- Email: [email protected]
Rainwater tanks in multi-unit buildings: A case study for three Australian cities
Resources, Conservation and Recycling 54 (2010) 1449–1452
Contents lists available at ScienceDirect
Resources, Conservation and Recycling journal homepage: www.elsevier.com/locate/resconrec
Rainwater tanks in multi-unit buildings: A case study for three Australian cities Erhan Eroksuz, Ataur Rahman ∗ School of Engineering, University of Western Sydney, Locked Bag 1797, Penrith South DC, NSW 1797, Australia
a r t i c l e
i n f o
Article history: Received 4 February 2010 Received in revised form 18 June 2010 Accepted 21 June 2010 Keywords: Rainwater harvesting system Rainwater collection Potable water savings Multi-unit building
a b s t r a c t Rainwater tanks have become popular in large Australian cities due to water shortage and greater public awareness towards sustainable urban development. Rainwater harvesting in multi-unit buildings in Australia is less common. This paper investigates the water savings potential of rainwater tanks fitted in multi-unit residential buildings in three cities of Australia: Sydney, Newcastle and Wollongong. It is found that for multi-unit buildings, a larger tank size is more appropriate to maximise water savings. It is also found that rainwater tank of appropriate size in a multi-unit building can provide significant mains water savings even in dry years. A prediction equation is developed which can be used to estimate average annual water savings from having a rainwater tank in a multi-unit building in these three Australian cities. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.
1. Introduction Australia is one of the driest inhabited continents on earth with highly variable rainfall. Although Australia has one of the highest per capital dam storage volumes in the world, reliability of water supply in most Australian cities is being questioned in recent years due to on-going droughts, climate change and increased public awareness towards water and environment. Water authorities in Australia are desperately looking for alternative sources of water including rainwater tanks in addition to recycling grey water, wastewater and use of desalination plants. Rainwater tanks have been a common water supply system in rural Australia for many years (EHAA, 1999). In 2007, 19.3%, or slightly more than 1.5 million households, reported a rainwater tank as a source of water (Australian Bureau of Statistics, 2010). This was an increase from 17.2% in March 2004 and 15.2% in June 1994. In recent years, rainwater tanks have re-emerged as an important alternative source of fresh water in Australian cities. Rainwater tank is an important component of water sensitive urban design (IEAust, 2006), which is a sustainable urban design practice. Rainwater tanks can save mains water significantly, provide on-site detention and reduce treatable urban runoff volume. The Building Sustainability Index (referred to as BASIX) has been introduced by New South Wales Department of Planning in Australia (NSWDP, 2005). It is a web-based tool that measures the potential performance of new residential dwellings against sustainability indices. BASIX requires all new houses in New
South Wales to save at least 40% potable water than the average one by adopting various water savings techniques including installation of rainwater tanks. There have been notable researches on rainwater tanks in Australia which have demonstrated that rainwater tanks can provide significant mains water savings (Coombes et al., 1999; Coombes and Kuczera, 2003; Chanan and Woods, 2006; Marks et al., 2006; Khastagir and Jayasuriya, 2010; Tam et al., 2010). Most of the local councils in Australia encourage installation of rainwater tanks. However, there have been limited researches on rainwater tanks in multi-unit buildings/developments in Australia. As reported by Australian Bureau of Statistics (2010), over one-quarter (25.9%) of separate houses had rainwater tanks installed, as opposed to only 6.2% of semi-detached or townhouses. Nearly one-quarter (24.9%) of family households had a rainwater tank installed compared with only 13.2% of group households or multi-unit houses. These data clearly show that the water savings from rainwater tanks in multiunit buildings/developments need to be demonstrated through research and also user-friendly tools should be made available to the public that can readily be used to estimate water savings and to find optimum rainwater tank size for multi-unit developments. As such, this paper investigates water savings potential of rainwater tanks in typical multi-unit single storey residential developments in the three cities of New South Wales State in Australia: Newcastle, Sydney and Wollongong.
2. Water savings potentials of rainwater tanks ∗ Corresponding author at: School of Engineering, University of Western Sydney, Building XB248, Kingswood, Locked Bag 1797, Penrith South DC, NSW 1797, Australia. Tel.: +61 247360145; fax: +61 247360833. E-mail address: [email protected] (A. Rahman).
