The Anaerobic Digester had previously been lined internally with a membrane system that extended over the top of the tank walls. Unfortunately Hydrogen Sulphide rich gas had been able to enter beneath this membrane, allowing biogenic sulphuric acid to form beneath resulting in attack and degradation of the tank structure.
We proposed the removal of defective concrete followed by reinstatement and protection with a suitable tank lining system. We proposed a choice of tank lining systems depending on the climatic conditions that might have been present at the time of application (February/March).
Due to the climatic conditions at the time of application the most suitable and cost effective tank lining system was Sikagard M790. This material can be applied in ambient and substrate temperatures as low as 5 Degrees Centigrade. By contrast other systems proposed required a minimum ambient temperature of 10 Degrees Centigrade.
This material is an excellent solution for aggressive waste water and digestion environments as it offers excellent chemical resistance and crack bridging, with an expected service life in excess of 20 years.
Initially the heavily degraded thickened sections on the pre cast concrete tank panel externals were isolated using diamond saws. These were then broken off using vibration damped breakers.
Termination points for the concrete repairs on the top of the tank wall were made using diamond saws prior to the removal of degraded concrete by method of high pressure water jetting (10,000 PSI).
All areas to be repaired were primed using Sika MonoTop 1010 applied by method of slurry brush to work the material into the substrate.
Sika MonoTop 612 was then laid ‘Wet on Wet’ into this bonding bridge for maximum adhesion.
The tank internals were then prepared by method of water entrained abrasive blasting. The pre cast concrete panels are extremely hard, with a compressive strength in excess of 70 N/mm, meaning that this was the best method to raise a surface profile for the tank lining to adhere to.
During the preparation works we discovered that the concrete had been stained through its service and to remove this staining required the degradation of the concrete surface that would necessitate further repairs before the tank lining could be applied.
To confirm if this was necessary, areas were blasted to a point that sufficient surface profile was present for adhesion of the tank lining but not removing the engrained staining. Sample areas of Sikagard P770 primer were then applied.
Once cured pull of test dollies were bonded in place and adhesion tests conducted. This confirmed that the staining was not detrimental to the adhesion of the tank lining, with all test results exceeding 3N/mm2.
This standard of preparation was then continued across all areas where the tank lining was to be applied.
Some localised areas were treated using Sika TopSeal 586 cementitious fairing coat to provide a surface suitable for a tank lining to be applied to.
Prior to coating works commencing and during application the climatic conditions were tested and recorded as part of our standard tank lining quality assurance to ensure compliance with the manufacturers recommendations.
This was again allowed to cure overnight, prior to the application of a second coat of Sikagard M790 in a contrasting grey colour at the same application rate as the previous coat. The reason that contrasting colours are used as per best tank lining practice is to aid identification during application and ensure full coverage.
Wet film thickness readings we taken throughout the application to ensure that the correct thicknesses were achieved
As part of our standard tank lining quality assurance the applied tank lining was tested for porosity by high voltage spark testing – correctly called DC Holiday testing. By running a copper brush earthed to a non coated area any breaks in the coating are identified. This is essential in an aggressive environment such as that found in an Anaerobic digester as a single pin hole will allow the attack of the concrete to recommence in these areas.
