Dear Editor,

I refer to the article “Water safety in above-ground drainage systems in Hong Kong” published in the September 2020 issue of Hong Kong Engineer. The article is well-written as it has attempted to address the drainage problems from a large number of angles. Nevertheless, as a building services engineer, I would like to point out the residual risk of using two-pipe system for drainage. For example, if in that system, the floor drain, bath tub and wash basin are all connected to a common stack, foul gas from the body waste of an infected person or his vomit (in case he vomits into the basin) may still find its way into the indoor environment. This is based on the scenario that when the floor drain is dry, foul gas from the common stack may escape into the room. Thus, the author is of the opinion that if any new buildings are to be designed with a two-pipe system, its primary purpose is to enable grey water recycling. If biological contamination is to be avoided, the floor drains should still adopt the W-trap design commonly found in public housing. Only through this means would we be certain that foul gas would not have a chance to come out from the floor drains.

Ir Dr Eddy W T LAU
Fellow of the HKIE