Measure Septic Tank Levels
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 (see links 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 the system.

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

Septic tank schematic showing scum and sludge layers (C) Carson Dunlop AssociatesSeptic 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.

Article contents

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:

  1. Failure to pump the septic tank frequently enough: leading to an early drainfield failure and costly repairs
  2. 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.
  3. 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

  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 an EFFLUENT 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.

What is The floating scum layer in a septic tank

Septic tank schematic showing scum and sludge layers (C) Carson Dunlop Associates

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 SEPTIC TANK LEVELS of SEWAGE 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:



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.


Continue reading at HOW TO MEASURE SEPTIC SCUM LAYER or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see SEPTIC TANK LEVELS of SEWAGE - 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.

Or see TOOLS for MEASURING SCUM & SLUDGE - how to actually measure scum & sludge thickness

Or see SEPTIC TANK OBJECTIVE DATA - actual data rather than using an arbitrary time-based septic tank pumping schedule

Septic Pumping Procedure & Pumper Truck Operation Articles


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MEASURE SCUM & SLUDGE at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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