Rainwater harvesting systems are a recognised technique for reducing reliance on potable mains supply and as a potential way to save money. However, the widespread uptake of these systems has been slow partly due to the level of uncertainty surrounding hydraulic and financial performance. Current methods of assessment tend to be simplistic, using generalised rather than site-specific data. Often little account is taken of financial issues other than capital costs, such as operating/maintenance and decommissioning expenses.

To overcome these shortcomings, a computer based modelling tool called RainCycle© was developed that has the capability to perform more detailed analysis than is possible with existing methods. The program includes a detailed hydraulic model of a typical rainwater harvesting system and also has the ability to explicitly account for all major costs associated with these systems.

Modelling a number of proposed designs revealed that many of the current methods overestimate the hydraulic efficiency and potential cost savings that are achievable. However, it was found that water and monetary savings are still possible under favourable conditions. Further, it was found that capital, maintenance and required mains top-up water account for the majority of the whole life costs, whilst energy and decommissioning costs represent a small fraction of the required lifetime expenditure.

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