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How and Why to Measure Septic Tank Floating Scum Thickness and Sludge Level in the Septic Tank
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InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.
Septic tank scum & sludge layers: this article explains the necessity of measuring the thickness of the floating scum layer
in a septic tank and of measuring the thickness of the settled scum layer in a septic tank as well.
We include descriptions of how to make septic sludge and scum measuring tools and how to use them. Whether your system is a conventional septic tank and drain field, an above ground septic system, or even a holding tank,
pumping and inspecting the septic tank is a critical septic tank maintenance and septic system maintenance chore.
Septic tank pumping frequency guidance is provided in a table at our website (seelinks listed at Related Topics .)
When the septic tank is pumped, measurements of the scum layer and sludge layer tell you the condition of
the system.
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.
MEASURE SCUM & SLUDGE - How and Why to Measure Septic Tank Scum and Sludge in the Septic Tank
 Septic tank sludge & scum thickness measurements, made or at least roughly estimated by the septic pumping contractor at the
time that a septic tank is pumped, provide key information that permits the home owner to know
1) whether or not the septic tank is being pumped often enough, and
2) whether or not there is evidence that the septic fields have been damaged by having not
pumped the tank soon enough.
At SEWAGE LEVELS in SEPTIC TANKS we explain how to interpret the meaning of high or low sewage levels in the septic tank as well as thick or thin scum or sludge levels.
Readers of this document should be sure to review
our table of septic tank pumping frequency.
This article is part of our series: Inspecting, Testing, & Maintaining Residential Septic Systems.
Also see Diagnosing Clogged Drains Is it a blocked drain or the septic system?
If you can't find your septic tank see How to Find The Septic Tank - tips on locating septic tanks for inspection and septic tank pumping.
A lot of septic pumping contractors and some other "experts" give a fixed rule of thumb that serves
their own interest, such as "pump your septic tank every year" or "pump your septic tank every two years" without
first having actually considered any information about the septic system capacity, level of
usage, age, or other conditions. There are two septic tank pumping
mistakes: pumping too infrequently, leading to an early drainfield failure and costly repairs, and
pumping too frequently, wasting money (though you're wasting a lot less money than the cost of a new drainfield).
The Frequency of Septic Tank Pumping Depends on These Factors
- Scum layer thickness: the actual observed accumulated thickness of the floating scum layer in the septic tank - a key factor that determines the septic tank retention time
- Sludge layer thickness: the actual observed accumulated thickness of septic tank sludge on the bottom of the septic tank - a key factor that determines the retention time
- Capacity of the septic tank - for the same level of septic system usage, a larger tank will need to be pumped less often as it will have a larger net free area and thus a longer retention time.
- Volume of wastewater (related to size of household) being placed into the tank daily - daily wastewater flow determines the load on the drain field or soakaway system, and the solids in the waste water affect the rate of accumulation of solids in the tank
- Amount of solids in wastewater (e.g. garbage disposals produce more solids) - not all wastewater places the same load on the septic system. Chemicals in waste water can also affect solid accumulation in the septic tank.
- Septic tank retention time: the effective septic tank effluent retention time, given the above parameters. Retention time is the time provided for solids to separate from the wastewater and thus to be retained in the septic tank. Inadequate retention time results in a higher level of suspended solids in the septic wastewater being sent to the drainfield or soakaway system. Sending solids to the drainfield shortens its life.
What is The floating scum layer in a septic tank
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The floating scum layer in a septic tank includes oil and grease which, if pushed into the leach field, will clog that component
of the septic system.
Oil and grease are particularly harmful to the aerobic portion of septic effluent treatment in the
soil absorption system.
Therefore we want to clean the septic tank if the scum layer has thickened to the point of risking pushing
grease and oil out of the tank.
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.
Septic tank schematic courtesy of Carson Dunlop Associates.
See SEWAGE LEVELS in SEPTIC TANKS for details on normal and abnormal levels and what they mean. |
What is the settled sludge layer in a septic tank
The sludge layer on the bottom of a septic tank includes various solids which are not dissolved in the septic effluent and
which are dense enough to fall to the bottom of the tank.
The septic tank bottom sludge is comprised of "settleable solids" and that portion of "suspended solids" which will, given enough time, also settle out. These accumulate at the bottom
of the septic tank until they are removed by a septic tank cleanout procedure.
Normally a septic tank should be pumped when the bottom layer of sludge is within 18 inches of the tank outlet.
WHEN TO PUMP the SEPTIC TANK - How thick can the septic tank sludge and scum layer be before septic tank cleaning is needed?
