LARGER IMAGE: when you can see the SEPTIC TANK during installation or after finding it for a pumpout, that's a good time to measure off and record the exact location of the tank and its cleanout openingsSeptic Tank Inspection - How to Inspect Septic Tank Condition

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How to inspect the condition of a septic tank: this document describes in detail how to inspect the condition of a septic tank - a key component in onsite wastewater disposal systems. We describe what to look for at three distinct septic tank inspection points: before pumping, during pumping, & after the septic tank has been emptied.

After discussing a list of things to watch for before, during, and after pumping or cleaning out a septic tank, we discuss specific septic system inspection details and concerns for steel septic tanks, concrete septic tanks, home made septic tanks, fiberglass or plastic septic tanks, and the condition of septic tank baffles.

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Inspecting Septic Tanks: How to Inspect the Condition of A Septic Tank

PHOTO of septic tank sludge being broken up for tank cleaningArticle topics:

  • Septic tank inspection checklist
  • Types of Septic Tanks: steel septic tanks, concrete septic tanks, fiberglass septic tanks, home made septic tanks - definitions and characteristics of various types of septic tanks
  • Inspection specifics each different type of septic tank
  • Inspection specifics for septic tank baffles, septic tank sludge and scum levels or thickness

Also see SEWAGE LEVELS in SEPTIC TANKS for an a discussion of: What Do the Levels of Sewage in the Septic Tank Mean about Septic Tank Condition, Septic Tank Leaks, & the Timing of Septic Tank Pumping.

Detail about septic tank tees and baffles is at SEPTIC TANK BAFFLES.

The purpose of the treatment tank or "septic tank" is to contain solid waste and to permit the beginning of bacterial action to process sewage into a combination of clarified effluent, settled sludge, or floating scum in the tank.

An intact, un-damaged septic tank is normally always filled with these materials.

However the inspector performing a "visual" check of the septic system needs to be alert for some important findings which we describe below.

How to Inspect the Septic Tank Before & After Pumping

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.

Photograph of a septic tank interior inspection tool using a pole, mirror, and flashlight

Septic Tank Safety Warnings

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

A Septic Tank Inspection Checklist

Inspecting the Septic Tank and Septic Tank Area Before Opening the Septic Tank

  • Subsidence (depressions or low areas in the soil) at the septic tank location - may risk dangerous, potentially fatal collapse
  • Evidence of recent work which may need to be investigated to understand the condition of the septic system
  • Evidence of backup or effluent breakout at the surface in the septic tank area (also see SEPTIC TANK GRASS or SNOWMELT)
  • PHOTO of an unsafe septic tank cover placed over collpasing concrete blocks Condition of the Septic Tank Covers: Condition and safety of the tank and access covers.

    In the photo shown here a round concrete septic tank cover was placed over collapsing concrete blocks stacked to provide an access to a septic tank. There was a septic tank collapse risk and a possible fatal hazard which at this property was an area only three meters from a children's playground.

The stacked concrete blocks were tumbling and the opening into the septic tank was larger than the cover. We covered the area with plywood, roped it off, and informed the appropriate parties including the property owner. A safe septic tank cover on a concrete tank is shown in a photo below where we discuss concrete septic tanks.

Inspecting the Septic Tank After Opening the Septic Tank but Before Pumping

PHOTO of the septic tank muck raking tool used to break up scum
and sludge layers during pumping.After Opening But Before Pumping the Septic Tank: 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:

