Corrosion Management of Anaerobic Digesters and Sludge Tanks

By Marina Silva and Tom Belli | January 03, 2019

It's no surprise that biomass energy is growing in popularity, as it satisfies two main needs of the modern generation—waste reduction, and sustainability. According to the International Energy Agency, biomass energy generation is forecast to expand as planned projects come online. The global installed capacity for biomass generation is expected to reach close to 140 gigawatts by 2026, which will be fueled primarily by expansion in Asia, using residues from food production and the forestry processing industry.

Like most industrial assets, some elements of a biomass plant can be affected by corrosion. The fermentation and oxidation processes used by a biogas plant’s anaerobic digesters, whether concrete or steel, create a perfect recipe for corrosion, both at the gas and liquid phase. Internally, steel digesters are typically protected by a glass lining. Sludge tanks face similar corrosion issues and maintenance challenges.

Steel Digesters
Glass-lined steel provides superior corrosion resistance to acids, alkalis, water and other chemical solutions (with the exception of hydrofluoric acid and hot concentrated phosphoric acid). As a result of this chemical resistance, glass lining can serve for many years in environments that would quickly render most metal vessels unserviceable.

At higher temperatures, glass is not as effective against alkalis, where an increase by 10 degrees Celsius (18 degrees Fahrenheit) can double the rate of attack on glass. Of course, as a glass lining provides protection to the extremely aggressive environments, its costs are directly proportional. In addition, they are very susceptible to impact damage and the repairs can be very costly.

Application errors and mechanical damage during installation can result in pinholes within the glass lining. Aggressive chemicals produced during the anaerobic digestion process then attack the steel, and corrosion can set in very quickly both at the liquid and gas phase.

We see defects the size of 1 centimeter in diameter, which quickly go through-wall, leading to the escape of liquid digestate or methane.

The biogas produced by a digester consists primarily of the combustible gases methane and carbon dioxide. At normal atmospheric conditions, the volume concentration of methane-in-air required to form an explosive mixture is between 5 lowest explosive limit and 15 percent upper explosive limit. This can be readily ignited by sparks from welding and maintenance operations, or even by static electricity discharge.
Potentially explosive atmospheres mean that welding repairs are out of the question. In addition, welding could damage the internal glass lining leading to its disbondment. Therefore, solutions that are applied and cure at ambient temperatures need to be considered. One hundred percent epoxy-paste-grade grade systems can be used to patch repair defects in the glass lining on the inside, or to bond plates on the outside of the AD tanks to seal live leaks.

Epoxies have long been known for their excellent adhesion to a variety of substrates, including both steel and the glass linings. The absence of solvent within the polymer matrix also makes them safe to use in confined spaces and prevents shrinkage during cure. Epoxy composites, such as Belzona 1111, are supplied as a two-part system with base and solidifier stored in separate modules. Once mixed, the hardening process starts and the material ultimately cures sealing leaks or preventing any defects from going through-wall. By their very nature, epoxies do not corrode and will protect the steel from any further corrosion attack.

In terms of using epoxy adhesives to restore integrity of the assets, prefabricated composite patches are now being considered in place of metal plates. The benefits of using a completely nonmetallic solution ensure the repair is lightweight and has zero potential for corrosion. This application is becoming a preferred choice for water treatment sludge tanks, among others, and may be considered for anaerobic digester maintenance.

Concrete Digesters
In concrete anaerobic digesters, complications can arise for one of three reasons: a cold start, as a new digester can be prone to corrosion before the necessary bacteria and conditions become established; having been out a service—anaerobic conditions are lost with the introduction of oxygen when offline, upsetting the pH and kick-starting corrosion; and the presence of volatile acids that cannot be effectively digested lowering the pH.

When these complications arise, the areas they impact the most are in the biogas phase, as well as at the surface of the effluent substrate.
At both points, it is the presence of moisture reacting with contaminants, such as hydrogen sulphide (H2S) that generates highly corrosive sulphuric acid. This was the issue affecting a digester at a sewage treatment plant in in southern California. The facility was suffering from severe corrosion occurring in vapor space at the top of the tank on the walls and the underside of the roof. In addition, the internal pipework was being attacked by corrosion.

The asset operator had looked at alternative coatings, previously selecting a coating system that lasted only two years. In search of something more permanent, they were introduced to Belzona 5811 (immersion grade), which has been implemented successfully in similar scenarios, with proven longevity. Since the installation of the coating in 2008, the vendor has periodically returned to inspect its performance. In 2012, the application was reported to be performing well and touch-ups to the coating have been carried out since to keep it in service until present day.

 Waste management industries are both expanding and aging around the globe. Assets like anaerobic digesters require corrosion management strategies that extend their lifetime, as well as maintenance-free periods. Nonmetallic solutions and high-performance coatings, in turn, are well-positioned to deliver benefits to the asset owners and operators.

Marina Silva
Marketing Supervisor, Belzona

Tom Belli
Marketing Executive, Belzona

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