Septic tank diagram shows normal sludge and scum thicknessesSeptic Tank Sewage Level FAQs

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Septic tank sewage level Q&As:

Here we answer common questions about the level of sewage in septic tanks, what it means, and what problems are created by too little liquid effluent net free area.

This article series 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:

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Septic Tank Sewage Height or Level Q&A

Example septic tank dimensions, adapted from Antigo Block Co., Antigo WI Tel: 715 623 4837 (C) & ABC

Recent questions & answers about the sewage levels in septic tanks, posted originally at SEPTIC TANK LEVELS of SEWAGE

On 2017-05-13 by (mod) - A single chamber tank will not purify the effluent more than very basically through sedimentation and flotation

Thank you for your helpful comment, Ulf. I'll be sure to keep it in this article series.

I agree that a single chamber septic tank has less capacity and requires more frequent cleaning than a larger multi-chamber tank, though in many countries, states, provinces, by far the simple single chamber tank is the most-common design found.

The ability of a single chamber tank to treat effluent, which it is designed to do, depends on the size of the net free area. If the sludge and scum layer occupy too much of the tank volume then there's not enough settlement time and solids can be pushed into the drainfield - ruining it.

Readers can search this website using the box just above to find SEPTIC TANK NET FREE AREA to read details about the success of effluent treatment and solids removal.

In all conventional septic tank and drainfield systems, single or multi-compartment tank, the level of treatment is about the same (If the tank is being properly maintained) and is about 45 % of the treatment required. The remaining effluent treatment occurs in the soil of a conventional tank and drainfield system.

On 2017-05-13 by Ulf

A single chamber tank will not purify the effluent more than very basically through sedimentation and flotation

Retain the sizes shown in the table and add at least 2 more 1/2or 1/3 size chambers where the biological cleaning takes effect to vastly extend the efficiency and reduce pumping

A 1 chamber tank is purely a holding tank not an effluent cleaning tank
I am talking from 45 years experience

On 2017-05-08 by (mod) - problems if septic tank outlet is higher than the inlet


It would be a serious problem if the outlet were above the inlet, but the reverse might not be a problem.

You should confirm that the tank is working acceptably by having it inspected at the inlet and outlet ends to double check that the septic tank baffles at the low end are not being flooded over the baffle top: if that's happening there would be a problem: solids clogging and ruining the drainfield. It's repairable by installing a tank tee to raise the baffle top above the sludge level.

If the outlet baffles are working then at most a tipped tank means that there is a small reduction in effective septic tank capacity - provided that the tank is otherwise set on solid soil and that it won't settle and tip further.

If the septic tank settles and tips further then it will break its plumbing connections and further repairs will be needed.

So check its condition again at the end of your wet season - (that depends on where in the world you live).

Keep me posted.

On 2017-05-08 by gmelch79

Two weeks ago we had a septic system installed. We have now discovered the cement tank was not placed level in the hole. It sets 4" higher on the inlet side. What if any are the future reprocussions of not having this properly leveled? Thank You

On 2017-03-29 by (mod) re: effluent retention time in septic tanks & multi-compartment septic tanks

Yeah Dave especially on my phone it has very different ideas about what I should be saying.

Your question about effluent retention time is important and I don't yet have an authoritative answer.

The general point is that with insufficient effluent retention time solids will not settle out before they get pushed out into the drainfield - ruining it.

A multi-chamber septic tank improves its performance in part by keeping more solids in the first chamber, some further settlement occurs in the second chamber - but how we would calculate effluent retention time for that design is an engineering question I'll need to ask.

In fact the first chamber in a multi-chamber tank will be smaller than a single compartment tank of the same size, so we lose retention time for that factor, but we gain the effective retention time by tank design that tries to hold solids back.

A clue we might find that I"ve not considered before is the question of how should we adjust the septic tank pumping schedule for multiple chamber septic tanks. Let's both research that.

Let me know what you find and I'll do the same.

On 2017-03-29 by Dave


I forgot to proof what I wrote and the automatic spell checked changed some words that made what I wrote gibberish. Sorry about that. I think my tank is pretty standard. Thanks for this really great resource.

