Water Online the Magazine gives Water & Wastewater Engineers and end-users a venue to find project solutions and source valuable product information. We aim to educate the engineering and operations community on important issues and trends.
Issue link: http://wateronline.epubxp.com/i/79777
Research Trends comprising more than 100 species. Other bacteria will oxidize elemental sulfur to a sulfate, which then reduces to form sulfuric acid: 2S0 + 3 O2 + 2H2) न 2(S2)-² + 4H+ Elemental sulfur oxidized to sulfate by SRB Sulfide-oxidizing bacteria (SOB) will convert hydrogen sulfide (H2S) to a sulfate: H2S + 2O2 न (SO4)-² + 2H With the following intermediate chemical species: SH- न S0 न (S2O3)-² न (S4O6)-² न (SO4)-² Researchers have found that the corrosion of both alloys and concrete in biological systems is many times greater than in the acids themselves. This is not a surprising, considering the complex chemistry occurring within these microbial communities. Thus, it is not sufficient to test a proposed This settles it! material used in sewer construction in sulfuric acid, for instance, when the exposure is biogenic sulfuric acid secreted within a microbial community of complex chemistry. Such an exposure would include oxidizing, reducing, and redox conditions as well as organic and inorganic chemistry. This fact points to the futility of laboratory simulations designed to duplicate the corrosive potential found in domestic sewer systems. If one were to try to culture in a laboratory all the microorganisms found within a sewer, one would quickly discover that only a fraction of the microbes can be successively cultured. Regrettably, the environment found within a sewer cannot be, at least presently, duplicated in a lab experiment, and the only meaningful exposure tests are those carried out within an active sewage collection or treatment system. Consider The Concrete An additional consideration for prolonging the life of a sewer system is the type of cement used to make the infrastructure. For many years, municipalities and their design-engineering firms specified the use of ASTM Type V portland cement for their pipe- lines and manholes. This was done based upon the belief that Type V would resist sulfuric acid better than Type I. Type V is a so-called "sulfate resistant" cement. It is more resistant to alkaline sulfate salts, such as sodium sulfate, magnesium sulfate, calcium sulfate, or potassium sulfate. These are the types of salts often found in groundwater and can cause degradation of Types I and III portland cement. It must be noted that these are alkaline salts. While Type V portland is more resistant to these alkaline salts, it is actually less resistant to sulfuric acid and other acid species, such as the SCFA pre- viously described, than either Type I or Type III. NEFCO, Incorporated 4362 Northlake Blvd, Ste 213 Palm Beach Gardens, FL 33410 Phone 561-775-9303 Fax 561-775-6043 www.nefcoinnovations.com 48 Water Online The Magazine, Wastewater Edition ■ wateronline.com Some researchers have carried out laboratory simulations showing that some cements are more resistant to the biogenic sulfuric acid than other types. For example, calcium alumi- nate cements have fared well in these tests, as well as in actual installations. Some manufacturers have done stud- ies that purport to prove that a cal- cium aluminate cement (CAC) binder blended with a fused calcium alumi- nate aggregate gives superior resis-