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On this page we discuss the depth at which septic tanks are installed.
This article is part of our septic system chapter which summarizes guidelines on the required septic tank size based on anticipated level of daily gallons of sewage wastewater flow.
The typical residential septic tank size required for a given average daily sewage wastewater flow in gallons is provided in a table of septic tank sizes.
This chapter also explains how to calculate septic tank volume based on septic tank inside dimensions measured in feet,
and we discuss the sizing, installation, and functions of septic tank tees to prevent septic system clogging.
Links to related septic system testing and design information are given.
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
SEPTIC TANK DEPTH - Notes on How Deep we Put the Septic Tank
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. (DO NOT add antifreeze to a septic system.)
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: 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 that we need to break down and process septic effluent.
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 PLANTS OVER SEPTICS
What if the septic tank is buried deep below ground surface?
Indeed we've seen septic tanks partially above ground, and others buried more than six feet deep. It's not necessarily a mistake, and deep septic components may be required by site conditions, but here are some considerations when the septic tank is more than a foot below ground surface:
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.
A large diameter riser is 24" or more in diameter. Small septic pumping risers that are just 4" or 6" in diameter are easy to install and cheap, but they do not permit inspection of the tank baffles, and their small diameter makes it difficult for the septic pumper to actually clean out the floating scum layer and sludge layer when the tank is pumped.
Deep drainfields mean poor septic effluent processing. If a deep septic tank means that the absorption area (leach field or drainfield are synonyms) is deep, such that the top of the soil trenches are more than a foot below ground surface, then the reduction in aerobic bacteria in the soil will limit the effectiveness of the drainfield in reducing pathogens and other environmental contaminants.
Additional septic effluent treatment methods can correct these conditions, but it's useful to keep in mind that there is a difference between successful septic effluent disposal and successful septic effluent treatment. A deep septic tank does not necessarily mean that the absorption area is also deep, since site conditions and use of septic effluent pumps may change the required depth of a drainfield.
Safety warning:Be sure that the septic tank riser and all septic tank covers are sound and secure since falling into a septic tank can be fatal. See SEPTIC & CESSPOOL SAFETY .
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