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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 openings. Septic Tank Condition
Inspect, Repair, or Re-Use Concrete Septic Tanks

  • SEPTIC TANKS, CONCRETE - CONTENTS: Characteristics of concrete septic tanks. Guide to the properties of different 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
  • POST a QUESTION or READ FAQs about concrete septic tanks: special problems, inspection, installation, troubleshooting, repairs, age, durability
  • REFERENCES
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Check the condition of a concrete septic tank:

This document describes how to inspect the condition of a septic tank, providing special considerations for inspecting concrete septic tanks. Inspecting concrete septic tanks is a key component in onsite wastewater disposal systems.



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Guide to Concrete Septic Tanks: properties, sizes, installation, maintenance, repair

PHOTO of a safe concrete septic tank cover being removed for tank pumping.

The photo shows a round concrete septic tank cover being removed to prepare for pumping a concrete septic tank. This is a safe cover and is rated thick enough to be driven-over by a car - but we do not recommend that practice.

Of course the area is quite unsafe while the septic tank cover is off - we would not leave the tank cover off and the area unattended. Concrete septic tanks at an existing septic installation are usually viable, but might have damaged baffles or cracks that permit seepage of groundwater in or septic effluent out around the tank.

Article Contents

Types of Damage Found at Concrete Septic Tanks

  1. Septic tank leaks & flooding: One of the most common problems found on concrete (and some other) septic tanks is tank flooding due to either a drainfield backup or due to surface runoff or groundwater entering the septic tank. Water also leaks into septic tanks from poorly-sealed pipe connections, leaky septic tank risers, and leaky septic tank covers.
    How do we know if the septic tank is flooded?
    See SEPTIC TANK LEAKS
    and
    See SEPTIC TANK LEVELS of SEWAGE
    and
    See SEPTIC TANK BACK FLOODING
    Also
    See SEPTIC DESIGN for FLOOD DAMAGE RESISTANCE
  2. Cracks in concrete septic tanks: Concrete tanks can crack or sections may separate causing leaks with the result of not only improper disposal of effluent (wrong location) but also subverting an attempt at a septic loading and dye test since when the system is un-used the tank liquid levels drop abnormally. Poorly-mixed concrete may also fail from spalling. Occasionally we've seen tanks made of poor-quality concrete (insufficient portland cement) which eroded badly. If the tank outlet or absorption system have been blocked, examination of the tank interior may show that effluent is or has been above the top of the baffles (see "baffles" below) thus indicating a system failure.

    The inspector may detect this condition only if there is a tank inspection port which is readily and safely accessible for before, during, and after inspection when running a loading and dye test.
  3. Damaged or lost septic tank baffles: while concrete, properly mixed and set, is quite durable, in some concrete septic tank installations we find cracked, broken, crumbled, or even totally-lost baffles. Without proper inlet and outlet baffles the septic tank sends solids to the drainfield or soakbed, much shortening its life; baffle loss at the tank inlet can lead to a sewer line blockage and sewage backup in the home. Below we give links to septic tank baffle repair or replacement information.
  4. Septic tank settlement: if not properly set on compacted soil, gravel, or another sound surface, a septic tank may settle or tip, causing poor operation or even leaks, blockages, broken pipes.
  5. Lost, missing, insecure septic tank access covers: permit someone to fall into the tank, usually fatal.
    Watch out: be sure to keep people away from the area of a septic tank of unknown condition or that lacks a safe secure cover.
    See SEPTIC TANK COVERS
  6. Septic tank damage visible during or after pumping is discussed in detail in this article series: SEPTIC TANK PUMPING PROCEDURE

Repairs to concrete septic tanks

Repairing damaged or lost concrete septic tank baffles

On occasion we find that the baffles at inlet or outlet ends of a septic tank have deteriorated, usually due to poor original concrete mix, and occasionally due to mechanical damage. A lost or damaged baffle at a septic tank is asking for sewage backup into the building or the passage of solids into the drainfield - substantially shortening its life.

To repair or replace a damaged septic tank baffle,
see SEPTIC TANK BAFFLES
And
see SEPTIC TANK TEES

Repairing leaks into a concrete septic tank

Besides leaks due to a crack in a concrete septic tank, we find leaks into the tank due to improperly algined or placed entry or exit piping or missing, damaged gaskets at those locations. To repair septic tank leaks
see SEPTIC TANK LEAKS.

In addition to sealing openings at tank piping and cracks or holes (described below) if your septic tank is being flooded from local groundwater or surface runoff, the flooded tank will also flood the drainfield or may cause a sewage back-up into the building. Some readers have suggested sealing the septic tank covers and access ports - but these need to be removable for service or repair, and really you may be treating the symptom, not the problem.

