Most structural assets within a WwTW are concrete. A potential risk to them is the attack from hydrogen sulphide (H2S). This will sit on surfaces of the gaseous zone of the structure – the area immediately above the waterline. When the H2S is mixed with water from the atmosphere and oxidises, it forms an aggressive concentration of sulphuric acid. This attacks and dissolves the concrete matrix, exposing further layers of concrete and also corroding the structure’s steel reinforcement.The problem can worsen when covers are placed over a particular structure and the H2S becomes contained, and limited airflow makes for an environment where bacteria can develop. Makers UK Ltd (MUL) has worked with UK water utilities to solve the problem.MUL is the infrastructure division of Keller’s Group. MUL acquired Accrete in 2002, which is a technical solution provider within the area of specialist coatings and structural refurbishment contracting, and has operated solely in the water industry since 1989. Under a recent group rebranding, all divisions within the group will function under
the Makers UK brand.
Preventing H2S attack
The solution to the H2S corrosion problem has been to repair all damaged areas and then, dependent on the area to be repaired, to apply a suitable coating to the internal surfaces – for example a hot-sprayed polyurethane membrane lining.
The benefit of this is that the membrane is rapid curing, resistant to the H2S chemical attack, and contains the properties that provide the flexibility to resist thermal and mechanical movement. Additionally, MUL has a patented corrosion inhibitor that can provide further protection to the steel reinforcement from external carbonation attack.
A major asset-refurbishment contract undertaken by MUL involved working with Southern to extend the operational asset life of eight wastewater tanks at one of its large southeastern facilities.
The problem facing Southern was that the asset was in need of structural repair. Additionally, when filled, it would contain wastewater, giving the potential of chemical attack from H2S in the gaseous stage, which would result in concrete corrosion. The structural integrity of the tank in key positions was also in doubt and needed strengthening.
The options available to Southern were to either demolish and rebuild the tanks or carry out structural refurbishment that would extend the asset life up to the required minimum of another ten years.
MUL has spent two years investigating solutions to H2S attack
and could offer refurbishment that provided Southern Water with a substantial saving over new build.
MUL incorporated the strengthening into its contract by acting as contract manager for the main sub-contract works for structural strengthening, with underpinning the preferred technical solution. The interior to tanks and inlet channels were refurbished with the areas of delaminated concrete being chased out and the damaged areas being concrete repaired. To help cope with the movement of the tanks when in use, MUL induced new movement joints into the structures.
As with many concrete liquid-retaining structures, in their original construction the movement joint areas are sealed with elastomeric mastic, which over a period of time becomes brittle, cracks and subsequently will be a source of leaks. These particular tanks had this problem and to resolve it the joints were waterproofed with a Hyperlon-over-banding system, which will flex within the structure’s natural movements.
To provide the main protection against H2S attack, all the eight tanks and inlet channels have been coated with hot-sprayed polyurethane. This provides a coating that protects potential areas of weakness from a variety of problems including chemical attack, thermal movement, mechanical movement or any other physical attack – for example from grit contained within the process.
To finalise the project, all scum boards and the associated securing brackets, which had also been attacked by corrosion, were renewed.
The project ran over 12 weeks and MUL worked in a three-way partnership with the incumbent framework contractor Morrison Brown and Root (MBR) and Southern Water.
The relationship has been very successful and the project completed within budget and programme with MUL, Southern Water (4D) and MBR together investigating further asset-refurbishment projects.
MUL is also a reservoir-refurbishment specialist. A reservoir environment will contain a large volume of steel pipe work of varying sizes. Due to the environment, this pipe work requires maintenance to remove external corrosion to maximise the pipe and valves asset life. MUL has worked with many of the UK water utilities on maintenance of the steel pipe and valves contained within reservoirs and pump houses.
Due to the aggressive nature of these types of environments, a specialist contractor with specialist knowledge of application of the coatings must be used to refurbish the external coatings on all the steel pipe work within the reservoir and valve chamber.
MUL has a partnering philosophy, which is applied throughout their supply chains. In both cases of the prementioned projects, this philosophy demonstrated many benefits in ensuring successful completion. Additional benefits were that MUL’s health and safety officers regularly visited the sites with the client’s team to ensure a maximisation and collaborative approach to all health and safety and environmental issues.
MUL also recommends that, prior to any structural work, a full structural survey be carried out. And, for this purpose, MUL has an in-house testing team.
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