How and Why to Measure Septic Tank Floating Scum Thickness and Sludge Level in the Septic Tank
MEASURE SCUM & SLUDGE - CONTENTS: When to Pump the Septic Tank - based on scum & sludge layer thickness - how and why to measure septic tank scum & sludge levels 5
POST a QUESTION or READ FAQs about the floating scum layer or settled sludge layer that forms inside of septic tanks: why we measure the thickness of these layers, what thickness means, why it is important.
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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 the "More Reading" links at the bottom of this article .)
When the septic tank is pumped, measurements of the scum layer and sludge layer tell you the condition of
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.
MEASURE SCUM & SLUDGE - CONTENTS: When to Pump the Septic Tank - based on scum & sludge layer thickness - how and why to measure septic tank scum & sludge levels
At SEPTIC TANK PUMPING SCHEDULE we discuss three septic tank pumping frequency
mistakes that can be avoided by either using a septic tank cleaning table based on actual wastewater production (number of occupants etc) or by making objective measurements which we discuss in this article. First the mistakes:
Failure to pump the septic tank frequently enough: leading to an early drainfield failure and costly repairs
Pumping or cleaning the septic tank too frequently, wasting money (though you're wasting a lot less money than the cost of a new drainfield.
Pumping the septic tank with the fantasy that doing so will "fix" a clogged or failed drainfield. All you really gain is a few days of toilet flushing before the tank has re-filled.
Actually inspecting the septic system, diagnosing any problems or failures, and inspecting conditions inside the septic tank will tell us whether the tank is being pumped at the correct frequency.
The "True" Need for 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 an EFFLUENT 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
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.
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
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)
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 (EFFLUENT RETENTION 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.
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.
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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.
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: email@example.com
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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."
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.)
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