The Thames Tideway Tunnel (TTT), is one of the main components of the London Tideway Improvements Programme, designed to capture combined sewer overflows (CSOs) which currently discharge untreated combined sewage into the tidal river Thames. CSO discharges would be stored in the tunnel until there is capacity at Beckton sewage treatment works (STW), to accept, for treatment, pumped sewage from the tunnel system. There is at present limited literature information on odour generation from stored combined sewage in CSO control systems; on sampling methods of actual combined sewage and continuous monitoring and sampling of gas and liquid phases of stored combined sewage. To better understand the relationship between odour and stored combined sewage, the TTT project team have designed and operated a septicity rig to evaluate the aspects of odour generation from the tunnel system during storage conditions. The key areas of research include: 1. Odour monitoring to measure the emission rates of H2S during tunnel storage conditions, 2. Assessment of the potential for the development of septic conditions and the generation of sulphide compounds that could cause odour problems, 3. Extensive investigation of the impact of biofilm formation and slime growth on the tunnel lining. The findings of the study have determined the levels of sulphide that could potentially be generated from the CSO tunnel system, during storage operations, to ensure adequate consideration in the design of the air management system and tunnel lining and potential for adverse impact on STW operations.