Potential Solutions for MIC in Septic Tanks

In my past few blogs I have discussed the mechanics of Microbialy Induced Corrosion of Concrete.  One thing that needs to be emphasized is this: MIC does NOT occur in all precast concrete.  The conditions that must be present in a septic tank result in this authors estimate that only about 1% of all precast concrete septic tanks installed in the United States show signs of MIC.  This statement is key to realizing that, while this issue is important, caution should be taken before assuming that all concrete is prone to this corrosion. The vast majority of precast concrete septic tanks perform very well for 15-25 years, an acceptable life span for this home waste treatment system.

The conditions that exist for MIC to occur are related to the incoming waste, the chemistry of the supplied water, and the design of the wastewater system.  The contractor who serves an area where the water hardness, sulfates, iron, and manganese levels are high may be seeing a a significant percentage of tanks with corrosion in his area.  With these conditions, they may wish to offer the homeowner with options to modify the tank in order to assure a full life span.  In these areas, the life span of a tank may be reduced to 5-7 years.  With knowledge, a contractor and/or homeowner can make a few wise economic choices to extend the life of their concrete septic tank.

Venting

Hydrogen sulfide gas feeds the aerobic bacteria responsible for creating the most damaging acid.  It is reasonable to assume that if the harmful H2S gas is exhausted from the air space, then the bacteria will not have an adequate food source.  This solution provides a promising solution.  There are several options on the market allowing for choice and competition.  My personal belief is that an "exchange" of air is necessary for venting to be most effective since H2S gas is heavier than air.

Coatings

There are many coatings on the market which may help protect concrete from the damaging effects of the acid.  In a perfect world, a coating is applied in a uniform thickness over the entire surface.  It is the presence of pinholes, fish eyes, out-gassing, etc. that leads to the potential failure of a coating system.  Most coatings require 28 day "cured" concrete before the coating is applied.  This, as well as the application instructions, needs to be evaluated.

Liners

A liner might be practical for a manhole or reinforced concrete pipe, but they really haven't gained acceptance in the onsite wastewater products.  They are relatively expensive, and extra labor is usually required for welding the seam.  About 10 years ago I talked to a company out of Massachusetts who wanted to manufacture pre-fit plastic core liners for septic tanks.  The company believed that the plastic bag concept worked because it was simple to use, economical to ship, and chemically resistant to most things seen in a septic system.  I think there is still work to be done in this area for septic tanks.

Crystalline Densifiers / Sealers

It is no secret that dense concrete is less prone to acidic attack than porous concrete.  Some admixtures and/or penetrating sealers chemically react with the by-products of cement hydration to form additional calcium silicate crystals in the concrete matrix.  In my experience, these products offer very good resistance to water penetration, and are somewhat effective in protecting concrete from mild acidic attack.  In sulfuric immersion testing, most of the crystalline densifying admixtures and sealers show increased life span of concrete exposed to acid.  The immersion testing is not standardized, and often varies from test to test.  Also, there is evidence that suggests that concrete which performs well in an immersion test will corrode in the field where MIC is known to be occurring.  My final thought: These products cannot hurt, and the concrete is likely to see an extended life as a result.

Antimicrobial Admixtures

I wrote in a previous blog about a concrete admixture technology that is scientifically proven to make concrete resistant to the growth and colonization of bacteria on the concrete surface.  This technology stops the process of life cycle of the anaerobic and/or aerobic bacteria that create hydrogen sulfide gas and convert the gas into sulfuric acid.  The technology is not all that new.  Dow Corning first patented SiQAT technology, the common name for the chemistry containing the active ingredient, in about 1970.  The antimicrobial biostatic can be found in all sorts of products we make contact with everyday.  The technology was first used in concrete in 1996, and it is now specified by several major metropolitan sewer districts for inclusion in concrete for municipal waste conveyance structures.  The chemistry is expensive yet is still more affordable than liners in most instances.  The addition to a small septic tank can easily double the price of the concrete.

Other Solutions

I don't think this is a "one size fits all" market.  There are probably more solutions than I have time and/or space to write about in this blog.  I want to conclude by reminding my readers that there is no substitute for good quality concrete made in a precast plant that follows a structured quality assurance plan.  All concrete is NOT the same.  The National Precast Concrete Association has provided a guide for best practices that a precaster needs to follow.  ASTM (American Society for Testing and Materials) has a specification for precast concrete septic tanks, ASTM C1227.  Both should be evaluated by the manufacturer and purchaser before making a purchase.  Consider utilizing one or more of the solutions available and on the market.  One of these by itself may not be the silver bullet, but together they may provide a septic tank product that will last 25 years or more in a known harsh environment.

MIC Explained - My Presentation

Over nearly two decades of research, I have uncovered a lot of interesting information regarding concrete corrosion due to microbially induced concrete corrosion.  Recently, I presented my research in the form of a FREE webinar for the Wastewater Education Association.  The link to the recorded presentation is:
http://www.youtube.com/watch?v=lRAbt9cy7zE
This presentation is a product of research that includes dozens of research papers on this subject that dates back to C.D. Parker in the 1950's.  There is still a lot we don't know about this condition that affects sewage conveyance systems around the world.  It is important to note that a relatively small amount of area geographically seems to be prone to this issue, so if you don't see this in your area, there is very little cause for concern.
For more information on this condition, email me at precastguru@gmail.com.  Use the phrase "MIC question" in the subject line.  I look forward to your comments and questions.