By Claude Goguen, P.E., LEED AP
It’s easy to associate corrosion with rusting steel on an old car. However, corrosion is not limited to steel. It is the deterioration of materials by a chemical interaction with their environment. This is why the term “corrosion” has been used for many years to describe the concrete degradation in sewer systems. The process of microbially induced corrosion, commonly known as MIC, does in fact include a few chemical reactions, but also includes biological processes as well. National Precast Concrete Association, with the support of its members and researchers at three universities, has been studying MIC since 2010. The original research, conducted at Purdue University starting in 2012, has led to two new studies that continue to unlock the chemical and biological processes behind MIC with a goal of breaking the chain of conditions that cause MIC to form.
It is important to note that MIC is by no means a rampant issue. Considering the amount of precast concrete pipe, manholes and septic tanks used worldwide, structures with MIC issues represent a miniscule percentage. However, that does not provide solace to those who deal with the issue.
In trying to break the chain of evolution at this stage, the most viable way may be to limit the bacteria’s main food source, hydrogen sulfide (H2S) gas, or to make the concrete surface inhospitable to its attachment. These two strategies are the basis for two NPCA-sponsored projects. One involves conducting field tests to examine H2S gas concentrations in the system before and after simple modifications. The other involves the use of antimicrobial admixtures in concrete to prevent attachment of sulfur-oxidizing bacteria.
Option 1: Limiting H2S Gas production in Tanks
The approach to limiting H2S gas production and concentrations in the tank can be achieved from a couple of different angles. One angle is to significantly reduce the release of H2S gas in the tank’s airspace, which is the basis for one NPCA-sponsored study underway in Wisconsin.
The inspiration to begin this study originated from independent testing that was being carried out in Sheboygan County, Wis. County Inspector Kevin Stange had come across signs of MIC when inspecting tanks in the area. Most of the issues he found were in pump tanks and in the septic tank near the outlet pipe. He studied the available information to better understand the causes of this phenomenon. He confirmed that the presence of a high concentration of H2S gas in the headspace of the tank was a primary contributor to the issue and started devising ways to lower that concentration. One idea was to configure piping in tanks to reduce turbulence. Turbulence will accelerate the release of H2S gas into the airspace. His other idea involved diverting any air coming back into the tank from downstream sources. He did this in a couple of different ways and found the H2S gas levels in the tank dropped drastically. He was introduced to NPCA’s MIC Task Force and soon after, three precast concrete producers tried his theory on their own structures and found similar results. By reducing turbulence, preserving the scum layer on the water and not allowing air to come into the septic tank from downstream components, they found the H2S gas concentrations were reduced measurably. This result was enough justification to enlist a university to conduct independent field studies.
In early 2017, NPCA enlisted University of Wisconsin-Stevens Point researchers to perform testing on units in the field and the program, “Investigating the Potential for Septic System Modifications to Reduce Microbial Induced Corrosion of Concrete,” emerged.
H2S that accumulates in the septic tank’s headspace is generated below the water line and is in an aqueous state. Many factors including turbulence will contribute to the release of that H2S into the headspace as a gaseous phase. Generally, reports from the field showed increased deterioration near the outlet of the tank. That led to suspicion of H2S gas released near the outlet and potentially also coming from downstream. Further supporting these findings were reports of distribution boxes with some advanced deterioration. With the absence of a significant volume of wastewater in those smaller boxes, the H2S gas must be coming from elsewhere, perhaps upstream. These are all assumptions of course, and the purpose of the study is to enhance our understanding of the source and generation of H2S gas under different system configurations.
The scope of the project includes the study of eight different septic systems that have signs of premature concrete deterioration. The research will quantify the effectiveness of design modifications to reduce H2S concentrations.
As of July 2018, two systems have been tested. Preliminary results support the effectiveness of significant H2S gas reduction through design modifications to the system. A larger campground system will be tested next. The project is expected to continue throughout the year with a final report expected in early 2019. If the report confirms these modifications work in reducing H2S gas in tanks, that information will be shared with septic system designers, which could be a very significant step forward in the reduction of MIC occurrences.
Option 2: Preventing Attachment
ASTM C13.03 Subcommittee on Determining the Effects of Biogenic Sulfuric Acid on Concrete Pipe and Structures has for many years sought funding to study MIC in concrete pipe and structures. Led by Mel Marshall, president of Mel C. Marshall Industrial Consultants, the committee finally received funding and selected Oregon State University for testing. In this research, concrete modified by additives and compounds will be studied to determine the effectiveness of the modified concrete to resist microbiologically induced sulfuric acid. OSU was selected based on many factors, one being that Jason Weiss, formerly of Purdue University and an advisor on the original MIC study, is now a researcher and faculty member at the university.
The project has three objectives:
- Develop a guide document that can be used in the coordination of standards relating to nomenclature, standard conditions and methods of concrete pipe exposed to biogenic sulfuric acid environments, and accelerated or service testing of such materials, components and combined assemblies. This proposal is intended to produce a draft guide document.
- Create test methods for inclusion in ASTM for:
- Measuring the efficiency of a concrete additive in neutralizing bacteria in solution.
- Measuring the bacterial activity and effectiveness of antimicrobial additives in concrete.
- Measuring the presence and dosage of antimicrobial additives in concrete.
- Modify existing acid immersion test ASTM C267, “Standard Test Methods for Chemical Resistance of Mortars, Grouts, and Monolithic Surfacings and Polymer Concretes,” or create a new test method that would be more applicable for use in testing precast concrete.
NPCA has partnered with many other associations and companies in sponsoring this project.
As of July 2018, a draft of the guide document progressed through its first review by the subcommittee and comments are being addressed. The intent is to submit this guide document for full ASTM Committee C13 ballot later this year.
NPCA Continues to Find Industry Solutions
Precast concrete tanks continue to play an important role in the decentralized wastewater industry. The large majority of tanks will not be compromised by the presence of MIC, but in cases where conditions can lead to issues, NPCA members are committed to finding solutions and are focused on ensuring the tradition of long-lasting, efficient precast concrete tanks in wastewater applications continues. The work to find mitigation strategies goes on, and many professionals in the health and environmental fields are also on the front lines. That spirit moves this industry forward, and NPCA is committed to being a part of the solution.
Claude Goguen, P.E., LEED AP, is NPCA’s director of technical education and outreach.