Thickness of the Floating Scum Layer & Bottom Sludge Layer in the Septic Tank Tell When to Pump Out the Septic Tank
This discussion explains how to decide when to pump & clean the septic tank based on an actual inspection of the septic tank sludge & scum layer thicknesses. An alternative that you can use if your septic tank is difficult to access or you don't have the ability to make these measurements (or to ask your septic contractor to make them) is to use the septic tank pumping schedule at SEPTIC TANK PUMPING SCHEDULE. That schedule considers the septic tank size and level of usage.
The following example was provided to us by Effluent Services, Ltd., an experienced septic service company in New Zealand. In New Zealand septic tank sizes range from 2000-6000 liters (500 gallons to 1,500 gallons U.S.) with the average being 3100 liters (about 800 U.S. gallons and below the minimum septic tank size permitted in most U.S. jurisdictions).
"Generally at a two year interval for septic tank pumping service the average septic tank in these size ranges will have a 400 mm scum layer with about a 200 mm sludge layer. With an average depth of 1600 mm, the solids content is about 600 mm thereby reducing the settling time by nearly 40%.
Therefore one may be right to suggest that the increase in total suspended solids exiting the tank will rise exponentially.
This may be a simplistic approach to calculating the frequencies between cleanings, however the evidence of it’s success is in the reductions of replacement leach fields."
This is very sound reasoned. Homeowners should use the scum and sludge layers found when their septic tanks are pumped as a way to determine whether or not the tank is being pumped often enough. In areas where most septic tanks are on the comparatively "small' side, a two year regular pumping schedule is reasonable for light-usage. In our experience, a 500-gallon septic tank in a home with a family of four is going to need very frequent pumping if the drainfield, leach beds, or soakaway system is to be protected.
US Department of Agriculture Guidance on What Scum or Sludge Levels Require Septic Tank Pumping
Similar rules of thumb offered by USDA on how much sludge or scum mean the septic tank needs cleaning include:
- Pump the septic tank when the total depth of scum plus sludge layers equals one-third of the depth of the tank
or
- Pump the septic tank when the bottom of the septic tank outlet baffle has less than three inches of clearance from the bottom of the scum layer (this may vary depending on the length of your outlet baffle or tee)
or
- Pump the septic tank when the bottom of the outlet baffle is less than 6 inches from the top of the sludge layer found on the septic tank bottom
Septic tank effluent net free area or effective septic tank working volume
Don't wait too long before removing septic scum and sludge
People who wait until their septic system stops working due to a clogged or over-filled (with sludge and scum) septic tank
have waited too long. As the bottom sludge layer increases in thickness, and as the top septic scum layer increases as well,
the remaining "net free area" or "effective septic tank volume" of effluent in the tank is reduced in volume.
When a septic tank is operating with a low volume of "net free area" of septic effluent, the system lacks adequate volume to provide adequate settlement time - time needed
for sludge to settle to the tank bottom and scum to coagulate at the tank top. In this circumstance, although drains in the building
seem to be just fine, the septic tank effluent is remaining in a constant state of stirred-agitation. In turn, that means that
the system is pushing floating debris into the leach field or other absorption system.
Simply put and quoting the USDA,
The smaller the [net free area] space, the greater amount of solids that leave the tank via the outlet. That is why septic tanks should be pumped out long before the sludge and scum layers build up to the point where they completely fill the [septic] tank and block the flow of wastewater.
... [ in contrast] The unpreventable failure of the soil-absorption system eventually occurs when growth of the organic material in the wastewater [the biomat (SEPTIC BIOMATS) that forms under and along the sides of a drainfield trench] becomes so large [thick] that they plug up the soil.
Watch out: Pushing grease, scum, and small solid debris
out of the septic tank and into the leach field reduces the future life of that expensive component of a septic system. In fact if a septic tank has become blocked or even nearly-blocked by solids, the system has already had a history of pushing solids into the drainfield and even if the system appears to still be working properly, the future life of the drainfield has been substantially reduced.
More Reading:
HOW TO MEASURE the SCUM LAYER THICKNESS in the SEPTIC TANK- How to measure the thickness of the floating scum layer in a septic tank
Details of how to measure the septic tank floating scum layer are at HOW TO MEASURE SEPTIC SCUM LAYER. Excerpts are just below.
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 This procedure is modeled after the procedure described in classes for obtaining a Massachusetts Title 5 Septic Inspectors License.
However other states and authorities describe a similar procedure. Some septic contractors have built and bring to the pumpout job
their versions of the equipment described below.
The septic tank scum layer probe shown at left (courtesy USDA and US FPS) is also illustrated in a photograph just below.
Safety concerns: The procedure described below involves opening a septic tank and probing to determine the thickness of settled sludge in a septic tank and
to measure the thickness of the floating scum layer in the tank. This procedure is dangerous, involving possible exposure to methane gas and possible
risk of falling into a septic tank. The procedure should be performed by a qualified septic contractor. No one should open, inspect, or
examine a septic tank while working alone.
