Best Material for Septic Tanks

They're usually made of reinforced concrete. FRP and even PE can also be used if they're not buried too deeply or are not carrying any heavy load above them.

Where are you?

We used precast concrete tanks, although you can get plastic tanks.

plain steel tanks, coated with bitumen seem to last 20+ years. I can't see why they last so long unless there is something that inhibits rust in the sewage. Lack of O2 maybe?

Lack of O₂ & the bitumen.

Concrete is in use near here. 35 years?

Engineering brick can also be used. 49 years for another installation nearby?

CaptMoosie-

GA from me, although I am a strong proponent of plastic (HDPE) tanks here in the Tropics, especially for remote areas. Two issues: concrete tends to dissolve in the ground (as much a quality control issue as an environmental effect), and, secondly, during heavy rains, ingress of water in concrete tanks (i.e., through leaky inspection covers, leaky pipes, whatever) results in flushing the active media into the leach field (assuming a proper leach field has been included!). Experience has taught me that plastic tanks are much easier to seal, and perform better in this climate.

No sngle solution is going to be optimum for every situation...

Hello cwarner7 11,

You are correct regarding your statements about concrete tanks falling apart due to aggressive soils, and infiltration due to leaky covers and pipes. However, at least here in upstate NY, certain county Health Departments (if they are present in the County government, then they usually have jurisdiction instead of the State Health Dept. and NYSDEC) have in the past dozen or so years begun to dictate that the tank interiors and exteriors be coated to prevent the concrete from being attacked by corrosive soils and sulfide gases. In respect to leaky pipes where they attach to the tanks, elastomeric pipe seals are now required and cement mortar is no longer allowed, thus preventing any leakage at those junctures. The leaky inspection cover (concrete plug) problem is solved with the installation of a sufficiently wide enough butyl rope sealant properly lapped at the ends.

In my experience in this climate, FRP and plastic tanks just don't cut it in regards to the buoyancy and rigidity problems. I've seen way too many failures of tanks constructed of these materials.

All too often I've seen where your typical homeowner (and subsequent owners) haven't a clue where the tank and the septic fields are located, and usually damage them by driving their cars or trucks or heavy equipment over them. This happens a lot when the homeowner hires a Contractor to re-roof the house or do other work on the house, and the Contractor's forces usually drive over the systems, hence destroying them. Also, some homeowners like to wash their vehicles in the vicinity of nearest outdoor hose bib that just so happens to be right next to the septic tank and the absorption field. I've found over the years in this region of our country that properly constructed (and protected) precast reinforced concrete septic tanks have proven themselves to be more than adequate for the task and have held up very well. Also, I must admit that we usually don't get the torrential rains like you do in the tropics, but we do have to deal with deep frost action in the soil mass each and every winter, which is this region can reach 4 feet or more in depth, depending on the type of soil, so RFP and plastic tanks typically just don't hold up long enough. Here, we expect the typical septic system, if properly designed, built, and maintained, to last at least 20 to 25 years before the absorption field has to be replaced. The septic tank and distribution box, as well as the piping had better last that long or longer. Pity the poor homeowner that doesn't have such a system and it fails miserably sort of it's intended lifespan, because it's very expensive to replace or rectify. In this neck of the woods, a typical homeowner now days can expect to pay an experience Contractor anywhere between $20,000 to $35,000 to replace a failed system, all of course the cost variations are highly dependent on many factors involved....

Have a great sunny day!

Pound for pound, I think PVC and HDPE outperform steel or concrete. Strength is an issue but can be designed for by the use of appropriate backfill and not loading the tank with heavy equipment after placement. Buoyancy is an issue because the tanks are lighter than concrete. Plastic and HDPE tanks often can be set with little effort whereas a concrete tank requires heavy equipment. Concrete is highly susceptible to corrosion due to H2S. Steel is highly susceptible to corrosion for this and other reasons. Coatings can be applied that will lengthen the life of both of these materials.

But it is all probably a moot point. Because if you get a septic system to last more than 20 years call yourself blessed. The surrounding soils will likely fail (plug) before the materials do. Many agencies require that you allow space between each line to construct new lines in the future. Thus doubling the expected life of the septic field with some expensive maintenance effort. In that case, the materials may not last as long as the septic field.

