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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