Septic Tank Inlet & Outlet Tee Size FAQs

Q&A on size, length, or dimensions ofseptic tank tees

Septic Tank Tee / Baffle Dimensions FAQs

Tip: See details at SEPTIC TANK BAFFLE or TEE DIMENSIONS - lengths, placement, elevation, depth of the tee extensions into the septic tank for both outlet and inlet tees or baffles

Also see SEPTIC TANK TEE LENGTHS vs SEWAGE LEVEL FAQs

Does the septic tank inlet tee need to stick down into the septic tank? We have septic odors in our house.

Is there a down spout within the septic tank, reason, I believe its come off. Smells coming into home On 2019-09-06 by Johnny Hall

Reply by (mod) -

Yes, Johnny, it's called a septic tank tee - see SEPTIC TANK TEES

However the Tee in the septic tank is not likely to be the cause or cure for septic or sewer gas odors indoors. Possible but not likely. There may be a blockage or a venting problem. Knowing when the issue began and what else was going on just before that may be diagnostic.

Watch out: never lean over or even work around a septic tank alone - falling in is quickly fatal and you can be overcome by methane gas. Ask for help from a septic expert.

How far should the septic tank tee stick down into the septic tank?

I had to repair the outlet baffle of my septic tank with pvc. I matched the length of the old concrete baffle which was about 36 inches is this ok? On 2020-01-15 by Rob

Reply by (mod) -

Rob

Probably you're fine, but I confess I don't have the full picture of your septic tank's design.

The right concept is that the bottom of the PVC tank tee extends deep enough that you're not sending floating scum out into the drainfield,

and the top of the PVC tank tee extends several inches above the highest liquid + floating scum level in the tank so that crud doesn't flow over the top and out into the drainfield.

And the bottom of the outlet tee is 2-3 inches lower than the bottom of the inlet tee

Details and example dimensions are in the article SEPTIC TANK TEES n the section titled

SEPTIC TANK BAFFLE or TEE DIMENSIONS

The outlet tee in the septic tank is longer than the inlet tee - is that OK?

What if my outlet tee is more than 2 to 3 inches - more like 10 inches longer than my inlet pipes. On 2011-07-23 by Ace

by (mod) - that's ok

It's fine for the outlet tee to extend downwards further into the effluent in the septic tank - that's preventing the solids in the floating scum layer from entering and clogging the outlet effluent line.

Septic Tank Inlet vs Outlet Tee Height Difference in the Septic Tank FAQs

Also see SEPTIC TANK TEE LENGTHS vs SEWAGE LEVEL FAQs

What are the recommended height difference between inlet and outlet Tees and distances from tees to sewage in the septic tank

How much lower should the outlet pipe be from the entry on a septic tank - On 2017-06-05 by David

What is the distance from the center of the septic tank to the discharge baffle - On 2017-06-05 by Gary

Reply by (mod) -

Good question, David. And there are opportunities to get confused.

Take a look at our Septic Tank Tee sketch just above. That red dashed line is illlustrating that the bottom of the inlet opening must be above the bottom of the outlet opening. But my research found varying opinions on just how much difference is needed.

The bottom of the outlet opening of a septic tank should be no less than 3" lower than bottom the inlet opening, - I got that number by examining a number of septic tank architectural drawings from septic tank manufacturers.

Really? That's not the whole story: take great care to look at the bottom of the vertical pipe of inlet baffle or tee will also be higher than the bottom of the outlet baffle tee.

That's illustrated by the green dashed line in my drawing.

Remember that in a properly-working septic tank the top of the waste in the tank will be at the level of the bottom of the outlet pipe that carries effluent to the drainfield, and the top of the waste will be (addressing yoru question) no less than an inch below the bottom of the inlet pipe, often more.

Watch out: If there's not enough difference between inlet and outlet opening heights in the septic tank, baffles or tees notwithstanding, the tank outlet can become blocked and the system will back-up into the building it serves.

Typically, if we set the minimum working septic tank depth as (D) (engineers often use 36" as "D")
then

D = working depth = tank bottom to top of wastewater in the tank

0.2D = minimum extension DOWN of the inlet tee below the bottom of the septic tank inlet

0.4D = minimum extension DOWN of the outlet tee below the bottom of the tank inlet into the wastewater

And

0.2D = minimum extension of BOTH inlet and outlet tee or baffle above the wastewater level in the tank

and

4" = typical inlet and outlet PVC pipe diameter and openings
These are minimum numbers in my view.

Here are more "rules of thumb" for septic tank metrics:

The outlet tee should extend 24" down into the wastewater (below the bottom of the outlet opening)

The outlet tee should extend 12" upwards above the wastewater top level (above the top of the outlet opening)

The top of the inlet and outlet tee should be at least 1" below the under-side of the septic tank cover or top or access cover

The inlet tee or baffle should be at least 1" above the top of the inlet opening to the tank and thus 1" above the top of the inlet waste pipe entering the septic tank

The bottom of the waste pipe entering the septic tank from the building should be at least 3" above the top of waste in the septic tank, otherwise velocity of entering sewage and wastewater may be too low and the waste may not be able to enter the tank through the floating scum layer.

Research on proper sewage levels & septic tank construction details

• Septic Tank Sewage Levels: https://inspectapedia.com/septic/Septic_Tank_Sewage_Levels.php
  
• Robert A. Schultheis Extension Agricultural Engineering Specialist, Residential Sewage Lagoon Systems: A Homeowner's Guide to Installation and Maintenance (2002) [Web article at UM] University of Michigan,
• Charles D. Fulhage and Donald L. Pfost Department of Agricultural Engineering, Sewage Treatment Plants for Rural Homes (2002) [Web article at UM] University of Michigan,
  
• 19 CSR 20-3.060, Minimum Construction Standards for On-Site Sewage Disposal Systems, Missouri Department of Health, February 1996.
  
• On-site Wastewater Treatment and Disposal Systems Design Manual, U.S. Environmental Protection Agency, 1980.
  
• CI Pre-cast septic tanks - http://www.caymanprecast.com/ci-precast-utility-products.html
  
• Coate septic products - http://www.coateconcreteproducts.com/septic-tanks-dayton-area.html
  
• Milan vault Inc http://www.milanvault.com/septic-tanks/design-installation-consideration/ - NOTE HERE is a 6" difference inlet-outlet
  
• OAS, building drawing guidelines, http://www.oas.org/cdmp/document/codedraw/sectionf.htm
  
• Grimm Building -http://www.grimmbldg.com/precast_catalog.php
  
• Jensen Pre-Cast Concrete Septic Tank Measurements, http://www.jensenprecast.com/Septic-Tanks/Residential-Septic-Tanks-p4060/