Relationship between hydrogen sulphide and odour units for London sewage

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. Hydrogen sulphide (H2S) is a dominant odorant for many waste processes and is particularly associated with sewage treatment and is often used as a surrogate ‘marker’ compound to approximate odour levels in wastewater based odours. 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. This relationship, which is presented in this paper, is based on testing samples of sewage at eight sites across London from November 2010 to October 2014. The relationship was derived by comparing the concentration of H2S in the gaseous phase (mg/m3) and laboratory olfactometry evaluation of the odour concentration (OUE/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 (3), other odorous compounds found to be dominant, which became insignificant at increased concentrations of H2S (ranging from0.5-5.0 mg/m3).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 impact of volatile organic compounds (VOCs) on the relationship was also investigated. The derived relationship has been applied to the odour measurement and dispersion modelling studies that were undertaken as part of the TTT project.