Performance of a large building rainwater harvesting system.

Rainwater harvesting is increasingly becoming an integral part of the sustainable water management toolkit. Despite a plethora of studies modelling the feasibility of the utilisation of rainwater harvesting (RWH) systems in particular contexts, there remains a significant gap in knowledge in relation to detailed empirical assessments of performance. Domestic systems have been investigated to a limited degree in the literature, including in the UK, but there are few recent longitudinal studies of larger non-domestic systems. Additionally, there are few studies comparing estimated and actual performance. This paper presents the results of a longitudinal empirical performance assessment of a non-domestic RWH system located in an office building in the UK. Furthermore, it compares actual performance with the estimated performance based on two methods recommended by the British Standards Institute - the Intermediate (simple calculations) and Detailed (simulation-based) Approaches. Results highlight that the average measured water saving efficiency (amount of mains water saved) of the office-based RWH system was 87% across an 8-month period, due to the system being over-sized for the actual occupancy level. Consequently, a similar level of performance could have been achieved using a smaller-sized tank. Estimated cost savings resulted in capital payback periods of 11 and 6 years for the actual over-sized tank and the smaller optimised tank, respectively. However, more detailed cost data on maintenance and operation is required to perform whole life cost analyses. These findings indicate that office-scale RWH systems potentially offer significant water and cost savings. They also emphasise the importance of monitoring data and that a transition to the use of Detailed Approaches (particularly in the UK) is required to (a) minimise over-sizing of storage tanks and (b) build confidence in RWH system performance.