Wastewater collection and treatment systems are a source of bad odors, the most prevalent coming from hydrogen sulfide (H2S), a toxic and corrosive gas with a characteristic rotten-egg smell. Often overlooked, though, is that H2S is also a precursor to the formation of sulfuric acid, H2SO4, which causes the destruction of metal and concrete substrates and appurtenances within wastewater facilities.
Biogenic sulfide corrosion is a bacterially mediated process of forming H2S gas and the subsequent conversion to H2SO4 that attacks concrete and steel within waste-water environments. Also referred to as microbiologically induced corrosion, biogenic sulfide corrosion is a big concern to engineers and practitioners because it contributes to the considerable cost of renovation of deteriorated sewer networks.
Fresh domestic sewage entering a wastewater collection system contains an abundance of sulfates, SO4=. In the absence of dissolved oxygen and nitrates, these sulfates are reduced by a sulfate-reducing bacteria, identified primarily from the obligate anaerobic species Desulfovibrio to form H2S (via an esoteric reaction). Conditions favorable for producing sulfide within wastewater conveyance/treatment processes are:
- Low dissolved oxygen content
- High-strength wastewater (in terms of biological oxygen demand)
- Low flow velocity and long detention times
- Turbulence/extensive pumping
- Elevated wastewater temperatures.
Sulfide generation is a bacterially mediated process that occurs in the submerged portion of sanitary sewerage systems. It begins with the establishment of a slime layer below the water level, composed of bacteria and other inert solids held together by a biologically secreted protein “glue” or biofilm called zooglea. When this biofilm becomes thick enough to prevent the diffusion of dissolved oxygen, an anoxic zone develops under the surface. A fully productive biofilm layer, complete with anaerobic zone, takes about two weeks to be established in a typical wastewater collection system.
Sulfate-reducing bacteria located within the anaerobic microniche use the ubiquitous sulfate ion, SO4=, a common anionic component of wastewater, as an oxygen source for the assimilation of organic matter. As SO4= is consumed for oxygen, the S= byproduct is released back into the wastewater where it establishes a chemical equilibrium under normal wastewater conditions as 50/50 bisulfide ion, HS-, and dissolved hydrogen sulfide, H2S(aq). Upon turbulence or aeration, wastewater releases the dissolved gas as free hydrogen sulfide gas and more bisulfide ion is transformed into the dissolved gas form to replace that lost to the atmosphere. The rate at which H2S leaves the aqueous phase is governed by the Henry's Law coefficient for that gas.