Household Rainwater Tanks and Peak Demand Factors Shirley A. Gato-Trinidad* 1 and Peter Roberts 2 *Presenting Author Rainwater tanks have been recognised as alternative sources of water supply in Greater Melbourne, Australia. Various studies reported that rainwater tanks resulted to decrease in usage of potable water coming from the main supply of water. This study determines the water savings from households in Greater Melbourne, Australia and the factors affecting the calculated average water consumption. The effect of household rainwater tanks on the diurnal patterns of water usage and on peak demand factors are also investigated. The study involves 100 households monitored by Yarra Valley Water from September 2010 to April 2012. Analysis of the data revealed that water usage of households with rainwater tanks is lesser than households without. The diurnal patterns showed almost the same peak in the morning due to non consumption from rainwater tanks. However the afternoon peak is lower in households with rainwater tanks due to garden watering from rainwater tanks. R I. Introduction AINWATER tank is an alternative source of water which has been widely practiced in Australia for many years (Eroksuz and Rahman 2010; Tam et al 2009). The Australian Bureau of Statistics (ABS 2010) reported that 1 Faculty of Science, Engineering and Technlogy, Swinburne University of Technology, John Street, Hawthorn, Victoria, Australia 3122, sgatotrinidad@swin.edu.au 2 Yarra Valley Water, Lucknow Street, Mitcham, Victoria, Australia, proberts@yarravalleywater.com.au 1
around 1.5 million of households in Australia had installed rainwater tanks and recognised it as a reliable source of water in 2007. Coombes, Kuczera and Kalma (2002) reported that rainwater tank could produce an annual reduction in mains water from 31 to 144 kl/household per year. Gato-Trinidad and Gan (2011) stated that rainwater tanks contributed to possible water savings of 74 to 139 kl per household per year. While savings on rainwater tanks installation have been studied and reported, only few papers report the effect of rainwater tanks on peak demand factors. Traditionally the design of water supply and wastewater supply systems are based on peak demand factors which are calculated as the ratio of the maximum demand to the average demand. The peak demand usually occurs in the afternoon due to garden watering (Gato et al 2011). With rainwater tanks most households tend to water their gardens with water from rainwater tanks. This will possibly reduce the peak demand which could reduce peak demand factor. As rainwater tanks become mandatory in new house buildings in Australia and with government rebates to install rainwater tanks, the reliance on rainwater tanks for garden watering will increase as well as other household usage such as toilet flushing and laundry. Taking this into consideration, there is a need to review peak demand factors being used for water and wastewater supply systems design for a more cost effective design of these systems. With the objective of determining the effect of rainwater tanks on peak demand factors and on diurnal patterns of household water consumption, the hourly usage of 100 households in Greater Melbourne, Australia under the service area of Yarra Valley Water have been analysed. II. Methodology To achieve the objectives of the study, the following approach have been adopted: 2
A. Data Collection and Analysis The data obtained from Yarra Valley Water includes hourly water usage of 100 households in Greater Melbourne from September 2010 to April 2012. The average consumption of households with rainwater tanks were compared with the average consumption of those without rainwater tanks to determine the advantage of rainwater tanks in terms of water consumption. B. Factors Affecting Water Consumption Factors affecting water consumption were determined and regression analysis was conducted to determine the significance of each factors. Factors considered were household size, garden size and tank capacity. C. Peak Demand Factors To determine the peak demand factors the average diurnal pattern of water usage of households with and without rainwater tanks were developed separately. This enabled the comparison of hourly water usage of those with and without rainwater tanks. From the average hourly water usage and the maximum hourly usage the peak demand factor was determined. III. Results and Discussions The following sections present the results of the analysis of the data in regard to the objectives of the study. A. Households Characteristics Yarra Valley Water monitored the water consumption of 100 households from September 2010 to April 2012. Of the 100 households monitored, 50 households have installed rainwater tanks. Of those with rainwater tanks, 8 have rainwater tanks connected to either toilet or toilet and laundry. 3