Fewkes (1999) studied the performances of rainwater tanks in a house in the UK, which produced a set of dimensionless design curves which enables estimation of the rainwater tank capacity
0921-3449/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.resconrec.2010.06.010
1450
E. Eroksuz, A. Rahman / Resources, Conservation and Recycling 54 (2010) 1449–1452
Table 1 Selected rainfall stations. City: Rainfall station name:
Sydney Sydney Observatory Hill
Newcastle Newcastle Nobbys Signal Station AWS
Wollongong Wollongong University
Station ID Period of record Record length (years) Average annual rainfall (mm)
66062 1858–2005 148 1204
61055 1862–2005 144 1140
68188 1970–2005 36 1320
required to achieve a desired performance level given the roof area and demand patterns. Vaes and Berlamont (2001) developed a model to assess the effect of rainwater tanks on the rainfall runoff using long-term historical rainfall data. Coombes and Kuczera (2003) evaluated the performance of 1–10 kL rainwater tanks in four Australian capital cities with mains water trickle top-up used to supplement mains water supply for domestic toilet, laundry, hot water and outdoor usages. They found that for individual dwelling with 150 m2 roof area and 1–5 kL tank size located in Sydney could achieve 10–58% mains water savings depending on the number of people living in the house. Depending on roof area and number of occupants in the household, the use of rainwater tanks resulted in an annual mains water savings ranging from 18 kL to 55 kL for 1 kL rainwater tank and 25 kL to 144 kL for 10 kL rainwater tank. Villarreal and Dixon (2005) investigated the water savings potential of rainwater harvesting system from large roof areas in Sweden. They found that 30% of mains water savings can be achieved from a 40 m3 tank if rainwater is used for toilet flushing and washing machine. Roebuck and Ashley (2006) discussed the development of a computer based modelling and assessment tool for rainwater harvesting system intended for domestic, commercial, industrial and public buildings. They argued that many of the methods of rainwater tank analysis overestimated the hydraulic efficiency and potential cost savings that could be achievable with rainwater tanks. Ghisi et al. (2007) investigated the water savings potential from rainwater harvesting system in Brazil and found that average potential for potable water savings range from 12% to 79% per year for the cities analysed. Ideal rainwater tank sizes for dwellings with low potable water demand range from about 2 kL to 20 kL depending on rainwater demand. For dwellings with high potable water demand, ideal rainwater tank sizes range from about 3 kL to 7 kL. The main conclusion drawn from the research was that the average potential for potable water savings in south-eastern Brazil is 41%. They concluded that rainwater tank capacity has to be determined for each location and dwelling as it depends strongly on potable water and rainwater demand. Ghisi et al. (2009) evaluated the potential for potable water savings using rainwater for washing vehicles in petrol stations located in Brasilia in Brazil. They found that the average potential for potable water savings using rainwater is 32%. Cheng and Liao (2009) presented a method of creating rainfall zones in northern Taiwan based on standardised rainfall data which would enhance rainwater harvesting applications. Tam et al. (2010) investigated cost effectiveness on the use of rainwater tanks for Australian residential environment. Seven cities were examined and it was found that using rainwater would be an economical option for households in Gold Coast, Brisbane and Sydney. Khastagir and Jayasuriya (2010) presented a novel methodology and a relationship for optimal sizing of rainwater tanks for Melbourne city in Australia considering the annual rainfall at the geographic location, the demand for rainwater, the roof area and the desired supply reliability. Imteaz et al. (2009) presented water savings from two large underground rainwater tanks in Melbourne city based on recorded daily rainfall data and irrigation water use. They found that these tanks are quite effective in savings water in wet, average and dry years.
For a multi-unit building, space for large rainwater tanks may be limited. However, it might be argued that if water savings is given higher priority, space for rainwater tank would not be a problem. For example, rainwater tank can be placed along the back fences of the house (say 0.50 m wide, 1.2 m high tank). Also, there have been examples that rainwater can be stored in foundation of the building, e.g. modular water POD storage system (RemTec, 2007). Also, instead of using a single large tank, a number of smaller tanks connected in series can be adopted. Rainwater tanks can be incorporated with the landscaping of multi-unit development projects with respect to size, location, shape and colour to make rainwater harvesting system aesthetically pleasing. 3. Study area and data For this study, three cities were selected from the east coast of New South Wales in Australia: Newcastle, Sydney and Wollongong. One rainfall station with long record was selected from each of the cities as shown in Table 1 and daily rainfall data were obtained from Australian Bureau of Meteorology. The study considers four different usages of water from rainwater tank: (a) toilet flushing, (b) laundry, (c) hot water (shower, laundry and kitchen) and (d) outdoor irrigation. The adopted average water demand values for these usages are shown in Table 2, which were based on Sydney Water recommended demand rates. Irrigation water demand varies with the season of the year, with 50%, 20%, 10% and 20% of total outdoor water use was assumed to be occurring in summer, autumn, winter and spring, respectively. Depending on the individual council requirements and preferences of the owners, a multi-unit building would have different features characterised by factors such as ratio of roof to site area, floor levels and ratio of pervious to impervious areas. This study assumes a multi-unit single storey residential building with the ratio of roof to site area of 0.5, a different ratio would affect the irrigation water usage and hence the efficiency of a rainwater tank. The roof/floor area of individual unit in a multi-unit single storey building would vary ranging from typically one bed room to three bed rooms units. It seems to be reasonable to assume that 1, 2 and 3 bed room units would have roof areas of 50 m2 , 75 m2 and 100 m2 , respectively, i.e. an average roof area of 75 m2 per unit assuming that there would be equal numbers of 1, 2 and 3 bed room units in the development. It also seems to be reasonable to assume that 1, 2 and 3 bed room units on average would occupy 1, 3 and 4 persons, respectively, which gives an average occupancy rate of 2.6 persons per unit which is equivalent to about 30 m2 roof area per person. A different occupancy rate would produce a different water savings rate from having the rainwater tank; however the assumed Table 2 Water usage rates for various purposes. Water usage type
Water usage rate
Toilet flushing Laundry Hot water Outdoor irrigation
36 L per person per day 36 L per person per day 62 L per person per day 28 L per person per day
E. Eroksuz, A. Rahman / Resources, Conservation and Recycling 54 (2010) 1449–1452
1451
Fig. 1. Average annual water savings in a multi-unit single storey residential development for various roof areas in Sydney.
