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Septic Tank Inlet and Outlet Tee Installation Details SepticAPedia © Enter your search terms Submit search form   Search This Website - InspectAPedia.com

This article discusses methods of septic tank repair by the installation of replacement septic Tank inlet and outlet tees. Use of this information at other websites, in books or pamphlets for sale is reserved to the author. Technical review by industry experts has been performed and is ongoing - reviewers are listed at "References." Comments and suggestions for content are welcome. This document is a chapter of Inspecting, Testing, & Maintaining Residential Septic Systems. Also see The Septic Information Website © Copyright 2008 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use the links at page left to navigate this document or to go to Other Website Topics. Green links at left show where you are in our document & website.

SEPTIC TANK TEES - Sizing and Installing Septic Tank Tees

This chapter discusses the requirements for septic tank inlet tees and septic tank outlet tees and explains why the length of the tee extensions is important to prevent septic tank or septic piping clogging and to protect the life of a drainfield. Links to related septic system testing and design information are given. We address septic waste line tee sizing and we include tank tee installation suggestions.

What is a Septic Tank Tee or Baffle or Gas Baffle?

Septic tanks may come with a baffle to prevent scum from clogging the inlet or outlet piping and septic fields. Often the baffle is insufficient in design, and on steel septic tanks the baffle may rust off, leading to blocked septic systems and septic field failure. A common repair or improvement is to install plastic pipe septic tees in the septic tank to perform the function of these baffles.

The septic tank baffle or tee is a device on the inlet or outlet of a septic tank which prevents sewage back-flow into the inlet or outlet pipe. The device may be made of concrete, steel, plastic, or other materials, but in all cases the septic tank tee or baffle forms a barrier between the septic tank and the inlet or outlet pipes to or from the septic tank.

Level the septic tank: The septic tank inlet tee is designed to be higher than the septic tank outlet tee. This helps assure that incoming sewage clears the baffle and enters the tank correctly, while outgoing effluent does not carry along floating solids, scum, or grease (which would clog the drainfield).

For this reason the when the septic tank itself is placed into the ground, the septic tank should be installed level. If the septic tank has settled and is no longer level it would be very difficult to re-position the tank, but it may be possible to make corrections at the tank inlet tee and outlet tee to cope with this change.

Septic tank inlet baffle inspection

The septic tank inlet tee is a pipe tee, typically of the same diameter as the incoming sewer line, placed at the inlet of a septic tank to prevent the inlet sewer line from being clogged by floating scum and waste in the tank (resulting in a blocked septic system).

On the inlet side the septic tank baffle is intended to permit waste to enter the tank, but to avoid clogging of the tank inlet. The septic tank inlet baffle accomplishes this job by blocking the floating scum layer, a thick layer of debris that forms naturally at the top of a working septic tank, from blocking the tank inlet pipe.

The septic tank inlet baffle also reduces the movement of gases from the septic tank backwards into the building that it serves. Any gases that do escape the tank in this direction should be venting above the building's roof through its plumbing vent system, provided that system is itself correctly installed.

Septic tank outlet baffle (gas baffle) inspection

The septic tank outlet tee blocks floating scum and sewage from flowing out of the tank where it would rapidly clog the drain field and effluent piping.

Located on the septic tank's outlet end where the tank is connected to piping intended to conduct clarified septic effluent to the absorption system (drainfield), the septic tank baffle or tee, also called a Gas Baffle in some literature or regulations, keeps solid waste and gas bubbles away from the septic tank outlet, thereby reducign the carry over of solid particles from the septic tank.

This job is critical since sending solid waste out of the septic tank into the drainfield (absorption area) will certainly lead to clogging of the soils there, shortening the life of the drainfield.

Septic Tank Tee or Baffle Specifications

Sample Specifications for a Septic Tank Inlet Tee

What you'd want to see at the inlet is that the top of the tee clears the tank cover when it's put on and that it's clear of the top of the floating scum layer - otherwise it'll get blocked.

The bottom of the tee is BELOW the BOTTOM of the floating scum layer when the tank is filled to its normal level. Otherwise your incoming line is going to be blocked by the scum layer.

The inlet tee also slows the water as it flows into the tank so that you don't stir things up inappropriately. In the informal drawings I have at hand the bottom of the inlet extends to just above the middle of the tank depth - I guess the presumption is that this clears the bottom sludge layer and the top scum layer.

Just as the inlet tee needs to extend below the floating scum so as not to become blocked by it, the outlet tee needs to extend 2 or 3 inches DEEPER than the bottom of the inlet tee. This keeps sewage and floating scum from running out the tank outlet where it would quickly clog the absorption system or drainfield.

So, and let's take a typical septic tank that is maybe 5 feet of internal depth.

The inlet pipe enters the tank perhaps 6 inches below the very top edge of the tank (top edge of the tank to top edge of the inlet hole) and is about 4 inches in diameter. So the bottom of the septic tank inlet opening (and outlet opening) in this example is about 10 inches from the top of the tank.

