- Email: [email protected]
Assessment of residential rainwater harvesting efficiency for meeting non-potable water demands in three climate conditions
Resources, Conservation and Recycling 73 (2013) 86–93
Contents lists available at SciVerse ScienceDirect
Resources, Conservation and Recycling journal homepage: www.elsevier.com/locate/resconrec
Assessment of residential rainwater harvesting efficiency for meeting non-potable water demands in three climate conditions Mohammad Hossein Rashidi Mehrabadi a,1 , Bahram Saghafian a,2 , Fereshte Haghighi Fashi b,∗ a b
Department of Technical and Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran Soil Conservation and Watershed Management Research Institute, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 19 August 2012 Received in revised form 30 December 2012 Accepted 24 January 2013 Keywords: Rainwater harvesting Roof Residential building Water supply Climate Iran
a b s t r a c t Global demand for clean water supplies is on the rise due to population growth. This is also true in most cities of Iran. Non-conventional water resources must be developed to partially offset the increasing demand. In this study, the applicability and performance of rainwater harvesting (RWH) systems to supply daily non-potable water were assessed. Storage of rain falling on the roofs of residential buildings and directed into installed tanks was simulated in three cities of varying climatic conditions, namely Tabriz (Mediterranean climate), Rasht (humid climate), and Kerman (arid climate). Daily rainfall statistics for a period of 53 years as well as the information on the contributing roof area, available tank volumes and non-potable water demand were collected in each city. Typical residential buildings with roof areas of 60, 120, 180 and 240 m2 with an average of four residents in each house were considered for the study. According to the results in humid climate, it is possible to supply at least 75% of non-potable water demand by storing rainwater from larger roof areas for a maximum duration of 70% of the times. For roofs with small surface area, the supply meets 75% of non-potable water demand for a maximum duration of 45% of the times. Moreover, for Mediterranean climate, it is possible to supply at least 75% of non-potable water demand in buildings with larger roof areas for a maximum duration of 40% of the times. It is also found that in arid climate, similar duration is only 23% of the times. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, water supply has become a vital issue globally. In some locations, water collected from roofs of residential or commercial buildings has been a practical way to supply water for irrigation and partially meet potable and non-potable water demand. Since major parts of urban areas are under roofs or covered by impervious material, the potential for harvesting rainwater is considerable in cities. These areas are not only a source of generating non-potable water, but also urban flooding may be reduced when rainwater is stored by installed tanks. Abdulla and Al-Shareef (2009) assessed the potential for potable water savings by using rainwater harvesting (RWH) in residential cities of 12 Jordanian governorates. The research provided suggestions regarding the improvement of both the quality and quantity of harvested rainwater and demonstrated the importance of roof rainwater harvesting systems for domestic water supply.
∗ Corresponding author. Tel.: +98 2177072309. E-mail addresses: hossein [email protected] (M.H. Rashidi Mehrabadi), b.saghafi[email protected] (B. Saghafian), [email protected] (F. Haghighi Fashi). 1 Tel.: +98 2177072309. 2 Tel.: +98 2144868407. 0921-3449/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.resconrec.2013.01.015
Sturm et al. (2009) developed an appropriate rainwater harvesting solution based on hydrological, technical, and social conditions in central northern Namibia. They provided appropriate RWH design guidelines for the required storage capacities. Results also revealed that it is economically feasible to implement RWH in the context of roof catchment systems. Ghisi et al. (2007) evaluated the potential for water saving by using RWH in Brazil and reported that average potential for potable water savings was 12–79% per year for the studied cities. In another research, Ghisi et al. (2009) studied rainwater harvesting as a way to optimize the supply and use of water resources. The results revealed that the average saving of potable water by RWHs was 32.7%, with the potential in the range of 9.2–57.2%. Eroksuz and Rahman (2010) evaluated rainwater harvesting systems in multi-unit buildings in Australia. They investigated the water saving potential of tanks fitted in three cities: Sydney, Newcastle and Wollongong. Results indicated that for multi-unit buildings, a larger tank size was more appropriate to maximize water saving. It was also found that these tanks provided significant water savings even in dry years. Khastagir and Jayasuriya (2010) investigated the reliability of rainwater tanks. They presented design charts of optimum tank sizes considering daily rainfall, water demand and roof area necessary to meet the demand with 90% reliability. Palla et al. (2011) investigated optimum performance of a rainwater harvesting
M.H. Rashidi Mehrabadi et al. / Resources, Conservation and Recycling 73 (2013) 86–93
Fig. 1. Schematic view of a rainwater harvesting system.
system. They developed a model that determined the efficiency of the system, over flow ratio and detention time. Performance was examined under various environmental conditions including three precipitation regimes and three water demand quantity. Imteaz et al. (2011b) developed a tool called eTank for the optimization of rainwater tank size. The study showed that, it is possible to achieve approximately 100% reliability with a roof size of 150–300 m2 and a tank size of 5000–10,000 L for a household of two occupants. Imteaz et al. (2012) presented several reliability curves for domestic rainwater tanks in relations to roof area, tank volume, number of residents in each house and total water demand supplied by the harvested rainwater under three climatic conditions. Their study showed that for a roof size of 100 m2 , 100%
87
reliability was not achievable even with a very large tank volume. They found that for a large roof size (200 m2 ), nearly 90% reliability can be achieved with a large tank volume (10,000 L) while larger volumes could yield 100% reliability, except under dry conditions. Overall, there is growing need to identify and promote alternative water supplies in light of growing demands. Rainwater harvesting systems in urban regions where the water supply is quantitatively, qualitatively or economically limited, could be a viable option. Harvesting rainwater may be considered as a suitable water resource in terms of cost and water quality. Although rainwater harvesting provides with safe water supply (Rygaard et al., 2011; Mun and Han, 2012), there is lack of information on the optimal tank volume in combination with roof area to reach the expected performance under different climatic conditions in Iran. The purpose of this study is to assess applicability and performance of rainwater harvesting systems for daily non-potable water supply in three different climatic areas located in Iran. The studied cities are Tabriz, Rasht and Kerman subject to Mediterranean, humid and arid climate conditions, respectively. Reliability of the system is determined based on the size of storage tank, roof area, and water demand. 2. Methodology In a rainwater harvesting system, firstly, rainwater is collected from a roof surface and directed to the storage tank by a down pipe. Surplus water is allowed to spill and flow to the surface drainage system or a waste water network. Collected rainwater may be stored in the tank and consumed as non-potable water. Fig. 1 shows a rainwater harvesting system tailored to a residential building.
