Plastic Septic Tank?

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    • #278702
      Avatar photoMasterPlumbers

        Anyone out there have any experience with a plastic septic tank? I thought that concrete was the only option, but the installer that’s replacing my (out-of-date-and-illegal) cesspool says that because my site is hemmed in by large trees and tough terrain that a plastic tank is the only viable option. (The usual truck with a sling or chain on a boom can’t get in and a small tank like the one for my design flow, in plastic, could be hefted by a couple of guys) I’m not necessarily opposed to the idea, but does anyone have any horror stories? The only thing I could think of is if I get the tank pumped a year or two down the line, and the surrounding ground is super-saturated (after a heavy rain or spring thaw) could the empty tank float in the surrounding groundwater and get dislodged and even possibly break through the ground? I’m not concerned about crushing because the tank will be close to the surface, as a result of the shallow pitch from the house to the tank location. Any experiences, good or bad, could help me feel good about this descision. Thanks!

      • #299099
        Avatar photoKen Zoeller

          Plastic tanks can be better than concrete. They do not leak and they do not fall a part over time due to the acids in the air/gas layer over the effluent. Concrete can float if pumped and not refilled timely. AK Industries makes a good one.

        • #299100
          Avatar photoJohn Aldrich1

            Harry Homeowner?, OK!, Harry, I have personnally installed several hundred rotationally molded, polyethylene plastic septic tanks in my career. I have shared your concerns regarding the possibility of the tank floating out of the ground if the ‘homeowner’ did something stupid, like having the contents of the tank removed when the ground around the tank was saturated. My advice to you is, don’t do that.

            I have witnessed concrete, sewage holding tanks which have floated out of the ground because they had been pumped in the fall of the year, no sewage flowed into them all winter, and the spring rains saturated the soil around them. It popped out of the ground like a cork.

            It is just a matter of displacement weight differential. If the weight of the water (sewage) that is removed is greater than the weight of the tank, and the soil over it, and the soil around the tank is saturated, then the tank will float like a boat, regardless of the material used in its construction.

            Plan on pumping the septic tank during a dry period in your area, and then fill the tank with water as soon as the pumper truck leaves the site.

            A polyethylene plastic tank is superior, in most instances, to a concrete tank in that it is watertight (no cracks), and it is impervious to attack from sulfuric acid. Hydrogen sulfide gas, created in the microbial digestion process in the tank, mixes with the water vapor, which condenses on the inside surfaces of the tank, above the water line. This combination produces sulfuric acid. The acid then reacts with the cement in the concrete, and soon the aggregate is separated from the cement, and the tank begins to crumble.

            The concrete in modern septic tanks is specially formulated to ‘resist’ this degradation, and has a very long service life, however, polyethylene will not be affected by the corrosive nature of sulfuric acid, EVER. In my view, an indefinite service life, is better than a service life of 50 to 100 years. This is called a ‘sustainable building practice.’

            Any way, it sounds like the site constraints on your property preclude the use of a concrete tank, so consider yourself fortunate that you are forced into the use of the best materials in the construction of your septic tank. Good Luck! JWA

          • #299101
            Avatar photofourth year

              The only thing to add to the above is not to plant any big trees by the septic tank. If have had concrete tank walls collapsed by the pressure of roots against them and plastic tanks deformed for the same reason. Also the roots will attack the inside of the tank through any available opening, whether it is the joint between the tank and lid, cleanout openings or the annular ring around the inlet and outlet pipes.

            • #299102
              Avatar photoSylvanLMP

                I personally like the tanks I installed being heavy gauge steel protected with hot applied coal tar enamel inside and out or the newer rosin types of Marine paints.

                Point of information think about cast iron class water main piping and No hub cast iron see how long they last UNDER GROUND with lots of fluids flowing through them? Yet they don’t rust or rot because of the protective coatings inside and out UNLIKE the plastic piping crap failures.

                These tanks do not float or crumble and cant be punctured like some plastic crap.

                We had installed some for the local utility where these under ground tanks are exposed to road salts and other pollutants PLUS we use these tanks inside large sky scrappers several stories below the public sewer system where we use triplex pumping stations.

                Think of STEEL ships and not plastic WHY? STRENGTH, durability especially today with all the possible coatings available to protect the base metal.

                I have used stainless steel LARGE capacity tanks for lots of commercial laundries where these tanks come in contact with all kinds of chemicals and high temperature discharge of hot wash water (140-160 degrees) Most plastic would start to lose shape from this type of thermo expansion and contraction and weight of the water in case of a severe blockage.

                I would look into other viable options using a PROFESSIONAL in your area rather then someone trying to push one product.

                Plumbing and drainage means OPTIONS including a proper mixture of concrete reinforced with rust resistant reinforcing bars.

                Steel tanks that are properly protected can offer you years of trouble free service.

                Many “steel” tanks are in use in some of the building I work in that are almost 100 years old and still showing no signs of rust or corrosion and these are used for domestic water (oxygen enriched) and sewerage and even STEAM condensate highly acidic concentrations of condensate.