This will define, roughly, the height top of the floating scum layer. (Even though the scum layer is blocked from flowing out of the tank by the outlet tee, it will rise up to about the same level as the bottom of the outlet tee, or slightly above it. Septic effluent will flow up the outlet tee from the bottom of the outlet tee up out of the septic tank, keeping the total sewage level in the tank from rising any higher.)

The bottom of the floating scum layer will vary depending on the thickness of that layer. Let's allow for an 8 inches thick floating scum layer. (A rental house septic tank which we examined recently had a 4 inche scum layer when it was pumped after 7 years of use by an average of 1 person occupancy.)

That means the bottom of the 8 inch thick floating scum layer would extend down in the tank to about 8 inches below the bottom of the tank inlet opening.

Your inlet tee had better be at least a couple of inches deeper than that or your scum layer, else the floating scum will clog and back up the septic system. So for these example dimensions I'd make my inlet tee extend down into the tank at least 12 inches below the bottom of the tank inlet at a minimum.

Should we make the inlet tee bottom extension longer? Yes. I'd bring it to perhaps a little above the mid-depth of the waste that's going to be in the tank - putting it between the bottom of the floating scum layer and the top of any sludge on the tank bottom. A bit longer means the tee does a bit better job of slowing the incoming wastewater down. So I might make my inlet tee extend down 14 inches to 16 inches into the tank.

Signs of Trouble With Septic Tank Baffles or Tees

Diagnosing septic odors in the drainfield area

Odors around the drainfield might be due to loss of the gas-baffle in the septic tank. Of course such odors may also be due to a failing drainfield, so further diagnosis is in order. Checking the presence and condition of the septic tank outlet baffle (gas baffle) is done at the septic tank and should be quick and easy. If the outlet baffle is lost it should be replaced, but you should also assume that the drainfield has a somewhat reduced future life.

Failing drainfields may have been caused in part by a previous loss of the septic tank outlet baffle

Diagnosing septic piping backups between building and septic tank

Backups at the waste line leaving the building, or apparent clogs in the pipe between the building and the septic tank could be caused by a lost septic tank inlet baffle which has led to clogging of the tank in that area, resulting in a slowed or even totally blocked flow into the tank.

If the plumber simply clears the line between house and tank, without repairing or replacing a lost septic tank inlet baffle, the repair will not be long-lasting.

Specification for a Septic Tank Outlet Tee

If you agree with the inlet tee dimensions discussed above, that means your outlet tee is going to extend down from the bottom of the tank outlet 16" to 18".

Add a Filter at Septic Tank Outlet to Protect the Leach Field

Now if you want to add protection for your drainfield, you might add a filter at the outlet tee. A number of companies sell these devices which will add life to your drainfield by reducing the level of suspended solids flowing out of the tank. If you buy a septic tank outlet filter you'll see that the filter product will typically be of about the dimensions of the outlet tee I cited above, or perhaps an inch or two deeper. See the links below for more information about septic filters and septic media filter systems.

More Reading:

• SEPTIC TANK BAFFLES - how to inspect septic tank baffles during septic tank pumping
• SEPTIC & GREYWATER FILTERS
• LEVELS: Inspecting the Level of Accumulated Solids, Sludge and Floating Scum in Treatment Tanks - Measuring the Sludge and Scum Levels in Septic Tanks
• SEPTIC MEDIA FILTERS
• SEPTIC SYSTEM DESIGN BASICS
• SEPTIC SYSTEM DESIGN ALTERNATIVES
• INSPECTING SEPTIC TANKS - how to inspect septic tanks, a checklist, and the proper sequence of inspection during septic tank pumping

The Septic Systems Online Book - Where Are We?

More Information About Septic Systems - Onsite Waste Disposal Design, Maintenance, Repair

• Return to the main septic system text: Inspecting, Testing, & Maintaining Residential Septic Systems
• The Septic Information Website our main information page on onsite waste disposal inspection, testing, maintenance, repair
• SEPTIC SYSTEM DESIGN BASICS: Choosing Septic Tank Size, Absorption System Size - basic septic system volume and absorption system design guides.
• How Big Should the Leach Field Be? - table of soil percolation rate vs. field size
• Septic Tank Pumping Guide: When, Why, How to Pump A Septic Tank.
• Septic Tank Pumping Procedure step by step photo guideline of how to pump and clean a septic tank - septic tank care
• Tank Location - How to Find the Septic Tank, how deep will the cover be, how to document its location
• Home & Outdoor Living Water Requirements
• The Home Buyer's Guide to Septic Systems
• Septic Systems - An Engineer's View - good explanation of what septic systems are, how they work, and why they fail
• Septic System Design Basics: Choosing Septic Tank Size, Absorption System Size

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