Septic tank design depth: To what depths are septic tanks or cesspools or seepage pits or drywells commonly buried?

Do we need to put the septic tank below the frost line to avoid freezing?

Here we describe the depth at which septic tanks are installed and we describe several factors that influence the actual depth to which a septic tank (or cesspool or drywell or soak-pit) will be buried.

A companion article linked-to at the end of this one explains the septic tank depth sketch shown at the top of this page and describes in detail how to find the actual depth of an existing installed septic tank.

Septic Tank Installation Depth

How Deep Should We Put the Septic Tank at Original Installation?

Septic tanks can be installed pretty much at any depth in the soil. 

Even in freezing climates, the septic tank serving an occupied home or even an unoccupied one is unlikely to freeze, partly because of latent heat the bottom of the septic tank receives from the earth and partly because of the heat generated by the bacterial action going on in the septic tank, and mostly from warm wastewater entering from the building served by the septic system.

Watch out: If your concern with septic tank depth is to avoid freezing, DO NOT add antifreeze to a septic system.

You'll kill the bacteria and destroy the drainfield as well as contaminate the local environment.

Factors Determining Septic Tank Depth

The principal factors that determine the actual depth at which a septic tank is likely to be buried (and thus how deep you may have to dig to find the septic tank) at a particular site include:

• Sewer line depth:
  
  The depth at which the lowest sewer line leaves the building which the septic tank serves.
  
  Since usually we rely on gravity to move sewage from the building to the septic tank, the tank will be lower than the exiting waste line of the building it serves
• Site conditions 
  
  of shape, rocks, obstructions: the location at which the contractor found site conditions suitable to bury the septic tank.
  
  If a site has bedrock or large boulders close to the surface, the tank may be located elsewhere;
  
  the further the tank is located from the building, if the system uses gravity to move sewage, the deeper the tank will be.
• Keep septic tanks high and assure proper sewer line or septic line slope or pitch
  
  we don't put the septic tank any deeper than necessary, since we are usually moving effluent from the septic tank to the drainfield also by gravity.
  
  Plumbers usually install sewer lines to slope down from inlet to outlet, at 1/8" per foot to 1/4" per foot of linear run of the waste pipe.
  
  We don't want the septic drainfield to be buried at unnecessary depth since the absence of oxygen deep in the soil will prevent some wanted bacterial action (the aerobic bacteria) that we need to break down and process septic effluent.
  
  but
  
  
• Be sure that the septic tank is deep enough for proper drain or sewer line slope
  
  Watch out: be sure that the septic tank is deep enough that the sewer drain line between building and septic tank can slope continuously at 1/8" to 1/4" per foot. We have found some septic tanks that were just a bit too shallow, resulting in inadequate slope between building and septic tank.
  
  The result is that solids remain in the sewer pipe and lead to a sewer line clog and risk a building sewage backup.
• If a sewage ejector pump or grinder pump system is used 
  
  to move sewage from the building to the septic tank, of course we can place the septic tank anywhere, including uphill from the building.
• If a sewage effluent pump is used 
  
  to move septic effluent from the septic tank to the drainfield, then of course we can place the tank "downhill" from the drainfield as well.
• Growing grass: 
  
  if the septic tank is just 2 or 3 inches below ground surface you may as well have left the top of the tank exposed, since you won't get grass to grow in such thin soil.
  
  Adding backfill to 6" to 12" may be enough to get grass to grow over the septic system - this is only a cosmetic issue, not a functional one.
  
  See  SEPTIC SYSTEMS, PLANTS OVER
• Manufacturer's recommendations:
  
  some advanced septic treatment system designs require very specific inspection & maintenance intervals by a trained system operator. For example, as we discuss
  
  at BAT MEDIA SEPTIC PLANTS, BAT septics (biologically accelerated treatment) are serviced or inspected at 6-month intervals.
  
  The manufacturer of that system (Jet Inc.) notes that:
  
  It is very important that the finished grade slope away from the plant.
  
  Also, grade must be at least 1” below the bottom of the access covers. (Jet ret. 2016).

