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Getting to Grips With... Tank Corrosion

Decades after enclosed digester tanks became the norm for UK wastewater treatment facilities, a previously undetected hazard is beginning to emerge - sulphuric acid corrosion

Coating a damaged digester tank with SPI polyureaCoating a damaged digester tank with SPI polyurea

by Marcus Lockett, Managing Director, SPI Performance Coatings

Between the 1960s and the 1990s, wastewater treatment facilities were encouraged to enclose their digester tanks in a bid to control the release of landfill gases into the atmosphere. In the absence of gaseous oxygen, conditions inside the tank become anaerobic - a completely natural process dramatically intensified by the newly covered conditions.

Whilst bio-fuel is a naturally occurring by-product of the digestion process, corrosion within the head-space of the tank is a common occurrence, affecting both the walls of the structure and the tank lid.

In recent years, however, a far more alarming corrosion hazard has emerged, which if left untreated will dramatically shorten the life expectancy of digester tanks throughout the UK. This hazard is sulphuric acid corrosion – and it can cause tank leakage, devastating asset failure or even contamination.

What is actually happening?

When lids were originally placed on digester tanks, the levels of hydrogen sulphide (H²S) trebled from 500ppm to 1500ppm, which at the time wasn’t considered a hazard.

Hydrogen sulphide is the by-product of sulphur-reducing bacteria found during the anaerobic decomposition of organic waste in a low oxygen environment. The presence of water within the enclosed tank allows the volatile hydrogen sulphide gas to produce sulphuric acid (H2SO4).

This highly corrosive, raw acid is denser than water, which means it sits within the water line, attacking the calcium carbonate within the concrete structure. This aggressive chemical corrosion is far greater than any corrosion experienced elsewhere within the digester and has even been responsible for causing significant damage to enamel coated steel modular tanks.

Why are we only learning about this now?

When lids were placed on digester tanks all those years ago, there were a number of different factors, like moisture levels and tank temperature, which were thoroughly investigated to ensure that the new anaerobic conditions were not hazardous. With so many variables to consider, the increased hydrogen sulphide levels were not seen as a potential risk factor.

Fast forward a couple of decades and the sulphuric acid has begun to take its toll on many tanks, meaning site operatives are beginning to see the effects of this so far undetected danger. For many sites, it is now a race against time to protect concrete digester tanks from irreversible degradation.

What is the extent of the problem?

Digester gas typically comprises 65% methane (CH4) and 35% carbon dioxide (CO2), so it is fair to assume there will be a certain level of concrete corrosion within the gas space. It is also safe to expect some natural degradation beneath the solids within the tank.

Add to this equation the newly discovered, and far more destructive, raw acid damage and the result is a series of corrosion rings moving down the inside of the tank. The most severe damage will be visible right where the sulphuric acid has settled on top of the organic matter.

Due to the infrequency with which digester tanks are emptied, cleaned and inspected, the raw acid corrosion is going undetected for some time. In most cases these tanks would only be emptied as part of plant extensions, meaning the damage is often only identified once leachate is seen escaping through the exterior of the tank wall.

The good news is that many sites have now started to remove the tank lids during planning maintenance, which is making the problem easier to detect.

What’s the solution?

There are a number of ways you can protect a concrete structure from chemical corrosion, as well as other common concerns like dynamic cracking from ground movement and concrete shrinkage cracking during its curing phase which can take in some cases many years. Performance coatings are an increasingly popular choice because they offer fast return to service – with some market leading polyurea coatings being inspection ready in minutes and ready for recommissioning in as little as 24 hours.

Temporarily decommissioning a digester tank is a costly task and will affect the performance of the entire wastewater treatment facility. The good news is that in some instances, with polyurea coatings, the tank can be taken out of service, emptied, cleaned, prepped, coating and tested, in under a week, which is considerably faster than any other form of corrosion protection.

About the author: Marcus Lockett is Managing Director of SPI Performance Coatings, which supplies plural component coatings, flexible ceramics, technical epoxies, primers, top coats, polyurethane foams and floor coatings. For more details on protecting digester tanks and other assets, see
www.spiperformancecoatings.com

Topic: Asset Management
Tags: tanks , corrosion

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