Study of rainwater tanks as a source of alternative water supply in a multistorey residential building in Sydney, Australia

Australia, the most urbanised population in the world, is also the driest inhabited continent with the most variable rainfall. Although Australia has the highest per capita dam storage volume in the world, reliability in water supply in Australian cities has come to question in recent years. Water has become a national issue in Australia with ever-increasing interest by politicians, media and the general public. The current storage level of the reservoirs of Sydney, which is the largest city in Australia with over 4.5 million people, is near 30%, which has resulted in the placement of Level 3 mandatory water restrictions. Sydney Water, similar to other water authorities in Australia, is desperately looking for alternative sources of fresh water including rainwater tanks in addition to recycling grey water, wastewater and the use of desalination plants. Rainwater is an alternative fresh water source that has received widespread attention in Australia in recent years. Although a rainwater tank can provide a significant volume of water to individual households and quality may not be a problem for non-potable purposes, the financial viability of a rainwater tank to individual house owners is yet to be established, in particular with the current level of subsidy for the tank provided by the government and with the current water price. There is a general lack of research on long term financial viability of rainwater tanks. This paper examines the viability of rainwater tanks in multistorey residential buildings in Sydney. Two different lot sizes (2000m2 and 4000m2) and three different scenarios were considered: 4 floors with 16 apartments and 48 persons (b) 6 floors with 24 apartments and 72 persons (c) 8 floors with 32 apartments and 96 persons. Also, the current planning method called ‘BASIX’ and the traditional approach (non-BASIX) were considered. The assumed tank size was 75kL. It has been found that is possible to achieve “pay back” for a rainwater harvesting system under some favourable scenarios and conditions. The most favourable financial conditions for the viability of the rainwater harvesting system are the 5% interest rate, the A$1.634/kL water price and the 4.5% water price inflation rate. This scenario produces a cost-benefit ratio of 72%. The worst case scenario is with an interest rate of 15%, an A$1.264/kL water price and a 2.6% water price inflation rate. The worst case cost/benefit produced from this scenario is 578%. The roof catchment size of the rainwater harvesting system also plays an integral part with the 2000m2 roof area outperforming the 800m2 roof area in terms of long term financial viability. The financial viability of a rainwater harvesting system is maximised when the water savings (or the quantity of water utilised from tanks) are high. The water savings increase with the water demand or number of occupants and whether or not the building is BASIX compliant. Thus the two best financial scenarios are for the eight-storey building and the non-BASIX approach. Capital and maintenance costs account for the majority of the expenditure over the whole life cycle cost of a rainwater harvesting system. Plumbing costs form the largest single component of the capital cost. Costs related to pump maintenance and replacement form a significant component of the total expenditure. Rainwater tank systems cannot be financially viable when the total mains top-up required is minimal i.e. when rainwater is utilised less.