The average household size of the 100 households monitored under this study is 3.12 with those with rainwater tanks having an average household size of 3.08 and 3.16 for those without rainwater tanks. This is higher than the average household size for Greater Melbourne of 2.13. B. Average Daily Household Water Consumption Based on the analysis of 100 households it was found that the average household consumption is 358 L/day. Households with rainwater tanks used an average of 349 L/day compared to 368 L/day average for households without rainwater tanks. This result revealed that there is a possible savings of 19 L/day (7 kl/year) per household for installing rainwater tanks. This is very much lower than the savings calculated in previous studies of 31 to 144 kl/household per year by Coombes, Kuczera and Kalma (2002) and of 74 to 139 kl per household per year by Gato-Trinidad and Gan (2011). The average daily household water consumption of households with rainwater tanks with indoor plumbing (connected to either toilet or toilet and laundry) was calculated to be 330 L/day compared to 349 L/day for households with rainwater tanks but without indoor connections. This means that an additional 19 L/day is possible if rainwater tanks are connected to either toilet or toilet and laundry. C. Diurnal Patterns The hourly consumption of the 100 households follow the same pattern of that of previous studies with two peaks, one in the morning and one in the afternoon (Linsley et al, 1992). A comparison of the diurnal patterns for those households with and without rainwater tanks is shown in Figure 1. The patterns are all showing two peaks over the 24 hours. The peak in the morning is created by water use related to preparation to go to work and/or school by the residents while afternoon peak is due to garden or lawn watering. In the early morning, the hourly use is at a minimum of 16% to 25% of the average hourly use before reaching the highest peak around 8 am of 189% of the average hourly use. The second peak in the afternoon occurred around 6 pm which is around 144% of the hourly use. 4
A comparison of households with rainwater tanks and those without showed that the afternoon peak of those without rainwater tanks is higher than those with rainwater tanks. While there is not much difference in the morning peaks and the timing of both peaks, the afternoon peaks of those without rainwater tanks is 1.62% of the hourly average while those with rainwater tanks is only 1.27% of the hourly average. This lower peak for those with rainwater tanks could be attributed to households using water from the tank to water their gardens thus lower demand from the mains. Figure 1. Comparison of hourly consumption of households with and without rainwater tanks Seasonal diurnal patterns were also developed from the average hourly consumption of the 100 households (Figure 2). It could be noticed that all the four seasonal diurnal patterns are all showing two peaks, one in the morning and one in the afternoon with the highest peak occurring in the morning. The summer pattern is in contrast with the average patterns of previous studies (Gato-Trinidad et al, 2011; Linsley et al, 1992) which showed that in summer period the highest peak occurred in the afternoon due to garden watering. The reason for this contrast is due to water restriction in garden watering in 2010 through to 2012. 5
Figure 2. Average Seasonal Diurnal Patterns from 100 Households A comparison of the summer diurnal patterns of households with rainwater tanks and those without rainwater tanks was also undertaken (Figure 3). While it has been shown that water restrictions during this period caused the afternoon peak to be lower than the morning peak, households with rainwater tanks have lower afternoon peak than those without. This shows that households used water collected in rainwater tanks to water their gardens than water from the mains. The similar morning peaks revealed that there is not much difference in households with rainwater tanks and without. This similarity can be attributed to the majority of household with rainwater tanks without indoor connections. The rainwater tanks are used solely for garden watering in the afternoon and not for morning use which are dominated by water use for shower, toilet and other related task in preparation for work or school. 6
Figure 3. Comparison of Summer Diurnal Patterns for Households with Rainwater Tanks and for those without To determine the effects of rainwater tanks in both morning and afternoon peaks, a comparison of diurnal patterns for both households with rainwater tanks with indoor plumbing (connections to toilet and or laundry) was undertaken (Figure 4). The analysis revealed that rainwater tanks connected to either toilet and or laundry reduces the highest morning peak at 8 am from 28.92 L (200% of its hourly average) to 25.37 L (184% of its hourly average). This shows that rainwater tanks connected to toilet and or laundry reduced both morning and afternoon peaks and reliance on mains water supply. 7
