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Continued commentary on concrete septic tank deterioration begs for more firsthand accounts from pumpers in the field

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I continue to receive information about concrete septic tank deterioration from various sources. I believe onsite professionals in the field who are observing conditions need to suggest solutions to solve the reported deterioration problem.

First, I will list several types of onsite sewage treatment systems and discuss them, including comments I have received.

1. A septic tank (not compartmented) without an effluent filter, or a filter that allows gas movement.

2. A septic tank (not compartmented) with an effluent filter that blocks gas movement.

3. A compartmented septic tank not properly vented between compartments.

4. A septic tank properly vented between compartments.

5. The first four systems, each discharging to a pumping station.

Of course there are many other possible combinations of tank configurations, use of effluent filters, gravity flow systems, and pumping tank systems.



The use of effluent filters, which blocked gas movement, was raised as a cause of septic tank deterioration. An effluent filter that did not allow gas to move through the outlet tee would keep the hydrogen sulfide inside the septic tank and cause excessive corrosion.

A similar comment from a reader stated that such a filter prevented oxygen from passing through the septic tank to the drainfield, and oxygen was needed for the drainfield to perform properly. However, there is no oxygen in the anaerobic environment of a septic tank. So it is not a valid observation that gas needs to flow from the septic tank to the drainfield. Also, oxygen needed in the drainfield must come from the surrounding soil.

A writer recommended using effluent filters for the outlet tee of a septic tank. In regard to the use of effluent filters, have any readers observed a difference in tank deterioration of systems No. 1 or No. 2 described above, either with or without effluent filters, or filters that prevent gas movement? I have difficulty understanding why effluent filters need to allow gases to pass through them. Excess gases generated in the septic tank should move through the roof vent of the plumbing system.



In regard to system No. 3, one onsite professional observed severe deterioration of the concrete in the second compartment of a septic tank. The point was made that the absence of a scum layer exposed more of the tank sidewall and an increased hydrogen sulfide level was present in the second compartment. However, the major bacterial action and hydrogen sulfide generation should logically take place in the first compartment.

Another onsite professional reported that if the compartment wall did not seal the flow of gases between the two compartments as in system No. 4, there was little or no tank deterioration. Some septic tanks are apparently constructed with a solid wall between the two compartments and an opening in the wall under the effluent surface. While this may make the tank construction process easier, it does not allow the entire septic tank to be vented.

The compartment wall should have at least one inch of opening under the tank to allow gases to move back and forth. An outlet tee must be installed in the compartment wall to allow effluent to flow into the second compartment.



Other information sent to me suggests bacterial action was the cause of the breakdown of the concrete. I do not agree with this idea. It is my understanding that hydrogen sulfide combines with water vapor to form a weak solution of sulfuric acid. Experience has shown sulfuric acid will break down concrete that is too porous because of an improper mix and/or improper curing.

Another writer asked the question, “Why is the gas (H2S) not reacting in pump tanks?’’ Their conclusion was the liquid level in the pump tank was always moving. However, I received a report from another onsite professional stating severe concrete deterioration takes place in the many concrete tank pumping stations he observes. These are certainly two different reports on the deterioration of pumping stations. Do you have any observations on deterioration of concrete tank pumping stations?

The case was made by several writers that hydrogen sulfide gas is heavier than air. This is true. However, as more hydrogen sulfide is generated in the septic tank and occupies the void space, some gas will move out the roof plumbing vent. Homeowners do report the rotten egg odor of hydrogen sulfide coming from improperly located plumbing system roof vents.

Another writer states hydrogen sulfide will not flow out of a roof vent, but must flow into the drainfield to save the septic tank. However, more onsite systems have pumping stations, so hydrogen sulfide cannot flow to the drainfield in such systems.



There is a general consensus in the reports I’ve received that concrete tank deterioration has been much greater for tanks made in the last 10 years than in previous years. I have difficulty understanding why the venting of tanks or the presence of pumping stations should affect the deterioration of a properly constructed concrete tank.

I have trouble accepting the idea that the home sewage of today generates more hydrogen sulfide and is the reason for concrete tank deterioration, as some have suggested. I tend to believe changes have been made by the concrete tank industry affecting the resistance of tanks to hydrogen sulfide. Why have many onsite professionals observed older concrete septic tanks in better condition than tanks made within the last 10 years?

The onsite industry is not going to escape from hydrogen sulfide occurring in septic tank systems. The gas is a byproduct of the anaerobic digestion of sewage wastes taking place in a septic tank.

In my opinion, the cement industry needs to take a close look at septic tank deterioration and concrete tank construction practices. Research must be done in the field by a qualified onsite professional talking to other active onsite professionals, viewing septic tanks, etc., and not from behind a desk sending out letters or questionnaires.



Tank materials not subject to deterioration are available to the onsite industry. A homeowner who is told their concrete septic tank has deteriorated in less than 10 years and needs replacement may ask, “Are septic tanks made of other materials that won’t deteriorate?”

Readers, please share your observations of concrete tank deterioration. Explain the type of onsite system used with the concrete tank. Send a message to me through We need to learn together if this problem is to be solved.


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