Evaluation of ‘Discharge Units’ for Domestic Washrooms in Hong Kong

Probable maximum drainage loads in domestic drainage stacks in high-rise residential buildings in Hong Kong and other cities are currently estimated using the ‘discharge unit approach’, which is based on the probability of discharging all outlets simultaneously, with the design discharge flow rate of an outlet running at a limiting failure rate. However, direct application of this approach to load estimation without proper consideration of the probable discharge patterns and occupant load variations might not optimize drainage stack design. With recent survey results of drainage discharge patterns in some domestic washrooms, this study investigated the probable maximum flow rate in a stack serving a number of typical domestic washrooms in Hong Kong. The results showed that the occupant load served by an appliance and the simultaneous discharge patterns of the appliances in domestic washrooms had significant influences on the probable maximum drainage demand in the stack. Following the adopted design approach, the equivalent ‘discharge unit’ for a typical washroom was evaluated. The results are applicable to drainage stack designs for high-rise residential buildings in Hong Kong and other cities.