Moving into 15 yr old home that does not have a sewer backflow check valve in the lateral line. The home sewer system is configured with a 4” PVC line that drains out 30 ft to the concrete septic tank.

As the tank fills up it raises the float on the sewer pump to empty the septic tank. The pump pushes the sewage out of the tank 100 linear ft to an elevated city-owned (pump station pit) that's 6 ft higher than the top of my septic tank. There are 5 other nearby homes that also empty into this pit.

The city pump station pumps this pit 300 linear ft, uphill 8 ft higher to where it FINALLY enters into a gravity-fed main line.

The bad news is, I am the lowest house on this totem pole, & scared about a possible backflow issue.

I talked to the city sewer dept, & they said definitely, by all means, I should install a 4” surface retrievable check valve & cleanout in my 30 ft lateral line out to my septic tank. I did find such a valve online.


  • Since I am downhill 14 ft from all the city lines & pump station, can I install (2) of these check valves, about 3 ft apart, in the lateral line to doubly lessen the chances of backflow?

  • Also, could these 2 check valves air lock or develop some other kind of issues?

Ecnerwal ; There is 3 ft of elevation between top of septic tank & (pier & beam) toilet closet flange for all 3 toilets ). So yes, the sewage should just run out on ground. But not sure if the pressue of backflo sewage would push the ( HUGE 80-90 lb, 2006 Deeter Foundry, Lincoln Nebraska, 2ft wide x 5/8” thick solid cast iron lid out of its cradle seat ( no holes in lid ) so it could run out on ground, before sewer rises 3 ft up into toilets. The lid is super heavy & takes a big crow bar, just to budge it out of the seat. I was planning on using a ( RectorSeal, “clean check” backwater valve, that has a flapper on the btm of 4” PVC, which slides down inside a 6” PVC barrel fittings that you glue in the existing PVC lateral line & rises up to top of ground and can be pulled for inspection in minutes. Can you please give some feedback on this issue. Hope I explained this issue good enough to understand. The main concern is the house is 14 ft below the City pump station & manhole sewer lines before it reaches a gravity main drain line. Please ask any questions, in case I need to be more sewer specific.


Ecnerwal crip659 FYI, I just contacted the company (RectorSeal) that makes these BackFlo check valves for sewer lateral lines which can be (pulled & inspected) from the surface, & the technician said there are no issues with 2 of these valves being used in series for your lateral lines. The tech said that the advantage is if the The tech said that if the flapper valve failed to fully seat due to closing on (paper, human solids or kids toys) then you may benefit from the second valve in the series. Also you calculated the pressure of the sewage backflo lifting the (24”) heavy cast iron tank, so thinking I can have one made out of aluminum.

  • 1
    It would be interesting if the six homes could agree to install a system that disables all their pumps and sounds an alarm if any of their check valves detects back flow. If you could agree and implement that, then you only have to worry about backflow of one single volume of the initial shared pipe, which is about 65 gallons.
    – jay613
    Feb 12, 2023 at 18:48
  • 2
    That 90 lb cover has an area of 452 square inches, being 2 feet in diameter. If it weighs 100 lbs, just over 6 inches of water pressure will lift it out of the hole. If the water gets up 3 feet, 587 lbs force pushing that thing out of the hole. It. Will. Move.
    – Ecnerwal
    Feb 13, 2023 at 0:06
  • 1
    If you were to install 2 check valves, I would install one between the house & septic and the other between the septic and the city side. One protects your house, the other protects your septic tank. I'm far from an expert, though, so don't take my word for it.
    – FreeMan
    Feb 13, 2023 at 16:14
  • FreeMan Yes, that would protect the house side lateral & also the Septic tank from over filling, & also keep the yard, & drive way clean from sewage running out of 24” manhole cover down the yard & drive way.
    – Oily Tex
    Feb 15, 2023 at 3:12
  • Ecnerwal, did suggest that there may already be a check in the septic tank discharge line, so need to look for one after I move in. If that’s the case, then only need 1 valve on the house side.
    – Oily Tex
    Feb 15, 2023 at 3:25

2 Answers 2


What is the elevation relationship between your septic tank top and your house? If that's (as it should be, since sewage only flows downhill until it's pumped) below your house fixtures, the hypothetical city sewage backup will come out the top of the septic tank onto the (presumably) lawn, not show up in your household pipes at all, even without a check valve between your house and the tank. Not nice, but nicer than a basement full of sewage.

Two check valves is normally not a useful thing, nor a good idea. While I'm more familiar with the problems of excessive check valves on the supply side, where they can cause problematic pressure spikes, (probably less of an issue in sewage, but I don't know) consider this:

At some point, one valve fails.

At that point, you have one failed valve and one working valve and no way to know that one valve failed. So, you have only one useful valve, until that fails, too.

You have no way to know that it failed as well and both have failed, until there's a failure at the city side (which may not have happened in 15 years, evidently) that sends sewage back your way and it comes though both of your failed valves. That would have to involve a failure of the tank vent and the manhole covers somehow being locked in place (possible if it does not have surface riser and they are all buried, I guess, but a tank in most civilized locations has been required to have at least one manhole brought to the surface for more than 20 years, so a 15 year old tank should.)

Seems like one reliable valve might be a better use of the money you'd spend on two valves, assuming you can determine "what's a more reliable valve for this purpose" and assuming it costs more than the most basic version (neither of which may be true.)

Presumably your pumped line (force main, I think is the usual term?) already has a check valve, which is between the city and your tank?

  • I can't see any reason to waste money on an aluminum lid - you have plenty of force available to dislodge the cast iron one you already own before sewage rises into your house, without your check valve(s.) Put the valve(s) in if that makes you happier, but don't waste money on a lighter lid. One reason lids should be heavy is to prevent small children from dying.
    – Ecnerwal
    Feb 13, 2023 at 16:02
  • Yes, didn’t think of the kids next door. Also learned that ( force = pressure x the area ), learning a lot here.
    – Oily Tex
    Feb 13, 2023 at 16:46

Two check valves should work, but usually not needed.

They are usually a simple design with low failure rate.

They also reduce the size of the pipe a bit where they are placed, so two might increase the chance of a blockage point.

  • crip 659 ; from what I can tell, the ( Clean Check ) bacj
    – Oily Tex
    Feb 12, 2023 at 18:41

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