occupancy rate would provide a reasonable result on water savings from having rainwater tanks. 4. Method In this study, a continuous simulation type water balance model was developed on a daily time step. The basic idea underlying the model is that the rainfall falling on a roof area initially discharges to a first flush device and then to the rainwater tank. Water is drawn from the rainwater tank for intended usages. If the water level in the rainwater tank goes below a set minimum value, the tank is topped up with mains water to keep a minimum volume of water in the tank, which was taken to be 5% of the tank volume in this study. When the rainwater tank is full, the excess rainwater entering into the tank overflows into the street drainage system. Ten different tank sizes (10 kL, 20 kL, 30 kL, 40 kL, 50 kL, 60 kL, 70 kL, 80 kL, 90 kL and 100 kL) and five different roof areas (500 m2 , 1000 m2 , 1500 m2 , 2000 m2 and 2500 m2 ) were considered.
Fig. 2. Average annual water savings in a multi-unit single storey residential development (site area = 4000 m2 , roof area = 2000 m2 ).
water savings increase with increasing roof area as expected; however, this rate of increase is relatively higher for larger tank sizes as evidenced by the increasing gradient of the individual curve (which represents a particular tank size) with increasing tank size. For smaller tanks increasing the roof area does not have as great an effect as for larger tanks because for larger tanks the tank does not fill and overflow as often as for smaller tanks. The rainwater that flows from the roof to the tank is more effectively collected and used when the tank is larger. A prediction equation was developed to estimate average annual water savings in a multi-unit development project fitted with rainwater tanks in the three cities:
5. Results
log(W )= − 6.309+0.779 log(A) + 0.318 log(T ) + 2.078 log(R)
Fig. 1 shows average annual water savings resulting from a rainwater tank for various roof areas in Sydney which shows that for smaller roof areas, an increase in rainwater tank size does not increase the water savings as much as for larger roof areas. For example, for roof area of 500 m2 , increase in tank size from 10 kL to 50 kL results in an increase in average annual water savings by 36%; however, for 1500 m2 roof area, the same tank size increment results in an increase in annual water savings by 100%. This is due to the fact that for smaller roof area increasing the tank size does not have as great an effect as for larger roof area because for small roof area the tank does not fill and overflow as often as for larger roof area. Fig. 1 also shows that for a given tank size, average annual
where W is average annual water savings in kL, A is roof area in m2 , T is rainwater tank size in kL and R is average annual rainfall in mm. The equation has a coefficient of determination (R2 ) = 96% and standard error of estimate of 2% of average annual water savings in log domain. The equation is valid for Sydney, Newcastle and Wollongong cities in New South Wales, Australia for roof areas in the range of 500–2500 m2 , an occupancy rate of 1 person for each 30 m2 roof area and when rainwater is used for toilet flushing, laundry, hot water and outdoor irrigation. To investigate the water savings potential from having a rainwater tank, a detailed analysis was undertaken for an example
Table 3 Annual water savings from rainwater tanks in a multi-unit single storey residential development in three different cities of New South Wales in Australia. Tank size (kL)
10 20 30 40 50 60 70 80 90 100
Table 4 Reliability of rainwater tanks, % of days when rainwater tank can provide all the required water (site area = 4000 m2 , roof area = 2000 m2 ). Tank size (kL)
Average annual mains water savings (%) Newcastle
Sydney
Wollongong
21 31 37 42 45 47 49 50 51 52
21 31 38 43 46 49 51 52 54 55
21 32 39 43 47 50 53 54 56 57
(1)
10 20 30 40 50 60 70 80 90 100
Average % of days in a year when no mains top-up is required Newcastle
Sydney
Wollongong
10 23 30 35 39 41 43 44 46 46
10 23 31 36 40 43 45 47 48 49
10 24 31 37 41 44 47 49 51 52
1452
E. Eroksuz, A. Rahman / Resources, Conservation and Recycling 54 (2010) 1449–1452
Table 5 Annual mains water savings in recent drought years (tank size 70 kL, Sydney). Year
Annual rainfall (mm)
Average annual rainfall (mm)
Ratio of annual rainfall and average annual rainfall
Annual water savings (kL)
Average annual water savings (kL)
2002 2004
860 995
1204
0.71 0.83
1324 1525
1852
development with a site area of 4000 m2 , roof area of 2000 m2 , outdoor irrigation area of 2000 m2 and 70 occupants (with 27 flats @ 2.6 persons per flat). For this development, the average annual mains water savings for various tank sizes for the three cities are presented in Table 3 and Fig. 2 which show that Wollongong provides highest water savings followed by Sydney and Newcastle. Generally, for this development, a 10 kL tank offers 21% mains water savings and a 50 kL tank offers 45% mains water savings. Table 3 also shows that for this example development project, a 40 kL tank is required to achieve at least 40% mains water savings as required by BASIX. Reliability of rainwater tank to provide all the required water (for toilet flushing, laundry, hot water and outdoor irrigation) was examined in Table 4 which shows that 10 kL tank can provide all the required water for only 10% of the days on average in a year. For 70 kL tank size, rainwater tank is able to provide all the required water for about 50% of the days. Reliability, as defined here may not have much significance to most of the house owners because the owners of tank would not be aware on which day top-up occurs. Their only concern is the overall amount of water saved. However, reliability would be important for a house owner who intends