Rainwater harvesting systems are commonly used as decentralised systems to reduce the need for potable water from water grid and counter the potential rise in water shortage in the future. Rainwater tanks are an established feature in individual households, with the Queensland Development Code MP 4.2 making it mandatory for all households constructed after 2007. A few studies exist on the life cycle costing of individual rainwater tanks whilst literature review on the life cycle costing for communal rainwater systems are difficult to obtain as it is an emerging rainwater harvesting approach in Australia and across the globe.Communal rainwater harvesting, collection and supply systems are planned and implemented for a group of houses at cluster or development scale. In such systems, the rainwater from individual homes flows through downpipes to a collection system and then to a communal storage tank. The rainwater is then supplied back to homes with or without treatment through a reticulation system based on its intended use. This study was aimed at developing an understanding and knowledge on the economics of scale of communal rainwater tank systems. The whole life cost of individual rainwater tanks was also conducted to compare household rainwater systems with communal systems. A net present value method of economics assessment was applied for assessing the costs for both individual and communal rainwater tanks, with main components comprising of the capital costs as well as the ongoing costs; which includes, maintenance, replacement and operation costs.The study showed that capital costs of individual rainwater tanks are the highest contributing cost component, relative to life cycle costs, followed by maintenance and replacement costs for individual rainwater tanks. Operational cost contributed the least, which is in agreement with another study (Stewart, 2011). Sensitivity analysis on discount rates showed lower rates affecting results more than higher rates.Analysis of contributing cost components for the communal systems showed capital cost to be a major contributor, followed by maintenance cost and, depending on the scale of the households, operation and replacement cost. Diseconomy of scale for pipe costs was observed with increasing households and was counterbalanced to some extent by the economics of scale from treatment units. The result highlighted an optimal housing scale observation between 192 and 288 households and minimum costs occurring at 192 dwellings considering the flat topography of the area and housing density of approximately 20 dwellings per hectare of the development conceptualised for the analysis..A desktop analysis to understand the influence of land topography on overall NPV showed that overall costs would reduce by 14% even with a small slope of 0.5%. Sensitivity analysis carried out on discount rates were in agreement with individual rainwater systems, with lower rates affecting results more than higher rates. However, no change was observed in the optimal scale of a communal rainwater tanks system. Removal of the treatment systems showed an overall reduction in NPV by 4%, at the optimal household level, although further analysis is required to investigate the impact of various parameters on the optimal scale of a communal system.The results presented in this report assists in providing a prediction of costs breakdown for both individual and communal rainwater systems as well as a methodology in carrying out future economics of scale analysis for different scenarios. Final results showed that although individual systems are lower in cost than communal systems, the latter’s ability to provide a source of potable water could outweigh the higher costs involved.
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