Problem of Air and Water Backflow in High-rise Building Drainage System

Virus transmission through vertical drainage stack is believed to be one of the hypotheses for the SARS outbreak in the Amoy Garden. After the outbreak a review of building drainage system is triggered in the community. This paper starts with a review on the contemporary building drainage system design in Hong Kong, and after that a field study in a 37-storeys high-rise building is reported. When sulphur hexafluoride tracer gas was injected in a flat in 16/F, the gas was detected at 21 floors above the flat under injection (i.e. 37/F), when a suction force was induced in the room due to the sole operation of exhaust fan. The result allows a better understanding on the risk of contaminated air transmission through building drainage system. In addition, a case study of water back flow at the lower floor of a building is also reported. Observed on the vertical stack design, the excessive bendings at the bottom of the vertical drainage stack was believed to induce hydraulic jump that can push the water in the trap seal back to the building. The study result reveals the importance of a proper design, operation and maintenance of drainage system in order to prevent spread of soil waste and contaminated air within buildings.