Proceedings

Abstract

The development of Thames Tideway Tunnel (TTT) aims to capture and convey to treatment combined sewage which presently overflows into the tidal River Thames.  The 25km long tunnel will pass beneath central London.  An important consideration, is the incorporation within the design, of an air release treatment system, to mitigate the potential risk of odour nuisance in the vicinity of tunnel shafts. The TTT team have established H₂S generation and liberation models to estimate the levels of H₂S within the air released from the system after treatment. To support this approach, a pragmatic understanding of the sensory perception of odours likely to be generated by the system has been sought and the relationship between odour and H₂S concentrations, for the effluent samples collected from the TTT catchment, has been investigated. The relationship was derived by comparing the concentration of H₂S in the gaseous phase (mg/m³) and laboratory olfactometry evaluation of the odour concentration (OU_E/m³). The relationship between H₂S and odour units was found to vary depending on the concentration of H₂S that is present. At relatively low concentrations of H₂S, other odorous compounds found to be dominant, which became insignificant at increased concentrations of H₂S. The results of the sampling programme, gave an acceptable correlation describing the relationship between H₂S and odour units for both high and low levels of H₂S.

The paper is focusing on:

• Studies from olfactory testing, to identify whether H₂S is the dominant odour and an acceptable indicator for odour generated in sewage systems.
• Extensive sampling and tests to derive more fully the relationship between H₂S and odour units.
• Studies to assess that the derived correlation can appropriately estimate the odour concentration at release points from a large CSO control tunnel system, to determine treatment requirements, to meet regulatory guidance and develop an air management plan that avoids nuisance odour complaints.