Fig. 2. Flowchart of simulation of a rainwater harvesting system.
88
M.H. Rashidi Mehrabadi et al. / Resources, Conservation and Recycling 73 (2013) 86–93
Fig. 3. Location of case study cities in Iran.
In order to model a rainwater harvesting system, the initial volume of water in the tank (V0 ) and overflow water (SP0 ) is taken as zero: SP0 = 0
(1)
V0 = 0
(2)
The volume of harvested rainwater from the roof may be expressed by: It = Rt × A × ϕ
(3)
where I is the daily volume of collectable rainwater (L), ϕ is the roof runoff coefficient (dimensionless), R is the daily rainfall depth (mm), t is the day number (T), and A is the area of the roof (m2 ). The water volume in the tank may be updated in any daily time step as follows: Vt = It + Vt−1 − Ot − SPt
(4)
where V is the water storage volume (L) and O is the daily nonpotable water withdrawal from the tank (L) (see Fig. 1). Water volume withdrawn from the urban piped system (U) and the total non-potable water demand (O) are related through the following conditions: If :
It + Vt−1 > Dt → Ot = Dt
If :
It + Vt−1 < Dt
Ot = It + Vt−1 Ut = Dt − Ot
(5)
3. Study area Case study cities were selected on the basis of a simple climate classification. Mediterranean, humid, and arid are climate types associated with Tabriz, Rasht and Kerman cities in Iran, respectively. Location of case study cities is shown in Fig. 3. Daily rainfall statistics were prepared over the 1956–2008 period (Table 1). The time series of mean daily rainfall of the study cities are shown in Fig. 4.
(6)
where D is the demand. Volume of overflowed water is determined as follows: SPt = Vt−1 + It − Ot − Vmax
rectangles and required parameters and input data are shown in the dashed right side rectangles. In this study, tank capacity was varied from 1000 to 15,000 L. Total land area taken up by residential buildings was selected in typical plots of 100, 200, 300 and 400 m2 of which roof area covers 60% of the total area, i.e. 60, 120, 180 and 240 m2 are roof area, respectively. Runoff coefficient for roofs of residential buildings was considered 0.8 in Kerman and Tabriz and 0.85 in Rasht based on construction material and slope of the roofs. The average number of occupants in each building was taken as four. Non-potable water demand was assumed in the range of 60–180 L per resident per day. Therefore, the total demand for each building was estimated at 240–720 L. The expected minimum requirement of the water supply system was set to 75%.
(7)
where SP is the volume of the tank overflow (L) and Vmax is the tank volume (L). In Fig. 2, computational steps are shown in the left side
Table 1 Average annual rainfall in three cities. Average annual rainfall (mm)
Period
City
288 1355 150
1956–2008 1956–2008 1956–2008
Tabriz Rasht Kerman
M.H. Rashidi Mehrabadi et al. / Resources, Conservation and Recycling 73 (2013) 86–93
Fig. 4. Variation of average daily rainfall depth over 1956–2008 period (starting from January 1).
4. Results and discussion Fig. 5 shows rainwater storage in various tank volumes in Tabriz for different roof areas. In Tabriz, rainwater storage for 60 m2 roof area is nearly constant at 13,700 L per year. The highest rainwater storage is associated with a roof area of 240 m2 , which could store a total volume of 36,000–55,000 L per year. Fig. 6 shows rainwater storage in Rasht. Rasht (with a humid climate) provides highest rainwater saving amongst three cities. The largest and the smallest average yearly rainwater storage volumes are 227,000 and 67,000 L, corresponding to roof areas of 240 and 60 m2 , respectively. Averages annual rainwater storage for small roof areas varies from 50,000 to 138,000 L, and for large roof areas from 80,000 to 270,000 L in Rasht. However, according to Fig. 7, the largest average storage for a roof area of 240 m2 is only 28,000 L
89
Fig. 7. Average annual water storage in tank for various roof areas in Kerman.
annually for larger than 5000 L tank volume in Kerman, while, averages of rainwater storage for small roof areas varies from 7000 to 14,000 L. Based on Fig. 8 for Tabriz, for a tank volume of over 3500 L, the ratio of average annual water stored by the tanks over the average annual harvested rainwater is approximately equal to unity. For smaller tanks, this ratio is 0.65–0.9. Based on Fig. 9, for a roof area of 240 m2 and tanks volume up to 11,500 L in Rasht, the ratio of average annual water stored by the tanks over the average annual harvested rainwater varies between 0.3 and 0.9. However, the ratio corresponding to 60 m2 roof area and tank capacity of over 5000 L approaches unity. Similar to Tabriz, for tank capacity of over 4000 L, the ratio is near one in Kerman (see Fig. 10).
Fig. 8. Stored water volume to total harvested rainwater for various roof areas in Tabriz. Fig. 5. Average annual water storage in tank for various roof areas in Tabriz.
Fig. 6. Average annual water storage in tank for various roof areas in Rasht.
Fig. 9. Stored water volume to total harvested rainwater for various roof areas in Rasht.
90
M.H. Rashidi Mehrabadi et al. / Resources, Conservation and Recycling 73 (2013) 86–93
Fig. 10. Stored water volume to total harvested rainwater for various roof areas in Kerman.
