MICROBIAL INDUCED CORROSION (MIC) has been increasingly evident in concrete manholes and related sanitary sewer structures for decades.
MIC occurs when sulfuric acid, generated from raw sewage, reacts with the properties of cement to diminish the integrity of concrete manhole bases and related structures. Hydrogen sulfide (H2S), in anaerobic and aerobic forms, has the capacity to severely damage concrete manhole structures. Utility operators, civil engineers and the precast concrete industry have accepted this as fact.
Since the US Clean Water Act (1980) mandated the elimination of certain toxic heavy metals – lead, chromium, mercury, arsenic, cadmium – from wastewater, effluent MIC has reached acute levels in sanitary and wastewater sewage systems; these heavy metals are toxic to humans and other life forms, including bacteria.
Prior to 1980, heavy metals in wastewater retarded the growth of bacteria in sewer systems, minimizing MIC. Now, bacterial colonies flourish and assist with the generation of H2S gas and, with oxidation, H2SO4. Acidic deterioration of sanitary sewer pipelines and manholes has increased substantially. Coating systems that once protected concrete structures now fail often within a few months. Splash and turbulent water flow exacerbate the destructive effect on concrete manholes.
These new conditions require revised design parameters for many components in the sewage transmission system, including manholes. Liner material, such as Fiberglass Reinforced Plastic (FRP) and Polypropylene (PP), can be imbedded in newly poured concrete to provide a dense, impervious and continuous plastic lining for the manhole base, walls and cover to protect the concrete substrate from destructive acid attack. This deteriorated concrete condition can be prevented with thermoplastic barriers that isolate the sewage from the manhole structure.
As evidenced by the anaerobic process, the eventual disintegration of a concrete-channeled manhole base is the result of a hydrogen sulfide attack. More aggressive corrosion can be expected when septic conditions exist, which may also result in leaking pipe connections. Other contributing factors, such as drop connections or a large number of entry points introduced into the structure, create more acid corrosion problems.