Around North America, deterioration of wastewater infrastructure due to H2S biogenic corrosion is a serious headache for owners and operators. Wastewater infrastructure is typically constructed with Portland cement concrete. Portland cement is a calcium silicate and its hydration inescapably liberates calcium hydroxide Ca(OH)2. Sewer bacteria excrete sulfuric acid H2SO4 which reacts with the liberated calcium hydroxide according to the following reaction:
Ca(OH)2 + H2SO4 ——> CaSO4 + 2H2O
This reaction produces gypsum and water. In a humid sewer environment, gypsum is dissolved. This ongoing disruptive phenomenon continually leaves a fresh layer of portland cement for attack.
The H2S biogenic corrosion mechanism is a well-known phenomenon but the specifics of the process are sometimes misunderstood. Surprisingly, wastewater itself is rarely corrosive. The corrosion begins with H2S created by the decomposition of the organic materials within the wastewater. This H2S builds in concentration in the areas of laminar flow. The H2S is then released into the sewerage network in areas of turbulent flow (outfall and force main type situations). Turbulent flow can occur in numerous areas of the system, including piping systems, manholes, pumping situations, treatment plants, etc. This turbulent flow causes the dissolved H2S to become an airborne H2S gas. The H2S gas is heavier than air and initially exists above the effluent level, dissolving in the moisture on the concrete surfaces above the flow level. As water is formed by the oxidation of the hydrogen, the H2S gas deposits elemental sulfur onto these surfaces. This elemental sulfur is a food source for naturally occurring bacteria present in the sewerage system. These bacteria, present generally in the slime layer, actually “eat” the elemental sulfur (as a source of oxygen). The byproduct of the bacteria’s digestion process is sulfuric acid. It is this sulfuric acid that is corrosive to wastewater structures, not the H2S gas itself.
Mechanism of sulfuric acid formation
Factors that can enhance this biogenic corrosion cycle include long retention times, high ambient temperatures, flat terrain, and low flow values. With the current growth of outlying suburban areas, feeding into the existing infrastructure of larger metropolitan areas, these factors are becoming increasingly prevalent throughout North America as treatment plants are commonly several miles from the city center, requiring very long distances to transport the effluent.
Contrary to the chemistry of Portland cement, the hydration process of calcium aluminate cement does not produce calcium hydroxide but liberates calcium aluminate hydrates and Al2O3.3H2O “gibbsite”. The gibbsite liberated from calcium aluminate cement hydration is not susceptible to H2S attack. At pH levels above 3.5 the gibbsite is insoluble and blocks the pores of the concrete, protecting it from the ingress of acid. Below a pH of 3.5 the gibbsite contributes to neutralizing the acid at the surface by the consumption of hydrogen ions:
2[Al2(OH)3]3- + 6H+ —-> 2Al3+ + 6H2O
The measure of an acidic pH is a measurement of the molecular concentration of hydrogen ions (H+). Therefore, the more H+ there are in solution, the lower the measured surface pH will be. In the equation above 6 H+ ions are removed from solution making them neutral. This is the “neutralization capacity” of a calcium aluminate. This neutralization reaction releases alumina ions (Al3+) which have an inhibitory effect on the metabolism of the bacteria creating the acid. By removing hydrogen ions from solution, the surface pH is locally raised. The released alumina ions react with the bacteria present to slow their activity. Calcium aluminates act as a Protective – Reactive Barrier, greatly reducing the corrosion of the concrete.
The more gibbsite available, the more corrosion resistant a calcium aluminate based product will be. A typical calcium aluminate mortar contains 20% – 35% calcium aluminate cement, with the remaining 65% – 80% being a natural aggregate system such as silica sand, limestone, granite, etc. While a calcium aluminate / natural aggregate material will perform better than a Portland cement based material, only the 20% – 30% cement portion will have the ability to neutralize acid and inhibit bacterial activity.
SewperCoat® by Imerys is a 100% calcium aluminate mortar (both cement and aggregate system), 100% of the product has the ability to neutralize acid and inhibit bacterial activity. SewperCoat® brings 30 years of proven performance protecting wastewater infrastructure assets against H2S biogenic corrosion.