 Septic Tank Sewage Levels & What They Mean
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Septic tank sewage levels: normal & abnormal: this document explains the significance of sewage levels inside of the septic tank and what the overall sewage level indicates about the presence of leaks into or out of the septic tank. We also explain how the thickness of the floating scum layer and bottom sludge layer give information about the necessary frequency of pumping or cleaning out the septic tank. Finally, we describe septic tank leak repair procedures.
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.
The Significance of Sewage Height or Level in Septic Tanks
At TANK INSPECTION PROCEDURE we describe how to inspect septic tanks. See MEASURE SCUM & SLUDGE for a detailed description of how we measure the thickness of septic tank floating scum and bottom sludge levels. See SEPTIC TANK LEVELS of SEWAGE for a discussion of the normal levels of sewage found in a septic tank. See Septic Tank Leaks - for an explanation of how and why septic tank leaks cause septic system failures.
A normally operating septic tank that is in use is always full of sewage: a mixture of solids, floating scum, and septic effluent. Our photo (left) shows septic dye (green) trying to enter the septic tank at the baffle.
Solids entering a septic tank are intended to remain there until pumped out during tank service. A large
portion of solids settle to the bottom of the tank as sludge.
Grease and floating scum remain at the top of the
sewage in the tank. Baffles (discussed above) help keep solids, scum, and grease in the tank. Bacterial action
in the tank make a modest reduction in the solids volume and begin the processing of sewage pathogens, a step
later completed by soil bacteria in the absorption fields.
Liquid septic effluent is what flows out of the septic tank and into the drainfield for final treatment and disposal.
A separate document, MEASURE SCUM & SLUDGE,
discusses how and why to measure septic tank scum and sludge
Normal septic tank sewage levels: If the liquid and waste level combined was near the top of the tank, that is, level with the bottom of the septic tank outlet pipe, then the tank is operating normally. High and low sewage levels and thick or thin sewage scum and sludge layer thickness are explained and diagnosed below.
At TANK INSPECTION PROCEDURE we explain that when the septic tank is opened before it has been pumped out or cleaned, important information about the condition of
the septic system is available, including the thickness of the floating scum and bottom sludge layers in the tank, the overall sewage level (how high is the sewage level in the septic tank), and other visible signs of problems with the septic tank, its SEPTIC TANK BAFFLES, piping, or problems with the septic drainfield.
What Does a High Level of Sewage in the Septic Tank Mean?
A high level of sewage in the septic tank is detected by observing that the top surface of the sewage in the septic tank is higher than the bottom of the septic tank outlet pipe. IF sewage is above this point, there is a problem with a blocked or damaged septic tank outlet pipe, a blockage at the distribution box, or a saturated, failing septic drainfield.
Further investigation is appropriate. If the problem is a blocked sewage pipe leaving the septic tank, or a tipped, blocked, or damaged distribution box, repair cost may be modest and the drainfield may have additional remaining life.
Watch out: Any time there is evidence that solid sewage has left the septic tank, say from a lost or damaged tank baffle (see SEPTIC TANK BAFFLES), the result is a reduced septic drainfield life because solids entering the drainfield speed clogging of its piping and its surrounding soil.
What Does a Low Level of Sewage in the Septic Tank Mean?
A low level of sewage in the tank is detected by observing, when septic tank is opened, that the top of the floating scum layer is one or more inches below the bottom of the septic tank outlet pipe.
Normally low levels of sewage in the septic tank may occur by transpiration - movement of moisture out of the tank by evaporation or vapor passage out through leaks in the cover if the septic tank has been un-used for months or longer.
Unusually low levels of sewage in the septic tank would be defined as sewage top below the tank outlet pipe bottom edge when a septic tank is in active use.
Low levels of sewage in the tank suggest that the septic tank has a leak. Low septic tank levels can have several causes depending on the tank age and the material from which it was built.
- Low Sewage Levels in Concrete septic tanks: If the tank is made of concrete
it should be pumped and cleaned thoroughly so that your contractor can inspect the tank for cracks or other damage.
