Sunday, June 16, 2024

Understanding Biogenic Corrosion

Imery’s SewperCoat® has been the solution of choice for asset owners to protect their structures from H2S biogenic corrosion

Around the World, deterioration of wastewater collection infrastructure due to H2S biogenic corrosion is a serious problem for owners and operators. Wastewater collection infrastructure such as gravity pipes, manholes, tanks, lift stations, control structures, and force mains are 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 sulphuric acid H2SO4 which reacts with the liberated calcium hydroxide with the 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.

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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 sulphur onto these surfaces. This elemental sulphur is a food source for naturally occurring bacteria present in the sewerage system. These bacteria use H2S gas as a nutriment, in presence of oxygen from air and the by-product of the bacteria’s digestion process is sulphuric acid. It is this sulphuric acid that is corrosive to wastewater structures, not the H2S gas itself.

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 the world as treatment plants are commonly several miles from the city centre, 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 neutralising 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+ ions 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 ‘neutralisation capacity’ of a calcium aluminate. This neutralisation 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% to 35% calcium aluminate cement, with the remaining 65% to 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% to 30% cement portion will have the ability to neutralise 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 neutralise acid and inhibit bacterial activity.

SewperCoat® is a cured-in-place, spray-applied cementitious liner that, it is claimed, delivers a unique, proven, and radically different trenchless repair strategy, trenchless repair with an easy-to- apply cementitious system able to resist the most severe biogenic corrosion conditions. SewperCoat® bonds well to moist surfaces, and provides rock solid structural rehabilitation within a few hours.

Calcium aluminate cement has been used to protect sewer structures since the 1950’s when it was applied in Perth, Australia. South African pre-casters have been lining precast pipes with calcium aluminate cement since the 1960’s. Since the late 1970’s Ductile Iron
Pipe (DIP) for wastewater has been lined with calcium aluminate mortar which became the EN standard. In 1991 the Sewpercoat® brand was born and it was applied for the first time to rehabilitate a manhole at the Hampton Roads Sanitation Department in Virginia, USA. In the UK, calcium aluminate cement was selected by Wessex Water to rehabilitate Shaft 13 at the Coastal Interceptor Sewer (CIS). After rehabilitating a few smaller assets to conduct due diligence on calcium aluminate cement, shaft 13 was scabbled back to the substrate, shotcreted by a make-up layer of SRPC concrete, and over-coated by a 40 mm veneer of Calcium aluminate cement for protection. Since 1991 SewperCoat® has been the solution of choice for asset owners to protect their structures from H2S biogenic corrosion around the world.

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