How to Measure Septic Tank Bottom Sludge Thickness Level
Significance of accumulated septic tank sludge for septic system operation & failure

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Septic tank bottom sludge:

This article explains how to measure the thickness of the layer of sludge on the bottom of a septic tank - a step in deciding if a septic tank needs to be pumped out and cleaned.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 .) 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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How to Measure the Bottom Sludge Layer Thickness in the Septic Tank: measuring the thickness of the settled sludge layer

Septic tank sludge measaurement method - USDA DJF

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.

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 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.

Readers of this document should be sure to review our table of septic tank pumping frequency.

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. A simple septic tank sludge measuring pole and towel shown at left (courtesy USDA).

  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.

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.

Where to Measure Septic Tank Scum & Sludge Levels

Septic tank scum sludge measuring routine - USDA DJF

Because of the typical sludge settlement pattern, & possibly also scum formation pattern, measure the septic tank sludge and scum layers near the septic tank outlet for a single-chamber septic tank, or at the outlet of the first chamber in a two or more chambered septic tank.

The USDA sketch (left) shows where sludge measurements are being made in a two-chamber septic tank. Make sure your septic tank access is over the outlet, not the inlet, when you are making these tests.

Here is a sketch of measuring the scum & sludge levels in a single-chamber septic tank.

When to Measure Septic Tank Scum & Sludge Levels

An optimum septic tank / soil absorption maintenance plan includes opening, inspecting, and measuring the sludge and scum levels annually. If your inspections never find baffle damage, evidence of tank flooding, tank leaks, tank backups, nor thick sludge or scum layers, you can try extending this interval to alternate years or longer.

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.

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%.   

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



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 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.

When to Pump the Septic Tank based on thickness of the septic tank sludge and scum layers

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%.   

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.

Beginning at SEPTIC TANK PUMPING PROCEDURE are the Steps in Septic Tank Cleaning Procedure in the order that they should be performed

Septic Tank Sewage Level Articles


Continue reading at SEPTIC TANK BACK FLOODING or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see SEPTIC TANK PUMPING MISTAKES - when not to pump a septic tank & what else can we foul-up

Or see SEPTIC TANKS - home


Suggested citation for this web page

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


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