- Low Sewage Levels in Plastic/Fiberglass septic tanks: after pumping the tank, look for a lost drain plug in the tank
bottom. Even pumping the tank can accidentally remove this plug - a condition you won't notice until the next time it's pumped.
- Low Sewage Levels in Steel Septic Tanks: Pump the tank completely, clean and inspect for rust holes - it's common for the
bottom of such tanks to rust completely away.
- Low Sewage Levels in Home Made or Site Built Septic Tanks: there is risk of tank collapse or leaks when septic tanks
are site-built such as using concrete blocks or stone. Leaks are likely
Check for Leaks Out of or Into the Septic Tank
Leaks out of the septic tank: As we explained above, a low level of sewage in a septic tank that has been in active use means there is a tank leak out. In a home occupied by a family of four people, an empty 1000 gallon septic tank (having just been pumped) would be expected to be full of liquid waste and sewage again in about a week or even less.
After the septic tank has been pumped out it may be useful to inspect its interior for evidence of cracks, settlement,
or damage to its baffles, or perhaps to confirm the tank size if most of the tank has remained buried.
If a septic tank has been serviced by removing a cover over the entire tank all of these conditions can
be seen easily. But more often the tank is pumped by access through a center cleanout port.
If there is no center cleanout port on a septic tank (some older concrete tanks) it is pumped preferably at the outlet end of the tank but
possibly at the inlet end. Septic pumpers may use a tool such as the one shown here,
a combination of a mirror at the end of a pole and a flashlight to look at the tank interior.
Look for Evidence of damage to the tank itself such as cracks, leaks, or additional evidence of damaged tank baffles.
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Leaks into the septic tank: At Septic Tank Leaks we explain how and why leaks into a septic tank cause septic system failures. But if a septic tank and the drainfield are working, a leak into the septic tank will not produce abnormally high levels of sewage in the septic tank - since excess groundwater running into the septic tank will continue onwards to the drainfield.
Repairing Septic Tank Leaks
If you have not already reviewed SEPTIC TANK SAFETY please
do so before continuing in this section. There are serious risks of injury, explosion, and death if safe procedures are not followed when working on septic systems.
Steel septic tanks that are leaky are usually doing so because the bottom of the tank has rusted through, or the tank may have rusted through at the sides, especially near the baffles, or at a point of mechanical damage. A rusted steel septic tank needs to be replaced.
Concrete septic tanks that are leaky can often be repaired. The septic tank is pumped clean, washed out, the washing water is also pumped out, and a trained professional, wearing an air supply tank, breathing apparatus, and protective clothing, enters the tank to inspect and repair cracks or holes using concrete patching material or special caulks. The repair person is monitored by at least one other expert who is similarly equipped but who remains outside the tank.
Septic Tank Safety Warnings - Never Enter the Septic Tank - it can be fatal
- Don't work alone: Falling into a septic tank or even leaning over a septic tank can be fatal.
Do not work on or at septic tanks alone - workers can become suddenly overcome by methane gas.
- Watch out: Do not ever go into a septic tank to inspect or repair it unless you are specially trained and are wearing the
special equipment and gear for that purpose, including self-contained breathing apparatus.
- Don't enter the septic tank to rescue someone: Never go into a septic tank to retrieve someone who has fallen in and was overcome by toxic gases
without a self-contained breathing apparatus (SCBA). if a SCBA is not available, call for emergency services and put a
fan at the top of the tank to blow in fresh air.
- Don't even lean over the septic tank openings: Do not lean over or stick your head into the septic tank to examine its interior - you could fall in to the
tank or become overcome by gases and fall into the tank, an event which is likely to be fatal.
- Don't ignite flames near the septic tank: Do not light a flame at or near the tank - methane gas is explosive. At one tank pumpout my client described
the explosion and burns received by the pumping contractor when he stood by the tank and lit a cigarette. Another exploded their septic tank by burning brush that was piled over the tank.
- Work area around the septic tank must be ventilated: Decomposing wastes in the septic tank produce toxic gases (such as methane) which
can kill a human in a matter of minutes. When working on a tank be sure the area is well ventilated.
