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.

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Microbial Induced Corrosion - Part 4

A few years ago, I purchased a red car.  I didn't really notcie how many red cars were on the road until after I owned one.  Now I seem to find them everywhere.  There must be a red car epidemic in our society.  When I hear people talk about the issue of this biogenic corrosion caused by the Thiobacillus bacteria, I wonder if the same thing isn't occuring.  A septic tank has the signs of corrosive activity in one locality, and it heightens our awareness.  We now become more sensitized to installations where this might be occuring and think it must be an epedimic.

While I do not want to minimize the impact that Microbial Induced Corrosion has, both financial and environmental, in our society, I want to point out that in a decade and a half of research on this topic, the number of actual cases is small compared to the number of precast systems installed and funtioning for many, many years.  The cases of MIC, as uncovered in the current research, tend to be concentrated in specific regions.  The area affected is possibly only 1% -2% of the land in the United States.  This may be true in other parts of the world as well. 

The research has pinpointed several characteristics that seem to be common at sites where septic tanks are known to be corroding.  The factors include, high amounts of sulfur in the water, extremely hard water, the presence of gas or oil wells in the area, iron content, venting, lack of venting, and chemicals in the waste stream.  The conditions that are necessary for the microbial induced concrete to occur are also difficult to replicate.  This make the process of designing resistent materials very difficult.

As this topic becomes more widely known, it will improve the information that is available for research.  The data from a larger population will lead to more information about the sites which will eventually isolate a common set of characteristics that are always present when concrete corrosion due to MIC is found.

Microbial Induced Corrosion: Part 3

LOW WATER TO CEMENT RATIO
In a harsh environment exposed to sulfates, chlorides, or acids, it is very important to use a high quality concrete mix with a low water to cement ratio.  According to the book Design and Control of Concrete Mixtures, "Decreased permeability improves concrete's resistance to freezing and thawing, resaturation, sulfate, and chloride-ion penetration, and other chemical attack."  It is very important to reduce the permeability to increase durability.  A water to cement ratio of 0.45 is good for most concrete products that are not exposed to harsh conditions.  If there is a potential that the concrete will be exposed to these harsh conditions, the water to cement ratio should not exceed 0.40.  The diagram below shows how important a low w/c is to reducing the permeability.

The water to cement ratio is simply the weight of the water divided by the weight of the cement.  The amount of water comes from two places: the water added to the mix and free water in the aggregates.  It is very important to calculate the free water on the aggregates in this calculation.  The amount of water on the aggregates contributing to the batch water can be 70 pounds or more.  This is enough water to change a 0.45 w/c ratio to a 0.57 w/c ratio.

SECONDARY CEMENTITIOUS MATERIALS
In addition to a low water to cement ratio, the use of pozzolanic and secondary cementitious materials can increase the density and lower the porosity.  Fly ash, slag, silica fume, and calcium aluminate cement are just a few of the options.  Using one or more of these mineral admixtures in the concrete mix design will increase the strength and density while lowering the porosity and improving chemical resistance.  There are also various liquid admixtures, such as silica, that can be added to the concrete mix or applied to the concrete after the pour to improve its mechanical properties.

ANTIMICROBIAL TREATMENT
While concrete densification is important to increasing the life of the concrete structure, maybe even by 100% or more, it will not stop the biological process that causes the Thiobacillus bacteria to secrete sulfuric acid.  This chain is broken by the addition and/or application of a material with an antimicrobial bound to a silane molecule.  The silane provide a mechanical bond with the concrete that will not leach out or wear out.  The antimicrobial portion is also chemically and mechanically bound to the concrete structure.  The technology name is QuaternaryAmmonium Silane, or Quat Silane.  The chemistry is considered a pesticide by the US EPA and by law can only be distributed by organizations having an US EPA registration and a pesticide registration in the state where it is sold.  One such product is ConBlock MIC from Concrete Sealants, Inc.

