocity creates only about 2’ of “friction head” per 100’ of pipe, so with my approximation of 10’ of static head we can consider 21 GPM at 12 TDH to be perfectly adequate.
The most cost-effective product Liberty could offer in this case would be the pre-assembled P372LE51 with an additional K001147 4” pipe seal for the additional inlet we are planning.
There you have it. I’ve tackled another plumbing question and now I know a lot more about a submersible sewage simplex pump. I hope you do to! But that’s in theory; I would still advise you to hire a reliable and experienced plumber to help you navigate.