4. MIC Attributes
Bacteria / Biofilm
Low dissolved oxygen in wastewater
Sulfates in the effluent
Warm Temperatures
Turbulence
Moisture on the walls above water line
Reactive compounds in concrete
Low effluent flow
5.
6.
7.
8.
9.
10. Link 1 – Hydrogen Sulfide
Wastewater
containing
sulfates
Anaerobic
bacteria reduce
sulfate to sulfide
Sulfide mixes with
water to form
hydrogen sulfide
11. Link 2 – pH Reduction
Carbon Dioxide
and water form
carbonic acid
Carbonic Acid
reacts with
concrete above
water level
pH of Concrete is
reduced to about
6.5
12. Link 3 – Bacteria Colonization
At ~ pH 6-6.5,
thiobacillus
bacteria colonize
Turbulence causes
H2S to rise into
airspace
H2S provides
nutrients for
aerobic bacteria
Different strains of
Thiobacillus live
and die off
Thiobacillus
consume H2S and
excrete sulfuric
acid
13. Link 4 - MIC
Sulfuric acid
reacts with
calcium
hydroxide
This reaction
forms gypsum
Gypsum further
reacts with the
aluminates
This reaction
forms ettringite
Ettringite is a gel
that expands with
water contact
14. People often wonder why MIC is
more of a problem today than it
was 30 years ago.
22. Unconsolidated
sand and gravel
aquifers at or near
the land surface.
Semiconsolidated
sand and gravel
aquifers.
Sand and gravel
aquifers of alluvial
and glacial origin
are north of the
line of continental
glaciation.
Microbial induced corrosion, unlike the other causes of deterioration, can affect high quality as well as low quality concrete. The primary difference is the length of time it takes for the deterioration to occur. MIC is caused by bacteria, specifically, the species Thiobacillus. The thiobacillus bacteria consume sulfur from hydrogen sulfide as food and secrete sulfuric acid.
There are dozens of strains of thiobacillus, non are more destructive than the strain named Concretivorous. This strain was named by C.D. Parker in 1945 and is derived from the Latin “concrete eating”. The name was later changed to acidothiobacillus.
For MIC to occur, most researchers agree that certain attributes must be present.
(read the list)
Remove any one of these and the chance of having MIC is reduced or eliminated.
MIC is the product of a chain of events. In the first link, the non-air breathing bacteria convert sulfates into sulfides. These sulfides form hydrogen sulfide gas in the wastewater.
Above the water line, carbon dioxide in the environment mixes with condensation inside the tank to form carbonic acid. The mild acid reacts with the concrete above the water line reducing the pH to around 6.5.
Thiobacillus can survive on concrete with a pH of 6.5 or lower. Turbulence releases hydrogen sulfide from the wastewater providing a source of food for the thiobacillus. The thiobacillus bacteria consume sulfur and secrete sulfuric acid. Different strains of bacteria survive at different pH levels.
This link is where the damage occurs. Sulfuric acid reacts with the free lime or calcium hydroxide forming calcium sulfate, also called gypsum. The gypsum reacts with the alumina in the concrete to form ettringite which can expand in the presence of moisture. This expansion causes the concrete to crack and spall. This leads to deeper penetration of acid, more cracking and spalling, and the cycle continues.