The way this technology works is not completely known, even though the chemistry was invented and first patented by Dow Corning in the late 1960's.  The most common explanation is that the Quat Silane is present on the surface of the concrete structure.  Through a positive charge present in the molecular structure, bacteria are attracted to the silane molecule where a carbon chain extends outward from the silane.  The carbon chain is sometimes compared to a sword that punctures the membrane of the cell wall.

The positively charged molecule is not destroyed by this process, and it is present for future antibacterial needs.  Since the bacteria cannot survive and colonize on the surface, the acid that is produced by them cannot be created biogenically.  If there is no acid, then there is no corrosion.  For this protection to completely protect the concrete structure, the antibacterial must be incorporated into the concrete as an admixture, and it must also be applied to the surface after all other embedments, attachments, or accessories are installed.

The Quat Silane will also provide some level of hydrophobic protection to the concrete as well, helping to reduce the absorption of water (as well as water soluble chemicals) into the concrete.  The result is that the concrete life will be extended.  Dense concrete, treated with an antimicrobial additive and coating, should last in excess of 100 years.  The longevity of this protection is under review, but if the products are used according to the manufactures label, there is no reason to expect anything different.

So in conclusion, a dense concrete is critical for chemical resistance and reduced permeability.  There are several products on the market which can and do enhance the product density.  In addition, for full protection against the bacteria that causes Microbial Induced Corrosion, an antibacterial called a Quat Silane, must be incorporated into the concrete design.  Following these procedure will increase the longevity of concrete structures for generations to come.

Microbial Induced Corrosion: Part 2

Not all concrete corrosion is a result of the biogenic process described in Part 1 of this blog series. Corrosion can result from chloride attack, internal reactions, or as a result of other acids. So the first step in the diagnosis is determining a root cause of the concrete deterioration.

In this blog, I would like to explain how an acid attacks the concrete. There are some compounds that are formed from the concrete/acid reaction that further react to form other compounds. Having a basic understanding of this link in the chain of events will be valuable in determining the best defense for preventing concrete deterioration.

During the hydration of cement, a compound is formed that provides the desirable properties of hardened concrete. This compound is calcium silicate hydrate (C-S-H). Typical hydrated cement forms about 50% C-S-H. Another compound is also formed in this phase, calcium hydroxide, which composes 15%-25% of paste by mass (Kosmatka, et.al, 2002). Some of the chemical reactions occur within minutes of cement hydration, other reactions take days, weeks, and even months. By day 3-7 of the curing process, the mass of the concrete is primarily composed of three compounds; C-S-H, calcium hydroxide, and calcium aluminoferrite hydrates.

Calcium hydroxide is hydrated lime. It does not provide any of the desirable properties to the concrete. Calcium hydroxide will easily react with acids and other compounds. The reaction with carbon dioxide forms calcium carbonate resulting in what is referred to as concrete carbonation. When sulfuric acid reacts with calcium hydroxide, the result is calcium sulfate and water. Calcium sulfate is a naturally occurring compound also known as gypsum. If you recall the previous blog, the evidence of MIC is a whitish foamy mass on the concrete: wet gypsum. Think of what drywall looks like when it is wet.

What happens next is the process that causes the concrete to disintegrate. In the hydration process of concrete, gypsum in the clinker reacts with Tricalcium Aluminate to form ettringite. Ettringite increases in volume, which is acceptable before the concrete hardens. When gypsum is formed on the walls of hardened concrete, it reacts chemically with the aluminates present in the mass of the hardened paste. This reaction forms ettringite, the expansive gel. As the ettringite reacts with water and expands it causes cracking and spalling to occur. The chain of events allows more penetration, access to calcium hydroxide, and a snowball of destruction.

Sources cited:
Kosmatka, S., Kerkhoff, B., and Panarese, W. (2002) Design and Control of Concrete Mixtures, 4th ed., p. 40-41. Portland Cement Association: Chicago, IL

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.