Microbial Induced Corrosion: Part 1

You just had your concrete septic tank pumped and the pumper noticed a wet chalky mass on the concrete above the water level. He power washed the foreign material off and found that the concrete is corroding. Now your upset and wonder what is happening to your septic system. Well, before you blame your wife for using all of those household chemicals, or blame your son for consuming so much junk food that their biological waste is destructive to concrete, consider the biological activity that occurs in the septic process first.

Waste in a septic tank is partially decomposed by bacteria and other naturally occurring processes. These bacteria live in the water and do not need oxygen to survive. The term for this type of bacteria is anaerobic. One of these species is Th. Desulfovibrio, a sulfate reducing bacteria(SRB), which converts the sulfate in the wastewater to hydrogen sulfide. The hydrogen sulfide is trapped below the scum layer in the septic tank. When there is turbulence, the hydrogen sulfide gas is released into the atmosphere above the waterline.

In the mean time, the natural environment is having an effect on the natural properties of the concrete. With a pH of 12.5-13.5, concrete is very basic. Water has a pH of around 7, and acids have a very low pH. Acids are reactive with the properties of concrete that provide this high amount of alkalinity. carbon dioxide, thiosulfuric acid, and other mild acids reduce the pH of the concrete to around 9. This process can take months or even years, depending on the concrete quality.

Once the concrete pH is near 9, a strain of the thiobacillus begins to colonize that is aerobic, or requiring oxygen. These are sulfate oxidizing bacteria (SOB), and they convert hydrogen sulfide into sulfuric acid. The weak sulfuric acid produced by this strain lowers the pH of the concrete until it dies off and another strain colonizes. Each strain of aerobic thiobacillus produces a stronger sulfuric acid than the previous one.

Eventually, a strain Thiobacillus Thiooxidan is present producing a 7% sulfuric acid that rapidly destroys the concrete. This strain is called Acidothiobacillus. At this point, concrete structures will loose 1/2" of mass per year. It can take as little as 2-3 years or as long as 10-15 years for this chain of events to reach this level of destruction depending on concrete quality.

While all concrete can be susceptible to this degredation, not all installations have the same environmental conditions that trigger the chain reaction. There is a significant amount of uncertainty as to what exact conditions must be present for this to occur. Some theories suggest a high amount of sulfur in the water supply, a high iron content in the water, very hard water, and chemicals introduced into the waste stream just to name a few. Sites that have natural gas or oil wells in the area seem to have a higher potential for the conditions. Research in this area is currently being funded and will provide some beneficial information for engineers when they plan a septic tank installation.

In the future parts of this series, I will explain how the sulfuric acid destroys the concrete, how concrete quality plays an important role in improving the resistance to corrosion, and finally the use of nano technology to break the chain of events that cause this biogenic process.