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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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?
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
Sludge layer thickness: As we discuss in more detail
Septic effluent retention time: As we explain
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.
Continue reading at EFFLUENT RETENTION TIME or select a topic from the More Reading links shown below.
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Frequently Asked Questions (FAQs)
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.
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.
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.
(Sept 23, 2011) devo said:
how far should the baffels be from the bottom of the tank
The question is not phrased properly. The septic tank baffle dimensions are related to the inlet and outlet ports and the scum and sludge levels not the absoute tank bottom. See SEPTIC TANK TEES
(Aug 27, 2012) Matt said:
Had a 1500 Gal. aerobic treatment unit installed. Used by a family of 2 adults and 4 children under 12yrs. Was told I have a 1" scum layer and 12" sludge layer and pumping is recommended. This unit has been in use for 9 months. No garbage disposal. I was told it would need to be pumped every 3 to 5 years depending on use. Is it normal to pump a tank at 9 or ten months with normal use?
(Sept 10, 2012) Dave said:
How far from the top.should the waste be? I got it pumped out in may & its about 6 inches from the top is that normal?
Details are at SEWAGE LEVELS in SEPTIC TANKS
Question: we used to climb into the septic tank
(Oct 1, 2012) Ed said:
I remember when I was younger (About 1960) my friend had an unused (never used) septic tank in his yard.
Watch out: Entering an in-use septic tank, even if it has been cleaned, is a quick way to die unless special equipment is used and the worker is properly trained in safe procedures - and is not working alone.
(Oct 3, 2012) Jim said:
My tank alert horn and light alarm are on. If it is because tank level is low, can I fill the tank to proper level to turn the alarm light off?
Sounds backwards - the septic tank alarm normally indicates that the septic tank level is abnormally high - because a pump has failed or the tank is flooding.
(Feb 2, 2013) David Kacey said:
I have built (very carefully!) a septic tank for our small new home and equally small restaurant. The design places the restaurant toilet atop the tank and the house toilet adjacent. Also, only solids and ladies liquid will flow into the tank. Men's urine and all "grey" water will go directly to a big drainfield (ie bath-, dish-, shower-water and men's urine). Is it essential to fill the tank at "going into service"?
No let the septic tank fill by normal use.
(Sept 20, 2014) Rob Hack said:
Hi, I had a pro flo 500slpt concrete aerobic system installed a little over a year ago a block or so from the Texas gulf coast.Elevation is 13' and the ground is a non-draining clay.(it use to be a rice field!)
I live in a pretty much permanent rv,collect my own water and use solar for power.My set-up is unique. Normally the aerator is running and the alarm powered. In order to pump it out I have to fire up the generator as it has quite the power draw.I can pump it down when the alarm sounds and the 2 sprinkler heads can take over 2 hours to complete the job. 5 days later the alarm is going off and I have to repeat the process. I live alone and use very little water.
On a very rainy day another week or so later I had all the breakers off during the day to conserve a little power. When I turn aerator and alarm on the alarm sounded.So as not to bother the neighbors I turned the alarm ONLY breaker off. The sprinklers didn't start when I tried to pump it out. I went back and turned the alarm breaker on. I looked over the fence again and it was
After a minute or two it stopped and the sprinklers kicked in.It wouldn't do anything without the alarm only breaker on which seems pretty weird.There is so much standing water here it doesn't go up or down so even if water was gushing into it you wouldn't see a change in the water level. The bubbling water was coming out by the outlet baffle I believe. I have a great photo with the standing water and the area the water was coming out circled.
My installer is let say.... an idiot.He has never given me any record of maintenance,terms of contract or anything.I'm sure he hasn't been coming as often as he is required. I'm disabled and usually home. He comes over,goes to the system and leaves when he's done! Never comes to the door. Then he sends me an email saying there was no power and the whimpy grass is too tall. I was waiting for him to turn it on so I could show him what it was doing.He wants 420.00 to fill in the dirt and put on 6" risers. I took a lot of photos when he installed the system and one shows the risers sitting on the ground next to the tank!
Guess he decided not to use them.Pretty sure I had already paid for those. I researched about 50 things an installer is suppose to do on an install and he violated about 20. Two key ones that he should know after at least 20 years of doing this are "The tank shall be designed and constructed so that all joints, seams, component parts, and fittings prevent groundwater from
I could fill in a 4" depression myself but this hole will take a dump truck full of sand or soil. Oh yeah.....the system was powered for a year because the electrician installed breaker on generator only side.
A maintenance provider coming every 4 months should have caught that. I finally figured it out myself and corrected it. How frustrating all this has been!
Sorry....correction.."the system was NOT powered
Question: toilet backups traced to septic tank flooding
11/2/2014 stacey said:
had a toilet back up twice. camera revealed no blockage. Look inside septic and water is flowing constantly. Completely shutoff all water to the home yet septic has constant water running into it?
11/4/2014 stacey said:
found it! leak at inlet=ground water!!
Nice going, Stacey. That was my guess. Now the questions are how to keep surface runoff or groundwater not only out of th septic tank but also out of th drainfield.
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