Van Goethem Septic Systems, Inc.

Concrete Treatment Tank Corrosion

Where does this corrosion occur?
Concrete corrosion occurs in many different areas in the United States. Many larger municipalities deal with concrete corrosion in their concrete sewer pipes. Locally, we are dealing with concrete corrosion in some of the concrete treatment (septic) tanks that are installed in many rural septic systems. Concrete treatment tank corrosion is an issue in certain geographical areas of eastern Wisconsin. Manitowoc County has some very defined “hot spots” where the corrosion issue is much more prevalent and progresses at a faster pace. Some of these areas are Northeim, the south side of Manitowoc near S. 15th Street, Shoto area to the Point Beach State Park and northern Manitowoc County near Tisch Mills. Brown County sees more occurrences of this issue in the southwestern part of the county, in the towns of Glenmore, Morrison, Wrightstown and Holland. Kewaunee County also deals with this issue, but the occurrences are more sporadic.

What causes this corrosion?
This corrosion is known as Microbial Induced Corrosion (MIC). MIC is a chain of events that occurs in the treatment tank when the necessary conditions are present. The first link in this chain of events appears to be a somewhat elevated level of sulfur in the water source that supplies the residence/facility. Once the water with the elevated level of sulfur is introduced into the treatment tank, the process of sulfate reduction begins in the effluent. Hydrogen sulfide gas is then released into the atmosphere, in the treatment tank, and then settles on the surface of the wastewater. The hydrogen sulfide gas then re-dissolves in the water of the slime that adheres to the concrete walls of the treatment tank. In this solution the hydrogen sulfide forms a weak sulfuric acid that will both attack the concrete directly and as well lower the pH of the concrete surface. This allows a new strain of bacteria to grow on the near pH neutral surface. This newly formed bacteria is called Thiobacillus, and it is the primary cause of MIC in concrete treatment tanks. The Thiobacillus bacteria consume hydrogen sulfide gas and excrete sulfuric acid as waste. The sulfuric acid is very corrosive to concrete. As the environment in the treatment tank becomes more acidic, more aggressive strains of the bacteria develop. These more aggressive strains of bacteria are most found in the upper areas of the treatment tanks. The areas of the concrete treatment tanks that are affected by the MIC are always above the water/effluent line of the treatment tank.

What can be done to combat the corrosion issue?
• In areas where concrete corrosion is known to be an issue, plastic treatment tanks can be installed. Plastic treatment tanks, though, are not an answer for every installation situation. Plastic treatment tanks are not as rigid as concrete treatment tanks so they are limited with the types of situations that they can be installed in. In our opinion, the plastic treatment tanks should only be installed in dry sandy type soil conditions with three feet or less of soil materials over the tank.
• Currently, we are installing a venting system on all our concrete treatment tanks to supply an avenue for the hydrogen sulfide gas to escape from the tank, hopefully breaking a link in the chain of events that causes the corrosion. We have also added some features inside the concrete treatment tanks to reduce the turbidity of the wastewater. Reducing the turbidity of the wastewater reduces the amount of hydrogen sulfide gas that is released into the atmosphere in the concrete treatment tank and thereby reduces the amount of “food” that the Thiobacillus bacteria feed on. We are in our fifth year of using this system and it is still a little to early to see how effective it is going to be. We have taken meter readings, on a few of our systems, to measure the levels of hydrogen sulfide gas that are escaping through the venting system, and we have been pleased with the numbers we are getting.

What other methods have been tried to combat the corrosion?
Our first remedy at combatting the concrete corrosion issue involved increasing the strength of the concrete and reducing the water/cement ratio during the production of the treatment tanks. The idea was to create a harder and denser concrete tank that would be more resistant to the corrosion. Over time, this proved to be somewhat more resistant to the corrosion, but definitely not a cure. Our second remedy involved the hardening and densifying of the concrete with an additional chemical admix being added to the concrete that would densify the concrete even more as it cured. This chemical admix would produce tiny crystals that would fill the small pore spaces of the concrete during the curing process. Again, the idea was to produce an even more dense concrete that would be even more resistant to the corrosion. This attempt did show some positive results but was still not a cure for the problem. Our current venting system is a bit different in that we are trying to break the chain of events that causes the corrosion rather than trying to build a tank that can withstand the corrosion. Within the next three to five years, we should be able to see how effective this venting system is at combatting the concrete corrosion issue.