Sanitary concerns: wear gloves when handling the septic probe and watch where you're swinging the pole around so as not
to splash septage on your assistant (never work alone on septic systems). The equipment should be washed after use if it is to
be retained for future use. Some septic contractors carry their poles in a long plastic-lined box which goes back on the truck.
Details of how to measure the septic tank floating scum layer are at HOW TO MEASURE SEPTIC SCUM LAYER. |
HOW TO MEASURE the BOTTOM SLUDGE LAYER THICKNESS in the SEPTIC TANK - How to measure the thickness of the settled sludge layer on the bottom of a septic tank
Details of how to measure the septic tank bottom sludge layer thickness are at HOW TO MEASURE SEPTIC SLUDGE LAYER. Excerpts are just below.
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 The same pole can be used for this purpose by removing the flapper, or a separate pole can be constructed. To re-use the scum pole,
just pull the hinge pin which will drop the flapper assembly, leaving half the hinge still bolted to the pole end for next use. A simple septic tank sludge measuring pole and towel shown at left (courtesy USDA).
- A towel, preferably a light color, is wrapped around the end of the pole and secured thoroughly. The towel does not
need to be a big bulge at the end of the pole but rather should be screwed or taped so that it simply wraps around the
pole for about three feet from the bottom of the pole up. (A sheet metal screw through the towel
into the pole makes sure you won't leave the towel behind in the bottom of the septic tank.
- Probe the septic tank to the bottom, just behind (not through) the outlet baffle, and keep the probe there for a minute or longer.
- Pull the probe back up from the tank bottom and observe the total height of sludge marked on the towel.
- Compare the sludge height from the tank bottom with the distance from the outlet baffle to the tank bottom.
- Sludge within 12 in. of the baffle or Tee or septic tank sludge closet than 18 in. to the actual point of effluent
outlet from the tank (the horizontal outlet pipe) means that the tank needs to be pumped.
Details of how to measure the septic tank bottom sludge layer thickness are at HOW TO MEASURE SEPTIC SLUDGE LAYER. Excerpts are above.
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USDA / US FPS design for Septic Tank Sludge & Scum Measuring Tube
Details about how to make and use this septic tank scum and sludge measuring tool are at ELECTRIC MONITOR for SCUM & SLUDGE. Excerpts are below.
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 The USDA/FPS sketch at left shows a tube and rubber ball design that can be used for measuring both the septic tank floating scum layer, and the septic tank bottom sludge layer thickness.
The tube can be constructed using acrylic plastic (as shown) or a PVC plastic pipe (more readily available. All you need is a rubber ball that is a bit larger than the inside diameter of the pipe, some string, a washer, cotter pin or wire, and a scrap of wood for a handle.
The length of pipe needs to be long enough to safely reach the tank bottom without you having to lean over or reach into the tank, but short enough to be easy to handle. Eight feet should be plenty. If you find that your septic tank bottom is just four or five feet below the ground surface you can later cut some excess off of the pipe.
Watch out: opening, leaning over, or falling into a septic tank is dangerous and can be fatal. Do not work alone. See the safety warnings at SEPTIC SYSTEM SAFETY WARNINGS.
Details about how to make and use this septic tank scum and sludge measuring tool are at ELECTRIC MONITOR for SCUM & SLUDGE. |
Electronic Monitors for Septic Tank Scum & Sludge Levels
Details about how to buy and use this electronic septic tank scum and sludge measuring tool or grease trap monitoring device are at ELECTRIC MONITOR for SCUM & SLUDGE. Excerpts are below.
 Below at References we also describe an electronic septic tank monitor or grease trap monitor from Worldstone. These devices can track sludge, scum, or grease levels in order to best schedule septic tank pumping or grease trap cleaning. This product is suitable for commercial installations and possibly for some residential septic tank systems.
According to the company, "Data from monitors can help establish appropriate service intervals, and document maintenance for regulatory compliance. Alarm features can help detect abnormal conditions and prevent costly backups."The company also produces an oil tank level monitor.
Thanks to reader Robert Shirley for this tip. Details about how to buy and use this electronic septic tank scum and sludge measuring tool or grease trap monitoring device are at ELECTRIC MONITOR for SCUM & SLUDGE.
Thanks to reader Robert Kaunitz for technical editing. |
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Frequently Asked Questions (FAQs)
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Questions & answers or comments about tools & methods to measure levels in the septic tank: measure the sludge level, scum level, & determining septic tank condition.
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Technical Reviewers & References
Related Topics, found near the top of this page suggest articles closely related to this one.
- Thanks to reader Robert Kaunitz for technical editing, January 2011.
- 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 (USDA), 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.
- 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.
- SEWAGE LEVELS in SEPTIC TANKS - details on normal and abnormal levels and what they mean.
