Can a lack of oxygen cause my septic tanks to deteriorate?

This was one of a list of questions recently asked by a homeowner concerning his septic system.

It is an interesting question since, as an industry, we see tank corrosion and have continuing discussions about the most common causes and remedies.

Corrosion problems with concrete have been documented throughout the wastewater treatment industry, not only in septic systems but also in municipal systems where raw sewage is conveyed in concrete pipe. Several of the engineering manuals used by wastewater professionals use the term “crown rot” to describe the corrosion and deterioration of the top of concrete sewer pipes.

Sulfate bacteria

With an agricultural background, I found this interesting because “crown rot” is a term used to describe a particular type of plant disease caused by a fungus that can be quite devastating to crops. In my brief survey, I was unable to determine if wastewater operators or plant pathologists were the first to use the term. The similarity between the conditions is striking in that crown rot destroys the growing point at the top of the plant and concrete corrosion that we are concerned with attacks the lid and the top of the outlet baffles of the septic tank.

So is it a lack of oxygen that causes the deterioration? The answer is no, but oxygen does play a role. In this case as in so many other aspects of our sewage treatment systems, bacteria and the presence or absence of oxygen determines whether our systems work the way they are supposed to. The simplified answer to how this happens in the septic tank is that hydrogen sulfide gas is generated by sulfate bacteria that are breaking down the organic waste in the septic tank as a part of the anaerobic digestion process.

Some hydrogen sulfide gas diffuses into the environment above the wastewater. Moisture evaporated from warm sewage condenses on unsubmerged walls and the lid of the tank in droplets. As a portion of the hydrogen sulfide gas and oxygen gas from the air above the sewage dissolves into these stationary droplets, they become a habitat for sulfur oxidizing bacteria.

Colonies of these aerobic bacteria metabolize the hydrogen sulfide gas to sulfuric acid. Since hydrogen sulfide gas is heavier than air, it will collect at the low spots in the tank that would be at the outlet. The concentration of gas becomes high enough for conversion to sulfuric acid to occur.

Leads to collapse

Sulfuric acid begins to interact with the concrete, changing the structure of the calcium hydroxide in the concrete, a larger molecule that causes pressure and spalling of the adjacent concrete and aggregate particles. The weakened concrete will ultimately deteriorate to the point of collapse. This is why in only a half-joking manner at workshops I like to point out if you see that deterioration, step back off the lid.

Where this condition is discussed in engineering and wastewater textbooks, two main prevention approaches are indicated. Sewage flows more rapidly through steeper gradient sewers, reducing time available for hydrogen sulfide generation. Providing good ventilation of sewers can reduce atmospheric concentrations of hydrogen sulfide gas and may dry exposed sewer crowns. Acid resistant materials like pipe materials are also suggested.

Speeding up the flow does not fit in the septic tank environment because we want the sewage to have a long enough detention time for solids to settle. So we set up the sewage to sit in a stale environment allowing plenty of time for the formation of the hydrogen sulfide gases. For our environment, the solution is to make sure the tank is properly vented back through the house-plumbing stack to the atmosphere so the hydrogen sulfide does not collect and build to levels where sulfuric acid is formed.

Two more comments are appropriate here. The sulfate reducing bacteria are naturally present in wastewater in general and in our septic tank environment. However, in areas where the groundwater is naturally high in sulfates – often corresponding to areas where the soils are high in gypsum – the water quality can worsen the problem. It can also result in the septic tank contents becoming acidic, reducing the ability of organisms in the tank to break down the solids. This, in turn, results in a lack of separation in the tank and increased organic loading to the drainfield. So if you are working in areas where the sulfates are naturally high, you may experience more problems.

Venting may help

Soils naturally high in sulfates can also be corrosive to concrete through a different set of processes. If sulfate-resistant concrete is not used, the tanks can deteriorate from the outside.

Finally, I have been getting a lot of reports about concrete distribution boxes showing excessive deterioration. Service providers report seeing this more often now than any time previously. One action we might want to consider is providing additional venting to our system. As we add things like effluent screens and other items to parts of the system, we may be interrupting the venting process, causing some additional problems.