Question:
Why do concrete risers, distribution boxes and lids deteriorate? I’m guessing it’s due to detergents, etc., but what are the ones homeowners should avoid? It is stuff like this that makes me love the plastic distribution boxes and risers. I’m not sure about water softeners either, as far as concrete deterioration goes. I have heard that water softeners raise the salinity of the effluent causing the sludge in the tank to sit lighter and fluffier.
Does using a garbage disposer or any other means of getting excess food to the septic tank raise the hydrogen sulfide level?
Answer:
There are two basic reasons concrete tanks and other concrete structures deteriorate: one is hydrogen sulfide gas, and the other is poor concrete construction of septic tanks and accessory parts like the manhole riser, drop boxes, etc.
Where does the hydrogen sulfide gas come from? This gas, which has the smell of rotten eggs, is a byproduct of anaerobic digestion of organic wastes flushed into the septic tank. The chemical symbol for hydrogen sulfide gas is H2S. When H2S combines with H2O water vapor, a chemical substance, H2SO4, is formed. You may recognize this formula as sulfuric acid.
What is so bad about sulfuric acid? Well, it just loves to break down porous concrete. And that’s what happens in your concrete distribution boxes (you should be using drop boxes), the manhole extensions of the concrete septic tank, and the portion of the septic tank above the liquid level. The concrete below the liquid level in the septic tank won’t have any sulfuric acid in contact.
The H2S gas can also move into other concrete structures of the septic system, like the distribution box, manhole extension, etc. When it combines with H2O water vapor, the sulfuric acid does its job on any porous concrete.
START WITH THE RIGHT CEMENT
Why is the concrete porous? The basic reason is the proper curing procedures and the proper cement have not been used. Sulphate-resistant cement must be used in the mix and the concrete must be properly cured. The same cement and curing procedures must be used for the concrete accessories, such as the septic tank cover, manhole extensions, drop boxes, etc.
I have observed tank manufacturers who used the proper cement and curing procedure for the septic tank, but did not use the same curing for the accessories. To speed up the process so the form could be removed sooner, the cement mixture was drier, and the concrete in the accessory was not cured properly. The tanks from this manufacturer would not deteriorate, but the smaller concrete parts would be subject to deterioration by H2S gas. You should consider using heavy-duty polyurethane products for these parts of the septic system.
National standards exist for the manufacture of concrete septic tanks, one of which is ASTM C1227, Standard Specifications for Precast Concrete Septic Tanks, as well as those created by IBCO Evaluation Service Inc. and the International Association of Plumbing and Mechanical Officials (IAPMO).
The National Precast Concrete Association has had many years of active involvement with ASTM to produce Standard C1227. This standard addresses specifications for septic tank volume, the number of compartments, liquid surface area, access openings for cleaning and inspection, inlets, outlets, baffling devices, and baffle submergence.
NPCA and the National Onsite Wastewater Recycling Association have produced a manual and video of best practices using these standards for the production of high-quality precast concrete septic tanks. For more information, contact NPCA at 800/366-7731 or go to www.precast.org.
DETERGENTS NOT AN ISSUE
H2S is formed when the aerobic bacteria in the septic tank do their thing on organic matter, such as human waste and the waste food from a garbage disposer. The more food for the bacteria, the more H2S will be generated. So, you are correct about the use of garbage disposers. These are not recommended with a septic system because of overloading of organic matter. Not only is more H2S generated, the sludge layer builds up faster.
I don’t have information to indicate detergents are responsible for more H2S. I don’t believe a detergent will break down even porous concrete.
In regard to water conditioners and salt, a water conditioner operating properly will not send much sodium chloride to the septic tank. Actually, a little salt is of benefit to bacterial action. Too much salt will slow down the bacterial action.
If the homeowner does not properly maintain the water conditioner, and iron is allowed to coat the resin beads, excess salt will be discharged to the septic tank each time the conditioner recycles.
However, my opinion is that a concrete septic tank made according to NPCA specifications will not be affected by the salt from even a poorly maintained water conditioner.
Hydrogen sulfide gas is going to be generated in every active septic tank. Sulfuric acid will be present above the liquid level of every septic tank. Unless the concrete septic tank and its accessories are properly made, they will deteriorate after time.
