- Email: [email protected]
Rainwater harvesting potential for southwest Nigeria using daily water balance model
Resources, Conservation and Recycling 62 (2012) 51–55
Contents lists available at SciVerse ScienceDirect
Resources, Conservation and Recycling journal homepage: www.elsevier.com/locate/resconrec
Rainwater harvesting potential for southwest Nigeria using daily water balance model Monzur Alam Imteaz a,∗ , Omotayo B. Adeboye b,c , Scott Rayburg a , Abdallah Shanableh d a
Faculty of Engineering and Industrial Sciences, Swinburne University of Technology, Melbourne, Australia Department of Agricultural Engineering, Obafemi Awolowo University, Ile-Ife, Osun State, Nigeria Department of Water Engineering (Land and Water Development), UNESCO-IHE Institute For Water Education, Delft, Netherlands d Department of Civil & Environmental Engineering, University of Sharjah, Sharjah, United Arab Emirates b c
a r t i c l e
i n f o
Article history: Received 11 October 2011 Received in revised form 7 February 2012 Accepted 9 February 2012 Keywords: Rainwater tank Water balance model Reliability Dry year and optimum tank size
a b s t r a c t For the performance analysis and design of rainwater tanks, a simple spreadsheet based daily water balance model was developed using daily rainfall data, contributing roof area, rainfall loss factor, available storage volume, tank overflow and rainwater demand. This water balance model was then used to design an optimum size of domestic rainwater tank to be used for southwest Nigeria. The optimisation criterion was set to provide uninterrupted intended demand from the selected rainwater tank during the critical (dry) months. For the tank water, two demand scenarios were assessed: (i) toilet flushing only; and (ii) toilet flushing and laundry use. Analysis was performed for a typical dry year (1998) in southwest Nigeria. Current analysis outcomes were compared with an earlier analysis using monthly average rainfall data. It is found that analysis using monthly average rainfall data overestimates the required rainwater tank size. In addition, the newly developed model was used to assess the reliability of domestic rainwater tanks in augmenting partial household water demand. This analysis showed that a reliability of 100% is possible to achieve with a tank size of 7000 L under low demand. However, with higher demand a bigger tank size (∼10,000 L) is required to achieve 100% reliability even though very high reliability could also be attained with a tank size of 7000 L. From overflow analysis, the results of this study showed that a large quantity of water is lost as overflow, even in a dry year with a tank size of 10,000 L. Thus, harvested rainwater could be used for other purposes if larger tanks are used as these would capture more of the excess rainwater which could then be tasked to other purposes without compromising the reliability of water availability for primary uses. © 2012 Elsevier B.V. All rights reserved.
1. Introduction With increasing population and changing climate regime, water supply systems in many cities of the world are under stress. To tackle this problem, Water Corporations and River Basin Authorities are adopting several measures including demand management, and identifying alternative water sources, such as stormwater harvesting, greywater and wastewater reuse and desalination. Among all the alternative water sources, stormwater (or rainwater) harvesting perhaps has received the most attention. Rainwater harvesting is not a new concept in water resources management. Indeed, the concept has been practised for a long period of time, even before the advent of large scale public water systems. In modern times, rainwater harvesting is actively being encouraged and promoted in China, Brazil, Australia and India and is beginning to gain traction in other parts of the world as well. The value of
∗ Corresponding author. E-mail address: [email protected] (M.A. Imteaz). 0921-3449/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.resconrec.2012.02.007
rainwater harvesting as a means of supplementing or replacing town water supplies has been demonstrated by numerous researchers. For example, rainwater has been reported to promote significant potable water savings in buildings (Hermann and Schimda, 1999; Fewkes, 1999; Appan, 2000; Handia et al., 2003). As a consequence of these findings, many cities have instigated large scale rainwater use projects in public facilities including in Japan (Zaizen et al., 1999), London, England (Hills et al., 2001) and Melbourne in Australia (Imteaz et al., 2011a). In Australia, government authorities have been promoting stormwater harvesting through media campaigns, as well as offering incentives and grants to promote water saving ideas and innovations. For example, in Victoria, rebates of up to $500 are available for those who install new rainwater tanks on their properties. This move to include rainwater harvesting as a vital component of urban water management also has secondary benefits as the expanded use of rainwater harvesting and other simple innovative technologies has the potential to reduce greenhouse gas emissions from water storage reservoirs and water treatment processes which contribute to climate change (Flower et al., 2007).