Fig. 13. Reliability curves for non-potable water supply for different demands and roof area of 180 m2 in Tabriz.
Figs. 11–22 present the reliability as the percentages of times in which at least 75% of non-potable water demand in various tank volumes, roof areas, and non-potable water demands is supplied. Figs. 11 and 12 correspond to 60 and 120 m2 roof areas, respectively, as non-potable water demand varies from 240 to 720 L in Tabriz (with a Mediterranean climate). For 60 m2 roof area, only in 12% of the times, at least 75% of 480 L of total non-potable water demand could be supplied while in 6% of times, a total demand of 720 L would be expected to be met. For the roof area of 120 m2 , maximum and minimum percentages of daily supply are 29% and 5%, respectively, corresponding to 240 and 720 L of daily demand. Similarly in Figs. 13 and 14 corresponding to 180 and 240 m2 roof areas in Tabriz, respectively, water supply reliability varies between 7%
and 44% for 180 m2 roof area, and between 10% and 59% for 240 m2 roof area, respectively. In case of 240 m2 roof area and 4500 L tank volume or larger, at least 17% and 50% of times water supply is sufficient to meet 720 and 240 L of total non-potable water demand, respectively. Figs. 15 and 16 correspond to roof areas of 60 and 120 m2 , respectively, in Rasht. For roof area of 60 m2 , maximum and minimum percentages of daily supply are 73% and 12%, respectively, corresponding to 240 and 720 L of daily demand. For roof area of 120 m2 , total duration that water is supplied by rainwater increases such that for large tanks and 240 and 720 L demands, average reliabilities were 82% and 39%, respectively. From tank capacity of 6500 L and larger, at least 40% of times, the non-potable water demand would be met. Figs. 17 and 18 correspond to roof areas of
Fig. 11. Reliability curves for non-potable water supply for different demands and roof area of 60 m2 in Tabriz.
Fig. 12. Reliability curves for non-potable water supply for different demands and roof area of 120 m2 in Tabriz.
Fig. 14. Reliability curves for non-potable water supply for different demands and roof area of 240 m2 in Tabriz.
Fig. 15. Reliability curves for non-potable water supply for different demands and roof area of 60 m2 in Rasht.
M.H. Rashidi Mehrabadi et al. / Resources, Conservation and Recycling 73 (2013) 86–93
Fig. 16. Reliability curves for non-potable water supply for different demands and roof area of 120 m2 in Rasht.
Fig. 17. Reliability curves for non-potable water supply for different demands and roof area of 180 m2 in Rasht.
180 and 240 m2 in the same city, in which for large tanks to supply 240 L of non-potable water demand, approximately 100% of times the demand is met. Figs. 19 and 20 correspond to roof areas of 60 and 120 m2 in Kerman (with an arid climate), respectively. For 60 m2 roof area, the percentage of non-potable water supply is constant and not dependent on the tank volume. The maximum and minimum daily water supply varied between 6.5 and 1% corresponding to 240 and 720 L of daily demand, respectively. For roof area of 120 m2 , total water supply duration is low at approximately 2% to 15%. According to Figs. 21 and 22, for roof areas of 180 and 240 m2 , large tanks and minimum non-potable water demand (240 L), days of supply were at most 23% and 31%, respectively.
91
Fig. 18. Reliability curves for non-potable water supply for different demands and roof area of 240 m2 in Rasht.
Fig. 19. Reliability curves for non-potable water supply for different demands and roof area of 60 m2 in Kerman.
Comparison of rainwater saving and reliability for various tank volumes and demands among the three cities are presented in Table 2. According to this table, Rasht provides the highest rainwater storage in the range of 49.6–270.1 m3 per year among the three cities for various roof areas. The minimum rainwater storage is associated with Kerman by less than 30 m3 per year. In Rasht, for the minimum non-potable water demand (240 L), the storage volume ranged from 38.7% to 98.6% while for the maximum nonpotable water demand (720 L), it ranged from 11.9% to 70.5%. In addition, the lowest reliability for different demands was obtained in Kerman.In general, in Mediterranean and humid climates, once the roof surface area and tank volume increase, rainwater storage is significant. In humid climate, it is possible to supply at least 75%
Table 2 Comparison of rainwater saving and reliability for various tank volumes, roof areas, and water demands in three cities. City
Roof area (m2 )
Tank volume (L)
Range of rainwater saving (m3 )
Range of reliability (%) for different demands 240 L
360 L
480 L
600 L
720 L
Tabriz
60 120 180 240
1000–15,000 1000–15,000 1000–15,000 1000–15,000
13.5–13.8 23.8–27.7 31.1–41.5 36.4–55.3
11.5–12.3 21.9–29.2 26.7–44.3 30.9–58.3
6.3–6.6 14.1–17.8 19.3–29.1 21.5–38.6
3.9–4.1 9.6–12.3 13.0–20.2 16.4–28.9
2.4–2.6 6.2–8.8 8.5–14.9 10.4–21.6
1.6–1.8 4.8–6.6 7.4–12.3 9.2–17.6
Rasht
60 120 180 240
1000–15,000 1000–15,000 1000–15,000 1000–15,000
49.6–69.1 68.7–138.1 79.1–205.8 86.1–270.1
38.7–72.3 48.7–93.6 52.7–97.5 55.9–98.6
27.5–48.2 37.4–78.5 42.0–88.6 44.3–92.9
19.9–34.8 28.1–65.3 32.0–78.3 35.3–85.2
14.9–27.0 20.8–54.2 24.6–69.7 25.6–77.2
11.9–21.3 17.8–45.5 21.1–61.5 23.2–70.5
Kerman
60 120 180 240
1000–15,000 1000–15,000 1000–15,000 1000–15,000
6.9–7.2 12.0–14.4 15.7–21.6 18.5–28.7
5.8–6.5 10.9–15.3 13.3–23.3 15.6–31.6
3.2–3.5 7.0–9.4 9.7–15.3 10.8–20.4
2.0–2.2 4.8–6.5 6.6–10.7 8.3–15.3
1.2–1.4 3.2–4.7 4.2–7.9 5.3–11.5
0.9–1.0 2.4–3.5 3.9–6.5 4.9–9.4
92
M.H. Rashidi Mehrabadi et al. / Resources, Conservation and Recycling 73 (2013) 86–93
Fig. 20. Reliability curves for non-potable water supply for different demands and roof area of 120 m2 in Kerman.