Figure 4. Comparison of Summer Diurnal Patterns for Households with and without indoor plumbing D. Peak Demand Factors According to Lucas et al (2010), design criteria for water supply infrastructure are traditionally based on the peak hour demand. Peak demand factor used in determining peak hour demand is calculated as the ratio of maximum demand to average demand from a long-term consumption records. In the absence of long term consumption records, Water Services of Australia (WSAA 2002) provides peak demand factor values. Based on the consumption of 100 households, the peak demand factors are calculated (Table 1). It is interesting to note that the calculated peak demand factors range from 1.69 (summer) to 2.55 (autumn) both from households with rainwater tanks and with connections to either toile and/or laundry. While it is difficult to determine the effect of rainwater tanks on peak demand factors, the calculated values are all below 5, which is value suggested by the WSAA (2002) for areas with population < 2000. The peak demand factor values for households with rainwater tanks especially those without indoor connections are higher than the others. These higher values could be attributed to lower average hourly 8
consumption due to garden watering from rainwater tanks but with maximum (unchanged) morning consumption. The maximum peak in the morning is unchanged but the average hourly consumption is reduced, this resulted to a higher peak demand factor. Table 1. Calculated Peak Demand Factors Average Spring Summer Autumn Winter All 1.90 1.92 1.84 1.96 2.06 Without RWTs 1.85 1.85 1.79 1.79 1.83 With RWTS 1.99 1.99 1.91 2.14 2.26 Without Indoor 1.99 2.05 1.95 2.10 2.49 Connections With Indoor 1.84 2.05 1.69 2.55 2.32 Connections E. Factors Affecting Consumption A regression analysis to determine the significant factors affecting water consumption was undertaken. The parameters considered are; household size, type of washing machine, garden size and installation of rainwater tanks. Based on the regression analysis undertaken it was revealed that that there is a poor correlation between water consumption and the four parameters considered with a coefficient of determination, R 2 of 30%. However, household size showed as a significant parameter having p-value < 0.01. 9
IV. Conclusion Based on the analysis of hourly consumption of 100 households within the service area of Yarra Valley Water in Greater Melbourne, it is hereby concluded that: 1. Rainwater tanks reduced water consumption of up to 19 L/day per household. A further 19 L/day per household can be achieved if rainwater tanks are connected to either toilet or laundry. 2. Rainwater tanks reduced average hourly consumption due to garden watering from the tanks however, peak demand factor increased due to unchanged morning peak. 3. The diurnal pattern of daily consumption with two peaks, one in the morning and one in the afternoon with the morning peak tends to be higher in summer and lower in winter has changed to morning peak higher in all seasons due to water restriction in place. 4. Diurnal patterns of water consumption from households with rainwater tanks have much lower afternoon peak due to garden/lawn watering from rainwater tanks. 5. Diurnal patterns of water consumption from households with rainwater tanks and connected to either toilet and or laundry have much lower morning peak due to toilet or laundry use from rainwater tanks. 6. Household size remains to be the significant factor of average daily consumption. Acknowledgements The data used in this analysis was provided by Yarra Valley Water, Australia. 10
References Coombes, P.J., Kuczera, G. and Kalma, J. D. (2002) Economic, water quantity and quality results from a house with rainwater tank in the inner city. Institution of Engineers Australia. Eroksuz, E. and Rahman, A. (2010) Rainwater tanks in multi-unit buildings: A case study for three Australian cities. Resources, Conservation and recycling, 54, 1449-1452. Gato-Trinidad, S. and Gan, K. (2011) Preliminary analysis of the cost effectiveness of rainwater tanks rebate scheme in Greater Melbourne, Australia. Water and Society, 153, 127-138. WIT Transactions on Ecology and the Environment. Gato-Trinidad, S., Jayasuriya, N. and Roberts, P. (2001) Understanding urban residential end uses of water. Water Science & Technology, 64.1, 36-42. Linsley, R. K., Franzini, J. B., Freyberg, D. L. and Tchobanoglous, G. (1992) Water Resources Engineering, 4 th ed, McGraw-Hill International Editions. Lucas, S. A., Coombes, P. J. and Sharma, A. K. (2010) The impact of diurnal patterns, demand management and rainwater tanks on water supply network design. Water and Science Technology: Water Supply WSTWS 10.1, 2010. IWA Publishing. WSAA (2002) Water Supply Code of Australia, Melbourne Retail Water Agencies edition: version 1, Water Services Association of Australia (WSA 03-2002). 11