  • Thickness of scum and sludge levels: Septic tank maximum scum and sludge buildup prior to pump out, and instructions for measuring the floating scum layer thickness and settled sludge layer thickness in a septic tank are available in a separate chapter at Septic Tank Pumping Guide. At SEWAGE LEVELS in SEPTIC TANKS we explain the meaning of thick or thin scum or sludge levels and high or low levels of sewage in the septic tank.
  • Back-flow of effluent into the tank during pumpdown - an indicator of flooded leach fields
  • Condition of the Septic Tank Baffles: damage to the tank baffles. Evidence of a broken concrete septic tank baffle is shown below at our discussion of home made site built tanks, and a rusted-steel septic tank baffle is shown in other photographs on this page. See SEPTIC TANK BAFFLES for an explanation of how to observe clues at the septic tank baffles or tees to look for signs of tank flooding when opening the septic tank for cleaning, pumping, or inspection.
  • Liquid and waste level in the tank: evidence of waste passing over the baffles - a flooded system, an indicator of septic system failure. Evidence of sewage flowing over the septic tank baffle is shown in a photo below where we describe septic tank baffles.
  • Unusually high levels of sewage in the septic tank - suggesting a blocked outlet or drainfield. The drainfield may be failing due a damaged or clogged pipe, a clogged, failing drainfield, or due to groundwater leaks into the septic tank or groundwater that saturates the drainfield.
  • Unusually low levels of sewage in the septic tank - suggesting that the septic tank has a leak, can have several causes depending on the tank age and the material from which it was built.
    • Low Sewage Levels in Concrete septic tanks: If the tank is made of concrete it should be pumped and cleaned thoroughly so that your contractor can inspect the tank for cracks or other damage.
    • Low Sewage Levels in Plastic/Fiberglass septic tanks: after pumping the tank, look for a lost drain plug in the tank bottom. Even pumping the tank can accidentally remove this plug - a condition you won't notice until the next time it's pumped.
    • Low Sewage Levels in Steel Septic Tanks: Pump the tank completely, clean and inspect for rust holes - it's common for the bottom of such tanks to rust completely away.
    • Low Sewage Levels in Home Made or Site Built Septic Tanks:

      Watch out: there is risk of tank collapse or leaks when septic tanks are site-built such as using concrete blocks or stone. Leaks are likely. Similarly, pumping out a site-built cesspool or drywall also risks fatal collapse hazards.

See SEPTIC TANK LEAKS - for an explanation of how and why septic tank leaks cause septic system failures.

See SEWAGE LEVELS in SEPTIC TANKS for an a discussion of: What Do the Levels of Sewage in the Septic Tank Mean about Septic Tank Condition, Septic Tank Leaks, & the Timing of Septic Tank Pumping.

Septic Tank Inspection During Tank Pumping

PHOTO of an septic tank during tank pumping.

During Septic Tank Pumping: if the pumper observes (or hears) septic effluent flowing back into the septic tank from the tank outlet pipe this is a sure indication that the drainfield or soil absorption system is waterlogged, and indicates a system failure needing further investigation.

The photo shows a concrete septic tank during pumping. As the effluent level dropped below that of the bottom of the tank baffles, we stopped pumping briefly to listen for the sound of effluent flowing back into the tank from its outlet.

Septic tank pumping is best performed from an access cover at the center of the tank if one is provided (as in this photo).

This gives best access to the pumper to clean sludge and debris from all areas of the tank bottom.

Septic Tank Inspection After the Septic Tank has Been Pumped Out

Only by pumping and visual inspection can actual tank capacity and condition be completely determined. Probing in the area of a tank, without excavation, is not recommended as the probe may damage a steel or fiberglass tank. When a tank is uncovered for pumping additional critical details may be observed before the pumping operation begins

PHOTO of an septic tank when pumping out has been nearly completed.

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.

SAFETY WARNING: Do not enter or lean down over or into any septic tank unless you're wearing special breathing apparatus and have a second worker watching you for safety - methane gas in the tank can cause fatal asphyxiation.

It should never be necessary to enter a septic tank. Any work to replace the baffles or repair the tank should be done from the outside.

SEPTIC TANK SOLIDS & SCUM - thickness, net free area, effluent retention time

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.

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

Net free area in the septic tank: If the sludge level becomes too high or the floating scum layer too thick, in addition to risking passage of solids out of the tank (damaging the absorption system), the remaining "net free area" of liquid in the tank is reduced. When the net free area becomes too small, there is insufficient time for waste entering the tank to settle out as bottom sludge or top floating scum.

