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Septic tank design depth:
To what depths are septic tanks or cesspools or seepage pits or drywells commonly buried? Here we describe the depth at which septic tanks are installed and we describe several factors that influence the acutal 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.
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. (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 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.
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fill over septic tank on a hill
(May 16, 2014) Anonymous said: We have a septic tank which is on a sloped hill, and are wondering how much fill we need to put between it and a retaining wall to keep it from freezing in the winter. One side will be completely buried. Thanks for any advice!
Anon, in some locations even an above ground septic tank survives moderate freezing locations without insulation, as being in-use it's warmed by inflowing wastewater. But a tank that's not in use or is in very cold areas indeed may need protection.The quantity of insulation needed to avoid freezing depends on how cold is the exposure - something you don't state. Take a look at the insulating values of earth found at SOIL R-VALUES - for help in deciding how much fill you need.
A very general comment is to take a look at the frost line depth for your area.
Watch out: even very large amounts of insulation won't prevent freezing of an inactive plumbing system or component in very cold areas. Rather the insulation slows the time to freeze but it can also slow solar or geothermal gain that counteracts freezing.
Also see this discussion about whether or not to turn off water in a winterized building: WATER TURN OFF?
Reader comment: we still want to insulate our septic tank
Grace said: Thanks for your response to my question yesterday!
We're in North Idaho, and are planning to build a retaining wall on the downhill side of the tank. Would you recommend insulating that side of the tank (which will then have fill (up to 1-3ft) with rigid foam? I've also understood that the moisture content of the soil greatly influences the insulative value and am planning to put tarp near the surface of the soil to drain surface water away from the tank.
Thanks for any other advice!
Use a solid closed cell foam insulating board that does not absorb water. 4-6" of solid closed cell insulating board might help.
Question: septic tank lid collapsed under soil weight
(Nov 28, 2014) Scott C. said:
I have a septic system with a power assisted pump tank. The lid collapsed under the weight of the soil it was buried under. There was about 3'2" of clay type soil that the installer covered it with. The lid thickness is 4". Is that up to standard for that much soil?
Scott, If you want to investigate the adequacy of the lid over a septic tank you need a bit of data. Septic tanks are indeed rated for different weights and loads. The thickness of the septic tank lid alone is not enough to know whether or not it was adequate - as we don't know if it contained steel reinforcement or if it did, on what spacing and using what material, nor do we know the quality of the concrete, nor its history (e.g. prior vehicle traffic over the tank) nor do we know the septic tank and size. See SEPTIC TANK DESIGN STRENGTH SPECS
Question: septic tank depth must permit adequate drain line slope
(Feb 12, 2014) Anonymous said:
If my sewer discharge pipe is 6ft+ below the ground surface does the septic tank inlet not have to be slightly lower in order for the waste to flow into the tank. Can a person use an effluent pump to discharge the waste to the field or do you use the effluent pump before the septic tank.
Anon. I agree completely. A waste line drains by gravity and slopes between 1/8" and 1/4" per linear foot. If your drain line had to run 20 feet from house outlet to tank inlet, the tank inlet has to be at least (20 x .125) " lower
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Septic Tank/Soil-Absorption Systems: How to Operate & Maintain [ copy on file as /septic/Septic_Operation_USDA.pdf ] - , Equipment Tips, U.S. Department of Agriculture, 8271 1302, 7100 Engineering, 2300 Recreation, September 1982, web search 08/28/2010, original source: http://www.fs.fed.us/t-d/pubs/pdfimage/82711302.pdf.JF
Pennsylvania State Fact Sheets relating to domestic wastewater treatment systems include
Pennsylvania State Wastewater Treatment Fact Sheet SW-161, Septic System Failure: Diagnosis and Treatment
Pennsylvania State Wastewater Treatment Fact Sheet SW-162, The Soil Media and the Percolation Test
Pennsylvania State Wastewater Treatment Fact Sheet SW-l64, Mound Systems for Wastewater Treatment
Pennsylvania State Wastewater Treatment Fact Sheet SW-165, Septic Tank-Soil Absorption Systems
Document Sources used for this web page include but are not limited to: Agricultural Fact Sheet #SW-161 "Septic Tank Pumping," by Paul D. Robillard and
Kelli S. Martin. Penn State College of Agriculture - Cooperative Extension, edited and annotated by
Dan Friedman (Thanks: to Bob Mackey for proofreading the original source material.)
Advanced Onsite Wastewater Systems Technologies, Anish R. Jantrania, Mark A. Gross. Anish Jantrania, Ph.D., P.E., M.B.A., is a Consulting Engineer, in Mechanicsville VA, 804-550-0389 (2006). Outstanding technical reference especially on alternative septic system design alternatives. Written for designers and engineers, this book is not at all easy going for homeowners but is a text I recommend for professionals--DF.
Builder's Guide to Wells and Septic Systems, Woodson, R. Dodge: $ 24.95; MCGRAW HILL B; TP;
Quoting from Amazon's description: For the homebuilder, one mistake in estimating or installing wells and septic systems can cost thousands of dollars. This comprehensive guide filled with case studies can prevent that. Master plumber R. Dodge Woodson packs this reader-friendly guide with guidance and information, including details on new techniques and materials that can economize and expedite jobs and advice on how to avoid mistakes in both estimating and construction. Chapters cover virtually every aspect of wells and septic systems, including on-site evaluations; site limitations; bidding; soil studies, septic designs, and code-related issues; drilled and dug wells, gravel and pipe, chamber-type, and gravity septic systems; pump stations; common problems with well installation; and remedies for poor septic situations. Woodson also discusses ways to increase profits by avoiding cost overruns.
Country Plumbing: Living with a Septic System, Hartigan, Gerry: $ 9.95; ALAN C HOOD & TP;
Quoting an Amazon reviewer's comment, with which we agree--DF:This book is informative as far as it goes and might be most useful for someone with an older system. But it was written in the early 1980s. A lot has changed since then. In particular, the book doesn't cover any of the newer systems that are used more and more nowadays in some parts of the country -- sand mounds, aeration systems, lagoons, etc.
US EPA Onsite Wastewater Treatment Systems Manual [online copy, free] Top Reference: US EPA's Design Manual for Onsite Wastewater Treatment and Disposal, 1980, available from the US EPA, the US GPO Superintendent of Documents (Pueblo CO), and from the National Small Flows Clearinghouse. Original source http://www.epa.gov/ORD/NRMRL/Pubs/625R00008/625R00008.htm Onsite wastewater treatment and disposal systems,
Richard J Otis, published by the US EPA. Although it's more than 20 years old, this book remains a useful reference for septic system designers.
U.S. Environmental Protection Agency, Office of Water Program Operations; Office of Research and Development, Municipal Environmental Research Laboratory; (1980)
"International Private Sewage Disposal Code," 1995, BOCA-708-799-2300, ICBO-310-699-0541, SBCCI 205-591-1853, available from those code associations.
"Manual of Policy, Procedures, and Guidelines for Onsite Sewage Systems," Ontario Reg. 374/81, Part VII of the Environmental
Protection Act (Canada), ISBN 0-7743-7303-2, Ministry of the Environment,135 St. Clair Ave. West, Toronto Ontario M4V 1P5 Canada $24. CDN.
Manual of Septic Tank Practice, US Public Health Service's 1959.
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