Research Looks for Ways to Slow Septic Tank Deterioration

New concrete mixes and component design changes may be keys to solving the issues caused by microbially induced corrosion.

Research Looks for Ways to Slow Septic Tank Deterioration

Jim Anderson, Ph.D., is an emeritus professor at the University of Minnesota Department of Soil, Water and Climate and recipient of the pumping industry’s Ralph Macchio Lifetime Achievement Award. Email Jim questions about septic system maintenance and operation at

As I mentioned last month, a long-running debate in the industry has centered on advanced corrosion in concrete sewage tanks. The culprit causing this corrosion has long been identified as hydrogen sulfide gas (H2S). The gas is generated by the decay of organic matter in anaerobic septic system sewage tanks. In addition, small quantities of the gas can be generated from organic material in piping to and from the tanks.

The National Precast Concrete Association has been looking at this issue for several years and continues to fund and conduct research to solve the problem. Corrosion in tanks occurs due to a process called microbially induced corrosion. There are three phases to the process where the pH of the concrete is first reduced due to the slightly acidic nature of H2S, then microbes can attach to the concrete, and finally sulfuric acid is generated causing accelerated corrosion.

The stock answer to questions about why it occurs and how to prevent it has been that there is lack of adequate ventilation to allow gases to escape the confines of the tank. So the recommendation is to improve the venting from the tank and the problem will be solved. Typically, for systems with gravity flow, tanks and piping are vented back through the house sewer and out the roof stack.


Any interruption in this free flow of air back through the vent leads to H2S accumulation typically around the outlet of the septic tank since this is the low spot in the tank air space. It appears through some of the research and a lot of service provider input that the H2S accumulations are not only due to the gas being generated in the tank, but also downstream from the tank outlet. This is consistent with the information that gas can be generated in the piping. It also helps explain why in some systems the distribution boxes show advanced corrosion. Another low point in the system — in this case much smaller than a tank — the concentration of H2S may be much higher, leading to rapid deterioration.

The National Precast Concrete Association has looked at concrete mixes, different additives and sealers. Better-quality concrete and these sealers can help delay the onset of the corrosion and should be a regular part of septic tank manufacture, but they are not the total solution.

As I mentioned in last month’s column, a colleague shared a couple of examples of the research National Precast Concrete Association is doing on this issue. The first involved corrosion in a pump tank at both the inlet and where the pressure pipe outlets from the tank. On the inlet side of the tank, the pipe was simply stubbed off so as the effluent comes from the septic tank, it creates a little waterfall effect causing turbulence in the tank, generating H2S.

This was interesting to me because I thought all pump tanks or chambers should have baffles on the inlet with the specific purpose to reduce or eliminate turbulence in the tank. I began to look at literature and installation guides about pump tanks and found that requiring baffles at the inlet often does not get mentioned as being a part of installation.

If the pump station was the second chamber in a tank, it was more likely indicated that an inlet baffle was needed. When an elbow and pipe extending to near the bottom of the pump tank was installed, the levels of measured H2S fell from 100 ppm to less than 10. I would encourage everyone to check their codes and requirements to make sure an inlet baffle is specified.


On the outlet side, the corrosion was noticed where the weep hole sprayed out against the wall. Concentrations of H2S in this area were about 100 ppm. The pipe was extended downward so the weep hole was positioned below the high-water elevation in the tank and again the concentration was reduced to less than 10 ppm. The weep hole should be placed where it will not spray the side of the tank and in an area to reduce turbulence.

Another example provided was in a gravity system, where the corrosion was found around the outlet baffle of a septic tank. Measured H2S concentrations were 700 ppm or more. This tank already had a separate vent with a charcoal filter to reduce odors, but there was still H2S accumulation and corrosion. This indicates the gas was coming back from the drainfield area.

When the effluent screen was removed and the outlet baffle was extended outside the tank, concentrations of H2S were reduced to less than 10 ppm. As I have surmised before, as our systems have become more complicated, it has become necessary to add additional venting to reduce the potential for H2S gas accumulations that could lead to corrosion.

National Precast Concrete Association has commissioned additional studies to look at different venting configurations to solve or reduce the problems due to microbially induced corrosion. I am sure we have not heard the last of this issue, but it looks like we may have a few more answers than we have in the past.


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