Septic tank schematic showing scum and sludge layers (C) Carson Dunlop AssociatesHow 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.
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Septic tank floating scum layer thickness measurement:

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

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 SEPTIC TANK LEVELS of SEWAGE 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:



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 SEPTIC TANK LEVELS of SEWAGE 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.

Septic Tank Sewage Level Articles


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