that various combinations of climate condition, roof area, tank volume, and water demand can lead to widely different reliabilities. A decision support system may be developed to take into account such variables in order to offer optimal plans (Imteaz et al., 2011a). It is necessary to document and quantify the expected water storage under various climate conditions and water demands to adopt cost-effective tank volume (Imteaz et al., 2011b). Finally, it is important to note that rainfall temporal variability is one of the most important issues in planning rainwater harvesting systems. Temporal variations are not properly reflected in the climate classification schemes which mainly rely on the average annual rainfall. Similar to large water storage systems such as dams, time distribution of water availability governs the volume of needed storage and the reliability of the storage in dry periods. Therefore, long-term average annual rainfall is not a sufficient factor for the estimation of storage tank volume and design of rainwater harvesting systems. Such an approach will demonstrate failure in seasonal and annual dry periods. Although rainfall temporal variability was properly simulated in this study, further works are needed to determine system reliability in dry vs. wet periods. 5. Conclusions
Fig. 21. Reliability curves for non-potable water supply for different demands and roof area of 180 m2 in Kerman.
of non-potable water demand for a maximum duration of 70% of the times by collecting water from large roof areas. Furthermore, cities subject to Mediterranean climate, it is possible to supply at least 75% of non-potable water demand for a maximum duration of 40% of the times in case of buildings with large roof areas. However, in arid cities under the same conditions, rainwater storage is not sufficient. It is found that in arid climate, only 23% of the times, the stored water can provide 75% of non-potable water demand in case of large roof areas. Overall, the water supply efficiency of RWH implementation under various roof areas and climatic conditions vary widely. Similar results were obtained by Jenkins (2007) who demonstrated that the climate condition of the study location imposes a major impact on the effectiveness of rainwater tank systems. It is expected
In this research, reliability of rainwater storage for supply of non-potable water demand was investigated based on climate condition, roof area of residential buildings and non-potable water demand in three Iranian cities. For cities with high rainfall, such as Rasht, rainwater storage was considerable as tank capacity and roof area increased. Average ratio of annual rainwater stored by tanks to the total harvested rainwater from large roofs in small tanks was much less than unity. By increasing the tank capacity, this ratio approached unity. In Kerman with low rainfall and arid climate, the effect of increasing tank capacity for the same roof area was not significant. Average ratio of annual storage by tanks to the total harvested rainwater was nearly one while it dropped to smaller values for small tanks. For cities with moderate rainfall, such as Tabriz, average annual rainwater storage in tanks to the total harvested rainwater from roofs was consistent with the tank capacity. If the tank capacity was small, this ratio would be much less than unity. In such a condition, the majority of the water would overflow. Overall, in cities with high rainfall, it is possible to supply majority of the non-potable water in all days of a year. Tank capacity is an important consideration to maximize rainwater storage. In order to determine the optimum tank volume, the amount of rainfall and roof area are critical factors. For cities with average rainfall, the performance of the system is moderate while in arid region, the rainwater harvesting system is incapable of providing sufficient part of the demand. References
Fig. 22. Reliability curves for non-potable water supply for different demands and roof area of 240 m2 in Kerman.
Abdulla FA, Al-Shareef AW. Roof rainwater harvesting systems for household water supply in Jordan. Journal of Desalination 2009;243:195–207. Eroksuz E, Rahman A. Rainwater tanks in multi-unit buildings: a case study for three Australian cities. Journal of Resources Conservation and Recycling 2010;54:1449–52. 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. Journal of Building and Environment 2007;42:1654–66. Ghisi E, Tavares DDF, Rocha VL. Rainwater harvesting in petrol stations in Brasilia: potential for potable water savings and investment feasibility analysis. Journal of Resources, Conservation and Recycling 2009;54:79–85. Imteaz MA, Ahsan A, Naser J, Rahman A. Reliability analysis of rainwater tanks in Melbourne using daily water balance model. Journal of Resources, Conservation and Recycling 2011a;56:80–6. Imteaz MA, Rauf A, Aziz A. eTank: a decision support tool for optimizing rainwater tank size. In: International Congress on Modelling and Simulation MODSIM; 2011b.
M.H. Rashidi Mehrabadi et al. / Resources, Conservation and Recycling 73 (2013) 86–93 Imteaz MA, Rahman A, Ahsan A. Reliability analysis of rainwater tanks: a comparison between South East and Central Melbourne. Journal of Resources, Conservation and Recycling 2012;66:1–7. Mun JS, Han MY. Design and operational parameters of a rooftop rainwater harvesting system: definition, sensitivity and verification. Journal of Environmental Management 2012;93:147–53. Jenkins GA. Use of continuous simulation for the selection of an appropriate urban rainwater tank. Australian Journal of Water Resources 2007;11(2): 231–46. Khastagir A, Jayasuriya N. Optimal sizing of rain water tanks for domestic water conservation. Journal of Hydrology 2010;381:181–8.
93
Palla A, Gnecco I, Lanza LG. Non-dimensional design parameters and performance assessment of rainwater harvesting systems. Journal of Hydrology 2011;401:65–76. Rygaard M, Binninga PJ, Albrechtsena HJ. Increasing urban water selfsufficiency: new era, new challenges. Journal of Environmental Management 2011;92(1): 185–94. Sturm M, Zimmermann M, Schütz K, Urban W, Hartung H. Rainwater harvesting as an alternative water resource in rural sites in central northern Namibia. Journal of Physics and Chemistry of the Earth 2009;34:776–85.