to use rainwater tank as his sole water supply source. Also, in the case of failure of mains water supply for any unforeseen reasons, reliability gives an indication of how dependable a rainwater tank is to meet the overall water demand. For the example development, Fig. 2 shows the annual average water savings for various tank sizes, e.g. for a 20 kL tank size, there is 100% increase in tank size as compared to 10 kL tank, which increases water savings by about 1.5 times. As the tank size increases further the rate of increase in water savings reduces, and beyond 70 kL tank size, the rate of increase in water savings is minimal. This paper thus assumes a 70 kL tank as the “optimum size” for this example development. It was then examined how much water savings can be achieved in drought years from the 70 kL tank. The long-term average annual rainfall for Sydney was found to be 1204 mm (based on the data from Sydney Observatory Hill Station). The annual rainfall in Sydney during two recent drought years 2002 and 2004 are 860 mm and 995 mm, respectively, which are 71% and 83% of the longterm annual average rainfall. For a 70 kL tank size, the mains water savings achievable for these two years (2002 and 2004) are 37% and 42%, respectively, which shows that even in drought years, a notable proportion of mains water savings can be achieved by having a 70 kL rainwater tank in the above multi-unit residential development. In Table 5, the reported annual water savings were calculated using the daily rainfall data of the corresponding year and the annual rainfall values here were used to identify the drought years. Table 5 shows that ratio of annual water savings has nearly complete correspondence with ratio of mean annual rainfalls in years 2002 and 2004. 6. Conclusions This paper examines the water savings potential from having rainwater tanks in multi-unit buildings in three cities of Australia: Sydney, Newcastle and Wollongong. It has been found that a larger
Ratio of water savings 0.70 0.82
tank size is more appropriate to maximise water savings in multiunit building. Rainwater tanks can provide significant water savings even in relatively dry years. It is also found that water savings in any given year from rainwater tanks have direct correspondence with the annual rainfall of the corresponding year. A prediction equation is developed which can be used to estimate average annual water savings from having a rainwater tank in a multi-unit single storey building in three Australian cities Sydney, Newcastle and Wollongong under some assumptions on-site, tank and water usage characteristics. Acknowledgements Authors would like to thank Bureau of Meteorology for providing the daily rainfall data and Mr. Michael Jeffrey for his input to the project. References Australian Bureau of Statistics. Environmental issues: people’s views and practices. Australian Government; 2010, http://www.abs.gov.au. Chanan A, Woods P. Introducing total water cycle management in Sydney: a Kogarah Council initiative. Desalination 2006;187:11–6. Cheng CL, Liao MC. Regional rainfall level zoning for rainwater harvesting systems in northern Taiwan. Resources, Conservation and Recycling 2009;53: 421–8. Coombes PJ, Argue JR, Kuczera G. Figtree place: a case study in water sensitive urban development (WSUD). Urban Water 1999;1(4):335–43. Coombes PJ, Kuczera G. Analysis of the performance of rainwater tanks in Australian capital cities. In: 28th International hydrology and water resources symposium, Wollongong, NSW; 2003. Environmental Heritage Aboriginal Affairs. Rainwater tanks: their selection, use and maintenance. Government of South Australia; 1999. Fewkes A. The use of rainwater for WC flushing: the field testing of a collection system. Building and Environment 1999;34(6):765–72. Ghisi E, Bressan DL, Martini M. Rainwater tank capacity and potential for potable water savings by using rainwater in the residential sector of southeastern Brazil. Building and Environment 2007;42:1654–66. Ghisi E, Tavares DF, Rocha VL. Rainwater harvesting in petrol stations in Brasilia: potential for potable water savings and investment feasibility analysis. Resources, Conservation and Recycling 2009;54:79–85. IEAust. Australian runoff quality: a guide to water sensitive urban design. Australia: The Institution of Engineers; 2006. Imteaz MA, Taylor J, Pateras M. Effectiveness and payback period analysis of rainwater tanks constructed within Swinburne University of Technology. In: International conference of society for sustainability and environmental engineering (SSEE); 2009. Khastagir A, Jayasuriya N. Optimal sizing of rain water tanks for domestic water conservation. Journal of Hydrology 2010;381:181–8. Marks R, Clark R, Rooke E, Berzins A. Meadows, South Australia: development through integration of local water resources. Desalination 2006;188:149–61. New South Wales Department of Planning (NSWDP). The Building Sustainability Index (BASIX); 2005, http://www.basix.nsw.gov.au/information/index.jsp. RemTec. The next generation aquacomb modular water POD storage system; 2007, http://www.remtec.net.au. Roebuck RM, Ashley RM. Predicting the hydraulic and life-cycle cost performance of rainwater harvesting systems using a computer based modelling tool. In: 4th International conference on water sensitive urban design, vol. 2; 2006. p. 699–706. Tam VWY, Tam L, Zeng SX. Cost effectiveness and tradeoff on the use of rainwater tank: an empirical study in Australian residential decision-making. Resources, Conservation and Recycling 2010;54:178–85. Vaes G, Berlamont J. The effect of rainwater storage tank on design storms. Urban Water 2001;3:303–7. Villarreal EL, Dixon A. Analysis of a rainwater collection system for domestic water supply in Ringdansen, Norrkoping, Sweden. Building and Environment 2005;40:1174–84.
