Case study of a rainwater recovery system in a commercial building with a large roof

This paper describes a prototype rainwater recovery system that has been fitted within a supermarket. In this system, rainwater is recovered from the roof structure, collected and then used to satisfy part of the domestic cold water requirements of the supermarket (for toilet flushing). The design and commissioning of the system installed is first described. The system has been fitted with a monitoring system, which has been used to record the water usage and to estimate the rainwater recovered for use in the supermarket over approximately an 8-month period. The results are used to calculate the collection efficiency of the system compared to potential gains determined from local rainfall data. This data has also been used to calculate the revenue cost savings for this system. The additional capital cost of this system has been estimated and this has been used to calculate a payback period of 12 years, based upon a collection efficiency of 57.4%. Alternative payback periods for different locations in the UK are also discussed. Finally, this paper considers the effects of alternative roof and tank sizes on system economics. This shows that tank size is critical and that different configurations may be used to achieve payback periods approaching 4 years.