Controlling the risk of cross-contamination from the building drainage system using the reflected wave technique to identify depleted water trap seals
Kelly, David A.
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The appliance trap seal remains the primary defence against cross-contamination from the foul air present within the building drainage system. As an identified vector in the spread of severe acute respiratory syndrome (SARS) in Hong Kong in 2003, trap seal failure has been confirmed as a significant, and potentially fatal, risk to public health. Prevention of trap seal failure depends upon both good design, to limit the air pressure transients propagated within the system, and good maintenance. However, current maintenance regimes rely solely on visual inspections which is time consuming and often impractical to implement in large complex buildings. This thesis documents the development of a novel approach to system maintenance whereby the threat of cross-contamination of disease is minimised by the remote monitoring of trap seal status. This was approached through the application and development of the reflected wave technique which is fundamentally based upon the characteristic reflection coefficients of system boundary conditions. An extensive programme of laboratory experiments and field trials were carried out to collect transient pressure data which, together with results from an existing mathematical model (AIRNET), developed by the Drainage Research Group at Heriot- Watt University, have been used to validate the proposed technique and to formulate a practical methodology which may be applied to any building drainage system. Automatic system diagnosis, which would in the future allow the proposed technique to be integrated as an automated system test, was provided by the development of the trap condition evaluator (TRACER) program by this author. Incorporating a time series change detection algorithm, the TRACER program accurately detects and locates a depleted trap seal by automatically identifying the return time of the trap’s reflection. The reflected wave technique has been demonstrated as a successful approach to depleted trap identification provided that the wave propagation speed is known and the dampening influence of the junction effect (which can delay the observed reflection return time) are taken into account. The reflected wave technique offers a remote and non-invasive approach to maintaining the building drainage system and provides, for the first time, a diagnostic tool to help prevent cross-contamination.