Life cycle cost analysis of rainwater tank in a multistorey residential building in Sydney

Rainwater being fresh in nature and easily available has emerged as an important alternative source of fresh water to meet the ever-increasing water demand in Australian cities. There has been notable research on quantity and quality aspects of rainwater but very little on its long term financial viability. This paper presents a life cycle cost analysis of a 75kL free standing rainwater tank in a hypothetical multi-story residential building in Sydney. For this study, items and prices of various components representing the capital, maintenance, repair, replacement and management costs for the tank were obtained from the tank retailers, plumbers, contractors, local councils and standard construction handbooks. The 75kL tank was found to meet the demand for about 70% of the days when rain water is used for outdoor irrigation, toilet and laundry purposes. The corresponding average annual water saving was close to 900kL. The 75kL tank can meet the demand for 99% of the days if rainwater is used for only toilet flushing. The financial analysis revealed that the rainwater tank costed far more than the expected return over its life cycle of 60 years at the current mains water price and current interest regime. Even the ongoing cost alone tended to outweigh the ongoing savings. The estimated cost-benefit ratios are in the range from 64% to 654% for various interest rates and conservative and less-conservative cost estimates. It was noted that lower interest rates and increased water price would make rainwater tanks more financially viable whereas higher interest rates are less favourable.