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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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• How and Why to Measure Septic Tank Scum and Septic Sludge Level in the Septic Tank
• Guide to septic tank scum and sludge layers in deciding when to pump a septic tank

This document 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. 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 pumping frequency guidance is provided in a table at our website (see links at page left.) When the septic tank is pumped, measurements of the scum layer and sludge layer tell you the condition of the system.

These 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. 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."

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 document is a chapter of 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.

© Copyright 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

MEASURE SCUM & SLUDGE - How and Why to Measure Septic Tank Scum and Sludge in the Septic Tank

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

1. 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
2. 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
3. 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.
4. 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
5. 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.
6. 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.

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. Septic tank schematic courtesy of Carson Dunlop. See SEWAGE LEVELS in SEPTIC TANKS for details on normal and abnormal levels and what they mean.

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.

Other measures of scum layer and sludge layer indicate when to pump the septic tank

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 reasoning. 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 pumpd 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.

More Reading:

• See SEWAGE LEVELS in SEPTIC TANKS for details on normal and abnormal levels and what they mean.
• Septic Contaminants, identifying - what are the contaminants produced by septic systems and what can be done about them if they escape initial treatment?
• Septic Tank Sludge and Floating Scum: what makes up the contents of residential sewage
• Safety: Septic System, Septic Tank, & Cesspool Safety Warnings for Septic Inspectors, Septic Pumpers, and Homeowners
• Sewage & Septic Backups and what to do about sewage spillage in buildings
• When Not to Pump a Septic Tank to avoid damage, unsafe conditions, or wasting money

WHEN TO PUMP - How thick can the septic tank sludge and scum layer be before septic tank cleaning is needed?

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.

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.

MEASURING SCUM THICKNESS in the SEPTIC TANK- How to measure the thickness of the floating scum layer in a septic tank

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.

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.

1. Make the septic tank probe:
   The tool shown here is one used by a septic contractor to probe the thickness of the scum and sludge layers. A 6x6" to 6x8" board is attached at the end of a (roughly) 8 ft. pole.

   I recommend using 2" poly piping for this purpose as it's easy to wash off. A simple door hinge is bolted to the end of the pipe and to the flapper board.

   While some pumpers let the board float to feel the points described below, I've seen others who used a board which was fixed at a 90-deg. angle to the pole.

2. Open the septic tank access port. If the tank is a single compartment septic tank this inspection is preferably at the tank outlet end as that's the point where risk of discharge into the absorption system is critical. However some tanks provide a ready access port only at the inlet side - less desirable but you can look there too. Watch out: if your tank is a two compartment type the solids, floating scum and settled sludge are accumulating at the inlet portion of the tank. Inspecting at the final outlet end will not discover sludge and scum early enough to prevent septic system damage. Such tanks may have a center inspection port which admits tank access at the outlet of the sludge/scum containing compartment. That's where to test in two-compartment septic tanks.
3. At the opened septic tank access port, probe with the flapper pole through the scum layer until you feel outlet baffle or sanitary tee. This means that the pole is turned so that the board will extend under and be able to feel the bottom of the baffle.
4. Mark the pole (chalk or pencil will work) to indicate that point. We line up the mark with something easy to reach such as the top of the access port which serves as a uniform reference point for the later measurements.
5. Now lift up the flapper pole to feel the resistance of the bottom of the floating scum layer.
6. Mark the pole again, using the same reference point as described above.
7. Scum distance to baffle bottom: The distance between the two marks gives the distance from the bottom of the scum layer to the (lower) bottom of the outlet baffle. If the bottom of the scum layer is within 3 inches of the bottom of the baffle the tank needs to be pumped.
8. Scum distance to baffle top: By visual inspection, if the top of the floating scum layer in the septic tank is within 1 inch of the top of the outlet baffle, the septic tank also needs to be pumped.

More Reading:

• See SEWAGE LEVELS in SEPTIC TANKS for details on normal and abnormal levels and what they mean.
• Septic Tank Sludge and Floating Scum: what makes up the contents of residential sewage
• Safety: Septic System, Septic Tank, & Cesspool Safety Warnings for Septic Inspectors, Septic Pumpers, and Homeowners

MEASURING SLUDGE THICKNESS in the SEPTIC TANK - How to measure the thickness of the settled sludge layer on the bottom of a septic tank

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.

1. 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.
2. Probe the septic tank to the bottom, just behind (not through) the outlet baffle, and keep the probe there for a minute or longer.
3. Pull the probe back up from the tank bottom and observe the total height of sludge marked on the towel.
4. Compare the sludge height from the tank bottom with the distance from the outlet baffle to the tank bottom.
5. 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.

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• Septic Tank Leaks - how and why septic tank leaks cause septic system failures
• Septic Contaminants, identifying - what are the contaminants produced by septic systems and what can be done about them if they escape initial treatment?
• Septic Tank Sludge and Floating Scum: what makes up the contents of residential sewage
• Safety: Septic System, Septic Tank, & Cesspool Safety Warnings for Septic Inspectors, Septic Pumpers, and Homeowners
• Components of Septic Tank Sludge and Floating Scum: what makes up the contents of residential sewage
• SEPTIC SYSTEM DESIGN BASICS
• Septic System Additives and "Helpers"
• Buyer's Guide Home Buyer's Guide to Septic Systems Inspection & Testing - What to Do, Step by Step.
• SEPTIC SYSTEM DEFINITIONS of septic system terms
• Planting Over Septic Systems: trees, shrubs, groundcover over the septic system: what can you plant over or near septic system components without causing a problem?
• Sewer Gas Odors diagnosing, finding, and curing septic tank and sewer line smells
• Ten Steps to Keeping a Septic System Working, suggestions from the U.S. EPA, edits and additions by DJF
• What is a Septic System An Engineer's View & Septic System FAQ's
• Sketches of the Septic System Components Private Sewage Disposal Systems - Septic Drawing Library
• SEPTIC SYSTEM DESIGN ALTERNATIVES
• SEWAGE LEVELS in SEPTIC TANKS - details on normal and abnormal levels and what they mean.

The Septic Systems Online Book - Where Are We?

• This Chapter: PUMPING FREQUENCY
• Next Chapter: SEPTIC PUMPING REPAIR
• First Chapter: SEPTIC SYSTEMS ONLINE BOOK

• 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 Alan Carson and Bob Dunlop, Carson Dunlop, Associates, Toronto, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education, publications, report writing materials, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.

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