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Need for sealing a septic tank?

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I had my septic tank cleaned earlier this year (it had to be located and un-covered for the cleaner). The fellow replacing the dirt over the tank put a man-hole with cover on top of the tank and left the lid off the tank so that when you remove the man-hole cover you can see down into the tank. We have had some issues just recently with a sewer odor in the house at one of the toilets (closest to the line going to the tank). I had a plumber come to replace the seal at the base of the toilet and he noticed a draft of air coming up from the sewer line. I am now being told by one septic-tank professional that the lid should have been replaced securely on the tank so the air pressure inside the tank would not be affected and cause the problem we are having of odor traveling into the system from the tank. The fellow who covered the tank says it is not necessary to put the secure cover on the tank since the inlet and outlet pipe are both under water and it should not cause the issue we are having. I would appreciate your thoughts.
asked in Plumbing by rstipp (120 points)
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1 Answer

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Hi Rstipp,

The two things may be related but not necessarily. Since the water inside the toilet’s base is the only seal (in most cases) that prevents sewer gasses from entering your house, removing the toilet opens the sewer pipe / there’s no p-trap like under the sink, tub, or shower.
Removing the toilet opens the passage of air between the bathroom and a plumbing vent stack that should be terminating above the roof surface, that’s probably the draft he noticed while replacing the wax ring.
There are also TEE’s at the end of an inlet and dispersal pipes inside the septic tank (again, in most cases / depending on design) with their bottom ends submerged and top open for venting (picture below).
Single compartment septic tank
With a slightly open lid this could also create a draft, and yes, the cover should be closed and sealed, but mostly to prevent odors from escaping (gravity tank), and of course safety reasons. 
Maintaining air pressure inside the tank is important in a pressurized system, the ones that include a pump.
answered by darekrudy (21,730 points)