The time allowed for sewage to separate and settle out as sludge or collect as floating scum is called septic tank retention time. Retention time is discussed further at EFFLUENT RETENTION TIME "Septic Effluent Retention time and Effective Septic Tank Volume - Why pump a septic tank before it is "full" of solids and grease?".

For an in-use septic tank with a small net free area, and therefore a short septic effluent retention time, the frequent entry of solid and liquid waste will keep the tank debris agitated, thus forcing floating debris into the absorption system where the life of that component will be reduced (due to soil clogging).

The importance of keeping an adequate net free area in a septic tank is the reason that tanks need to be pumped at regular intervals. Building owners who never pump a tank until it is clogged have already damaged the absorption system and reduced its future life expectancy.

Continue reading at : SEWAGE LEVELS in SEPTIC TANKS

Below at in the form of individual detailed articles are the Steps in Septic Tank Cleaning Procedure in the order that they should be performed

More Reading

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Frequently Asked Questions (FAQs about septic tank inspection procedures:

Question: My septic tank was cleaned out, then received about two feet of wastewater - the level never dropped. Is something wrong?

I had my septic tank emptied and left the lid off so I could monitor what was going on . soon there was 2 feet of liquid in it .. I then left town for 2 weeks and when I came back , the 2 foot level had not dropped at all ... if this tank is a 2 compartment tank does that mean that the water level must get a lot higher to spill over to the part of the tank that heads to the leach field ? also , if that is true then by putting in liquid that would help clear up my leach lines wouldn’t do any good until the level is much higher ? can you give me any comments about this please ? thanks for the help. P.D.


A competent onsite inspection of the septic tank by an expert usually finds additional clues that help accurately diagnose a problem with the tank, its baffles, piping, or the septic drainfield. That said,

Watch out: you should never leave the lid off of a septic tank. Doing so risks killing someone. Anyone, adult or child, who falls into an open septic tank is likely to die very quickly due to asphyxiation from methane and other gases in the tank. Details are at SEPTIC SYSTEM SAFETY WARNINGS

Now, a properly functioning septic tank is water tight. The liquid or sewage or wastewater level in the septic tank won't drop below the tank outlet opening unless the tank is damaged and leaking. A normal septic tank is always full of waste up to a level just below the outlet opening.

In a two compartment septic tank the wall separating the two compartments will have an opening that allows liquid effluent to flow into the second compartment, keeping floating scum and settled sludge in the first compartment (mostly). The entire tank, both compartments, will need to be filled with wastewater before any effluent will begin to flow out of the septic tank and into the drainfield or soakaway bed.

So when you observed about two feet of waste in the septic tank, then left the system unused, you'd expect to find exactly the same amount in the tank weeks later. Only a very slight drop in level might occur, less than an inch - caused by evaporation - because you left the tank open (and dangerous).

Please see SEWAGE LEVELS in SEPTIC TANKS for details about how to interpret abnormal levels of sewage found in the septic tank (too high or too low).

Question: I am concerned that my septic pumping chamber is being eaten away above the water line - the concrete is chipping

i have a concern my pump chamber is eating away above the water line and the concrete is chipping away have any idea and the tank is only nine years old i never noticed this 3 years ago but my neighbor whose house was built about 3 months after mine said he noticed his after six months can u help me as to what may have cause this. - Joseph 4/15/12



You are describing a concrete septic tank or in this case a concrete effluent or sewage pumping chamber. I'm doubtful that the chipping away (probably spalling) is due to anything you are putting into the septic tank via your wastewater, certainly not in normal use.

If the damage to the concrete tank is significant, you may have a concrete tank that was poured out of a bad mix.

If the damage is superficial, say 1/2-inch or less into a 4" or thicker concrete tank wall, it's not a near term issue.

When the septic tank is pumped, ask the pumper to note the condition of the tank, evidence of cracks, holes, damage, damaged baffles, excessive spalling, or a lot of concrete scrap on the tank bottom.

See SEPTIC TANK INSPECTION PROCEDURE for help in understanding how a septic tank is cleaned and inspected, and for examples of defects that can be found on inspection of the septic tank.


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