Rainfall Catch Efficiency for Domestic Water Supply

Increasing demand on urban water supplies due to increasing population densities is making rainwater tanks a more viable proposition for supplementing domestic water supply. First flush devices are frequently recommended to reduce the amount of faecal bacteria, chemicals and sediment entering the rain tank. First flush devices have a storage volume, which when filled, allows all further runoff to be diverted to rainwater tank storage. A bleed valve empties the reservoir and resets it for the next rainfall event. This study reports on the hydraulic performance of first flush devices fitted to the Healthy Home project on the Gold Coast, which measures water and energy sufficiency in an urban setting (Gardner et al, 2002). Average catch efficiency for a 22,000 litre tank connected to 124 m2 of roof area was of the order of 60% for a 8-month period between Sep 2001 to April 2002. Measured hydraulic performance of the first flush device was used to construct a water balance model running at 10-min intervals to predict the catch efficiency of the rainwater system. Predicted and measured catch was in close agreement with error < 3%. The dominant source of loss is the continuous leakage at approximately 1 litre per minute during the rainfall event. Of the total loss of 40 %, the leakage accounts for about 30%, with the remaining loss of 10% due to the combined effect of initial loss due to roof and gutter interception and a fixed first flush volume of 23.8 litre for the first flush device.