If that is the case, I still highly recommend at least considering plastic. But do be aware of the other issues mentioned. Yes, plastic has its issues, but if installed properly, after 30 years it will still be operational. You will likely have to replace concrete or steel by that time.

Good Luck

Hello ccoop610,

Base on my experience and following normal NYSDOH and NYSDEC minimum standards and proto-call, I still strongly recommend the use of precast concrete septic tanks because on WORSE CASE BASIS, DESIGNWISE, as accidents do happen where people do drive over septic tanks all the time...the tanks are buried and people normally forget were they are located if they even know in the first place, and most do homeowners not......please see my previous posting. You practically have to install steel bollards filled with concrete all around the septic tanks so as to prevent people's vehicles (and building renovation Contractors too) of of them! In my 32 years in the Civil Engineering field I have investigated dozens of septic tank structural failures, and most of them were either FRP, PVC and steel tanks. Ask any County Health Dept. Sanitary Engineer in New York State how many times they've seen people crush their septic tanks and you'll be very shocked by the horror stories you will hear! I'm not making this up. If you don't believe me I can supply their names and telephone numbers by the handful....likewise with other Licensed Professional Engineers that I have either worked with or had contact with in the past!

I regards to longevity concerns, from my past experience. and it is extensive, I've seen concrete septic tanks that were properly coated with asphaltic (tar based) coatings applied inside and outside last well over 40 years, with some even lasting 50+ years. The newer epoxy coatings in the market place today which have mostly replaced the older asphaltic coatings provide even better protection!

Need I say any more? If you want to go cheap and disregard most Health Dept. standards as well as forego a required Building Inspection of the installed system, then that's your business and you'll most likely pay the piper (or the homeowner will most likely be doing the paying...) due to an untimely failure. After all, you get what you paid for in the long run.

Have a great sunny day!

except... ...except that, in functionality terms, no septic tank, long-lived or otherwise, can outlive its leach field (drain field). So if it's "Bury it and forget it (for generation or more, say)," then tis better to match the tank to the expected service (and leach field) life requirement.

If the intention, otherwise, is to reclaim and rebury as successive leach fields become exhausted ... then it would seem that ease of removal & and transport intact my figure equally with tank material.

So it seems that "go with what's standard for your region" might be the way to go??

Just a thought.

Hello Guest,

Unfortunately, I got carried away, as usual, in my reply herein....sorry!

Well, at least here in New York the State Health Dept regulations require that an undisturbed area must be reserved parallel to and immediately next to the existing absorption field. The requirement states that this reserve area must be at least equal to a minimum of 50% of the original absorption field area. In the event that if any of the existing absorption field trenches should fail (and they do, usually after 20 to 25 years typically, but it wholly depends on a lot of household factors), then it will be fairly easy to install new absorption trenches parallelling the existing ones, and in this way it is equally easy to just extend the solid wall pipe leader lines from the existing distribution box to the high end (or beginning) of the new absorption trenches. When this method is utilized, there would be no need to replace or even move the existing septic tank unless it was either leaking or had somehow failed structurally. Of course, a lot of factors have to be evaluated to determine that.

Many PE's that I know personally and likewise, myself included, who design septic systems usually provide more that the minimum 50% reserve area on there design plans. I personally like to provide 100% or more for a simple absorption field, and roughly 150% to 200% for a raised bed system so as to accommodate the new basal area and to account for the effects of the bed side-slopes. Basically, all of this is done to avoid problems down the road in the future when it's time to rehab a failed system. It just takes a little forethought on the part of the design engineer to plan for nearly all eventualities. I find this part of the engineering of the system a quite a thrill actual and challenge to second guess the homeowner or commercial property owner on what he or she is going to do or improve their property over a period of years....like where is the house or building addition going to be built if they do so, or where is the future garage and driveway or parking lot going to be placed? That sort of thing, etc. The biggest problem I find with septic systems is where the homeowner may plant trees and shrubs in the future. Some just don't get it that it's a big no-no to do so in a subsurface wastewater disposal field because the plant roots will eventually clog the distribution laterals and crushed stone base, etc.. Hell, even my own mother does it and I keep reminding her not to do that! Does she listen to her own son??? NO!!! To her I'm still a little kid that knows jacksh*t, and besides, her plants look beautiful, and that's more important, what with all that wonderful free "fertilizer" that's flowing under the grass!!! ***LOL*** Surprised that I haven't torn out all my remaining hair over this!!! LMAO