We agree that faced with a high cost of site drainage corrections, sealing the septic tank lid may be an appealing solution. First make sure that the flooded septic tank is due to surface runoff or groundwater, not a backing-up or failed drainfield, or you're simply fixing the wrong problem.

It makes sense to direct surface runoff away from the septic tank, or if necessary, install an intercept drain to keep ground water and surface water away from the tank.

Reparing cracks & holes in a concrete septic tank

It is possible to repair a crack or hole in a concete septic tank using concrete patching compounds and some foundation repair compounds, epoxies, and crack sealers. Key considerations are

Watch out: Safety - never enter a septic tank without special training, equipment, and assistance as gases are likely to be quickly fatal even if the tank has just been pumped and washed out

Concrete surfaces to be patched need to be clean of sewage and dirt or debris, and for some patch products, dry as well

See both Seal Cracks in Concrete, How To

and Seal Cracks by Polyurethane Foam Injection

How to Convert a Concrete Septic Tank for Other Uses

East Germany concrete septic tank converted to useful space (C) InspectAPedia.com KH

Question: convert a concrete septic tank into a room, shed, boat house or other use - East Germany

2007/10/14 I live in the former East Germany and stumbled upon your excellent website about septic tanks.

I have a really disgusting question for you but I think I have to give you one or two sentences of context first: We live on a lake deep deep in a forest which is a nature reserve and we are not allowed to build any new structures. We are just having a biological septic system installed which purifies waste through a series of chambers (the second of which has special plants,...).

We were debating about whether to collapse and fill the old huge spetic tank (about 6 feet high X 10 feet wide X 15 feet long) or whether to remove it. A friend suggested that we CONVERT it into a room or a shed or a boat house or a wine cellar or something. At first we laughed but now we have been wondering if that would even be possible. It is located under our lawn which ends with a steep bank overlooking the lake so it would be easy to access the "room" via a door from the lake side.

So my question is whether you think that it would be possible to clean the old (cement) tank well enough and then treat/paint, etc. the walls such that in the end it would be safe a healthy to actually go in there?

I can't picture having this as a guest room "Oh yes, this is under the lawn because it used to be the septic tank, sleep well!" but I can imagine using it as a shed for our tools or a place to store the boats -- Lord knows no one would steal the stuff because who would think to look for a shed under the lawn!! I would greatly aappreciate your brief take on the idea. - Regards, Anonymous by private email

Reply: steps in cleaning and preparing a used septic tank for other purposes

RE purposed septic tank converted to underground storage space (C) InspectApedia.com KH

Above: nearing completion in 2016, this East German concrete septic tank has been cleaned and sealed to permit re-purposing it as an underground storage facility. The concrete pillars left in place help protect against a collapse of the septic tank cover.

It sounds worth trying - but

Watch out: BE CAREFUL Do not ever enter a septic tank, new or old - there could be fumes that can kill you quickly

To convert a septic tank to some other storage or possibly occupancy use you'd need to be sure it is clean, safe to enter, and sound; you'll also need to be sure you are successfully keeping surface runoff or groundwater from entering the tank.

  1. remove the tank cover
  2. open the end of the tank facing to outdoors and downhill
  3. power-wash the tank interior
  4. disinfect all surfaces
  5. coat or paint with a sealant and top coat
  6. perhaps using a bactericide in your paint coating

and also

  1. direct suface runoff away from the septic tank entry or opening
  2. provide a safe easy means of entry and exit

If the costs of all of these steps, including probably adding additional windows for light and ventilation make all of this worth the trouble it's worth considering. But I wonder if it wouldn't be less costly to collapse and remove the remains of the tank and build something a bit larger and more comfortable.

Any mistake could certainly make someone sick so be thorough. You could probably take some bacterial sample swabs from the interior surfaces and have them checked for bacterial level by a local lab to be sure the surfaces are safe. Let me know what happens, and send along photos if you try this project - I may be able to make other suggestions.

Reader follow-up: 2007/10/22

Since writing to you I had been considering removing the old tank and replacing it with a new one (basically, the same suggestion you made). We'll look into costs of getting or building something new (and maybe a little bigger) and of doing the steps you suggested and then decide. It won't be until Spring and I will write again and let you know what we do (including photos). Once again, I very much appreciate your taking your time to advice!

Reader follow-up: 2009/10/02 Septic tank converted to outdoor storage space

East Germany concrete septic tank converted to useful space (C) InspectAPedia.com KH

Maybe you remember me. I wrote to you almost 2 years ago to ask for your opinion about the idea of converting a large septic tank into a storage cellar and you kindly replied with some suggestions.

