Septic tank diagram shows normal sludge and scum thicknessesSeptic Tank Sewage Levels & What They Mean

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Septic tank sewage levels:

Normal & abnormal: this document explains the significance of sewage levels inside of the septic tank and what the overall sewage level indicates about the presence of leaks into or out of the septic tank.

We also explain how the thickness of the floating scum layer and bottom sludge layer give information about the necessary frequency of pumping or cleaning out the septic tank. Finally, we describe septic tank leak repair procedures. We discuss:

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The Significance of Sewage Height or Level in Septic Tanks

Septic dye in the baffle box (C) D FriedmanArticle contents

  • What does the height of sewage inside the septic tank tell about its condition
  • What do the thickness of scum and sludge levels mean in a septic tank?
  • Evidence of leaking septic tanks - leaks out of the tank vs. leaks into the septic tank and what they mean
  • When and how to repair septic tank leaks & Inspection specifics for septic tank baffles, septic tank sludge and scum levels or thickness

A normally operating septic tank that is in use is always full of sewage: a mixture of solids, floating scum, and septic effluent. Our photo (left) shows septic dye (green) trying to enter the septic tank at the baffle.

Solids entering a septic tank are intended to remain there until pumped out during tank service. A large portion of solids settle to the bottom of the tank as sludge.

Grease and floating scum remain at the top of the sewage in the tank. Baffles (discussed above) help keep solids, scum, and grease in the tank. Bacterial action in the tank make a modest reduction in the solids volume and begin the processing of sewage pathogens, a step later completed by soil bacteria in the absorption fields.

Liquid septic effluent is what flows out of the septic tank and into the drainfield for final treatment and disposal.

A separate document, MEASURE SCUM & SLUDGE, discusses how and why to measure septic tank scum and sludge

Normal septic tank sewage levels: If the liquid and waste level combined was near the top of the tank, that is, level with the bottom of the septic tank outlet pipe, then the tank is operating normally. High and low sewage levels and thick or thin sewage scum and sludge layer thickness are explained and diagnosed below.

At Septic TANK INSPECTION PROCEDURE we explain that when the septic tank is opened before it has been pumped out or cleaned, important information about the condition of the septic system is available, including the thickness of the floating scum and bottom sludge layers in the tank, the overall sewage level (how high is the sewage level in the septic tank), and other visible signs of problems with the septic tank, its SEPTIC TANK BAFFLES, piping, or problems with the septic drainfield.

What Does a High Level of Sewage in the Septic Tank Mean?

A high level of sewage in the septic tank is detected by observing that the top surface of the sewage in the septic tank is higher than the bottom of the septic tank outlet pipe. IF sewage is above this point, there is a problem with a blocked or damaged septic tank outlet pipe, a blockage at the distribution box, or a saturated, failing septic drainfield.

Further investigation is appropriate. If the problem is a blocked sewage pipe leaving the septic tank, or a tipped, blocked, or damaged distribution box, repair cost may be modest and the drainfield may have additional remaining life.

Explanation of how to diganose abnormal septic tank sewage levels is found

Watch out: Any time there is evidence that solid sewage has left the septic tank, say from a lost or damaged tank baffle the result is a reduced septic drainfield life because solids entering the drainfield speed clogging of its piping and its surrounding soil.

What Does a Low Level of Sewage in the Septic Tank Mean?

PHOTO of an septic tank when pumping out has been nearly completed.A low level of sewage in the tank is detected by observing, when septic tank is opened, that the top of the floating scum layer is one or more inches below the bottom of the septic tank outlet pipe.

Normally low levels of sewage in the septic tank may occur by transpiration - movement of moisture out of the tank by evaporation or vapor passage out through leaks in the cover if the septic tank has been un-used for months or longer.

Unusually low levels of sewage in the septic tank would be defined as sewage top below the tank outlet pipe bottom edge when a septic tank is in active use.

Low levels of sewage in the tank suggest that the septic tank has a leak. Low septic tank levels can have several causes depending on the tank age and the material from which it was built.

  • Low Sewage Levels in Concrete septic tanks: If the tank is made of concrete it should be pumped and cleaned thoroughly so that your contractor can inspect the tank for cracks or other damage.
  • Low Sewage Levels in Plastic/Fiberglass septic tanks: after pumping the tank, look for a lost drain plug in the tank bottom. Even pumping the tank can accidentally remove this plug - a condition you won't notice until the next time it's pumped.
  • Low Sewage Levels in Steel Septic Tanks: Pump the tank completely, clean and inspect for rust holes - it's common for the bottom of such tanks to rust completely away.
  • Low Sewage Levels in Home Made or Site Built Septic Tanks: there is risk of tank collapse or leaks when septic tanks are site-built such as using concrete blocks or stone. Leaks are likely

Check for Leaks Out of or Into the Septic Tank

Leaks out of the septic tank: As we explained above, a low level of sewage in a septic tank that has been in active use means there is a tank leak out. In a home occupied by a family of four people, an empty 1000 gallon septic tank (having just been pumped) would be expected to be full of liquid waste and sewage again in about a week or even less.