Contents lists available at SciVerse ScienceDirect
Resources, Conservation and Recycling journal homepage: www.elsevier.com/locate/resconrec
Assessment of residential rainwater harvesting efficiency for meeting non-potable water demands in three climate conditions Mohammad Hossein Rashidi Mehrabadi a,1 , Bahram Saghafian a,2 , Fereshte Haghighi Fashi b,∗ a b
Department of Technical and Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran Soil Conservation and Watershed Management Research Institute, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 19 August 2012 Received in revised form 30 December 2012 Accepted 24 January 2013 Keywords: Rainwater harvesting Roof Residential building Water supply Climate Iran
a b s t r a c t Global demand for clean water supplies is on the rise due to population growth. This is also true in most cities of Iran. Non-conventional water resources must be developed to partially offset the increasing demand. In this study, the applicability and performance of rainwater harvesting (RWH) systems to supply daily non-potable water were assessed. Storage of rain falling on the roofs of residential buildings and directed into installed tanks was simulated in three cities of varying climatic conditions, namely Tabriz (Mediterranean climate), Rasht (humid climate), and Kerman (arid climate). Daily rainfall statistics for a period of 53 years as well as the information on the contributing roof area, available tank volumes and non-potable water demand were collected in each city. Typical residential buildings with roof areas of 60, 120, 180 and 240 m2 with an average of four residents in each house were considered for the study. According to the results in humid climate, it is possible to supply at least 75% of non-potable water demand by storing rainwater from larger roof areas for a maximum duration of 70% of the times. For roofs with small surface area, the supply meets 75% of non-potable water demand for a maximum duration of 45% of the times. Moreover, for Mediterranean climate, it is possible to supply at least 75% of non-potable water demand in buildings with larger roof areas for a maximum duration of 40% of the times. It is also found that in arid climate, similar duration is only 23% of the times. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, water supply has become a vital issue globally. In some locations, water collected from roofs of residential or commercial buildings has been a practical way to supply water for irrigation and partially meet potable and non-potable water demand. Since major parts of urban areas are under roofs or covered by impervious material, the potential for harvesting rainwater is considerable in cities. These areas are not only a source of generating non-potable water, but also urban flooding may be reduced when rainwater is stored by installed tanks. Abdulla and Al-Shareef (2009) assessed the potential for potable water savings by using rainwater harvesting (RWH) in residential cities of 12 Jordanian governorates. The research provided suggestions regarding the improvement of both the quality and quantity of harvested rainwater and demonstrated the importance of roof rainwater harvesting systems for domestic water supply.
∗ Corresponding author. Tel.: +98 2177072309. E-mail addresses: hossein [email protected] (M.H. Rashidi Mehrabadi), b.saghafi[email protected] (B. Saghafian), [email protected] (F. Haghighi Fashi). 1 Tel.: +98 2177072309. 2 Tel.: +98 2144868407. 0921-3449/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.resconrec.2013.01.015
Sturm et al. (2009) developed an appropriate rainwater harvesting solution based on hydrological, technical, and social conditions in central northern Namibia. They provided appropriate RWH design guidelines for the required storage capacities. Results also revealed that it is economically feasible to implement RWH in the context of roof catchment systems. Ghisi et al. (2007) evaluated the potential for water saving by using RWH in Brazil and reported that average potential for potable water savings was 12–79% per year for the studied cities. In another research, Ghisi et al. (2009) studied rainwater harvesting as a way to optimize the supply and use of water resources. The results revealed that the average saving of potable water by RWHs was 32.7%, with the potential in the range of 9.2–57.2%. Eroksuz and Rahman (2010) evaluated rainwater harvesting systems in multi-unit buildings in Australia. They investigated the water saving potential of tanks fitted in three cities: Sydney, Newcastle and Wollongong. Results indicated that for multi-unit buildings, a larger tank size was more appropriate to maximize water saving. It was also found that these tanks provided significant water savings even in dry years. Khastagir and Jayasuriya (2010) investigated the reliability of rainwater tanks. They presented design charts of optimum tank sizes considering daily rainfall, water demand and roof area necessary to meet the demand with 90% reliability. Palla et al. (2011) investigated optimum performance of a rainwater harvesting
M.H. Rashidi Mehrabadi et al. / Resources, Conservation and Recycling 73 (2013) 86–93
Fig. 1. Schematic view of a rainwater harvesting system.
system. They developed a model that determined the efficiency of the system, over flow ratio and detention time. Performance was examined under various environmental conditions including three precipitation regimes and three water demand quantity. Imteaz et al. (2011b) developed a tool called eTank for the optimization of rainwater tank size. The study showed that, it is possible to achieve approximately 100% reliability with a roof size of 150–300 m2 and a tank size of 5000–10,000 L for a household of two occupants. Imteaz et al. (2012) presented several reliability curves for domestic rainwater tanks in relations to roof area, tank volume, number of residents in each house and total water demand supplied by the harvested rainwater under three climatic conditions. Their study showed that for a roof size of 100 m2 , 100%
87
reliability was not achievable even with a very large tank volume. They found that for a large roof size (200 m2 ), nearly 90% reliability can be achieved with a large tank volume (10,000 L) while larger volumes could yield 100% reliability, except under dry conditions. Overall, there is growing need to identify and promote alternative water supplies in light of growing demands. Rainwater harvesting systems in urban regions where the water supply is quantitatively, qualitatively or economically limited, could be a viable option. Harvesting rainwater may be considered as a suitable water resource in terms of cost and water quality. Although rainwater harvesting provides with safe water supply (Rygaard et al., 2011; Mun and Han, 2012), there is lack of information on the optimal tank volume in combination with roof area to reach the expected performance under different climatic conditions in Iran. The purpose of this study is to assess applicability and performance of rainwater harvesting systems for daily non-potable water supply in three different climatic areas located in Iran. The studied cities are Tabriz, Rasht and Kerman subject to Mediterranean, humid and arid climate conditions, respectively. Reliability of the system is determined based on the size of storage tank, roof area, and water demand. 2. Methodology In a rainwater harvesting system, firstly, rainwater is collected from a roof surface and directed to the storage tank by a down pipe. Surplus water is allowed to spill and flow to the surface drainage system or a waste water network. Collected rainwater may be stored in the tank and consumed as non-potable water. Fig. 1 shows a rainwater harvesting system tailored to a residential building.