                Before I jump on the plastic band wagon check out steel tanks and some of the newer fiberglass options like we use for oil storage tanks. The (ASSE) American society Of Sanitary Engineers another place to ask for a suggestion. Also check the possibility of checking the American Petroleum Society (API) for the great materials they use for under ground storage of “toxic” oils and gasoline.

                They must have a clue to what non metallic materials hold up well.

                Your local soil conditions may also play a key factor in finding the right product for this application.

                But hey I’m only a “plumber” so we plumbers think of several options rather then figuring one product does it all.

                If you need help ask your local building department for their suggestion or feel free to E mail me.

                You could always opt for a steel tank with an outer encasement of concrete to hold it in place with LEAD WEIGHTS ( Ingots dippped in thermoplastics ) on the bottom for ballast :-)

              • #299103
                Avatar photoSylvanLMP

                  EVEN Concrete can be protected by coal tar applications.

                  We installed several precast manhole openings and clean out junctions weighing in a several tons and again these are PROVEN methods unlike the state OF THE ART plastic under slab failures that were supposed to have lasted a 1,000 years.

                  Yet with in a few short years FAILURE TIME with everyone pointing a finger at each other BUT the home owner was the final one who bit the bullet.

                  There is a whole lot more to sewerage disposal then digging a hole and planting a box.

                  Its your money spend it wisely.

                • #299104
                  Avatar photoRichard

                    How can you be sure that your polyethylene tank will last “indefinately”. Surely you don’t plan on practicing your “trade” for eternity, much less live that long. I will make you an offer, if you can back up your words, install one for me with an eternal guarantee. If and WHEN it fails, I will dig your corpse up to make you stand by your end of the deal.

                    [Edited by Richard on 15 October 2000]

                  • #299105
                    Avatar photoJohn Aldrich1

                      Harry, I think that you will find that the ISDS regulations, in most jurisdictions, specifically prohibit the use of steel tanks for use as septic tanks. The company which manufactures the steel tanks which Sylan is promoting, may apply to the local health department to gain approval of their tank for use as a septic tank. Think about how excited the steel tank company will be to make this application, so that they ‘may’ sell one tank. What do you suppose the price will be of this specially coated, steel septic tank?

                      The next issue is to determine the time required to gain approval of a specific steel tank for use as a septic tank. Compare that time requirement with the required time, dictated by the health department in which you have to improve your sewage treatment system.

                      It is true that the inside walls, and ceiling of a concrete tank, which are susceptible to degradation by sulfuric acid, can be protected by tar, or some other sort of bitumastic material. The problem is finding a concrete tank precaster that is willing to provide the coating on their septic tanks.

                      Most importantly, a concrete tank is not appropriate for use on this site because of a lack of adequate access for the large, heavy equipment required to handle a very heavy, concrete tank. Sylvan gets to rambling, goes off on unrelated topic tangents, almost breaks his arm patting himself on the back, and forgets the details of the problem revealed in the origial inquiry. That is his nature, and no criticism of his opinion will be tolerated, nor will any amount of criticism change his nature.

                      So that brings the choices down to Rotomolded Polyethylene Plastic tanks, and Fiberglass tanks.

                      I have installed several fiberglass septic tanks in my career. The quality of these tanks, in my opinion, was not the best, frankly they were less than adequate, but they were the best fiberglass septic tanks available on the market at the time. The fiberglass tanks were difficult to handle, and install. I installed the tanks to the best of my ability, only because they were specified by an engineer on various projects. The first fiberglass tank that I installed was one that I specified for use in a single family home septic system. I have not specified another fiberglass tank since.

                      Polyethylene septic tanks are also difficult to install if done properly. Much care must be taken in the backfilling operation. I always backfill a plastic tank by hand with a shovel, and with selected soils. Also be sure the tank is full of water prior to backfilling. Despite these difficult installation procedures, if performed correctly, I think that a high quality, polyethylene plastic septic tank is your best option, given the time, and site constraints that you face.

                      I will not respond to the comments provided by Richard, for his opinions and proposals are ridiculous as usual. A response to his comments will not address your original questions, Harry. It seems that he thinks that he is a comedian, and constantly states that he is not a plumber.


                    • #299106
                      Avatar photoRichard

                        Hey John “gravedigger”, you forget that whenever you speak about polyethylene, you are speaking of a polymer made by CHEMISTS, not plumbers. When you talk about sulfuric acid resistance, again, that is CHEMISTRY. Where would plumbing be without chemistry making advances in coating, polymers (including plastics), metalurgy, and welding rods. Even water is an interesting chemical, do you know its the only compound where the solid state is LESS dense than the liquid state. When you see ice-cubes float in water, that is actually a strange behavior for liquid-solid interfaces. I would explain it, but I doubt a grave digger knows about hydrogen bonding, van der waals forces, or 112 degree bond angles due to the electronegativity of oxygen and repulsion of the hydrogen.

                        Think where your grave digging job would be without the work of chemists.

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