Deep septic tanks should have a service riser installed. 

Septic risers are large-diameter "wells" that are placed over a septic tank inlet baffle access port (and possibly outlet) to permit easy access for septic tank pumping, inspection, and baffle repair.

If the septic tank happens to be buried more than just a few inches below ground surface, good practice includes installing a septic riser, a large diameter pipe that gives good access to the septic tank for inspection and cleaning.

Details are at SEPTIC TANK RISERS.

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

How far below the local groundwater table can I put a septic tank?

How deep is a septic tank be below ground water table ? On 2020-11-11 by Kedir

by (mod) - warnings about septic tanks below groundwater levels

Fair question Kedir but that's not a specification I've seen.

The top of a septic tank is usually close to ground surface, sometimes 2-4 deep, with variations as we explain in the article above.

Certainly the bottom of the tank might be in seasonal groundwater, but at the very least the tank must be installed and sealed to be darn sure that groundwater never leaks into the septic tank nor its piping.

Watch out: if seasonal high water tables in your area are above the septic tank piping openings or access openings, those must be perfectly sealed against groundwater leaks into the septic tank, otherwise the tank will flood and may cause a sewage backup into the building or a septic drainfield failure.

Watch out: if the septic soakaway or drainfield or leachfield is also ever being flooded or is less than 24" above the seasonal high ground-water level, then the system cannot successfully treat pathogens and is likely to contaminate the envrionment including nearby water wells or waterways.

Watch out: : if your septic tank is lightweight plastic, polyethylene, or fiberglass, and it is empty such as just after installation or just after pumping, there's risk that the septic tank may float up out of the ground, damaging the system and even causing a sewage backup into the building.

Details are at SEPTIC TANKS FLOATING UP

How to improve toilet and other drains if the slope to the septic tank is not enough

the drop from toilet and other household drains to the septic tank isn't very much. I was wondering if there is an inline pump to help push water and waste to the septic tank On 2017-06-29 by thefiddlist69@aol.com

Reply by (mod) - use a sewage ejector pump

Sure, Fiddle; as long as the house drains are properly sloped (1/8" to 1/4" per foot) and if the only problem is the drain between the building and the septic tank, you could convert your house-to-septic-tank connection by converting it to a forced main or pumped line using a sewage ejector pump sized big enough to handle the building.

See details at SEWAGE EJECTOR / GRINDER PUMPS - home

Does the septic tank have to be level?

Does a septic tank have to be set level or could it be set at a downward pitch ? The house is built on the upside of a sloping lot On 2021-06-11 by Mark

Reply by inspectapedia.com.moderator (mod) - septic tank should be level, not sloped

@Mark,

The septic tank needs to be level; if it's out slightly that's probably not an issue but inches out of level invites uneven solids settlement and increases the risk of pushing solids out into the drainfield, damaging it and leading to need for early and costly drainfield replacement.

Consider that for proper septic tank operation and to avoid sewer line clogging, the septic tank inlet opening in the tank itself is a few inches higher than the outlet opening.

Watch out: If the septic tank is sloped down towards the inlet it's going to clog and you risk a blockage and sewage backup in the building.

Watch out: If the septic tank is sloped down towards the outlet it may lose capacity, treat solids inadequately, and you risk pushing solids into the drainfield, clogging it and leading to costly repairs.

Here is just one example of a manufacturer specifying that the tank needs to be level:

NORWESCO SEPTIC TANK INSTALLATION INSTRUCTIONS [PDF]

see step #1 "The tank should be installed level."

Do I need to buy a new septic tank and install it less deep in the soil?

My septic tank is 3 ft to the lid, and 4 ft to the tip of the outlet pipe.

My drain field is broken, and I am gonna replace it. Should I buy a new tank and not bury so deep so I don't have to bury the drain lines 5 or 6 ft deep? On 2021-04-05 by Michael

by danjoefriedman (mod) -

@Michael,

Not necessarily.

You will need to ask your installer to look at the slope of the drain between the house and the tank. It may be that your tank needed to be that deep in order to drain from the house.