Fig. 2. Flowchart of simulation of a rainwater harvesting system.
88
M.H. Rashidi Mehrabadi et al. / Resources, Conservation and Recycling 73 (2013) 86–93
Fig. 3. Location of case study cities in Iran.
In order to model a rainwater harvesting system, the initial volume of water in the tank (V0 ) and overflow water (SP0 ) is taken as zero: SP0 = 0
(1)
V0 = 0
(2)
The volume of harvested rainwater from the roof may be expressed by: It = Rt × A × ϕ
(3)
where I is the daily volume of collectable rainwater (L), ϕ is the roof runoff coefficient (dimensionless), R is the daily rainfall depth (mm), t is the day number (T), and A is the area of the roof (m2 ). The water volume in the tank may be updated in any daily time step as follows: Vt = It + Vt−1 − Ot − SPt
(4)
where V is the water storage volume (L) and O is the daily nonpotable water withdrawal from the tank (L) (see Fig. 1). Water volume withdrawn from the urban piped system (U) and the total non-potable water demand (O) are related through the following conditions: If :
It + Vt−1 > Dt → Ot = Dt
If :
It + Vt−1 < Dt
Ot = It + Vt−1 Ut = Dt − Ot
(5)
3. Study area Case study cities were selected on the basis of a simple climate classification. Mediterranean, humid, and arid are climate types associated with Tabriz, Rasht and Kerman cities in Iran, respectively. Location of case study cities is shown in Fig. 3. Daily rainfall statistics were prepared over the 1956–2008 period (Table 1). The time series of mean daily rainfall of the study cities are shown in Fig. 4.
(6)
where D is the demand. Volume of overflowed water is determined as follows: SPt = Vt−1 + It − Ot − Vmax
rectangles and required parameters and input data are shown in the dashed right side rectangles. In this study, tank capacity was varied from 1000 to 15,000 L. Total land area taken up by residential buildings was selected in typical plots of 100, 200, 300 and 400 m2 of which roof area covers 60% of the total area, i.e. 60, 120, 180 and 240 m2 are roof area, respectively. Runoff coefficient for roofs of residential buildings was considered 0.8 in Kerman and Tabriz and 0.85 in Rasht based on construction material and slope of the roofs. The average number of occupants in each building was taken as four. Non-potable water demand was assumed in the range of 60–180 L per resident per day. Therefore, the total demand for each building was estimated at 240–720 L. The expected minimum requirement of the water supply system was set to 75%.
(7)
where SP is the volume of the tank overflow (L) and Vmax is the tank volume (L). In Fig. 2, computational steps are shown in the left side
Table 1 Average annual rainfall in three cities. Average annual rainfall (mm)
Period
City
288 1355 150
1956–2008 1956–2008 1956–2008
Tabriz Rasht Kerman
M.H. Rashidi Mehrabadi et al. / Resources, Conservation and Recycling 73 (2013) 86–93
Fig. 4. Variation of average daily rainfall depth over 1956–2008 period (starting from January 1).
4. Results and discussion Fig. 5 shows rainwater storage in various tank volumes in Tabriz for different roof areas. In Tabriz, rainwater storage for 60 m2 roof area is nearly constant at 13,700 L per year. The highest rainwater storage is associated with a roof area of 240 m2 , which could store a total volume of 36,000–55,000 L per year. Fig. 6 shows rainwater storage in Rasht. Rasht (with a humid climate) provides highest rainwater saving amongst three cities. The largest and the smallest average yearly rainwater storage volumes are 227,000 and 67,000 L, corresponding to roof areas of 240 and 60 m2 , respectively. Averages annual rainwater storage for small roof areas varies from 50,000 to 138,000 L, and for large roof areas from 80,000 to 270,000 L in Rasht. However, according to Fig. 7, the largest average storage for a roof area of 240 m2 is only 28,000 L
89
Fig. 7. Average annual water storage in tank for various roof areas in Kerman.
annually for larger than 5000 L tank volume in Kerman, while, averages of rainwater storage for small roof areas varies from 7000 to 14,000 L. Based on Fig. 8 for Tabriz, for a tank volume of over 3500 L, the ratio of average annual water stored by the tanks over the average annual harvested rainwater is approximately equal to unity. For smaller tanks, this ratio is 0.65–0.9. Based on Fig. 9, for a roof area of 240 m2 and tanks volume up to 11,500 L in Rasht, the ratio of average annual water stored by the tanks over the average annual harvested rainwater varies between 0.3 and 0.9. However, the ratio corresponding to 60 m2 roof area and tank capacity of over 5000 L approaches unity. Similar to Tabriz, for tank capacity of over 4000 L, the ratio is near one in Kerman (see Fig. 10).
Fig. 8. Stored water volume to total harvested rainwater for various roof areas in Tabriz. Fig. 5. Average annual water storage in tank for various roof areas in Tabriz.
Fig. 6. Average annual water storage in tank for various roof areas in Rasht.
Fig. 9. Stored water volume to total harvested rainwater for various roof areas in Rasht.
90
M.H. Rashidi Mehrabadi et al. / Resources, Conservation and Recycling 73 (2013) 86–93
Fig. 10. Stored water volume to total harvested rainwater for various roof areas in Kerman.
Fig. 13. Reliability curves for non-potable water supply for different demands and roof area of 180 m2 in Tabriz.
