You might say “restoring corroded manholes is easy: just re-line the manhole with a good inert material!” However, there are further considerations if the manhole is to be restored for the longer term.
Expanding a sewerage system to keep up with urban development sometimes has detrimental consequences on existing concrete sewer manholes. Additional hydrogen sulphide (H₂S septicity) can be generated in the original sewers when the newer outlying sewers deliver older sewage from further afield.
In higher concentrations, the presence of H₂S gas in sewers can result in hazardous work environments, odour complaints and accelerated corrosion of assets such as sewers, pump station wells, discharge manholes and treatment plants. These problems generally occur after the infrastructure has been extended. Therefore, operators are typically the front line when the manholes begin to corrode, and H₂S gas is detected.
Western Bay of Plenty District Council and Pattle Delamore Partners Ltd have just completed a two year field trial of four alternative manhole restoration options. The trial aimed to identify the options with the best value for money and the best projected long-term performance in a highly corrosive environment (89 ppm H₂S). We discovered from the trial that retrofit manhole corrosion restorations require a careful design-install approach (compared to new installations).
This paper discusses the very high concentrations of H₂S gas and manhole corrosion faced by Western Bay of Plenty District Council in one of its trunk mains, and how the best restoration solutions were identified and chosen. It also briefly addresses the possible causes of the high H₂S gas levels in the particular delivery main.