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.
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
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.
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.
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.
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.
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.
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
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.
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.
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.
(May 23, 2015) Susan said:
How far down should the sewage be below the rim of the outside sewage tank?
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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(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
(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.
We used to climb down inside it with a ladder.
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!)
When it rains I have standing water up to the base of the access caps.It must be leaking in.
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
bubbling/blowing out water I think....or maybe air?...on the concrete side from under the edge of the lid like a strong natural spring(The installer backfilled the tank LEVEL and uncompacted so now with settling there is at least a foot of water all the way around system.) Water looked/smelled clean.
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
entering the tank, and prevent wastewater from exiting the tank, except through
designed inlet and outlet openings." and
"The backfill material shall be
mounded over the excavated area so that the center of the backfilled area slopes down to
the outer perimeter of the excavated area to allow for settling."
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
for a year"
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.
(Nov 3, 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?
If water is flowing in the septic tank and all water in the building has been shut off, including toilets or tanks that could be draining by gravity into the building drain system, then the water is from an outside source, such as ground water leaking into the septic tank or a flooded drainfield back-flowing into the septic tank.
(Nov 3, 2014) stacey said:
found it! leak at inlet=ground water!!
Reply: 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.
Nov 24, 2014) Joe said:
Our one toilet is flushing slowly and other upstairs was between slow and flushing. So I checked the septic tank which was replaced 5 months ago. The sludge-liquid line is about 6 inches below the outlet pipe where the filter is. There is also still water at the bottom of the pipe that the filter is in. Shouldn't this water drain out of the pipe into the new tank ? I have a picture I can email you but do not have your email address ? Thank You
Joe to send photos use the email found at the CONTACT US link at page top right.
Normally the tank liquid level will fill to about the bottom of the outlet pipe. Floating scum will be above that point.
If liquid waste is below the septic tank outlet and the septic tank has been in recent use, there is probably a leak in the tank itself. So you could still have a clogged failed drainfield that causes toilet backups when the plumbing system is in more active use.
Or there may be a partly blocked drain line between house and tank.
(Jan 30, 2015) Bonnie said:
I live in a house that was built in the 70s (there may have been another dwelling on the property earlier)after educating myself through your site, I was able to determine that the septic probably needed cleaning. We had no idea where the tank was and once again, we turned to your site. The site's great advice REALLY helped us locate the tank.
Here's the question, a local company came out to pump the tank and he said the tank was SO old that there were no service ports, rather than going through the inlet my son told me the septic people made a hole in the top of the septic tank and pumped it out, then covered it with a cinder block and concrete powder? Is this going to cause problems? Can we just cover the tank with walkway bricks?
PS Thank you sooooo much for all of the great information and photos, it really helped us!!
I commend your septic pumper for keeping your costs to a minimum.
If the septic tank is a steel unit it may be possible to cut holes in the lid to permit pumping, installing and sealing a riser at each of the cutouts and providing a cover on the riser.
Watch out: But I'd be worried about a few other things:
1. If the tank is steel the lid and tank may be unsafe - old, rusted, ready to collapse - so you'd need to plan for replacement
2. If the tank cover is not child proof and someone could fall in the system is unsafe right now,
3. If the Concrete block over the lid is not sealed against surface runoff the tank could fail from surface runoff water flooding
4. Ultimately you'll want to remove the cover and inspect the tank condition to determine if it's safe and functional. For example if the inlet and outlet baffles are broken or lost and you don't repair them you speed the demise of the drainfield or soakaway bed.
(Feb 18, 2015) AA said:
I have a 500 gallon steel septic. I had this inspected for a sale of the cabin and when inspected the water level went down .25" over a 24 hour period. he said this is fine. is this true?
I got a second opinion and they just said water level was low and it needed to be replaced. even though I told them our place has been vacant for 2-3 years.
just trying to figure out if I need to replace this tank for my buyers. thanks
1/4" is not much but can indicate trouble - a failing drainfield or. Rusted through, leaking steel tank. A new system is in your future at any rate. No currently code will find a 500g tank big enough. A true inspection should have commented as well on the tank baffles and starting tank level. High suggests failing drainfield as well.
(Apr 15, 2015) Anonymous said:
What is low effluent level
Picture 3 layers in the septic tank: a bottom layer of settled sludge, a middle section of effluent (septic liquid waste), and a top crust or floating scum layer of floating solids.
In a properly working and not-leaking septic tank the sewage effluent level in the tank is normally at the same level as the bottom of the septic tank outlet pipe - inspected at the outlet end of the tank.
Because the floating scum layer may rise higher than the bottom of the outlet pipe, a baffle keeps that crud from flowing out into and clogging the drainfield.
So to actually see the effluent level we need to look at the outlet end (or inlet end) of the septic tank, down through the baffle, to see the actual liquid effluent level in the tank.
If the effluent is somewhere below the bottom of the outlet pipe, then the effluent in the tank is "low" and the suggestion is that the tank is leaking.
(May 16, 2015) Charles said:
I have a tank that is suspected of leaking because the effluent level was lower than expected. Everything I read indicates that the levels are low for an active tank. If the system had been unused for over a year, could the levels be lower simply from evaporation?
Charles I can't guess from your brief e-text, but generally the evaporation rate from a closed septic tank is trivial. When the tank is pumped and cleaned have its interior inspected (by a professional - NEVER enter a septic tank or even lean over it or that can be quickly fatal) for damage. Also check for leaks around the inlet and outlet piping.
Effluent that has fallen less than an inch below a septic tank that has NOT been in use for several months or longer, may be due to evaporation or transpiration, but for shorter periods of dis-use or for effluent levels that have fallen further the tank is probably leaking.
(May 23, 2015) Susan said:
What causes sewage to plug up in outlet pipe?
Questions & answers or comments about problems with the operation of aerobic septic systems
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Thanks to reader Charles Kaplan whose question suggested clarification on the meaning of and urgency of septic tank leak repairs 07/13/2010.