Well here we are 2 years later and fairly close to completion and at this point I am thinking and feeling that this was a good idea after all!

The new biological septic tank was up and running last summer so the old septic tank was empty over the winter. A few months ago, we took the dirt off part of the top of it to figure out exactly how big it actually is and thus access the feasibility of the conversion. It is pretty large but positioned in a way that made uncovering half of the roof and later accessing it via a door and spiral staircase -- not hard.

So, we decided to proceed. As the tank was emptied last summer when we switched to the new system, it did not smell bad. The sanitation guys came and power cleaned it well and measured the gasses with their meters and found zero bad gasses. It was baking sunny hot for several weeks and we left the tank open so it could thoroughly air out.

The sanitation guys here said that it takes 30 days to be sure that all bacteria are dead. It's been open and empty a lot longer than that now. (I will still probably take a culture to a lab at the end before we start really going down there and touching the walls, etc.).

Next, we removed the interior walls with the exception of the support posts and a 20" cross in the middle that further support the support posts and keep them from shifting. That's where we are now. It is really a good size now that I see it = The ceiling is a generous 7 feet up and it is (round) just over 12 feet in diameter.

We plan to use it to store all our annoying stuff like extra fence materials and garden tools, as well as possibly for a small wine cellar. Our East German friends (who were very used to creatively improvising and rigging things to work with odd materials and strange substitutions -- under the old East German system) have lots of ideantergarden s and advice -- like, for example, using it as a refrigerator. Anyways, it was one of these guys who suggested converting it to begin with so I am more than happy to listen to their ideas!

East Germany concrete septic tank converted to useful space (C) InspectAPedia.com KH

Last steps [ in septic tank conversion to occupied use ] now are to

1. install the spiral staircase along the wall and -- my husband's idea -- a PULLEY system for lowering and raising heavy items and to

2. paint it with a sealant and

3. add the walls over the opening and the access door with roof. Plus electricity.

I am attaching a few photos, as promised. First (9744) there is one that shows how the tank is positioned vis a vis the lawn and the driveway and the lake, etc. Then (9746) you can see what the internal structure looked like at first. In photo 1842, you can see what it looks like with the internal walls removed but the support pillars left in place (in this photo, they are just shortening the cement cross in the middle of the structure so that it is just about 20" high instead of the 5 feet it was originally.

It is now perfectly easy to get around and bright enough and airy enough not to be horrible. Of course, the wall and door will eliminate a lot of the light and airiness but we are going to usé a transparent roofing material like wintergarden plastic or something.

All in all, it will have cost about 2500. for all the work (cleaning, knocking down the walls and installing the stairs and walls, etc.) and materials (paint/sealant and cement and wood and plastic whatever for the external walls/roof.

Removing this thing would have been cost-prohibitive and it would not have been possible to install something any bigger since we are talking about a space that is under our lawn!

Also, the nature preserve laws prohibit building something new.

So, I just wanted to follow-up as promised and send you some photos with an update. Thanks for your suggestions and well, now you know,.... not that I imagine that you will have hundreds of US customers wanting to follow-suit but with the current economic crisis,... who knows! I'll send you a final photo when the fence and door and roof are built and the interior is painted and all pristine looking, if you are interested.

Reader follow-up: 2016/06/02

I just happened upon your email from some years ago.
Maybe you remember me: I live in Germany and converted my old septic tank into a storage cellar.
I'm not sure if this email is still good for you but if you reply, I would be happy to send you a brief update and photos. We are glad we did this.

Reply:

Yes please do send along photos - as we've heard from several readers who have considered making alternative use of old septic tanks or other old concrete, fiberglass, or plastic structures.

Reader follow-up on conversion of an East Germany septic tank to usable space: 2016/06/04

Update since 2009:

- Anon. 2016/06/04

Update: 2016/07/18: below is the cleaned, sealed interior of this concrete septic tank converted to an underground storage facility:

Interior of concrete septic tank converted to storage room (C) InspectApedia.com KH

Below: seen from above-grond is a diamond-plate hinged cover assembly providing access to the underground storage facility converted from the re-purposed septic tank. Remaining tasks include installation of gas charged lift supports to make opening the access cover easy and safe, and perhaps a security lock to keep the underground storage room child-safe.

Access cover for concrete septic tank converted to underground storage room (C) InspectApedia.com KH Access cover for concrete septic tank converted to underground storage room (C) InspectApedia.com KH

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Or see SEPTIC TANKS - home

Or see SEPTIC TANK, HOW TO FIND

Or see SEPTIC TANK PUMPING SCHEDULE

Or see SEPTIC TANK SAFETY

Or see SEPTIC TANK SIZE

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