Contents lists available at ScienceDirect
Resources, Conservation and Recycling journal homepage: www.elsevier.com/locate/resconrec
Rainwater tanks in multi-unit buildings: A case study for three Australian cities Erhan Eroksuz, Ataur Rahman ∗ School of Engineering, University of Western Sydney, Locked Bag 1797, Penrith South DC, NSW 1797, Australia
a r t i c l e
i n f o
Article history: Received 4 February 2010 Received in revised form 18 June 2010 Accepted 21 June 2010 Keywords: Rainwater harvesting system Rainwater collection Potable water savings Multi-unit building
a b s t r a c t Rainwater tanks have become popular in large Australian cities due to water shortage and greater public awareness towards sustainable urban development. Rainwater harvesting in multi-unit buildings in Australia is less common. This paper investigates the water savings potential of rainwater tanks fitted in multi-unit residential buildings in three cities of Australia: Sydney, Newcastle and Wollongong. It is found that for multi-unit buildings, a larger tank size is more appropriate to maximise water savings. It is also found that rainwater tank of appropriate size in a multi-unit building can provide significant mains water savings even in dry years. A prediction equation is developed which can be used to estimate average annual water savings from having a rainwater tank in a multi-unit building in these three Australian cities. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.
1. Introduction Australia is one of the driest inhabited continents on earth with highly variable rainfall. Although Australia has one of the highest per capital dam storage volumes in the world, reliability of water supply in most Australian cities is being questioned in recent years due to on-going droughts, climate change and increased public awareness towards water and environment. Water authorities in Australia are desperately looking for alternative sources of water including rainwater tanks in addition to recycling grey water, wastewater and use of desalination plants. Rainwater tanks have been a common water supply system in rural Australia for many years (EHAA, 1999). In 2007, 19.3%, or slightly more than 1.5 million households, reported a rainwater tank as a source of water (Australian Bureau of Statistics, 2010). This was an increase from 17.2% in March 2004 and 15.2% in June 1994. In recent years, rainwater tanks have re-emerged as an important alternative source of fresh water in Australian cities. Rainwater tank is an important component of water sensitive urban design (IEAust, 2006), which is a sustainable urban design practice. Rainwater tanks can save mains water significantly, provide on-site detention and reduce treatable urban runoff volume. The Building Sustainability Index (referred to as BASIX) has been introduced by New South Wales Department of Planning in Australia (NSWDP, 2005). It is a web-based tool that measures the potential performance of new residential dwellings against sustainability indices. BASIX requires all new houses in New
South Wales to save at least 40% potable water than the average one by adopting various water savings techniques including installation of rainwater tanks. There have been notable researches on rainwater tanks in Australia which have demonstrated that rainwater tanks can provide significant mains water savings (Coombes et al., 1999; Coombes and Kuczera, 2003; Chanan and Woods, 2006; Marks et al., 2006; Khastagir and Jayasuriya, 2010; Tam et al., 2010). Most of the local councils in Australia encourage installation of rainwater tanks. However, there have been limited researches on rainwater tanks in multi-unit buildings/developments in Australia. As reported by Australian Bureau of Statistics (2010), over one-quarter (25.9%) of separate houses had rainwater tanks installed, as opposed to only 6.2% of semi-detached or townhouses. Nearly one-quarter (24.9%) of family households had a rainwater tank installed compared with only 13.2% of group households or multi-unit houses. These data clearly show that the water savings from rainwater tanks in multiunit buildings/developments need to be demonstrated through research and also user-friendly tools should be made available to the public that can readily be used to estimate water savings and to find optimum rainwater tank size for multi-unit developments. As such, this paper investigates water savings potential of rainwater tanks in typical multi-unit single storey residential developments in the three cities of New South Wales State in Australia: Newcastle, Sydney and Wollongong.
2. Water savings potentials of rainwater tanks ∗ Corresponding author at: School of Engineering, University of Western Sydney, Building XB248, Kingswood, Locked Bag 1797, Penrith South DC, NSW 1797, Australia. Tel.: +61 247360145; fax: +61 247360833. E-mail address: [email protected] (A. Rahman).
