The collection of rainwater from roofs and its storage and subsequent use make a significant saving in the use of potable water. The capacity of the rainwater store is important because it affects both system and initial costs. The investigation concentrates upon the use of behavioural models to simulate the performance of rainwater collectors. The input data, in time series form, is used to simulate the mass flows through the model, and will be based upon time intervals of a minute, an hour, a day or a month. A preliminary mapping exercise is described that evaluates the accuracy of behavioural models for the sizing of rainwater collection systems using different time intervals and different reservoir operating rules applied to a range of reservoir capacities and collection areas. The preliminary analysis indicated that the yield after spillage (YAS) reservoir operating algorithm based on a hourly time interval could be used as a standard against which other models could be compared and calibrated. The detailed analysis enabled constraints to be proposed for the application of hourly, daily and monthly models expressed in terms of storage fraction, S/AR, where S = storage capacity (m 3 ), A = roof area (m 2 ) and R = annual rainfall (m). Hourly models can be used for sizing small stores with a storage fraction below or equal to 0.01. Daily models can be applied to systems with storage fractions within the range 0.01 to 0.125. Monthly models are only recommended for use with storage fractions in excess of 0.125.
List of symbolsA Roof area (m3) D Annual demand (m3) A Demand (m3) during time interval t ET Water saving efficiency Ml Mains supply makeup (m3) during time interval t 0, Overflow from store (m3) during time interval t Q Rainvater runoff (m3) during time interval t R Annual rainfall (m3) R, Rainfall (m) during time interval t RCS Rainwater collection sizing S Store capacity (m3) t Time interval (h, day, month) V, V_l Volume in store (m3) during time interval t, t -1 YAS Yield after spillage YBS Yield before spillage Y,
For a sustainable urban future, society must move towards the goal of efficient and appropriate water use. Reuse of domestic greywater and rainwater has a significant role to play in this task. In this study, rainfall time series have been used in conjunction with estimates of domestic water appliance usage generated by the Monte-Carlo simulation technique to predict long term system performance. Model results show that changes in the attributes of household occupancy, roof area, appliance type and storage volume affect the water saving efficiency of a single store reuse system. Considering greywater and rainwater in combination, the greatest rate of increase of efficiency with storage size occurs in the range 0-100 litres. Further analysis of small volume storage and reuse indicates that savings of up to 80% of the WC flush water can be made with less than 50 litres storage. However, the collection of rainwater in addition to greywater in a single store reuse system offers little improvement in water saving efficiency. Small volume domestic water reuse systems lend themselves to application in the urban housing environment and therefore offer potential in the move towards a more sustainable city.
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