- Rope off Dangerous Septic Tanks, Cesspools, Drywells, Work Sites: If your inspection discover that there are dangerous conditions, such as
an unsafe tank cover, tank collapse, or a home-made septic tank or cesspool (which are at increased risk of sudden collapse)
such areas should be roped off and clearly marked as dangerous to prevent access until proper evaluation and repairs can be made.
- See SEPTIC TANK SAFETY for more about septic tank dangers and hazards and safe inspection or service and repair procedures.
What Does a Thick Layer of Floating Scum or Bottom Sludge in the Septic Tank Mean?
Scum layer thickness: If the floating scum layer and or the septic tank bottom sludge layer are thick, then the septic tank needed to be cleaned or pumped out. Just how thick is "thick"? As we discuss in more detail at MEASURE SCUM & SLUDGE, the septic tank needs to be pumped when the floating scum layer has accumulated to reach 3
inches of the bottom of the outlet baffle or tee.
Sludge layer thickness: As we discuss in more detail at MEASURE SCUM & SLUDGE, normally a septic tank should be pumped when the bottom layer of sludge is within 18 inches of the tank outlet.
Septic effluent retention time: As we explain at EFFLUENT RETENTION TIME, it would be better for the life of the drainfield to pump the septic tank sooner than this, depending on the septic tank size, depth, and general shape. That's because a small net-free area, the space between the bottom of floating scum and top of bottom sludge, means that the septic tank will have a reduced net retention time, or reduced settling time - so we are more likely to push floating solids out into the drainfield where its life is then reduced by that debris.
What Does a Thin Layer of Floating Scum or Bottom Sludge in the Septic Tank Mean?
If the liquid and waste level combined was near the top of the tank - normal, as stated just above, but the thickness of the floating scum layer or thickness of the sludge layer on the bottom of the tank or both were thin - that is, if there was not much solid waste in the tank but the combined solid and liquid level was normal, then the tank was operating very well and/or in only light use, and you can safely wait a bit longer than the recommended septic tank pumping frequency in our tank pumping table.
Frequently Asked Questions (FAQs) about normal & abnormal levels of sewage found in the septic tank
Question: I saw stuff floating about 4" from the top of my septic tank and concluded the septic tank needed pumping. Is that normal?
I have a 1200 gallon tank that was installed new 6 years ago. It is taking care of a rented duplex unit (2 family) I recently had it pumped out. When I looked into the tank I saw water and other things floating within approximately 4 inches from the top.
When I commented to the tank truck guy that it looks as though the tank was full and needed pumping out he said yes. When I mentioned to someone else that the tank was full I was told that the tank is always operating with the water up to within inches of the top of the tank.
So I ask you if I remove the cover from the septic tank at anytime will the water level always be that high? Thank you in advance Bill - William Rebman
Reply: a properly operating in-use septic tank will always be full to just the height of the bottom of the outlet effluent pipe. Higher or lower levels indicate a problem.
Bill,
Remember that a normally operating septic tank is always full, right up to the level of the exit pipe that drains off effluent to the drainfield or soakaway bed.
So yes, if you open a septic tank at any time the wastewater will be high - roughly up to the outlet pipe.
Definition of a "normally full" septic tank
Let's refine this "septic tank full level" definition just a little bit.
When you open a working septic tank, what you see as the very top level of the wastewater in the septic tank is the upper surface of the floating scum layer - the brown line in our sketch at above left. You cannot really see the top of the effluent in the septic tank except for a brief period after the septic tank has been pumped clean and refilled with wastewater - before the new floating scum layer has formed.
Floating scum layer top - brown line in sketch: The upper surface of the floating scum area in a working septic tank will be somewhere between the height of the bottom of the outlet pipe and the top of the inlet and outlet pipe baffles or tees - depending on the thickness of the scum layer.
When a septic tank has been in use and not pumped for some time, the increased thickness of the floating scum layer may cause the absolute top of all the waste material in the septic tank to be above the wastewater exit pipe, as you can see by the brown line in our sketch. That is not necessarily an indicator of a problem.
Remember that you are looking down onto the septic tank contents from the top, not from the side as shown in our sketch. This higher level of the floating scum layer is OK provided that the septic tank baffle or tee is intact or in place to keep the floating solids from flowing out of the septic tank (where they would clog the drainfield).
