Control concrete sewer corrosion via the crown spray process

The crown spray process involves the spray application of a high‐pH mixture to the crown area of the sewer. The aim is to deactivate the sulfur‐ or sulfide‐oxidizing bacteria (SOB) and neutralize the acid at the sewer crown; to render the sewer crown environment unfavorable for regrowth and recolonization of SOB; to leave a residual alkalinity on the sewer crown to neutralize the acid produced as the SOB recolonize the sewer, and, consequently, control the crown corrosion process. A number of different chemicals, both hazardous and nonhazardous, were investigated for use with the process. The use of biocides as additives to enhance the deactivation potentials of the various treatment mixtures was also evaluated. To gauge the effectiveness of the treatment, surface pH of the sewer crown is measured before and after treatment. Immediately after spraying the surface, pH of the crown generally increases from less than 2 to greater than 9 and then gradually decreases with time to background levels. The mechanism of deactivation and regrowth of the organisms is discussed. Magnesium hydroxide slurry, a nonhazardous chemical, is the most effective treatment chemical tested to date. Application of a 50% slurry to the crown of a highly corroded active sewer has maintained the surface pH at or greater than 9 for approximately 9 months. Microbiological assays indicate a reduction in population of SOB from 107 to 102 cells/g of corrosion product (below detectable levels). Approximately 9 months after treatment, the number of SOB are still 1 to 2 orders of magnitude below the background population level. The operational aspects, economics, and the mechanism of the crown spray process are discussed. The data indicate the crown spray process is the most economical corrosion control technology of eight other methods previously evaluated by the districts.