Mechanisms and Processes of Concrete Corrosion in Sewers

Concrete corrosion in sewers is caused by the combination of chemical and biological processes including sulfide and carbon dioxide generation and partition in wastewater, sulfide oxidation, neutralizing reactions of carbon dioxide, hydrogen sulfide, and its oxidation products (mainly sulfuric acid) with concrete. Wastewater is a sulfate-rich environment with sufficient carbon sources. The metabolism of sulfate-reducing bacteria leads to the formation of hydrogen sulfide in wastewater under anaerobic conditions. During wastewater transport through the sewers, depending on the dissolved oxygen concentrations and pH in wastewater, hydrogen sulfide can be chemically or biologically oxidized in wastewater, or partition into sewer gas in gravity sewers. Due to the alkaline and porous nature of concrete sewer pipes, the hydrogen sulfide reacts with intact concrete and reduces the concrete pH, lowering the concrete surface pH through chemically induced corrosion. The additional outgassing of CO2 from the wastewater further accelerates these processes. The reduction of surface pH facilitates the colonization of sulfide-oxidizing microorganisms on concrete surfaces. Sulfide-oxidizing microorganisms can further biologically oxidize hydrogen sulfide into sulfuric acid, leading to microbiologically influenced concrete corrosion. This chapter describes and discusses the mechanisms of these processes in sewers.