Integrated dual-mode roofwater collection system for non-potable uses in the NTU complex

The Republic of Singapore has an annual average rainfall of 2250 mm and a limited land area of about 630 km(2). Almost 60% of the water is imported and hence there is a continuous search for new sources. One such source is the collection of roofwater from high-rise buildings which account for almost 84% of the population. The main objectives of this paper are to study the feasibility of tapping the roofwater from the north spine of the Nanyang Technological University (NTU) Campus and utilizing the water for non-potable uses. A simple input-output model has been used which has a dual-mode system (DMS) of operation that ensures that non-potable requirements are primarily met with by the collected roofwater. In the case study involving a roof area of 38,714 m(2), an integrated collection and supply system has been worked out resulting in delivery of non-potable water for flushing of toilets, laboratory uses etc., The design parameters were optimized leading to a rainwater storage tank that is 41.0m x 15.5m x 4m depth with cut-in and cut-outs placed at depths of 1m, 1.5m and 3.5m from the floor of the storage tank. Incorporation of such a DMS leads to a monthly saving of S$18,400.00 which amounts to about 12.4% of the average monthly expenditure for water.