Microbial induced concrete corrosion in sewage and water treatment systems is a “natural enemy” of conventional plants, frequently causing damage to concrete and metal elements that is expensive to repair. As a result, it is not uncommon for wastewater system components to be refurbished or replaced every 10 years. Toxic gases released during biogenic processes, such as hydrogen sulfide, also pose a significant health risk, causing a range of symptoms from irritation to respiratory failure and death.
An interdisciplinary group of researchers from Graz University of Technology (TU Graz) and the University of Graz has outlined strategies aimed at preventing microbial induced concrete corrosion (MICC).
Understanding microbial induced concrete corrosion
MICC in wastewater treatment facilities results from a sequence of biogenic sulfate reduction reactions, followed by reoxidation. Initially, sulfate in pressure pipelines or standing wastewater is reduced by bacteria under anaerobic conditions, forming hydrogen sulfide. This pungent, highly poisonous gas escapes into the air and diffuses into sewer pipes and manholes. There, reoxidation by autotrophic bacteria takes place on concrete walls that do not even come into contact with wastewater.
These microorganisms produce sulfuric acid, which reacts with concrete construction elements. This leads to the vigorous formation of a biofilm on the surface of the concrete, a reduction of the pH value to below two, and extensive formation of new minerals, mainly in the form of gypsum. The combination of these processes results in the rapid destruction of the concrete, explained Günther Koraimann of the Institute of Molecular Biosciences at the University of Graz.
“MICC often corrodes the conventional types of concrete used in wastewater treatment plants at a rate of a centimetre or more per year,” said Cyrill Grengg of the Institute of Applied Geosciences at TU Graz. “Accordingly, the concrete elements can be destroyed in a matter of only a few years, causing significant damage to wastewater systems.”
The Graz-based scientists worked on a holistic solution using an interdisciplinary research approach that looked into the micro-structural and microbiological processes, as well as the development of new MICC-resistant materials. In this context, geopolymer concrete proved to be particularly well suited to withstand acid corrosion.
When developing this building material, resistance to acid was an extremely desirable property, as were highly anti-bacteriostatic surfaces, preventing the microorganisms that trigger the initial oxidation process from settling on surfaces in the first place. This, in turn, prevents the formation of sulphuric acid.
“We achieved some very promising results with materials that have a far greater lifespan than conventional types of concrete,” said Florian Mittermayr of the Institute of Technology and Testing of Construction Materials at TU Graz. “Use of these long-lasting materials would allow operators to refurbish damaged wastewater systems, significantly extending their service life and reducing the financial burden.”
Read the researchers’ latest findings on MICC prevention in the May 2018 issue of Water Research.
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