Severe concrete attack from biogenic sulfide corrosion in the head-space of a grit chamber has removed up to 1½ inches of concrete. Photos: Tnemec
The effects of biogenic sulfide corrosion of concrete are identified by the whitish decomposition products calcium sulfate and ettringite.
Sulfuric Acid Production
Once the H2S gas diffuses into the headspace environment above the waste-water, a sulfur oxidizing bacteria—primarily genus Thiobacillus aerobic bacteria, which colonize on pipe crowns, walls, and other surfaces above the waterline in wastewater pipes and structures—metabolizes the H2S gas and oxidizes it to H2SO4. This oxidizing process can take place where there is an adequate supply of hydrogen sulfide gas greater than 2 parts per million (ppm), high relative humidity, and atmospheric oxygen. These conditions exist in the majority of wastewater systems for at least some portion of the year.
In fact, hydrogen sulfide generation in municipal wastewater collection and treatment systems has always been present. Up until the late 1970s, H2S levels stayed generally below 10 ppm. Since then it has become common to see H2S levels spike well above 50 ppm. Many of the concrete surfaces experiencing severe biogenic sulfide corrosion were measured and found to have a pH of 0.5, which equates to a 7% H2SO4 solution.Concrete Corrosion
The effect of biogenic sulfide corrosion and the formation of a 7% sulfuric acid solution on concrete surfaces exposed to the sewer environment are devastating. Entire pump stations and manholes and large sections of collection interceptors have collapsed due to the loss of structural integrity in the concrete.
Sulfuric acid attacks the matrix of the concrete, which is commonly composed of calcium silicate hydrate gel, calcium carbonate, and un-reacted calcium hydroxide. The primary product of concrete decomposition from biogenic corrosion is calcium sulfate, more commonly known by its mineral name, gypsum. It usually is present in sewers and structures as a pasty white mass on concrete surfaces above the waterline.
Another damaging effect of biogenic corrosion is the formation of a mineral called ettringite, chemically known as calcium sulfoaluminate hydrate. When gypsum or ettringite form, these expansive products increase internal pressure in the concrete and result in formation of small cracks (especially in protective liners). These corroded materials can be removed by the flow of sewage, accelerating the corrosion. This erosion can be particularly evident at the high waterline in sewerage structures where diurnal or other high flows intermittently scour the walls.
Protecting concrete structures against biogenic sulfide corrosion in wastewater collection and treatment systems is vitally important to meeting the structure's anticipated design life (see sidebar on using coatings to reduce corrosion). One effective prevention measure is the use of high-performance lining systems that can resist the permeation of H2S gas and the chemical attack of H2SO4
—Vaughn O'Dea is the technical sales manager, water/wastewater for Tnemec Co. Inc., Kansas City, Mo.