What I read on your very complete site, is very clear to me except the EFFLUENT RETENTION TIME in a tank with a partition. I would think that when my 1500 gallon tank inlet partition gets completely filled with sludge, as long as my outlet partition has no sludge, I would still have the EFFLUENT RETENTION TIME of a 750 gallon tank. Wouldn't that mean I could still be OK as long as I don't put more than 750 gallons/day of liquid into the tank?

On 2017-03-27 by (mod)

I don't fully understand your system, Dave, so all I can say is rather speculative.

Generally we look at BOTH scum thickness (on tank bottom) and sludge thickness (on tank top).

Please take a look at the article series on measuring these that starts at MEASURE SCUM & SLUDGE


That's pretty thorough - I'd be glad to hear further from you with critique, questions, suggestions.


On 2017-03-27 by Dave

Thanks for the really fast response. Your answer implies that the levels on the outlet compartment have no bearing on when top pump. Am I reading that correctly?

The total distance from the bottom of the tank to the top of the liquid is 4 feet 3 inch. Since there is no scum layer, under the 40% rule, I think I can give it another 6 years before I check again. Does that sound about right to you?

On 2017-03-26 by (mod) - how long to wait to pump the septic tank?

The answer to how long you might want to wait depends on the septic tank size and shape.
For example if the tank were one of those (IMO horrible) flat wide septic tanks installed over rocky areas, it might be just 3 feet tall and one foot of sludge is already enough that the tank needs pumping.

On 2017-03-26 by Dave

14 year old 1500 gallon, two compartment tank in California. Pumped and certified 8 years ago. Last 8 years only two adults using home that don't use garbage disposal and dB don't put fats and oils down drain and use outdoor shower so very little soap and shampoo goes into tank.

I built tool of clear plastic pipe to capture a column of water in each compartment. Inlet compartment had very little if any scum layer and about one foot of sludge.

Outlet compartment has no sludge and no scum just murky water. I was expecting to see enough to justify pumping this year. Should I just wait another 8 years and then check it again?


On 2017-03-03 6 by (mod) - septic tank loses liquid by evaporation?

It's not likely that a tank would lose 18" of liquid by evaporation even over 5 months. You could have the tank pumped and its interior inspected by a pro.
DO NOT try leaning over nor looking into the tank - that can be fatal.

On 2017-03-03 by Ron

We're selling our house and it's been vacant for about 5 months. Septic inspection shows the liquid level is 18 inches below the invert of the outlet line, indicating that the tank is leaking at the midseam. Since the house has been empty for 5 months, could this be due to evaporation/transpiration instead of a leak? Is there a way to tell which it is? Thanks for any help.

On 2016-12-22 2 by (mod) - problems if new septic tanks are already full of water even though not connected to building


If the septic system is not hooked up to the home and two tanks are full of water I can think of these explanations:

1. the tanks are lightweight fiberglass, were not anchored in the ground, so the installer, to prevent the tanks from floating up out of the ground in event of wet weather, filled them (this would be unusual, anchoring is preferred, and where would they have obtained the water?)

2. There is a leak somewhere and groundwater has entered the system through a leak such as you suspect: a damaged tank or a damaged pipe leading to a tank.

I think I'd inspect the tanks and piping, pumping the water out if needed, and using an inspection camera system in piping

On 2016-12-22 by Bonnie

We have had a sand mount new septic system installed, plastic septic system and dosing tank with a pump. We have not hooked anything up to the house yet and they are both full of fresh water.
The certified installer says he does not know why.

We know these tanks are S/b be water tight. The installer does not see anything wrong with this. We refuse to hook up to the house until this is resolved because the pump will keep running and burn out. The installer is threatening small claims court because we are holding back 5,000 and have thus paid him 15,000. He has not put any fresh water into the tanks for testing nor has done any testing.

Might you have any ideas why this is happening. We suspect the backhoe person might have damaged the tank or one of the intake or outage valves is not sealed.

