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Research Trends Corrosion In Wastewater Systems: More Than One Bug Already susceptible to old age, wastewater infrastructure systems are also threatened by corrosive bacteria – but they needn't be. by Heather Ramsey, John Davis, and Gary Hall he role of various bacteria in the destruction of concrete in wastewater systems has been recognized since 1945. Parker1 described the role of an acid- producing bacterium that he called Thiobacillus concretivorous, which he had isolated from corroded concrete from a Melbourne, Australia, sewage system. Th. concretivorous was described as a sulfate-reducing bacterium (SRB) T which converts hydrogen sulfide (H2S) and uses thionates, polythionates, and elemental sulfur as sources of energy, and in the process secretes sulfuric acid as a metabolic byproduct. This class of bacteria is collectively referred to as SRBs. The acids they secrete are often referred to as biogenic, as they were produced by a biological process. Th. concretivorous was later reclassified as Th. Thiooxidans. In 2000, Bergey's Manual of Systematic Bacteriology2 reclassified this species as Acidithiobacillus Thiooxidans. Several other microbes that are thought to be involved in the microbiologically-influenced corrosion (MIC) processes were reclassified at the same time. Severe corrosion of concrete in sewage systems has been reported in Australia, Iraq, Israel, Ireland, the United States, the UK, Lebanon, Germany, and Mexico, among others. In many instances, the corrosion rate has been catastrophic, often resulting in total collapse of a concrete appurtenance, pipeline, or other structure. Concrete corrosion rates as great as 10 inches (25.4 cm) in less than four years have been described. With declining federal budgets and tightening local funding, municipalities and sewage authorities are faced with the necessity of protecting their expensive, difficult-to-replace infrastructure. In order to determine the most effective means of protecting these critical assets, it is beneficial to understand the corrosion process, a process called Microbiologically Influenced Corrosion. Recent studies have found that the processes involved in MIC in wastewater collection and treatment systems are more complex and the organisms involved more diverse than originally thought. Further, it has 46 Water Online The Magazine, Wastewater Edition ■ wateronline.com been determined that the microbial species involved require the establishment of synergistic/mutualistic communities, in addition to certain non-biological chemical reactions, in order for the process to proceed. Studies have also confirmed that in addition to bacteria certain fungi are also implicated in the processes. MIC Processes The crown section of this 14-foot tunnel displays severe degradation. Ubiquitous in the wastewater system are microbes, consisting of bacteria and fungi, molds, and yeasts. Some of these bacteria produce acidic metabolic byproducts secreted as waste products onto the surfaces upon which these microbes form their communities. The fungi digest their food externally and secrete enzymes in order to do so. These enzymes are amino acids and can cause corrosion on susceptible substrates. Additionally, fungi, molds, and yeasts secrete short chain fatty acids (SCFA). It is now recognized that the establishment of these synergistic communities is critical to the overall corrosion process. Most articles published on the subject of MIC in wastewater collection and treatment systems have focused upon the role of certain bacteria, notably the Acidithiobacillus thiooxidans. These bacteria cannot cause the observed corrosion without contributions from other species and some inorganic chemistry. Decaying solid waste in sewage gives off H2S gas, which is poorly soluble in water. Sulfate-reducing anaerobic bacteria, such as Desulfovibrio, convert sulfates in the solid waste into sulfides, including hydrogen sulfide. The H2S escapes easily from the sewage with turbulence caused by flow through the sewer system. Points of increased turbulence increase the amount of H2S evolved. The H2S then dissolves in a thin layer of water that condenses on the crown of the structure. This condensate layer will have a high pH due to the pH of the virgin concrete on which it is condensed, which is 12 to 13. Tests have shown that distilled water will obtain a pH of 12.5