Fewkes (1999) studied the performances of rainwater tanks in a house in the UK, which produced a set of dimensionless design curves which enables estimation of the rainwater tank capacity
0921-3449/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.resconrec.2010.06.010
1450
E. Eroksuz, A. Rahman / Resources, Conservation and Recycling 54 (2010) 1449–1452
Table 1 Selected rainfall stations. City: Rainfall station name:
Sydney Sydney Observatory Hill
Newcastle Newcastle Nobbys Signal Station AWS
Wollongong Wollongong University
Station ID Period of record Record length (years) Average annual rainfall (mm)
66062 1858–2005 148 1204
61055 1862–2005 144 1140
68188 1970–2005 36 1320
required to achieve a desired performance level given the roof area and demand patterns. Vaes and Berlamont (2001) developed a model to assess the effect of rainwater tanks on the rainfall runoff using long-term historical rainfall data. Coombes and Kuczera (2003) evaluated the performance of 1–10 kL rainwater tanks in four Australian capital cities with mains water trickle top-up used to supplement mains water supply for domestic toilet, laundry, hot water and outdoor usages. They found that for individual dwelling with 150 m2 roof area and 1–5 kL tank size located in Sydney could achieve 10–58% mains water savings depending on the number of people living in the house. Depending on roof area and number of occupants in the household, the use of rainwater tanks resulted in an annual mains water savings ranging from 18 kL to 55 kL for 1 kL rainwater tank and 25 kL to 144 kL for 10 kL rainwater tank. Villarreal and Dixon (2005) investigated the water savings potential of rainwater harvesting system from large roof areas in Sweden. They found that 30% of mains water savings can be achieved from a 40 m3 tank if rainwater is used for toilet flushing and washing machine. Roebuck and Ashley (2006) discussed the development of a computer based modelling and assessment tool for rainwater harvesting system intended for domestic, commercial, industrial and public buildings. They argued that many of the methods of rainwater tank analysis overestimated the hydraulic efficiency and potential cost savings that could be achievable with rainwater tanks. Ghisi et al. (2007) investigated the water savings potential from rainwater harvesting system in Brazil and found that average potential for potable water savings range from 12% to 79% per year for the cities analysed. Ideal rainwater tank sizes for dwellings with low potable water demand range from about 2 kL to 20 kL depending on rainwater demand. For dwellings with high potable water demand, ideal rainwater tank sizes range from about 3 kL to 7 kL. The main conclusion drawn from the research was that the average potential for potable water savings in south-eastern Brazil is 41%. They concluded that rainwater tank capacity has to be determined for each location and dwelling as it depends strongly on potable water and rainwater demand. Ghisi et al. (2009) evaluated the potential for potable water savings using rainwater for washing vehicles in petrol stations located in Brasilia in Brazil. They found that the average potential for potable water savings using rainwater is 32%. Cheng and Liao (2009) presented a method of creating rainfall zones in northern Taiwan based on standardised rainfall data which would enhance rainwater harvesting applications. Tam et al. (2010) investigated cost effectiveness on the use of rainwater tanks for Australian residential environment. Seven cities were examined and it was found that using rainwater would be an economical option for households in Gold Coast, Brisbane and Sydney. Khastagir and Jayasuriya (2010) presented a novel methodology and a relationship for optimal sizing of rainwater tanks for Melbourne city in Australia considering the annual rainfall at the geographic location, the demand for rainwater, the roof area and the desired supply reliability. Imteaz et al. (2009) presented water savings from two large underground rainwater tanks in Melbourne city based on recorded daily rainfall data and irrigation water use. They found that these tanks are quite effective in savings water in wet, average and dry years.
For a multi-unit building, space for large rainwater tanks may be limited. However, it might be argued that if water savings is given higher priority, space for rainwater tank would not be a problem. For example, rainwater tank can be placed along the back fences of the house (say 0.50 m wide, 1.2 m high tank). Also, there have been examples that rainwater can be stored in foundation of the building, e.g. modular water POD storage system (RemTec, 2007). Also, instead of using a single large tank, a number of smaller tanks connected in series can be adopted. Rainwater tanks can be incorporated with the landscaping of multi-unit development projects with respect to size, location, shape and colour to make rainwater harvesting system aesthetically pleasing. 3. Study area and data For this study, three cities were selected from the east coast of New South Wales in Australia: Newcastle, Sydney and Wollongong. One rainfall station with long record was selected from each of the cities as shown in Table 1 and daily rainfall data were obtained from Australian Bureau of Meteorology. The study considers four different usages of water from rainwater tank: (a) toilet flushing, (b) laundry, (c) hot water (shower, laundry and kitchen) and (d) outdoor irrigation. The adopted average water demand values for these usages are shown in Table 2, which were based on Sydney Water recommended demand rates. Irrigation water demand varies with the season of the year, with 50%, 20%, 10% and 20% of total outdoor water use was assumed to be occurring in summer, autumn, winter and spring, respectively. Depending on the individual council requirements and preferences of the owners, a multi-unit building would have different features characterised by factors such as ratio of roof to site area, floor levels and ratio of pervious to impervious areas. This study assumes a multi-unit single storey residential building with the ratio of roof to site area of 0.5, a different ratio would affect the irrigation water usage and hence the efficiency of a rainwater tank. The roof/floor area of individual unit in a multi-unit single storey building would vary ranging from typically one bed room to three bed rooms units. It seems to be reasonable to assume that 1, 2 and 3 bed room units would have roof areas of 50 m2 , 75 m2 and 100 m2 , respectively, i.e. an average roof area of 75 m2 per unit assuming that there would be equal numbers of 1, 2 and 3 bed room units in the development. It also seems to be reasonable to assume that 1, 2 and 3 bed room units on average would occupy 1, 3 and 4 persons, respectively, which gives an average occupancy rate of 2.6 persons per unit which is equivalent to about 30 m2 roof area per person. A different occupancy rate would produce a different water savings rate from having the rainwater tank; however the assumed Table 2 Water usage rates for various purposes. Water usage type
Water usage rate
Toilet flushing Laundry Hot water Outdoor irrigation
36 L per person per day 36 L per person per day 62 L per person per day 28 L per person per day
E. Eroksuz, A. Rahman / Resources, Conservation and Recycling 54 (2010) 1449–1452
1451
Fig. 1. Average annual water savings in a multi-unit single storey residential development for various roof areas in Sydney.