Figs. 11–22 present the reliability as the percentages of times in which at least 75% of non-potable water demand in various tank volumes, roof areas, and non-potable water demands is supplied. Figs. 11 and 12 correspond to 60 and 120 m2 roof areas, respectively, as non-potable water demand varies from 240 to 720 L in Tabriz (with a Mediterranean climate). For 60 m2 roof area, only in 12% of the times, at least 75% of 480 L of total non-potable water demand could be supplied while in 6% of times, a total demand of 720 L would be expected to be met. For the roof area of 120 m2 , maximum and minimum percentages of daily supply are 29% and 5%, respectively, corresponding to 240 and 720 L of daily demand. Similarly in Figs. 13 and 14 corresponding to 180 and 240 m2 roof areas in Tabriz, respectively, water supply reliability varies between 7%
and 44% for 180 m2 roof area, and between 10% and 59% for 240 m2 roof area, respectively. In case of 240 m2 roof area and 4500 L tank volume or larger, at least 17% and 50% of times water supply is sufficient to meet 720 and 240 L of total non-potable water demand, respectively. Figs. 15 and 16 correspond to roof areas of 60 and 120 m2 , respectively, in Rasht. For roof area of 60 m2 , maximum and minimum percentages of daily supply are 73% and 12%, respectively, corresponding to 240 and 720 L of daily demand. For roof area of 120 m2 , total duration that water is supplied by rainwater increases such that for large tanks and 240 and 720 L demands, average reliabilities were 82% and 39%, respectively. From tank capacity of 6500 L and larger, at least 40% of times, the non-potable water demand would be met. Figs. 17 and 18 correspond to roof areas of
Fig. 11. Reliability curves for non-potable water supply for different demands and roof area of 60 m2 in Tabriz.
Fig. 12. Reliability curves for non-potable water supply for different demands and roof area of 120 m2 in Tabriz.
Fig. 14. Reliability curves for non-potable water supply for different demands and roof area of 240 m2 in Tabriz.
Fig. 15. Reliability curves for non-potable water supply for different demands and roof area of 60 m2 in Rasht.
M.H. Rashidi Mehrabadi et al. / Resources, Conservation and Recycling 73 (2013) 86–93
Fig. 16. Reliability curves for non-potable water supply for different demands and roof area of 120 m2 in Rasht.
Fig. 17. Reliability curves for non-potable water supply for different demands and roof area of 180 m2 in Rasht.
180 and 240 m2 in the same city, in which for large tanks to supply 240 L of non-potable water demand, approximately 100% of times the demand is met. Figs. 19 and 20 correspond to roof areas of 60 and 120 m2 in Kerman (with an arid climate), respectively. For 60 m2 roof area, the percentage of non-potable water supply is constant and not dependent on the tank volume. The maximum and minimum daily water supply varied between 6.5 and 1% corresponding to 240 and 720 L of daily demand, respectively. For roof area of 120 m2 , total water supply duration is low at approximately 2% to 15%. According to Figs. 21 and 22, for roof areas of 180 and 240 m2 , large tanks and minimum non-potable water demand (240 L), days of supply were at most 23% and 31%, respectively.
91
Fig. 18. Reliability curves for non-potable water supply for different demands and roof area of 240 m2 in Rasht.
Fig. 19. Reliability curves for non-potable water supply for different demands and roof area of 60 m2 in Kerman.
Comparison of rainwater saving and reliability for various tank volumes and demands among the three cities are presented in Table 2. According to this table, Rasht provides the highest rainwater storage in the range of 49.6–270.1 m3 per year among the three cities for various roof areas. The minimum rainwater storage is associated with Kerman by less than 30 m3 per year. In Rasht, for the minimum non-potable water demand (240 L), the storage volume ranged from 38.7% to 98.6% while for the maximum nonpotable water demand (720 L), it ranged from 11.9% to 70.5%. In addition, the lowest reliability for different demands was obtained in Kerman.In general, in Mediterranean and humid climates, once the roof surface area and tank volume increase, rainwater storage is significant. In humid climate, it is possible to supply at least 75%
Table 2 Comparison of rainwater saving and reliability for various tank volumes, roof areas, and water demands in three cities. City
Roof area (m2 )
Tank volume (L)
Range of rainwater saving (m3 )
Range of reliability (%) for different demands 240 L
360 L
480 L
600 L
720 L
Tabriz
60 120 180 240
1000–15,000 1000–15,000 1000–15,000 1000–15,000
13.5–13.8 23.8–27.7 31.1–41.5 36.4–55.3
11.5–12.3 21.9–29.2 26.7–44.3 30.9–58.3
6.3–6.6 14.1–17.8 19.3–29.1 21.5–38.6
3.9–4.1 9.6–12.3 13.0–20.2 16.4–28.9
2.4–2.6 6.2–8.8 8.5–14.9 10.4–21.6
1.6–1.8 4.8–6.6 7.4–12.3 9.2–17.6
Rasht
60 120 180 240
1000–15,000 1000–15,000 1000–15,000 1000–15,000
49.6–69.1 68.7–138.1 79.1–205.8 86.1–270.1
38.7–72.3 48.7–93.6 52.7–97.5 55.9–98.6
27.5–48.2 37.4–78.5 42.0–88.6 44.3–92.9
19.9–34.8 28.1–65.3 32.0–78.3 35.3–85.2
14.9–27.0 20.8–54.2 24.6–69.7 25.6–77.2
11.9–21.3 17.8–45.5 21.1–61.5 23.2–70.5
Kerman
60 120 180 240
1000–15,000 1000–15,000 1000–15,000 1000–15,000
6.9–7.2 12.0–14.4 15.7–21.6 18.5–28.7
5.8–6.5 10.9–15.3 13.3–23.3 15.6–31.6
3.2–3.5 7.0–9.4 9.7–15.3 10.8–20.4
2.0–2.2 4.8–6.5 6.6–10.7 8.3–15.3
1.2–1.4 3.2–4.7 4.2–7.9 5.3–11.5
0.9–1.0 2.4–3.5 3.9–6.5 4.9–9.4
92
M.H. Rashidi Mehrabadi et al. / Resources, Conservation and Recycling 73 (2013) 86–93
Fig. 20. Reliability curves for non-potable water supply for different demands and roof area of 120 m2 in Kerman.
