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Research Trends within four days of exposure to the concrete. At high pH levels, H2S forms HS- or S2- ions. These serve to allow more H2S to enter the condensed water layer. As the pH of the concrete decreases, the H2S concentration increases. In the presence of oxygen, the H2S reacts to form elemental sulfur and partially oxidized sulfur species. (See equation below.) CO2, which is also ubiquitous in the sewage system, also dissolves in the water vapor condensate that accumulates above the sewage to form carbonic acid (H2CO3). These acids combine to reduce the pH of virgin concrete to ~8 to 9. It is felt by most cement technologists that the primary acidification reaction is due more to the carbonate formation. Oxidation of H2S to S by autotrophic bacteria 2H2S + O2 न 2S0 + 2H2O At this pH level, certain fungi and bacteria can begin to proliferate. Fungi grow best where there is an abundant supply of decaying, preformed organic matter and plenty of moisture, although they are desiccant resistant and will grow at humidity levels as low as 65%. Most fungi are saprobic, meaning they obtain their nutrients from dead organic matter. They are described as being chemoheterotrophic, or more specifically, chemo organotrophic, i.e. they obtain their energy from oxidation of organic matter. Fungi secrete specific enzymes onto their nutrient source. Enzymes are designed to facilitate one very specific chemical reaction. The enzymes break down the nutrients into smaller molecules and the fungi absorbs the digested meal. Most fungi grow at a pH of 5 to 6. Most fungi species grow best at a temperature of 25°C (77°F), except pathogens, which grow best at body temperature, 37°C (98.6°F). Various fungi also secrete several short chain fatty acids (SCFA), such as acetic, citric, formic, butyric, glutaric, propionic, oxalic, and lactic acids. These organic acids will attack alkaline substrates such as concrete as well as susceptible metallic components. It appears that the enzymes, even though acidic, do not participate much in the corrosion process, as they are very specific reaction facilitators. Assessing the overall corrosion process of concrete in a wastewater environment must take into account the presence of these fungi and their acidic secretions. As noted earlier, the reaction of water, hydrogen sulfide, and carbonic acid will reduce the pH of virgin concrete. Raw, i.e. untreated, sewage has a near neutral pH of 6 to 8.5, which is considerably more acidic than the concrete. Exposure to raw sewage will further serve to decrease the pH through direct neutralization of the free lime in concrete and through demineralization of alkaline chemical species within the concrete by the sewage. Fungi will grow in a pH range of ~4.5 to 8.3. As the pH drops below 8.3, fungi of many genera and species begin to grow. See Table 1 below for a partial listing of fungi found in sewage. As these organisms proliferate, the SCFAs and enzymes secreted will attack concrete (and susceptible metallic alloys) further decreasing the surface pH. Table 1. Partial listing of fungi associated with municipal sewage. Alternaria Aspergillus Aureobasidium Absidia Botrytus Candida Cephalosporium Chaetomium Cladosporium Coniothyrium Cryptococcus Epicoccum Epidermophyton Fusarium Geotrichum Gliocladium Gliomastix Monialiales Mucor Paecilomyces Penicillium Phialophora Phycomyces Phoma Prototheca Pyrenochaeta Rhiocladiella Rhizopus Rhodotorula Sepedonium Septoria Torulopsis Trichoderma Tricophyton Tricosporon Verticillium As the substrate pH decreases, conditions become more favorable for acidophilic and acid-secreting bacteria. The formation of the biofilm begins with the initial approach and attachment of the bacteria in a random pattern. The second phase is consolidation and is indicated by the appearance of microbial communities. The final stage is maturation, which is characterized by cells embedded in a matrix of expolymers where the distribution of the microorganisms is well-established and biodiversity increases significantly over earlier stages. The expolymers, comprised primarily of polysaccharides, serve to anchor the biofilm and to stratify the biofilm community. These stratified communities serve to control component concentration (O2, P, S, etc.,), density, absorption, diffusion, and the biomass activity. It is at this point, where acidophilic and acid-secreting bacteria, such as Acidithiobacillus thiooxidans, begin to dominate. Sulfur-oxidizing bacteria, such as Desulfovibrio, oxidize elemental sulfur and sulfides to a sulfate. (See equations below.) Sulfate-reducing bacteria (SRB), such as Acidithiobacillus thiooxidans, then convert sulfates to sulfuric Deteriorated concrete is shown in a wet-well. acid (H2SO4). It is common in microbial communities to find such mutualism, where the changes produced in the local environment by one microbe's metabolism serve to facilitate the growth of another species, which returns the favor. Mutualism has been observed in microbial communities wateronline.com ■ Water Online The Magazine, Wastewater Edition 47