Sewage Holding Tank Falls Correction, Nottinghamshire

THE PROBLEM

The thermoplastic sewage holding tank had been installed incorrectly, with the tank falling away from the sump. This was resulting in sewage becoming septic resulting in unpleasant odours on site and additional frequent cleaning and tanker costs.

THE SOLUTION

We were contacted and asked to develop a solution that would correct the falls within the tank. A particular challenge when working with thermoplastics is gaining adhesion as these materials have a low surface energy which make them challenging to bond to.

Following pumping out of the tank as far as possible we employed a specialist confined space cleaning company to conduct a tank entry clean under full breathing apparatus with a rescue team present. Given the known issue of septicity it was highly probable that high levels of toxic hydrogen sulphide would be encountered during cleaning operations.

Once clean and dry all surfaces to be adhered to were abraded using 110V sanders to raise a surface profile. Following preparation all surfaces were vacuumed clean to prevent dust disrupting the bond to the plastic substrate.

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Prior to and during all material application climatic monitoring was instigated and heating introduced in order to ensure compliance with manufacturers recommendations.

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To gain adhesion to the plastic Corrolastic SG was applied by slurry brush. This polymer rich slurry is one of the few materials that will bond to thermoplastics such as HDPE and PVC.

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Once cured all previously coated surfaces were primed using a damp tolerant epoxy primer and a quartz aggregate broadcasted into the wet resin to create a mechanical anchor for the subsequent polymer modified screed to adhere to.

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Any loose our unbound quartz was removed using soft brooms and vacuums.

Falls were then installed using a polymer modified screed so that sewage would flow towards the sump as originally intended.

To accelerate the incoming flows a steeper ramp detail was installed below the inlet using a 10:1 epoxy mortar.

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The polymer screed was allowed to hydrate prior to the application of coating materials. This moisture content was tested prior to the application of any coating materials to ensure proper adhesion of the tank lining materials.

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All surfaces were primed using a moisture tolerant polyurethane primer, followed by the application of a solvent free crack bridging polyurethane tank lining to a nominal thickness of 1000 microns.