Microbially Induced Corrosion in Septic Tanks pt 1

Microbially Induced Corrosion or MIC has been a problem with the concrete industry for years; however, it's impact on the precast industry has really been brought to the forefront recently with the discovery of rapid corrosion in septic tanks.  Septic tanks that normally have a service life 30-50 years are failing in a matter of years.  There are many aspects of this problem from how does the MIC happen to quality of concrete to actual relevance in an already struggling infrastructure.  Although it is my intent to cover most of these issues in most parts I would first like to start with an introduction.
MIC is a form of corrosion that is very dependant on bacteria.  Naturally occuring bacteria in septic tanks exrete hydrogen sulfide gas (rotten egg smell in sewerage).  Now this gas naturally stays dormant in septic tanks, until a form of turbulence is introduced.  When the turbulence is introduced the gas is released and floats in the air space of the tank.  Another occuring bacteria the thiobacillus bacteria converts the hyrogen sulfide gas into hydrosulfuric acid which eats concrete.  So looking at this from the outside you say ok eliminate the sulfur production or the thiobacillus bacteria and everything should be ok.  However it is those two aspects that make this problem one of the hardest to figure out.  Where does all the extra sulfur come from? Medications? Foods? Cleaners? Fertilizers?  How do we kill the thiobacillus bacteria?  There are modern additives that can be put in concrete to kill the bacteria, however I have not had the chance or heard from a first person perspective how effective the additive is.  Another approach is to realize that the thiobacillus bacteria cannot strive on surfaces with pH's greater than 9.  Fortunately or unfortunately, concrete is very basic pH 12-14.  It is the progression of the wastwater becoming more and more acidic that allows the thiobacillus to grow more prevalent on the concrete surface.  So as you can see, this problem just from the beginning is really complex.  In future blogs I plan on touching on topics such as quality concrete, coatings, additives in concrete, impact on infrastructure, the future of MIC, and other topics so stay tuned.   I leave you with a few pics to see how bad this problem is.  Thanks...Aaron