Microbial Induced Corrosion (MIC) in wastewater collection and treatment systems is a severe and on-going problem in the United States. A 1991 US EPA report to congress cites a national cost, in 1991 dollars, for sewer rehabilitation at $6 billion. In addition to the direct cost of replacing corroded infra-structure, there are hidden costs in lost time and labor which are diverted to emergency and repair activities, which are not spent on core wastewater collection system operations.
Evoqua engaged in a study to determine the impact of hydrogen sulfide concentration on the strength and integrity of concrete in wastewater collection systems.
Over a period of two years concrete test samples were deployed at the discharges of two force mains. Both force mains were similar in terms of potential for sulfide generation and hydrogen sulfide release. One site was treated with a nitrate double salt solution to prevent the formation of sulfide in the wastewater, while the other was left untreated. Concrete test samples were compared at six-month intervals to assess the impact of hydrogen sulfide gas on the strength
and integrity of the material.
After two years of exposure in a manhole conveying an untreated stream, the concrete samples were exposed to an average of 68.5 ppmv hydrogen sulfide; those samples lost 5.4% of their original mass and 13% of their original strength.
The samples deployed downstream of the treated site were exposed to an average of 3.6 ppmv hydrogen sulfide, and showed a 0.2% reduction in mass and a 9% increase in compressive strength.
The primary conclusion from the trial is that minimal corrosion of concrete occurred at hydrogen sulfide concentrations below 5ppmv.
Treated sample (left) and untreated (right) after 24 months
showing corrosion of concrete.