Our septic system sketch (above left) shows these protective "septic tank tees" as white pipes at the inlet and outlet ends of the septic tank. Actually, in a healthy septic tank working normally, the septic effluent will be up inside the tee or behind the tank baffle up to a level just even with the bottom of the septic tank outlet pipe. More about septic tank baffles and tees and their inspection, repair or replacement is at SEPTIC TANK BAFFLES.
Liquid effluent level top - pink line in sketch: When you open and look into a normally operating septic tank the effluent level - the liquid waste level - in the septic tank will be just below the bottom of the septic tank effluent outlet pipe - the pink line in our septic tank sketch above. As we explained above, if there is a floating scum layer it will be atop the liquid effluent in the septic tank - so you may not see the liquid effluent itself.
But provided that the septic tank baffles and tees are intact and in place, it is the location of the top of the effluent layer that tells us if the septic tank and drainfield are in trouble or not - as we elaborate just below.
Depending on the thickness of the floating scum layer, the effluent level will be either exactly at the bottom of the outlet pipe, or it may be pushed down by the floating scum layer to be ever slightly below that point. When new wastewater enters the septic tank, that increased liquid water volume will cause the effluent level to rise and liquid effluent will flow out of the septic tank exit pipe.
Definition of "abnormal septic tank sewage levels"
Abnormally high sewage effluent levels in the septic tank are any effluent levels above the outlet pipe - indicating a blockage, clog, or flooded drainfield. Remember we're talking about the effluent, not the floating scum that we discussed just above.
Abnormally low sewage levels are any level of wastewater below the outlet pipe, usually indicating that the tank has a leak.
Exceptions to low levels include, of course, the period right after the septic tank has been pumped. Depending on septic tank size and the level of its usage, number of building occupants, amount of wastewater sent daily into the septic tank, it can take a few days for it to refill after pumping.
Given that the normal height of an effluent wastewater pipe is a foot or more below the very top of the septic tank, wastewater up to 4" from the top of your septic tank may mean that the outlet pipe opening itself was clogged or that there is a blockage or flooding in the septic drainfield - possible signs that a costly repair will be needed soon.
Further, if during the septic tank pumpout you heard wastewater running backwards from the drainfield and through the septic tank exit pipe back into the septic tank that would be a sure indicator that the drainfield is flooded.
"The septic pumper agreed that because the tank was full it needed to be pumped": is therefore not quite correct. The tank is normally always "full". But remember that many septic pumper truck operators may not have been English majors in school and may not communicate precisely. Or the septic pumper may have been referring to the formation of a thick floating scum or settled sludge layer, either or which would indeed mean that the septic tank may have been past due for a cleanout.
Either the SEPTIC TANK PUMPING SCHEDULE or a thick scum layer or thick settled sludge layer (MEASURE SCUM & SLUDGE) indicate when a septic tank should have been pumped.
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Technical Reviewers & References
Related Topics, found near the top of this page suggest articles closely related to this one.
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
 The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
Or choose the The Home Reference eBook for PCs, Macs, Kindle, iPad, iPhone, or Android Smart Phones. Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter INSPECTAEHRB in the order payment page "Promo/Redemption" space.
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- Inspecting Septic Systems: Online Book, Inspection, Test, Diagnosis, Repair, & Maintenance: our Online Septic Book: Septic Testing, Loading & Dye Tests, Septic Tank Pumping, Clearances, details of onsite waste disposal system inspection, testing, repair procedures.
- 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.
Design Manuals for Septic Systems
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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)
- Eco John® Innovative Toilet Solutions, Global Inventive Industries, Fountain Valley CA, PDF, product brochure
- "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.
Onsite Wastewater Disposal Books
- Onsite Wastewater Disposal, R. J. Perkins;
Quoting from Amazon: This practical book, co-published with the National Environmental Health Association,
describes the step-by-step procedures needed to avoid common pitfalls in septic system technology.
Valuable in matching the septic system to the site-specific conditions, this useful book will help you install a reliable system in
both suitable and difficult environments. Septic tank installers, planners, state and local regulators, civil and sanitary engineers,
consulting engineers, architects, homeowners, academics, and land developers will find this publication valuable.
