Selecting Materials of Construction to Resist And Control Biogenic Sewer Corrosion

The estimated cost of corrosion damage to the United States wastewater infrastructure is over 2.1 billion dollars annually. This is based upon the rehabilitation cost of known corroison and is a low estimate if ALL corroison damage is included. This high cost places a significant strain on the maintenance budget for most utilities. The constantly wet environment in sewers containing a variety of cleaning products, chemicals and industrial discharges can present an aggressive corrosion environment for any sewer, but combined with hydrogen sulfide produced in the sewers by the sewage itself the corrosion can be devastating.Hydrogen sulfide gas (H2S) is known to be the most corrosive compound found in sewers. This is not because the H2S itself is particularly corrosive but because of what happens to it in the sewer environment. Researchers over the years have discovered that specialized bacteria in the genus Acidithiobacillus thrive on the walls of sewers and other wastewater structures, where they consume H2S gas and secrete a 7% sulfuric acid solution as a waste product. With high concentrations of H2S, these bacteria can rapidly drop the pH of concrete and other sewer surfaces to 0.5 or less, resulting in rapid loss of concrete and structural stability. The acid from these bacteria can also quickly destroy and damage ferrous and non-ferrous metals if not protected.Many methods of dealing with the corrosive impact of H2S, and the acid it produces, have been tried with varying degrees of success. Chemicals are often added to remove H2S before it is generated in the sewer, while other approaches cover the exposed and sensitive sewer surfaces with coatings or plastic liners. Some sewer pipes are plastic and naturally inert to acid. There is also a myriad of concrete coating materials of many different formulations and application methods.This presentation discusses the common products that are used for corroison control and protection and examines them for their “true” corrosion resistance value, including concrete, gunnite, high-alumina cements and other cementitious products. As concrete is the most common sewer construction material the presentation examines concrete mix designs that have been found to provide sewer corrosion protection, including special admixtures and additives touted to control concrete corrosion. Even the aggregates used in concrete impact the rate of corroison under certain situations which are examined. The presentation also includes analyses of metals and specific grades of stainless steel that can resist the lowest pH ranges of the bacteria without damage. The presentation includes an innovative process on how to use various inert materials to actually remove H2S gas from the sewer without damage.To aid the utility professional in their quest for corrosion abatement, Mr. Joyce presents a detailed examination of the lifecycle of Acitidhiobacillus in sewers and how they use H2S gas. He buildss upon the acid generation process by including technical assessments of common corroison protection chemicals, concrete admixtures, coatings and liners. The presentation stresses the proper evaluation of the corrosion potential and selection of appropriate materials for both rehab and new sewer construction.Armed with this detailed information Mr. Joyce explains how to manipulate the sewer environment to achieve both odor and corrosion control using innovative applications of standard products. With a detailed understanding of how the sewer corrosion process works and how corrosion resistant materials respond, a utility manager is better able to focus their limited maintenance budget on the products that work, and how to select the proper corrosion-proof materials for future capital sewer construction projects and avoid future sewer corrosion.