On the other hand if it's a significant advantage to raise the tank and raise the drainfield then you might find it worthwhile to do so even if that means you need to have a grinder pump to move sewage from the house to the tank.

Or you may need an effluent pump to move effluent from the septic tank to a new drainfield or mound system.

What's the code on maximum septic tank depth or soil cover-over?

In Tucson. Is a fiberglass tank.supposed to be buried no more than 18 inches deep? On 2020-05-27 by Billy

by danjoefriedman (mod) - Here's the code for maximum or minimum depth of cover fill over a septic tank & requirement for septic tank risers

Billy

I don't think you will find that 18" specific septic tank cover depth in any building code.

But there are requirements specifying the maximum or recommended depth of soil cover over septic tanks in Arizona.

Clearly and permanently marked with the manufacturer’s name or registered trademark, or both, the month and year of manufacture, the maximum recommended depth of earth cover in feet, and the design liquid capacity of the tank.

The tank is manufactured to protect the markings from corrosion so that they remain permanent and readable for the operational life of the tank. - R18-9-A314. Septic Tank Design, Manufacturing, and Installation for Onsite Wastewater Treatment Facilities

Here are two examples from Pima County in Arizona. However Arizona's septic code for septic tanks or septic design includes, for septic tanks,

PIMA COUNTY PDEQ SEPTIC INSPECTION for PIPE & GRAVEL SYSTEMS [PDF] Pima County Arizona Government, original source: https://webcms.pima.gov/cms/one.aspx?pageId=63828

The word "depth" appears fifteen times in this specification including:

• Document the depth of the septic tank cover
• Septic drainfield gravel trench depth set to 11ft MAX and trench width typically is 12"
• Form C sets the minimum and maximum depths for septic drainfield trenches
• Septic tank riser shall be installed if there is more than 6" of cover over the septic tank.
  
  See details at SEPTIC TANK RISERS

Nowhere is an 18" depth to septic tank cover mentioned.

PIMA COUNTY PDEQ SEPTIC INSPECTION for CHAMBER SEPTIC SYSTEMS [PDF]

Similar to the gravel system regarding depths of septic components, though trench depths are different.

• Septic drainfield chamber system trench depth maximum is 6' 5" and trench width is 3'
• Form C often sets minimum and maximum depths – stay within limits or contact the designer and Development Services for a revision
• Maximum depth of septic tank cover must be documented
• Septic tank riser shall be installed if there is more than 6" of cover over the septic tank.

Typical septic codes may require that a septic riser shall be installed if there is more than 6" of cover over the septic tank.

Watch out: Note that the septic tank manufacturer may indeed specify a maximum depth of fill over the septic tank cover.

See Arizona's septic regulations under the AZ link found

at SEPTIC & DESIGN CODES & SPECIFICATIONS U.S.A

What can I do do if the septic or sewer line is higher than the building's main drain pipe?

What do i need to do if septic waste connection is higher than the waste line running from the house? On 2019-06-16 by Anonymous

by danjoefriedman (mod) -

Anon

You will need a sewage ejector pump -

See SEPTIC SYSTEM PUMPS - topic home. Be sure to review the guidance given there.

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Continue reading at SEPTIC TANK DEPTH to find the depth to the cover of an existing septic tank or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see SEPTIC TANK DESIGN DEPTH FAQs - questions and answers posted originally on this page

Or see these

Recommended Articles

• SEPTIC CLEARANCE DISTANCES
• SEPTIC DRAINFIELD LOCATION
• SEPTIC DRAINFIELD SIZE
• SEPTIC TANK COVERS
• SEPTIC TANK DESIGN STRENGTH SPECS
• SEPTIC TANK & DRAINFIELD FREEZE PROTECTION
• SEPTIC TANK, HOW TO FIND
• SEPTIC TANK RISERS
• U.S. SEPTIC AUTHORITIES & DESIGN SPECIFICATIONS

Suggested citation for this web page

SEPTIC TANK DESIGN DEPTH at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

Or see this

INDEX to RELATED ARTICLES: ARTICLE INDEX to SEPTIC SYSTEMS

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