Another problem is improper maintenance by the homeowner....like never pumping out the septic tank sludge that should usually be done every 2 or 4 years (depends on the actual household...everyone is different!), or flushing cooking grease and oils down the drain. Ditto with caustic household chemicals and cleaners. One thing that can really mess up a septic system fast is consistently using a garbage disposal or grinder installed at the kitchen sink....it appears to me that the suspended fines in the waste stream that are a product of the food and garbage grinder are so small and light that they do not adequately settle out in the settling side of the septic tank and thereby past over the internal tank baffle and eventually find their way into the absorption field, thus resulting in premature clogging of the trench crushed stone.

A good practice to follow is flush down the toilet a box of live active yeast every month. That yeast will help digest the soluble and insoluble solids in the waste stream, especially in the septic tank. Of course you can by a bottle of the that "super bug" stuff at the local Lowe's, Home Depot or supermarket. Yes it works, but so does the box of yeast at a far cheaper cost to you and it accomplishes the same end. Besides, when you buy the bottle of bug juice as I like to call it, you're also paying a premium for Madison Ave market hyperbole, the high percentage of the cost going towards the water contents, and the plastic bottle itself! I could go on and on about this stuff and resultant waste of money while standing on my 'Lil 'Ole soapbox!

Hope this helps! Have great sunny day!

Dear CaptMoosie

Would there be any truth in what many old engineers say: "Throw a lot of old tires in your sceptic tank and see the magic." How would old tires help degrade bio material? By allowing bio-agents to colonize better?

Dear Guest,

You pose an interesting question. I've never seen heard or seen it being done in a septic tank. I believe what you're driving at is giving the bacteria more surface area to grow on? Is that your assertion?

Although I'm not a microbiologist (even though I had college courses in Microbiology and wastewater treatment), I think there may be a common misunderstanding what the purpose of a septic tank is principally used for.

(1). First off, it's used to settle out large pieces of waste, including sand and any junk the little kids flush down the toilet! Try living with a very young Autistic boy (my step-son, who is now age 19 and finally outgrown the habit...) and see what that very expensive plumber fishes out of the waste lines or the waste removal service pumps out of the septic tank. and you'd be totally shocked! Ever think that a standard sized can of hair spray would ever get crammed-through the toilet siphon into the waste line? Well it happened here on several occasions thanks to my step son! He turned out to be a very expensive kid to raise because of all the times the plumber had to visit us over the years!!!! I can look back and laugh now, but not then! *LOL* Sorry for digressing!

(2). Second, it's used to allow flotation, and hence to a small degree grease removal.....the opening in the baffle plate between the two chambers (if there are any at all) allows for this because it's below the surface level of the supernatant.

(3). Third, and most importantly, the biological process utilized is a septic tank is Anaerobic Digestion....meaning the waste is very slowly digested by anaerobic bacteria that do NOT use air (oxygen component) in their metabolism of the food, or nutrients that comprise human waste and food scraps.

Usually, by providing a larger surface area for bacteria to survive on is generally a Aerobic Digestion process where the bacteria utilize the oxygen in the air in their metabolic processes...that's why Rotating Biological Contactors (RBC's) are used in wastewater unit operations as a secondary treatment, much in the same manner as the Complete Mix Aeration process (Activated Sludge Process), but is less energy intensive. Aerobic bacteria digest waste (nutrients like N & P) at a much higher rate and efficient rate than Anaerobic bacteria. Think of Anaerobic metabolism as a way to digest very hard-to-break down wastes, like large volumes of solids, sludges and grease.

Frankly, I'd be very leery of adding rubber tires inside a septic tank because it'll lessen the storage volume of the tank to accept solids, and hence sludge over a period of a few years. I honestly do not seen any short term or long term benefit to adding said tires. By adding automotive tires you'll just end up pumping out the septic tank more often! Do you like to burn $$$$$???? That's exactly what you'd be doing.

Hope that helps you with your question. Have a great sunny day!

having spent 10 years or so inspecting instalation of septic systems in NYC ( I was a Public Health Sanitarian a fancy name for health inspector ) I would recommend using reinforced concrete. Steel will eventually rust. I guess you could use fiberglass but I don't know how strong or how leak proof it would be. As I said, I would recommend reinforced concrete.