52
M.A. Imteaz et al. / Resources, Conservation and Recycling 62 (2012) 51–55
Coombes and Kuczera (2003) found that for an individual building with a 150 m2 roof area and 1–5 kL tank in Sydney, Australia can yield 10–58% mains water savings (depending on the number of people using the building). According to Coombes and Kuczera (2003), depending on roof area and the number of occupants, rainwater tank use can result in annual mains water savings of 18–55 kL for 1 kL sized tanks and 25–144 kL for 10 kL sized tanks. In Sweden, Villarreal and Dixon (2005) investigated water savings potential of stormwater harvesting systems from roof areas. Villarreal and Dixon discovered that mains water saving of 30% can be achieved using a 40 m3 sized tank (toilet and washing machine use only). Ghisi et al. (2007, 2009) investigated the water savings potential from rainwater harvesting systems in Brazil (South America) and found that average potential for potable water savings of 12–79% per year for the cities analysed. Despite positive outcome from many studies, there remains a general community reluctance to adopt stormwater harvesting on a wider scale. Part of the reason for this reluctance can be attributed to lack of information about the effectiveness of a stormwater harvesting system and the optimum storage size required to satisfy the performance requirements under the specific site conditions (Imteaz et al., 2011a). A proper in-depth understanding of the effectiveness of any proposed on-site stormwater harvesting system is often lacking. Vaes and Berlamont (2001) developed a model to determine the effectiveness of rainwater tanks and stormwater runoff using long term historical rainfall data. Coombes (2007) conducted studies on the modelling of the rainwater tanks and the opportunities for effective retention storage using the PURRS (Probabilistic Urban Rainwater and Wastewater Reuse Simulator) water balance model. Jenkins (2007) developed a computer model for the continuous simulations of amount of rainwater stored in the tank, amount of rainwater used, amount overflowed and amount of mainwater used to top up the tank for household rainwater tanks. Jenkins (2007) concluded that the climate characteristics of the site have a significant influence on the effectiveness of the rainwater tank. Eroksuz and Rahman (2010) investigated the water savings potential of rainwater tanks in multi-storied residential buildings for three cities in eastern Australia. They have concluded that rainwater tank of appropriate size in a multi-storied building can provide significant water savings even in dry years. Imteaz et al. (2011b) have presented a range of reliability charts for residential rainwater tanks in Central Melbourne that serve to illustrate the benefits of rainwater harvesting even in a highly variable climatic regime like that experienced in Australia. The preponderance of data and modelling associated with rainwater harvesting is focussed on developed countries, where often, water scarcity is a matter of economics rather than basic human health. Consequently, there is a need to consider how rainwater harvesting might improve conditions in the developing world where access to fresh water is often lacking with significant implications for quality of life. Like many developing countries, Nigeria cannot satisfy its domestic water needs and in 2007 only 47% of the total population had access to water from improved sources (Aladenola and Adeboye, 2010). The country is already facing water supply shortages in both urban and rural areas despite the abundant land and water resources that are available in various climatic zones (Adeboye and Alatise, 2008). Rising population and urbanisation coupled with climate change may further increase the scarcity of water. If the abundant rainfall in the country particularly in southwest Nigeria is well harnessed, however, it might help to solve the perennial water scarcity being experienced in the entire country. In Nigeria, data on available rainwater harvesting for domestic use are very scanty and not well documented thereby making it difficult for objective assessment despite the fact that rainwater constitutes more than 90% of surface water. Similarly, the use
of rainwater for water closet (WC) flushing and laundry has not been estimated for any city or region in Nigeria. This paper presents analyses of household rainwater harvesting potential in Abeokuta (Nigeria) using a daily water balance model. Findings of the current model were compared with an earlier study conducted by Aladenola and Adeboye (2010) using monthly average rainfall data. Eventually, the model was used to prepare several rainwater tank reliability charts under different scenarios. These data provide an estimate of the benefits to be derived from rainwater harvesting in the developing world and can help to provide justification for rainwater harvesting in water management plans for these areas. 2. Study area Abeokuta is located in the humid tropical rainforest zone of the Southwestern part of Nigeria between Latitude 7◦ 5 N to 7◦ 20 N and Longitude 3◦ 17 E to 3◦ 20 E. The mean daily temperature is about 28 ◦ C. The city is drained mainly by the River Ogun which is dendritic in pattern. It covers a geographic area of 1256 km2 and has a population of 449,088 inhabitants. The water supply from Ogun State Water Corporation (OSWC) is inadequate to meet the daily domestic and industrial demands of the growing population; hence, there is a need to develop other water sources. Alternate water sources used in the city include borehole and shallow wells; however, these are prone to contamination (Orebiyi et al., 2008) and so are not of sufficient quality to meet the growing demand. However, rainwater could complement available water sources in Abeokuta and other parts of Nigeria if effectively harnessed. 