Interceptor Install Case Study
Project Background
We were approached by a national infrastructure contractor after concerns were raised as to the compliance of an interceptor located on a utility project and the validity of an assessment that had already been conducted.
A national environmental consultancy firm had previously been commissioned to inspect the interceptor and whilst they had rightfully identified that the interceptor was not fit for purpose, they had misidentified the faults of the interceptor.
There had been very little maintenance conducted on the interceptor and the assessment conducted by the environmental contractor had classified the interceptor as a 7 chamber, brick built, class II, bypass interceptor. Their report concluded that it was not fit for purpose due to the fact it discharged to a storm water system and therefore needed to be replaced by a class I interceptor.
GPT were contracted to re-assess the interceptor’s suitability and compliance with current British Standards and the site utility operator’s own technical specifications. We conducted an integrity inspection of the interceptor and identified several issues making the interceptor non-compliant with the British Standard BSEN 858 as well as the site operator’s technical standards.
During the inspection the interceptor was found to actually be 2 separate 4 chamber interceptors end to end with a shared final chamber. Not a 7-chamber interceptor as thought by the previous consultancy firm. The 2 brick built interceptors were class II systems meaning that it was not suitable for discharging to a storm water system.
The interceptors were constructed from unlined engineering brick. Failure points were identified throughout the internal brickwork surfaces which our assessor believed could be allowing oil to escape from the interceptor and leaching to ground causing contamination in the soil surrounding the system.
The interceptors also had bypass system installed which should only be used in low risk catchment areas. Due to the high risk nature of the catchment area a bypass system was non-compliant.
Our recommendation following the inspection was to replace the interceptor with a prefabricated, class I, full retention interceptor; as well as undertaking drainage alterations to link the 2 drain lines upstream.
Interceptor Install
We specified and supplied an interceptor which was suitable for the size of the catchment area whilst considering risks within the area and site discharge requirements.
The first stage was to remove the existing interceptor, this was broken out and sent for disposal. As suspected due to the existing system not being fit for purpose it had allowed oil to escape the system and contaminate the soil surrounding the interceptor. Our inhouse team of contaminated land specialists were able to conduct investigative sampling that confirmed that there was hydrocarbon contamination present. However waste classification in line with Technical Guidance WM3 determined that the contamination levels were below the hazardous threshold so the contaminated soil could be treated as non-hazardous reducing costs significantly.
Once the remediation was complete, excavation for the new system was undertaken. Utilities drawings had been provided which showed no electrical cables present in the excavation area and our team also followed their safe excavation methodology . Thanks to our safe system of work electrical cables not shown on the drawings were discovered within the excavation area. Worked was stopped and electrical utility engineers confirmed the cables were inactive before removing the cables from the install location. The remainder of the excavation was undertaken using a hand dig methodology.
Once the excavation was completed the interceptor was successfully installed as per the manufacturer’s guidelines.
Drainage alterations were undertaken to combine the 2 previous inlets into 1 shared drainage run.
A prefabricated concrete penstock chamber was installed downstream of the interceptor for use in isolating the site’s discharge point in a spill scenario.
The excavation was reinstated around the interceptor and the interceptor filled in line with the hydrodynamic loading requirements. A concrete wash down bunded area was also installed above the interceptor as per the clients requirements.
Finally the system, automatic closure device and high oil alarm were commissioned. The client was extremely satisfied with the project and had no snagging when the project was signed off.