occupancy rate would provide a reasonable result on water savings from having rainwater tanks. 4. Method In this study, a continuous simulation type water balance model was developed on a daily time step. The basic idea underlying the model is that the rainfall falling on a roof area initially discharges to a first flush device and then to the rainwater tank. Water is drawn from the rainwater tank for intended usages. If the water level in the rainwater tank goes below a set minimum value, the tank is topped up with mains water to keep a minimum volume of water in the tank, which was taken to be 5% of the tank volume in this study. When the rainwater tank is full, the excess rainwater entering into the tank overflows into the street drainage system. Ten different tank sizes (10 kL, 20 kL, 30 kL, 40 kL, 50 kL, 60 kL, 70 kL, 80 kL, 90 kL and 100 kL) and five different roof areas (500 m2 , 1000 m2 , 1500 m2 , 2000 m2 and 2500 m2 ) were considered.
Fig. 2. Average annual water savings in a multi-unit single storey residential development (site area = 4000 m2 , roof area = 2000 m2 ).
water savings increase with increasing roof area as expected; however, this rate of increase is relatively higher for larger tank sizes as evidenced by the increasing gradient of the individual curve (which represents a particular tank size) with increasing tank size. For smaller tanks increasing the roof area does not have as great an effect as for larger tanks because for larger tanks the tank does not fill and overflow as often as for smaller tanks. The rainwater that flows from the roof to the tank is more effectively collected and used when the tank is larger. A prediction equation was developed to estimate average annual water savings in a multi-unit development project fitted with rainwater tanks in the three cities:
5. Results
log(W )= − 6.309+0.779 log(A) + 0.318 log(T ) + 2.078 log(R)
Fig. 1 shows average annual water savings resulting from a rainwater tank for various roof areas in Sydney which shows that for smaller roof areas, an increase in rainwater tank size does not increase the water savings as much as for larger roof areas. For example, for roof area of 500 m2 , increase in tank size from 10 kL to 50 kL results in an increase in average annual water savings by 36%; however, for 1500 m2 roof area, the same tank size increment results in an increase in annual water savings by 100%. This is due to the fact that for smaller roof area increasing the tank size does not have as great an effect as for larger roof area because for small roof area the tank does not fill and overflow as often as for larger roof area. Fig. 1 also shows that for a given tank size, average annual
where W is average annual water savings in kL, A is roof area in m2 , T is rainwater tank size in kL and R is average annual rainfall in mm. The equation has a coefficient of determination (R2 ) = 96% and standard error of estimate of 2% of average annual water savings in log domain. The equation is valid for Sydney, Newcastle and Wollongong cities in New South Wales, Australia for roof areas in the range of 500–2500 m2 , an occupancy rate of 1 person for each 30 m2 roof area and when rainwater is used for toilet flushing, laundry, hot water and outdoor irrigation. To investigate the water savings potential from having a rainwater tank, a detailed analysis was undertaken for an example
Table 3 Annual water savings from rainwater tanks in a multi-unit single storey residential development in three different cities of New South Wales in Australia. Tank size (kL)
10 20 30 40 50 60 70 80 90 100
Table 4 Reliability of rainwater tanks, % of days when rainwater tank can provide all the required water (site area = 4000 m2 , roof area = 2000 m2 ). Tank size (kL)
Average annual mains water savings (%) Newcastle
Sydney
Wollongong
21 31 37 42 45 47 49 50 51 52
21 31 38 43 46 49 51 52 54 55
21 32 39 43 47 50 53 54 56 57
(1)
10 20 30 40 50 60 70 80 90 100
Average % of days in a year when no mains top-up is required Newcastle
Sydney
Wollongong
10 23 30 35 39 41 43 44 46 46
10 23 31 36 40 43 45 47 48 49
10 24 31 37 41 44 47 49 51 52
1452
E. Eroksuz, A. Rahman / Resources, Conservation and Recycling 54 (2010) 1449–1452
Table 5 Annual mains water savings in recent drought years (tank size 70 kL, Sydney). Year
Annual rainfall (mm)
Average annual rainfall (mm)
Ratio of annual rainfall and average annual rainfall
Annual water savings (kL)
Average annual water savings (kL)
2002 2004
860 995
1204
0.71 0.83
1324 1525
1852
development with a site area of 4000 m2 , roof area of 2000 m2 , outdoor irrigation area of 2000 m2 and 70 occupants (with 27 flats @ 2.6 persons per flat). For this development, the average annual mains water savings for various tank sizes for the three cities are presented in Table 3 and Fig. 2 which show that Wollongong provides highest water savings followed by Sydney and Newcastle. Generally, for this development, a 10 kL tank offers 21% mains water savings and a 50 kL tank offers 45% mains water savings. Table 3 also shows that for this example development project, a 40 kL tank is required to achieve at least 40% mains water savings as required by BASIX. Reliability of rainwater tank to provide all the required water (for toilet flushing, laundry, hot water and outdoor irrigation) was examined in Table 4 which shows that 10 kL tank can provide all the required water for only 10% of the days on average in a year. For 70 kL tank size, rainwater tank is able to provide all the required water for about 50% of the days. Reliability, as defined here may not have much significance to most of the house owners because the owners of tank would not be aware on which day top-up occurs. Their only concern is the overall amount of water saved. However, reliability would be important for a house owner who intends