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Rainwater Harvesting Systems Could be Cheaper

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The rate of uptake of rainwater harvesting (RWH) in the UK has been slow to date, but is expected to gain momentum in the near future. A number of factors have so far contributed to the lack of progress: ambiguity in the financial viability, lack of experience and the absence of well-run demonstration sites. Although some technical guidance is available, the costing information provided is sketchy and there is limited advice on the appropriate system sizing methods to use. However, RWH is now explicitly mentioned in key government documents such as the Building Research Establishment's Environmental Assessment Method and the Code for Sustainable Homes.

In an article in Water Science and Technology the authors recommend that continuous simulation methods should be adopted, since the simple tools currently used lead to the oversizing of tanks and excessive capital costs.

The authors evaluate the designs of two different new-build RWH systems using three different methods within a modelling tool: (1) a continuous simulation which can utilize daily rainfall and demand time series based on the yield-after-spill approach; (2) a simplified version of the AR/D (catchment area times average rainfall divided by average demand) approach; and (3) a simple rule-of-thumb method recommended by the Environment Agency (EA) which sizes the tank based on a user-defined percentage of average annual rainfall or demand (whichever is the lower).

The two case studies were a university building and a housing development; the RWH systems were shown to meet 46% and 36% of WC demand, respectively. It was found that design methods (2) and (3) overestimated tank sizes. Despite this, it was found that average annual financial savings for method (2) was equivalent to that of method (1). However, payback periods would be significantly longer for method (2) due to the higher capital costs of larger tanks. Continuous simulation (method (1)) therefore provides a better assessment of tank size in terms of cost-benefit analysis for a particular demand. In addition, the authors found that levels of demand met were limited by the catchment area size, which also had implications for financial savings. This indicates not enough consideration is given to the catchment size when designing a RWH system.

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