- Mark Cramer Inspection Services Mark Cramer, Tampa Florida, Mr. Cramer is a past president of ASHI, the American Society of Home Inspectors and is a Florida home inspector and home inspection educator. Mr. Cramer serves on the ASHI Home Inspection Standards. Contact Mark Cramer at: 727-595-4211 mark@BestTampaInspector.com
- John Cranor is an ASHI member and a home inspector (The House Whisperer) is located in Glen Allen, VA 23060. He is also a contributor to InspectApedia.com in several technical areas such as plumbing and appliances (dryer vents). Contact Mr. Cranor at 804-747-7747 or by Email: johncranor@verizon.net
 Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. The firm provides professional home inspection services & home inspection education & publications. Alan Carson is a past president of ASHI, the American Society of Home Inspectors. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book & illustrations from The Illustrated Home. Carson Dunlop Associates' provides:
- Commercial Building Inspection Courses - protocol ASTM Standard E 2018-08 for Property Condition Assessments
- Home Inspection Education Courses including home study & live classes at eleven colleges & universities.
- Home Inspection Education Home Study Courses - ASHI@Home Training 10-course program.
Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on these courses: Enter INSPECTAHITP in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
- The Home Reference Book, a reference & inspection report product for building owners & inspectors.
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.
- The Home Reference eBook, an electronic version for PCs, the iPad, iPhone, & 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.
- The Illustrated Home illustrates construction details and building components, a reference for owners & inspectors.
Special Offer: For a 5% discount on any number of copies of the Illustrated Home purchased as a single order Enter INSPECTAILL in the order payment page "Promo/Redemption" space.
- The Horizon Software System manages business operations,scheduling, & inspection report writing using Carson Dunlop's knowledge base & color images. The Horizon system runs on always-available cloud-based software for office computers, laptops, tablets, iPad, Android, & other smartphones.
- Thanks to Richard at Effluent Services, Ltd., Cambridge 3434 NZ for the suggestions regarding the significance of the septic tank floating scum layer and settled sludge layer. New Zealand readers can contact this septic system service company at 07 827 0436 or Waikato Wide: 0800 EFFLUENT.
- Thanks to reader Robert Shirley, Calgary, for discussing alternative ways to measure septic tank sludge, including the relatively low cost electronic system on the market from Worldstone Inc. (SepticWatch) - a system to monitor sludge and scum and grease levels in grease traps or septic tanks - 04/20/2010
- Worldstone, http://www.worldstoneinc.com/ electronic monitors, including via internet, for grease traps, septic tanks, oil tanks - manufactured by F.O.G. Solutions,
1348 Delta Drive, Saginaw, MI 48638, 866 279-2824, 989-928-1806, Fax 480-247-4983. The Company also makes an oil tank level monitor.
"Worldstone Inc. is a Braintree Massachusetts based company that specializes in separation tank monitoring. Using established ultrasonic technologies, Worldstone’s GREASEwatch, SEPTICwatch and OILwatch monitors capture critical tank information and transfers data to city officials, end users or service providers creating a safer, cost effective solution to manual inspections. Data from monitors can help establish appropriate service intervals, and document maintenance for regulatory compliance. Alarm features can help detect abnormal conditions and prevent costly backups.
Worldstone monitors are currently assisting over 200 major restaurants, hospitals, prisons, casinos and city officials across the United States."
Septic Tank Monitoring:
"The SEPTICwatch monitor provides continuous monitoring of the sludge, scum, temperature and liquid levels [in a septic tank]. ... real-time information on actual changes as a percentage of permissible floating solids, bottom solids and total solids. It also provides the immediate status of the liquid level within the tank in inches. Using the monitor's programmable Control Unit, the tank's optimum level for pumping is set, so the tank can be pumped when it's actually needed. The [septic tank] monitor's Control Unit can also be programmed to warn of dangerously high solid levels and lower than normal/higher than normal liquid levels - emergency conditions that could indicate system failure."
Grease Trap Monitoring:
"...GREASEwatch monitor uses ultrasonic transducers and an embedded microprocessor to continuously monitor the sludge, scum, liquid levels and temperature in your grease trap. ... get real-time information on incremental changes in the levels of floating solids, bottom solids and total solids at the touch of a button. ... the immediate status of the liquid level in the trap in inches. The Control Unit can be programmed to indicate how full the tank should be before pumping, so there's no more guessing when it's time for service. The GREASEwatch monitor lets you plan a pumping level that meets your needs and protects your investment."
- The Home Buyer's Guide to Septic Systems
- Septic Tank Pumping Guide: When, Why, How to pump the septic tank
- Ten Steps to Keeping a Septic System Working, suggestions from the U.S. EPA, edits and additions by DJF
- 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.)
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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