After the septic tank has been pumped out it may be useful to inspect its interior for evidence of cracks, settlement, or damage to its baffles, or perhaps to confirm the tank size if most of the tank has remained buried.

If a septic tank has been serviced by removing a cover over the entire tank all of these conditions can be seen easily. But more often the tank is pumped by access through a center cleanout port.

If there is no center cleanout port on a septic tank (some older concrete tanks) it is pumped preferably at the outlet end of the tank but possibly at the inlet end. Septic pumpers may use a tool such as the one shown here, a combination of a mirror at the end of a pole and a flashlight to look at the tank interior.

Look for Evidence of damage to the tank itself such as cracks, leaks, or additional evidence of damaged tank baffles.

Look for leaks into the septic tank:

At SEPTIC TANK LEAKS we explain how and why leaks into a septic tank cause septic system failures. But if a septic tank and the drainfield are working, a leak into the septic tank will not produce abnormally high levels of sewage in the septic tank - since excess groundwater running into the septic tank will continue onwards to the drainfield.

Repairing Septic Tank Leaks

If you have not already reviewed SEPTIC TANK SAFETY please do so before continuing in this section. There are serious risks of injury, explosion, and death if safe procedures are not followed when working on septic systems.

Steel septic tanks that are leaky are usually doing so because the bottom of the tank has rusted through, or the tank may have rusted through at the sides, especially near the baffles, or at a point of mechanical damage. A rusted steel septic tank needs to be replaced.

Concrete septic tanks that are leaky can often be repaired. The septic tank is pumped clean, washed out, the washing water is also pumped out, and a trained professional, wearing an air supply tank, breathing apparatus, and protective clothing, enters the tank to inspect and repair cracks or holes using concrete patching material or special caulks. The repair person is monitored by at least one other expert who is similarly equipped but who remains outside the tank.

Septic Tank Safety Warnings - Never Enter the Septic Tank - it can be fatal

  • Don't work alone: Falling into a septic tank or even leaning over a septic tank can be fatal. Do not work on or at septic tanks alone - workers can become suddenly overcome by methane gas.
  • Watch out: Do not ever go into a septic tank to inspect or repair it unless you are specially trained and are wearing the special equipment and gear for that purpose, including self-contained breathing apparatus.
  • Don't enter the septic tank to rescue someone: Never go into a septic tank to retrieve someone who has fallen in and was overcome by toxic gases without a self-contained breathing apparatus (SCBA). if a SCBA is not available, call for emergency services and put a fan at the top of the tank to blow in fresh air.
  • Don't even lean over the septic tank openings: Do not lean over or stick your head into the septic tank to examine its interior - you could fall in to the tank or become overcome by gases and fall into the tank, an event which is likely to be fatal.
  • Don't ignite flames near the septic tank: Do not light a flame at or near the tank - methane gas is explosive. At one tank pumpout my client described the explosion and burns received by the pumping contractor when he stood by the tank and lit a cigarette. Another exploded their septic tank by burning brush that was piled over the tank.
  • Work area around the septic tank must be ventilated: Decomposing wastes in the septic tank produce toxic gases (such as methane) which can kill a human in a matter of minutes. When working on a tank be sure the area is well ventilated.
  • Rope off Dangerous Septic Tanks, Cesspools, Drywells, Work Sites: If your inspection discover that there are dangerous conditions, such as an unsafe tank cover, tank collapse, or a home-made septic tank or cesspool (which are at increased risk of sudden collapse) such areas should be roped off and clearly marked as dangerous to prevent access until proper evaluation and repairs can be made.
  • See SEPTIC TANK SAFETY for more about septic tank dangers and hazards and safe inspection or service and repair procedures.

What Does a Thick Layer of Floating Scum or Bottom Sludge in the Septic Tank Mean?

Scum layer thickness: If the floating scum layer and or the septic tank bottom sludge layer are thick, then the septic tank needed to be cleaned or pumped out. Just how thick is "thick"? As we discuss in more detail
at MEASURE SCUM & SLUDGE, 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.

Sludge layer thickness: As we discuss in more detail
at MEASURE SCUM & SLUDGE, normally a septic tank should be pumped when the bottom layer of sludge is within 18 inches of the tank outlet.

Septic effluent retention time: As we explain
at EFFLUENT RETENTION TIME, it would be better for the life of the drainfield to pump the septic tank sooner than this, depending on the septic tank size, depth, and general shape. That's because a small net-free area, the space between the bottom of floating scum and top of bottom sludge, means that the septic tank will have a reduced net retention time, or reduced settling time - so we are more likely to push floating solids out into the drainfield where its life is then reduced by that debris.

What Does a Thin Layer of Floating Scum or Bottom Sludge in the Septic Tank Mean?

If the liquid and waste level combined was near the top of the tank - normal, as stated just above, but the thickness of the floating scum layer or thickness of the sludge layer on the bottom of the tank or both were thin - that is, if there was not much solid waste in the tank but the combined solid and liquid level was normal, then the tank was operating very well and/or in only light use, and you can safely wait a bit longer than the recommended septic tank pumping frequency in our tank pumping table.


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SEWAGE LEVELS in SEPTIC TANKS at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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