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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

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
at SEPTIC TANK BACK FLOODING

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.
See SEPTIC TANK BAFFLES

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.

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

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.

What are the "Normal" or "Correct" Sewage Levels in a Septic Tank?

Reader Question: I saw stuff floating about 4" from the top of my septic tank and concluded the septic tank needed pumping. Is that normal?

I have a 1200 gallon tank that was installed new 6 years ago. It is taking care of a rented duplex unit (2 family) I recently had it pumped out. When I looked into the tank I saw water and other things floating within approximately 4 inches from the top.

When I commented to the tank truck guy that it looks as though the tank was full and needed pumping out he said yes. When I mentioned to someone else that the tank was full I was told that the tank is always operating with the water up to within inches of the top of the tank.

So I ask you if I remove the cover from the septic tank at anytime will the water level always be that high? Thank you in advance Bill - William Rebman

Reply: a properly operating in-use septic tank will always be full to just the height of the bottom of the outlet effluent pipe. Higher or lower levels indicate a problem.

Septic tank diagram shows normal sludge and scum thicknessesBill,

Remember that a normally operating septic tank is always full, right up to the level of the exit pipe that drains off effluent to the drainfield or soakaway bed.

So yes, if you open a septic tank at any time the wastewater will be high - roughly up to the outlet pipe.

Definition of a "normally full" septic tank

Let's refine this "septic tank full level" definition just a little bit.

When you open a working septic tank, what you see as the very top level of the wastewater in the septic tank is the upper surface of the floating scum layer - the brown line in our sketch at above left. You cannot really see the top of the effluent in the septic tank except for a brief period after the septic tank has been pumped clean and refilled with wastewater - before the new floating scum layer has formed.

Floating scum layer top - brown line in sketch: The upper surface of the floating scum area in a working septic tank will be somewhere between the height of the bottom of the outlet pipe and the top of the inlet and outlet pipe baffles or tees - depending on the thickness of the scum layer.

When a septic tank has been in use and not pumped for some time, the increased thickness of the floating scum layer may cause the absolute top of all the waste material in the septic tank to be above the wastewater exit pipe, as you can see by the brown line in our sketch. That is not necessarily an indicator of a problem.

Remember that you are looking down onto the septic tank contents from the top, not from the side as shown in our sketch. This higher level of the floating scum layer is OK provided that the septic tank baffle or tee is intact or in place to keep the floating solids from flowing out of the septic tank (where they would clog the drainfield).

Our septic system sketch (above left) shows these protective "septic tank tees" as white pipes at the inlet and outlet ends of the septic tank. Actually, in a healthy septic tank working normally, the septic effluent will be up inside the tee or behind the tank baffle up to a level just even with the bottom of the septic tank outlet pipe. More about septic tank baffles and tees and their inspection, repair or replacement is at SEPTIC TANK BAFFLES.

Liquid effluent level top - pink line in sketch: When you open and look into a normally operating septic tank the effluent level - the liquid waste level - in the septic tank will be just below the bottom of the septic tank effluent outlet pipe - the pink line in our septic tank sketch above. As we explained above, if there is a floating scum layer it will be atop the liquid effluent in the septic tank - so you may not see the liquid effluent itself.

But provided that the septic tank baffles and tees are intact and in place, it is the location of the top of the effluent layer that tells us if the septic tank and drainfield are in trouble or not - as we elaborate just below.

Depending on the thickness of the floating scum layer, the effluent level will be either exactly at the bottom of the outlet pipe, or it may be pushed down by the floating scum layer to be ever slightly below that point. When new wastewater enters the septic tank, that increased liquid water volume will cause the effluent level to rise and liquid effluent will flow out of the septic tank exit pipe.

Definition of "abnormal septic tank sewage levels"

Abnormally high sewage effluent levels in the septic tank are any effluent levels above the outlet pipe - indicating a blockage, clog, or flooded drainfield. Remember we're talking about the effluent, not the floating scum that we discussed just above.

Abnormally low sewage levels are any level of wastewater below the outlet pipe, usually indicating that the tank has a leak.

Exceptions to low levels include, of course, the period right after the septic tank has been pumped. Depending on septic tank size and the level of its usage, number of building occupants, amount of wastewater sent daily into the septic tank, it can take a few days for it to refill after pumping.

Given that the normal height of an effluent wastewater pipe is a foot or more below the very top of the septic tank, wastewater up to 4" from the top of your septic tank may mean that the outlet pipe opening itself was clogged or that there is a blockage or flooding in the septic drainfield - possible signs that a costly repair will be needed soon.

Further, if during the septic tank pumpout you heard wastewater running backwards from the drainfield and through the septic tank exit pipe back into the septic tank that would be a sure indicator that the drainfield is flooded.

"The septic pumper agreed that because the tank was full it needed to be pumped": is therefore not quite correct. The tank is normally always "full". But remember that many septic pumper truck operators may not have been English majors in school and may not communicate precisely. Or the septic pumper may have been referring to the formation of a thick floating scum or settled sludge layer, either or which would indeed mean that the septic tank may have been past due for a cleanout.

Either the SEPTIC TANK PUMPING SCHEDULE or a thick scum layer or thick settled sludge layer (MEASURE SCUM & SLUDGE) indicate when a septic tank should have been pumped.

Reader Question: How far down should the sewage be below the rim of the outside sewage tank?

(May 23, 2015) Susan said:
How far down should the sewage be below the rim of the outside sewage tank?

Reply:

Example septic tank dimensions, adapted from Antigo Block Co., Antigo WI Tel: 715 623 4837 (C) InspectApedia.com & ABC The liquid effluent level, seen through the baffle or tank tee, will be just at the bottom of the septic tank outlet pipe.

The floating scum layer may be several inches higher than that and should be blocked by the tank baffle.

If you see sewage that is flowing over the top of the baffle or effluent that is higher than the bottom of the outlet pipe then the outlet piping is blocked or the drainfield has failed.

Typically (septic tank dimensions vary) the very top of the septic tank walls will be 12-18" higher than the bottom of the 4-inch to 6-inch diameter septic tank outlet pipe opening.

Our illustration at left, adapted from a sketch from Antigo Block Company, 230 Milton St, Antigo Wisconsin (a producer of concrete septic tanks Tel: 715-623-4837), shows some example septic tank dimensions. Septic tanks produced by other manufacturers will vary in total capacity, dimensions, wall thickness and other features but this tank is fairly typical.

[Click to enlarge any image]

Here are some dimensions from Antigo Block's septic tank. The brown arrows show sewage entering the septic tank at the left end, moving down through the inlet baffle into the septic tank, and at the right side of the tank, clarified effluent (settled sludge and floating scum have separated) moves up through the septic filter and out through the septic tank outlet opening.

OK so with these example dimensions, where will the top of the liquid effluent be in this septic tank if the system is working properly and the septic tank is in active use? It will be 40" above the tank bottom - visible at the outlet tee.

How does a septic pumping company actually measure the sludge, scum, and effluent thicknesses when we can't see down through all that floating crud?
See MEASURE SCUM & SLUDGE.

What's Clarified Effluent? See EFFLUENT RETENTION TIME

More about septic tank baffles is at SEPTIC TANK TEES

More about septic filters and how they extend drainfield life is at FILTERS SEPTIC & GREYWATER

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Or see SEPTIC TANK SEWAGE LEVEL FAQs questions & answers posted originally at this article

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Septic Pumping Procedure & Pumper Truck Operation Articles

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