THE PROBLEM
During an environmental audit the clients effluent pit had been identified as suffering from degradation after over 40 years in service without any significant maintenance.
Effluent Pit Lining, Cambridgeshire
THE SOLUTION
Given the tanks age and below ground construction we proposed the application of Sika Sikagard M790 as given the risk of external ground water and the risk of osmotic blistering if an impermeable tank lining were to be applied.
What causes osmotic blistering in below ground tank linings?
Osmotic blistering in below-ground tank linings is primarily caused by the movement of water or moisture through the lining system due to differences in concentration gradients, leading to the accumulation of water beneath the coating. Here’s a breakdown of the key causes of this phenomenon:
- Water Ingress and Permeation:
Moisture from soil: Below-ground tanks are typically exposed to moisture from the surrounding soil. If the tank lining is not fully impermeable, moisture can permeate through micro-cracks, pinholes, or areas where the coating is thinner or has voids.
Groundwater pressure: The pressure from groundwater, especially in regions with high water tables, can drive moisture into the tank lining system.
- Osmotic Gradient:
Salt Contamination: Osmotic blistering often involves the presence of soluble salts (such as chlorides, sulphates, or other contaminants) either in the substrate (tank surface) or within the lining material itself. When moisture comes into contact with these salts, it forms a solution with a higher concentration than the surrounding water in the soil or groundwater.
Osmotic pressure: Water will move from an area of lower salt concentration (external environment) to an area of higher concentration (beneath the coating) to balance the osmotic gradient. This causes water to accumulate under the lining, leading to pressure buildup.
Following draining down and cleaning of the pit our concerns were confirmed as we witnessed significant ground water ingress.
Before any preparation works could commence we had to resolve these leaks by method of resin injection using Sika Inection – 107 foaming polyurethane resin. This forms a permanently elastic seal for waterproofing cracks and voids.
Once active leaks had been resolved all surfaces to be coated were prepared by method of vacuum controlled diamond grinding to profile and ensure a clean surface for optimum adhesion of the new tank lining.
The pit internals were then tanked using SikaTop 586 Seal – a cementitious waterproofing system that also remove the irregularities in the brick courses to provide a surface suitable for tank lining application. This was left with a brush finish to promote adhesion of the resin elements of the tank lining. This was allowed to set and hydrate naturally.
Prior to and during application of the resin element of the tank lining climatic monitoring was instigated in order to ensure the correct climatic conditions were present as per the manufacturers recommendations – part of our standard quality assurance.
Wet film thickness were taken during application as per correct tank lining and protective coating practice to ensure that the required thickness was achieved.
Once cured all coated surfaces were tested for porosity using a DC holiday spark tester to identify pin holes in the new tank lining. A single pin hole in an aggressive effluent environment will allow aggressive chemicals such as hydrogen sulphide and biogenic sulphuric acid to reach the substrate and attack recommence, undermining the new lining.
Any pin holes identified were then made good using the same material applied by brush.
Our team received a commendation from our client about their professionalism whilst undertaking these works, which at times given the extensive water ingress were at points particularly challenging.