Sewer pipes and wastewater treatment plants are an unglamorous but critical asset to society. Keeping pollutants out of the environment, they safeguard human life and form an essential part of the infrastructure we all depend on.

However, the corrosive and infectious fluids they corral safely away from us also attack the construction materials that contain them. This is biogenic corrosion.

What causes the problem and how big a threat is it?

More accurately described as ‘biogenic sulphide corrosion’, it is caused by naturally occurring bacteria that generate hydrogen sulphide gas. Humid, enclosed spaces such as wastewater structures provide ideal conditions for this process.

The gas is biochemically oxidised in the presence of moisture to form sulphuric acid, which then reacts with the concrete and steel surfaces in wastewater environments. In the USA alone, corrosion is causing sewer asset losses estimated at $14 billion per year. This cost is expected to increase as the aging infrastructure continues to fail.

In New Zealand we face a similar threat to the environment from aging wastewater assets, which require serious ongoing investment.

The obvious solution would appear to be specialised coatings to protect the concrete exposed to such relentless attack. But this turns out to be harder than it first appears.

The first – and last – line of defence

Almost all the various products put forward over many years to protect against biogenic corrosion use their own ability to resist mineral acids to provide a seamless layer of protection over the underlying concrete structure.

The folly of this approach is that once the seamless barrier is breached then the underlying concrete is vulnerable to biogenic attack through the cracks or holes that develop in the protective barrier. Movement, abrasion and even long-term weathering of these membrane systems mean that ultimately, they fail.

At BBR Contech we have seen examples of poorly applied coatings failing after 18 months with disastrous consequences. Even well-applied polymer coatings after 10 years of service have started to degrade.

Only SewperCoat passes biogenic corrosion test

SewperCoat® works in quite a different way. Based on calcium alumininate cements and aggregates (CAC), this patented concrete product was developed in France to address the vulnerability of polymer-based systems. Because it is a 100% CAC mortar, it avoids the issues of corrosion spreading once the initial barrier is breached.

SewperCoat’s outstanding durability in the most severe sewer environments relies on the unique ‘bacterio-static effect’ of calcium aluminates.

The metabolism of acid-generating bacteria is inhibited on calcium aluminate surfaces, thus maintaining a pH level above 3. Even if cracks form in the overlying SewperCoat layer, it still provides protection to the substrate.

As a cementitious material SewperCoat offers a significant advantage over all other repair options, as it can be applied in wet and damp conditions when other systems need dry environmentally stable conditions. This has been an important consideration on many of the projects we have undertaken where short windows of opportunity of repair and live sewer conditions mean that achieving perfect application conditions for a traditional coating would be an impossible requirement.

BBR Contech is the sole New Zealand distributor of SewperCoat. We have investigated its capabilities and implemented SewperCoat repairs in many high-profile wastewater assets. In our opinion, it is the most appropriate material to mitigate the risk of biogenic corrosion in New Zealand’s critical wastewater management systems.