- Onsite Wastewater Treatment Systems, Bennette D. Burks, Mary Margaret Minnis, Hogarth House 1994 - one of the best septic system books around, suffering a bit from small fonts and a weak index. While it contains some material more technical than needed by homeowners, Burks/Minnis book on onsite wastewater treatment systems a very useful reference for both property owners and septic system designers.
- 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
- Soil Percolation Tests soil perc testing guide and instructions
- Percolation Testing Manual, CNMI Division of Environmental Quality, PO Box 501304, Saipan, MP 96950
- Planting Over Septic System Component", Daniel Friedman (author/editor, InspectAPedia.com), The Innovator, Winter/Spring 2008, BCOSSA, British Columbia OnSite Sewage Association, 201-3542 Blansard St., Victoria BC V8X 1W3 Canada
- Save the Septic System - Do Not Flush These Items Down the Toilet, Daniel Friedman, InspectAPedia.com - PDF document, printable
- SEPTIC STANDARDS
- SEPTIC MAGAZINES
- Septic System Owner's Manual, Lloyd Kahn, Blair Allen, Julie Jones, Shelter Publications, 2000 $14.95 U.S. - easy to understand, well illustrated, one of the best practical references around on septic design basics including some advanced systems; a little short on safety and maintenance. Both new and used (low priced copies are available, and we think the authors are working on an updated edition--DF.
Quoting from one of several Amazon reviews: The basics of septic systems, from underground systems and failures to what the owner can do to promote and maintain a healthy system, is revealed in an excellent guide essential for any who reside on a septic system. Rural residents receive a primer on not only the basics; but how to conduct period inspections and what to do when things go wrong. History also figures into the fine coverage.
- Test Pit Preparation for Onsite Sewage Evaluations, State of Oregon Department of Environmental Quality, Portland OR, 800 452-4011. PDF document. We recommend this excellent document that offers detail about soil perc tests, deep hole tests, safety, and septic design. Readers should also see Soil Percolation Tests and for testing an existing septic system, also see Dye Tests
- Grass is Always Greener Over the Septic Tank, Bombeck, Erma: $ 5.99; FAWCETT; MM;
This septic system classic whose title helps avoid intimidating readers new to septic systems, is available new or used at very low prices.
It's more entertainment than a serious "how to" book on septic systems design, maintenance, or repair. Not recommended -- DF.
- US EPA Onsite Wastewater Treatment Systems Manual 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
- Water Wells and Septic Systems Handbook, R. Dodge Woodson. This book is in the upper price range, but is worth the cost for serious septic installers and designers.
Quoting Amazon: Each year, thousands upon thousands of Americans install water wells and septic systems on their properties. But with a maze of codes governing their use along with a host of design requirements that ensure their functionality where can someone turn for comprehensive, one-stop guidance? Enter the Water Wells and Septic Systems Handbook from McGraw-Hill.
Written in language any property owner can understand yet detailed enough for professionals and technical students this easy-to-use volume delivers the latest techniques and code requirements for designing, building, rehabilitating, and maintaining private water wells and septic systems. Bolstered by a wealth of informative charts, tables, and illustrations, this book delivers:
* Current construction, maintenance, and repair methods
* New International Private Sewage Disposal Code
* Up-to-date standards from the American Water Works Association
- Wells and Septic Systems, Alth, Max and Charlet, Rev. by S. Blackwell Duncan, $ 18.95; Tab Books 1992. We have found this text very useful for conventional well and septic systems design and maintenance --DF.
Quoting an Amazon description:Here's all the information you need to build a well or septic system yourself - and save a lot of time, money, and frustration. S. Blackwell Duncan has thoroughly revised and updated this second edition of Wells and Septic Systems to conform to current codes and requirements. He also has expanded this national bestseller to include new material on well and septic installation, water storage and distribution, water treatment, ecological considerations, and septic systems for problem building sites.
- The NSFC Products List has an excellent list of design manuals/modules available from their website or by telephone 800-624-8301
- Submissions welcome. send us a suggested document link or request an exchange of website links
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