to use rainwater tank as his sole water supply source. Also, in the case of failure of mains water supply for any unforeseen reasons, reliability gives an indication of how dependable a rainwater tank is to meet the overall water demand. For the example development, Fig. 2 shows the annual average water savings for various tank sizes, e.g. for a 20 kL tank size, there is 100% increase in tank size as compared to 10 kL tank, which increases water savings by about 1.5 times. As the tank size increases further the rate of increase in water savings reduces, and beyond 70 kL tank size, the rate of increase in water savings is minimal. This paper thus assumes a 70 kL tank as the “optimum size” for this example development. It was then examined how much water savings can be achieved in drought years from the 70 kL tank. The long-term average annual rainfall for Sydney was found to be 1204 mm (based on the data from Sydney Observatory Hill Station). The annual rainfall in Sydney during two recent drought years 2002 and 2004 are 860 mm and 995 mm, respectively, which are 71% and 83% of the longterm annual average rainfall. For a 70 kL tank size, the mains water savings achievable for these two years (2002 and 2004) are 37% and 42%, respectively, which shows that even in drought years, a notable proportion of mains water savings can be achieved by having a 70 kL rainwater tank in the above multi-unit residential development. In Table 5, the reported annual water savings were calculated using the daily rainfall data of the corresponding year and the annual rainfall values here were used to identify the drought years. Table 5 shows that ratio of annual water savings has nearly complete correspondence with ratio of mean annual rainfalls in years 2002 and 2004. 6. Conclusions This paper examines the water savings potential from having rainwater tanks in multi-unit buildings in three cities of Australia: Sydney, Newcastle and Wollongong. It has been found that a larger
Ratio of water savings 0.70 0.82
tank size is more appropriate to maximise water savings in multiunit building. Rainwater tanks can provide significant water savings even in relatively dry years. It is also found that water savings in any given year from rainwater tanks have direct correspondence with the annual rainfall of the corresponding year. A prediction equation is developed which can be used to estimate average annual water savings from having a rainwater tank in a multi-unit single storey building in three Australian cities Sydney, Newcastle and Wollongong under some assumptions on-site, tank and water usage characteristics. Acknowledgements Authors would like to thank Bureau of Meteorology for providing the daily rainfall data and Mr. Michael Jeffrey for his input to the project. References Australian Bureau of Statistics. Environmental issues: people’s views and practices. Australian Government; 2010, http://www.abs.gov.au. Chanan A, Woods P. Introducing total water cycle management in Sydney: a Kogarah Council initiative. Desalination 2006;187:11–6. Cheng CL, Liao MC. Regional rainfall level zoning for rainwater harvesting systems in northern Taiwan. Resources, Conservation and Recycling 2009;53: 421–8. Coombes PJ, Argue JR, Kuczera G. Figtree place: a case study in water sensitive urban development (WSUD). Urban Water 1999;1(4):335–43. Coombes PJ, Kuczera G. Analysis of the performance of rainwater tanks in Australian capital cities. In: 28th International hydrology and water resources symposium, Wollongong, NSW; 2003. Environmental Heritage Aboriginal Affairs. Rainwater tanks: their selection, use and maintenance. Government of South Australia; 1999. Fewkes A. The use of rainwater for WC flushing: the field testing of a collection system. Building and Environment 1999;34(6):765–72. Ghisi E, Bressan DL, Martini M. Rainwater tank capacity and potential for potable water savings by using rainwater in the residential sector of southeastern Brazil. Building and Environment 2007;42:1654–66. Ghisi E, Tavares DF, Rocha VL. Rainwater harvesting in petrol stations in Brasilia: potential for potable water savings and investment feasibility analysis. Resources, Conservation and Recycling 2009;54:79–85. IEAust. Australian runoff quality: a guide to water sensitive urban design. Australia: The Institution of Engineers; 2006. Imteaz MA, Taylor J, Pateras M. Effectiveness and payback period analysis of rainwater tanks constructed within Swinburne University of Technology. In: International conference of society for sustainability and environmental engineering (SSEE); 2009. Khastagir A, Jayasuriya N. Optimal sizing of rain water tanks for domestic water conservation. Journal of Hydrology 2010;381:181–8. Marks R, Clark R, Rooke E, Berzins A. Meadows, South Australia: development through integration of local water resources. Desalination 2006;188:149–61. New South Wales Department of Planning (NSWDP). The Building Sustainability Index (BASIX); 2005, http://www.basix.nsw.gov.au/information/index.jsp. RemTec. The next generation aquacomb modular water POD storage system; 2007, http://www.remtec.net.au. Roebuck RM, Ashley RM. Predicting the hydraulic and life-cycle cost performance of rainwater harvesting systems using a computer based modelling tool. In: 4th International conference on water sensitive urban design, vol. 2; 2006. p. 699–706. Tam VWY, Tam L, Zeng SX. Cost effectiveness and tradeoff on the use of rainwater tank: an empirical study in Australian residential decision-making. Resources, Conservation and Recycling 2010;54:178–85. Vaes G, Berlamont J. The effect of rainwater storage tank on design storms. Urban Water 2001;3:303–7. Villarreal EL, Dixon A. Analysis of a rainwater collection system for domestic water supply in Ringdansen, Norrkoping, Sweden. Building and Environment 2005;40:1174–84.