On 2016-06-24 by (mod) product to eat away the scum in a septic tank


The top of water in the toilet tank should be around an inch below the top of the overflow tube. Just about every toilet fill valve has an adjustment to set the water height.

Typically if a toilet flush tank is dirty I turn off water to the toilet, hold up or remove the flush valve flapper to let ALL of the water out of the tank, then I scrub the tank interior with a cleansing powder.

Let the tank re-fill, then hold the flapper fully open to fully empty the tank again,

Then let it work normally

On 2016-06-24 by frank

my toilets back up .i check the tank was high in water.pump out some water and now toilets work ok .

i pump the water from between the 2 baffles and the scum staid in the tank part .i tried to pump in the scum part but my pump would not remove any im going to try it again and see what happens .

if the drainage field is filled with scum is there any product that will clear or eat away at the scum thanks for your time and knowledge would appreciate if you can help

On 2016-02-29 by (mod) - legal action against septic installer who put in wrong system?


I'm no lawyer so cannot offer legal advice.

But you might want to start with your building department. Find out if the system plans were filed and approved. Then find out if what was filed and approved was what was installed.

"Wrong septic system" needs some clarity and specifics. Perhaps your system is not working because of something as simple as a broken pipe. So an onsite inspection by an expert sounds appropriate as well.

On 2016-02-28 by Richard

I was wondering is there any leagal action you can take against a system installer who put in the wrong system, my system should have been a advanced system, but a standard was installed don't work right.

On 2016-01-16 by (mod) - low levels due to cracked septic tank - still need to pump?


I would have the septic tank pumped and thoroughly cleaned so that my septic contractors could inspect the tank to determine where damage is present as well as whether or not repair is cost - reasonable.

Watch out: never lean over nor attempt to enter a septic tank as doing so good result in death.

On 2016-01-16 by Hilda Key

liquid level low due to suspected crack in septic tank, should we still have the sludge pumped?

On 2015-11-05 by (mod) - how long to wait to pump a "full" septic tank


NOt to worry: as long as you're not having a sewage backup issue, you can certainly wait nine days.

It is perfectly normal for the septic tank to be always "full" to a few inches from the top of the tank. That's how septic tanks work. The tank is always full (except for a few days right after pumping); when you send wastewater into the septic tank, the entering waste pushes a like volume of effluent out of the septic tank at its outlet pipe where that effluent passes on to a soil absorption system, drainfield, leachfield, soakbed, or soakpit.


to read details.

On 2015-11-05 by Tank Level 8# headspace

I have a tank that is 8'4" long by 5' wide. It hasn't been pumped in 4 years. The liquid level
under the tank access is 8" to the top of the tank. The septic company is telling me not to worry that I can wait until the 14th which is 9 days away. Two people living here, moderate to light water use.

Should I pay up for emergency pump out or wait.

Question: how far should the baffels be from the bottom of the tank

(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

Question: Is it normal to pump a tank at 9 or ten months with normal use?

(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?


No. Either the tank is improperly sized or something else is wrong. Or the pumping is not necessary.


(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?



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.
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.

Question: tank alert horn and light alarm are on

(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.

Question: Is it essential to fill the septic tank at "going into service"?

(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.

Question: water and sewage bubbling up out of septic tank

(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."

"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"

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.

Question: toilets back up, sewer lines are not blocked

(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?

Reply: look for a flooded drainfield

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.

Question: slow flushign toilets, septic tank level is below the outlet pipe

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.

Question: septic tank with no service ports - how do we pump it?

(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.

Question: 500 gallon steel septic tank leaks

(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.

Question: definition of "low septic effluent level" in the septic tank

(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.

Question: can low effluent in the septic tank be due to evaporation

(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.

Question: What causes sewage to plug up in outlet pipe of the septic tank?

(May 23, 2015) Susan said:
What causes sewage to plug up in outlet pipe?


  1. A lost baffle that was supposed to keep solids in the septic tank and out of the outlet pipe
  2. A failed drainfield that is backing up, causing sewage to flow over the top of an intact septic tank baffle


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