Septic tank schematic showing scum and sludge layers (C) Carson Dunlop Associates How to Measure Septic Tank Floating Top Scum Layer Thickness

  • HOW TO MEASURE SEPTIC SCUM LAYER - CONTENTS: How to measure septic tank scum layer to determine when the septic tank needs cleaning. 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
  • POST a QUESTION or READ FAQs about how, when, where, & why to measure the scum layer thickness in a septic tank & what the thickness means about tank condition & the need for pumping.

Septic tank floating scum layer thickness: this document explains the tool and the procedure for measuring the thickness of the floating scum layer in a septic tank as an aid in determining septic tank condition and whether or not the septic tank needs to be pumped and cleaned. We include descriptions of how to make septic sludge and scum measuring tools.

Whether your septic system is a conventional septic tank and drain field or soakaway bed, an above ground septic system, or even a sewage holding tank, pumping and inspecting the septic tank is a critical septic tank maintenance and septic system maintenance chore.

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How to Measure the Scum layer thickness in a septic tank: How to measure the thickness of the floating scum layer in a septic tank

Septic Tank Scum Probe - USDA DJF

When the septic tank is pumped, measurements of the scum layer and sludge layer tell you the condition of the system. 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.

Page top septic tank sketch courtesy Carson Dunlop Associates.

Septic tank scum layer thickness measurements
and septic tank bottom sludge layer thickness measurements
(HOW TO MEASURE SEPTIC SLUDGE LAYER), 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
  2. Whether or not there is evidence that the septic fields have been damaged by having not pumped the tank soon enough.

Watch out: Safety concerns when measuring septic tank scum and sludge levels: 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 when measuring septic tank scum and sludge levels: 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.

How to Make the septic tank probe for measuring scum & sludge layer thickness

Photograph of  a septic tank sludge and scum layer thickness measuring tool

The septic tank scum and sludge measurement 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.

We 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. (See illustration above).

Also see TUBE for MEASURING SCUM & SLUDGE for a tool that can make both scum and sludge thickness measurements with a single device.

Septic tank sludge and scum measurement in a single chamber tank - USDA DUF
  • Open the septic tank access port. If the tank is a single compartment septic tank (USDAsketch at left) 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 septic tank 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.
  • Insert the septic tank measuring tool: 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.
  • 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.
  • Now lift up the flapper pole to feel the resistance of the bottom of the floating scum layer.
  • Mark the pole again, using the same reference point as described above.
  • 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.
  • 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.

A similar sludge & scum measuring procedure is provided in 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.

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.

See SEWAGE LEVELS in SEPTIC TANKS for details on normal and abnormal levels and what they mean.

WHEN TO PUMP - 3 Rules on How thick can the septic tank sludge and scum layer be before septic tank cleaning is needed?

The following is an excerpt from What Scum Sludge Thickness = Pump Needed.

"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%.   

Similar rules of thumb offered by USDA on how much sludge or scum mean the septic tank needs cleaning include:

  • 1/3 of tank depth: Pump the septic tank when the total depth of scum plus sludge layers equals one-third of the depth of the tank


  • Less than 3" between scum bottom & outlet baffle: 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)


  • Less than 6" from sludge top outlet baffle bottom: 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

Watch out: 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.

Also see TUBE for MEASURING SCUM & SLUDGE for a tool that can make both scum and sludge thickness measurements with a single device.

For deep or hard to access or commercial septic tanks that may need close monitoring,
see ELECTRIC MONITOR for SCUM & SLUDGE and also take a look at Other Measures Scum / Sludge. Septic tank pumping pumping frequency guidance is provided in a table at our website (seelinks listed at the "More Reading" links at the bottom of this article .)

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.

Below at in the form of individual detailed articles are the Steps in Septic Tank Cleaning Procedure in the order that they should be performed


Continue reading at HOW TO MEASURE SEPTIC SLUDGE LAYER or select a topic from the More Reading links shown below.


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HOW TO MEASURE SEPTIC SCUM LAYER at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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