that various combinations of climate condition, roof area, tank volume, and water demand can lead to widely different reliabilities. A decision support system may be developed to take into account such variables in order to offer optimal plans (Imteaz et al., 2011a). It is necessary to document and quantify the expected water storage under various climate conditions and water demands to adopt cost-effective tank volume (Imteaz et al., 2011b). Finally, it is important to note that rainfall temporal variability is one of the most important issues in planning rainwater harvesting systems. Temporal variations are not properly reflected in the climate classification schemes which mainly rely on the average annual rainfall. Similar to large water storage systems such as dams, time distribution of water availability governs the volume of needed storage and the reliability of the storage in dry periods. Therefore, long-term average annual rainfall is not a sufficient factor for the estimation of storage tank volume and design of rainwater harvesting systems. Such an approach will demonstrate failure in seasonal and annual dry periods. Although rainfall temporal variability was properly simulated in this study, further works are needed to determine system reliability in dry vs. wet periods. 5. Conclusions
Fig. 21. Reliability curves for non-potable water supply for different demands and roof area of 180 m2 in Kerman.
of non-potable water demand for a maximum duration of 70% of the times by collecting water from large roof areas. Furthermore, cities subject to Mediterranean climate, it is possible to supply at least 75% of non-potable water demand for a maximum duration of 40% of the times in case of buildings with large roof areas. However, in arid cities under the same conditions, rainwater storage is not sufficient. It is found that in arid climate, only 23% of the times, the stored water can provide 75% of non-potable water demand in case of large roof areas. Overall, the water supply efficiency of RWH implementation under various roof areas and climatic conditions vary widely. Similar results were obtained by Jenkins (2007) who demonstrated that the climate condition of the study location imposes a major impact on the effectiveness of rainwater tank systems. It is expected
In this research, reliability of rainwater storage for supply of non-potable water demand was investigated based on climate condition, roof area of residential buildings and non-potable water demand in three Iranian cities. For cities with high rainfall, such as Rasht, rainwater storage was considerable as tank capacity and roof area increased. Average ratio of annual rainwater stored by tanks to the total harvested rainwater from large roofs in small tanks was much less than unity. By increasing the tank capacity, this ratio approached unity. In Kerman with low rainfall and arid climate, the effect of increasing tank capacity for the same roof area was not significant. Average ratio of annual storage by tanks to the total harvested rainwater was nearly one while it dropped to smaller values for small tanks. For cities with moderate rainfall, such as Tabriz, average annual rainwater storage in tanks to the total harvested rainwater from roofs was consistent with the tank capacity. If the tank capacity was small, this ratio would be much less than unity. In such a condition, the majority of the water would overflow. Overall, in cities with high rainfall, it is possible to supply majority of the non-potable water in all days of a year. Tank capacity is an important consideration to maximize rainwater storage. In order to determine the optimum tank volume, the amount of rainfall and roof area are critical factors. For cities with average rainfall, the performance of the system is moderate while in arid region, the rainwater harvesting system is incapable of providing sufficient part of the demand. References
Fig. 22. Reliability curves for non-potable water supply for different demands and roof area of 240 m2 in Kerman.
Abdulla FA, Al-Shareef AW. Roof rainwater harvesting systems for household water supply in Jordan. Journal of Desalination 2009;243:195–207. Eroksuz E, Rahman A. Rainwater tanks in multi-unit buildings: a case study for three Australian cities. Journal of Resources Conservation and Recycling 2010;54:1449–52. 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. Journal of Building and Environment 2007;42:1654–66. Ghisi E, Tavares DDF, Rocha VL. Rainwater harvesting in petrol stations in Brasilia: potential for potable water savings and investment feasibility analysis. Journal of Resources, Conservation and Recycling 2009;54:79–85. Imteaz MA, Ahsan A, Naser J, Rahman A. Reliability analysis of rainwater tanks in Melbourne using daily water balance model. Journal of Resources, Conservation and Recycling 2011a;56:80–6. Imteaz MA, Rauf A, Aziz A. eTank: a decision support tool for optimizing rainwater tank size. In: International Congress on Modelling and Simulation MODSIM; 2011b.
M.H. Rashidi Mehrabadi et al. / Resources, Conservation and Recycling 73 (2013) 86–93 Imteaz MA, Rahman A, Ahsan A. Reliability analysis of rainwater tanks: a comparison between South East and Central Melbourne. Journal of Resources, Conservation and Recycling 2012;66:1–7. Mun JS, Han MY. Design and operational parameters of a rooftop rainwater harvesting system: definition, sensitivity and verification. Journal of Environmental Management 2012;93:147–53. Jenkins GA. Use of continuous simulation for the selection of an appropriate urban rainwater tank. Australian Journal of Water Resources 2007;11(2): 231–46. Khastagir A, Jayasuriya N. Optimal sizing of rain water tanks for domestic water conservation. Journal of Hydrology 2010;381:181–8.
93
Palla A, Gnecco I, Lanza LG. Non-dimensional design parameters and performance assessment of rainwater harvesting systems. Journal of Hydrology 2011;401:65–76. Rygaard M, Binninga PJ, Albrechtsena HJ. Increasing urban water selfsufficiency: new era, new challenges. Journal of Environmental Management 2011;92(1): 185–94. Sturm M, Zimmermann M, Schütz K, Urban W, Hartung H. Rainwater harvesting as an alternative water resource in rural sites in central northern Namibia. Journal of Physics and Chemistry of the Earth 2009;34:776–85.