Why Do Septic Tanks Deteriorate?

White crumbling concrete. Rusty streaks. Septic professionals have been seeing the same problem for years – deteriorating septic tanks. Learn what causes this damaging issue, and find out how to address the problem.
Why Do Septic Tanks Deteriorate?
A deteriorating septic tank shows white fiber reinforcements becoming visible.

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It may show up as white crumbling concrete around an outlet or as rusty streaks on the side of a tank, but it comes down to the same problem septic professionals have been seeing for years – a deteriorating septic tank. 

Research is underway to provide answers to the problem, and in the meantime there are steps any septic professional can take to investigate and address the problem. 

Regulating leakage

There is a reason so much conversation about the issue surfaced within the last few years, says Dave Gustafson, an engineer with the Water Resource Center at the University of Minnesota Extension. 

“I think that’s when it started to be identified as a hydrogen sulfide issue. I think prior to that it was still a problem,” he says. 

Hydrogen sulfide gas is produced naturally by anaerobic bacteria – the bacteria that don’t use oxygen to digest waste and survive in septic systems without aeration. 

Hydrogen sulfide gives sewage and septage the characteristic smell of rotten eggs, and it is the combination of the gas and moisture that causes deterioration in concrete tanks. When hydrogen sulfide combines with moisture in the air inside a tank and moisture coating the tank walls above the septage level, it forms sulfuric acid. That is recognized as one of the most powerful acids, and it attacks the concrete. 

Depending on when the lid is pulled for an inspection, the deterioration may still be small, just a bit of white decay around the outlet.

If the acid has reached the reinforcing rod the tank sides will show rusty streaks, indicating serious structural problems. 

What brought this on, in part, were new regulations requiring tanks not leak septage, Gustafson says. For the environment the rules are good because they keep untreated septage from filtering into the ground.

In terms of the hydrogen sulfide problem the rules are not so good because the new tightness of concrete tanks means hydrogen sulfide gas cannot seep out along with septage. 

Poor combinations

Early on there was talk that poor concrete mixes led to deterioration, but this idea did not stand up to what contractors saw in the field. In one subdivision all the homes had concrete tanks made by the same company, and all homes drew water from the same source, yet only one-third of the tanks showed deterioration, Gustafson says.

Septic tank deterioration has been a topic in only a few places in the country, notably Oregon and Wisconsin. Gustafson wonders if the problem isn’t more widespread but hasn’t been found because no one is actively looking for it. 

Ty Gable, president of the National Precast Concrete Association (NPCA), believes it is a strictly limited problem. The organization has an extensive system for members to report problems. Based on those reports – the association has members in all 50 states, nine Canadian provinces, and 13 other countries – deterioration is localized and rare, he says. 

The NPCA has been concerned about tank deterioration for some time and six months ago launched a research project in connection with Purdue University. That institution has one of the best and best-equipped concrete materials laboratories in the world, Gable says. 

“We have about 2,000 years of experience with concrete, so we understand what results in really good, structurally sound, watertight concrete – and what happens if you don’t do those things,” Gable says. 

Root of the issue

Good concrete practice is only a starting point, however. Nature adds other factors to the mix. To learn what may be promoting deterioration the NPCA project is looking at three broad areas: soil, water chemistry, and concrete mix design. 

Mix design will look at whether there should be any changes or special recommendations in the concrete used for septic tanks. (In Germany several years ago researchers developed an acid-resistant concrete for power plant cooling towers. Their mix was designed to minimize the amount of binder and maximize the dense packing of aggregate.) 

Other researchers on the team will look at the soil — how porous it is and also its chemical composition to see what may aid deterioration. The same will happen with water chemistry, analyzing it and seeing how it fits into the larger picture. 

There’s a bacteriologist on the project, too, who will study the mix of microorganisms in septage. The question to be answered here is whether some alteration in the microscopic ecosystem is promoting a larger population of hydrogen-sulfide-generating bacteria, or whether those bacteria themselves have changed. 

These are only the initial questions. As in other fields of science, answers are likely to bring more questions. 

“The scope of the project continues to expand as we get more curious,” Gable says. “The more we learn the more we want to try.” 

The hope is to have some preliminary conclusion in about a year, he says. 

While they wait for clearer answers and solutions, septic professionals may find hope in doing what they do every day. This is one case, Gustafson says, where regular maintenance can identify a problem before it becomes a disaster. 

Do your customers experience tank deterioration? What are the main causes in your area? Have you been able to repair the problem? Send your thoughts to kim.peterson@colepublishing.com and I promise to respond.


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