Question:
I read your article, “Tank Troubles”, (October 2010) and have come across a bad tank. I have a 10-year-old, two-chamber septic tank. The top of the second chamber has deteriorated to a thickness of 1 inch. This is obviously a safety issue as I am afraid it will fall through if a lawn mower goes over it. I was told this is due to hydrogen sulfide gas coming from the septic field. The first chamber is fine and I was told that’s because the gas is allowed to release through vent pipes that lead to the roof. I have not seen this before. Is there a fix?
Answer:
You did not include details about the construction of the compartmented tank showing severe deterioration of the cover over the second compartment. I will need to make some assumptions about the construction. I am including a sketch of a properly compartmented septic tank for your information and reference.
First of all, the purpose of a two-compartment septic tank is to have the major bacterial action in the first compartment. The solids settle to the bottom of the first compartment and materials such as detergent scum, cooking fats, etc., form the floating scum layer at the top of the liquid layer.
The effluent from the first compartment flows into the second compartment slowly and more settling of fine sewage solids takes place. The result is a cleaner effluent being discharged to the soil treatment field.
The opening through the compartment wall should be vented and the air space between the first and second compartments should be connected. If the tank is made properly, excess gases in both compartments should be discharged through the plumbing vent on the roof.
Most of the bacterial action and generation of hydrogen sulfide will take place in the first compartment, as that is where most of the sewage solids are located. There will be some bacterial action in the second compartment, but very little bacterial action in the drainfield.
I do not agree with the assertion that the hydrogen sulfide gas is coming from the septic field. First of all, much more hydrogen sulfide gas will be generated in the septic tank than in the drainfield.
Secondly, very little, if any, gas or air movement will go from the drainfield to the septic tank. Also, any gas or air movement from the drainfield should also move through the septic tank and to the roof vent.
Is it possible the second compartment has a separate cover that was not properly made and cured to prevent breakdown by sulfuric acid? It seems to me that the cover over the first compartment would have as much or more exposure to hydrogen sulfide gas than the cover over the second compartment.
You also stated that the thickness of the tank cover in the second compartment has been reduced to 1 inch. I am wondering if there were reinforcing rods in this part of the cover and if they have been exposed. You did not indicate there were any reinforcing rods or mesh.
In my opinion, and for whatever reason, the portion of the cover over the second compartment was not made properly by the tank manufacturer. It is difficult to imagine why this would be, but I can think of no reason why one part of the septic tank cover should deteriorate and the other part not have any problems. It should not be caused by more hydrogen sulfide gas on one side or the other.
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This is the second question I’ve received recently about deterioration of concrete septic tanks. Does anyone else have comments on this problem or experienced similar or different problems with concrete septic tanks? Maybe together we can collect information that will help increase the life of concrete septic tanks.
