Concrete is an invaluable component in many septic systems. Its usefulness in custom tank capacities and thicknesses make it a go-to solution for many installers. But with that, comes inevitable deterioration over time. 

Mark Hayes, owner of Minnesota Geotechnical Services in Maple Lake, Minnesota, has pondered the puzzle of tank rot for years and believes to have discovered its main cause. As for a solution to the problem, Hayes is still working on it.

So what exactly is tank rot? “It’s the name for a concrete tank that disintegrates due to biological and chemical reactions,” says Hayes. 

A trifecta of trouble

As with many complex issues, tank rot occurs for a multitude of reasons. All of them must converge for this type of concrete disintegration to happen.

In the case of tank rot, its occurrence is based on a trifecta of trouble. In plain language, three things have to occur inside a septic tank for rot to occur.

The first culprit is the presence of “sulfur fixing bacteria,” more properly known as sulfate-reducing bacteria. These microorganisms thrive in anoxic or low-oxygen environments, converting sulfates into sulfides. SRBs are the reason hydrogen sulfide is found in septic tanks, well water and poorly aerated fish ponds.

The second culprit is a medium on which SRBs can grow, namely one where sulfur is present. Vulcanized rubber or synthetic rubber (also known as ethylene propylene diene monomer, or EPDM) used in tank boots. 

“These materials provide a flexible, watertight seal for pipes entering and exiting the septic tank,” Hayes says. “The problem with this product is that the vulcanization process uses sulfur in amounts of 1-5% of the rubber’s total weight. When exposed to heat or other environmental factors, this rubber can undergo a process called ‘sulfur blooming,’ where the sulfur migrates to its surface. You know this is happening when you see a whitish-gray powder form on its normally black exterior.”

When a sulfur bloom occurs due to an up to 5% presence of sulfur in the vulcanized rubber, the pH on the rubber’s surface drops to as low as 0.87. This creates an excellent growing environment for SRBs — as long as they have access to water (which they do) and food (effluent).

When this happens, the sulfur feeds the SRBs. In turn, they convert the sulfates into sulfides, generating hydrogen sulfide gas. “When hydrogen sulfide gas touches the moist wall of the tank, sulfuric acid is formed, which affects the concrete,” says Hayes. “Once the erosion of the concrete begins, the bacteria may advance up the wall as the area for growth media increases — but further in-depth research in this area is needed as to the direct process.”

It’s worth noting that the sulfuric acid doesn’t actually dissolve the concrete. Instead, it converts the concrete into gypsum, the soft sulfate mineral used in drywall boards. Unlike concrete, gypsum has no structural strength. As a result, “when this happens, the structural integrity of the tank walls or lid can be severely compromised due to the conversion of concrete into gypsum,” Hayes says. “That’s what leads to the tank falling apart — or rotting.”

The third culprit for concrete rot is the aggregate used in the concrete mix. “Aggregates come from a wide variety of sources, and sometimes the source rocks may contain sulfur minerals,” says Hayes. “Aggregate high in sulfur minerals will trigger the chemical reaction almost immediately.”

This sort of rot occurs on the walls of the tank where the cream coat is removed. When this happens, the tank integrity is compromised at some level.

No easy solution

Aerating the septic tank to add oxygen may slow down tank rot, but it won’t stop it. “If the rot is from the aggregate in the concrete, aeration won’t help much at all,” Hayes says. “You can tell when it’s a mineral problem because the cream coat on the tank’s inside walls will be missing when it has been pumped out and you can see inside.”

Trying to find low-sulfur aggregate sounds like a good idea. Unfortunately, this content level can vary widely even within a single gravel pit, so it’s not simply a matter of finding ‘safe sources.’ “You could literally fire your aggregate before it goes into the mix to burn off its sulfur minerals, which is called ‘roasting,’” he says. “But when you’re making a few hundred or even a thousand tanks annually, roasting your aggregate would be incredibly expensive. A chemical wash might be an option, but I don’t foresee that being any cheaper in the long run.”

However, there is one preventive step that might reduce the chances of tank rot. “If you’re getting a tank delivered to the site and its rubber is gray and not black, you’ve got sulfur bloom,” Hayes advises. “The solution to that is one of two things: either put some rubbing alcohol on a rag and just rub the sulfur off, or take petroleum jelly and rub the sulfur off. I prefer petroleum jelly because it actually leaves a film on the rubber, which prevents the sulfur from blooming.” 

As for signs of tank rot to look out for? “If you see chunks of concrete falling off inside a 25- to 35-year-old tank, it could be tank rot,” concludes Hayes. “If you see an odd triangular shape forming above the tank outlet this is the start of tank rot. Therefore, the key is to make sure your rubber tank boots are good and black when you install a new tank and rub them with petroleum jelly to inhibit the growth of SRBs. While this isn’t a complete solution to tank rot, it can significantly improve your chances of preventing it.”