3. Methodology A spreadsheet based on a daily water balance model was developed considering daily rainfall, contributing catchment (roof) area, losses due to leakage, spillage and evaporation, storage (tank) volume and water uses. The daily runoff volume is calculated from daily rainfall amount by multiplying the rainfall amount with the contributing roof area and deducting the losses. Based on a guideline produced by Thomas and Martinson (2007), a 15% deduction from the produced runoff was applied to account for losses (including leakage, spilling and evaporation) for hard roof in humid tropics. Generated runoff is then diverted to the connected available storage tank. Available storage capacity is compared with the accumulated daily runoff. If the accumulated runoff is higher than available storage volume, excess water (overflow) is deducted from the accumulated runoff. Then, the amount of water use(s) is deducted from the daily accumulated/stored runoff amount (if sufficient water is available in the storage). When sufficient water is not available in the storage, the model assumes that the remaining water demand is supplied from the town water supply. Thus, the model calculates daily stormwater use, daily water storage in the tank, daily overflow and daily town water use. In addition, the model calculates accumulated annual stormwater use, accumulated annual overflow and accumulated annual town water use. The overall process can be mathematically described as follows: Cumulative water storage equation, St = Vt + St−1 − D
(1)
St = 0, for St < 0
(2)
St = C, for St > C
(3)
where St is the cumulative water stored in the rainwater tank (L) after the end of tth day, Vt is the harvested rainwater (L) on the tth day, St−1 is the storage in the tank (L) at the beginning of tth day, D
M.A. Imteaz et al. / Resources, Conservation and Recycling 62 (2012) 51–55
is the daily rainwater demand (L), and, C is the capacity of rainwater tank (L), Townwater use equation, TW = D − St , for St < D
(4)
where TW is the townwater use on tth day (L). Overflow equation, OF = St − C, for St > C
(5)
53
Table 1 Required tank size: a comparison of analyses using monthly average data and daily recorded data. Demand (m3 ) per month
High = 2.45 Low = 1.80
Required rainwater tank size (L) Analysis with monthly average data
Analysis with recorded daily data
9720 5510
6000 4500
where OF is the overflow on tth day (L). Reliability is calculated with the equation, Re =
N−U × 100 N
(6)
where Re is the reliability of the tank to be able to supply intended demand (%), U is the number of days in a year the tank was unable to meet the demand, and N is the total number of days in a particular year. 4. Analysis and results Current analysis is based on a daily water balance (i.e., using daily rainfall and water uses data). However, daily rainfall data for different regions of Nigeria is quite scarce. For the current study, daily rainfall data was collected from Abeokuta city for a typical dry year, 1998. It is to be noted that the long-term annual average rainfall for the city is 1156 mm, whereas in 1998 the total annual rainfall was 567 mm. Fig. 1 shows the rainfall time series for the year 1998, starting from 1st January. As the preliminary objective was to compare the outcomes of the daily water balance model with typical average monthly data, other relevant data were taken from the study conducted by Aladenola and Adeboye (2010) who used monthly average rainfall data for their analysis. An average household roof area for the area was taken as 80 m2 . To avoid rainwater quality issues, harvested rainwater was not assumed to be used for drinking and/or cooking. Based on estimation of Aladenola and Adeboye (2010), two typical demand scenarios were selected, namely: (a) low demand; for toilet flushing only, which is 1.8 m3 in a month per household; and (b) high demand; for combined toilet flushing and laundry, which is 2.45 m3 in a month per household. As average monthly rainfalls during March to October is well above the above-mentioned demand, the critical months (November to February) were considered for current analysis. With the selected year’s daily rainfall data, the daily water balance model was simulated starting from March to February in the following year. The model was simulated from March, to allow for the reality that before the onset of the dry period (November), the tank may have some residual water from the previous months. The model was simulated to achieve an uninterrupted supply (i.e., rainwater tank size is sufficient to supply the intended demand) of
Fig. 1. Daily rainfall data for the year 1998 from Abeokuta city.
Fig. 2. Reliability curves under different demand scenarios.
rainwater. Table 1 shows the comparisons of the findings of the daily water balance model with the analysis considering average monthly rainfall. It is found that analysis with the average monthly data overestimate the tank sizes under both scenarios. It is to be noted that the daily water balance model is expected to produce more realistic outcomes. With the same dry year’s rainfall data, several analyses were performed using a whole year’s rainfall data to provide detail insights of performances for different tank sizes. Fig. 2 shows the reliability curves varying with tank size, under different demand scenarios. It is found that a 100% reliability can be achieved for a tank size of 7000 L for the low demand scenario and for a tank size of 10,000 L for the high demand scenario. Fig. 3 shows the overflow loss (annual) curves varying with tank sizes, under different demand scenarios. It is found that overflow losses can be reduced significantly with an increase in tank size. It is also found that even in a high demand scenario, with a tank size of 10,000 L, there will be an annual overflow loss of about 6000 L in a typical dry year. This loss would be more than 10,000 L under a low demand scenario. Fig. 4 shows the total (annual) rainwater savings varying with tank
Fig. 3. Cumulative overflow loss curves under different demand scenarios.
54
M.A. Imteaz et al. / Resources, Conservation and Recycling 62 (2012) 51–55
Fig. 4. Total annual rainwater savings under different demand scenarios.
demand. From presented reliability chart it is clear that a reliability of 100% is possible to achieve with a tank size of 7000 L under low demand. However, with higher demand a larger tank size (∼10,000 L) is required to achieve a 100% reliability. Nonetheless, under high demand ∼100% reliability is achievable even with a tank size of 7000 L. From overflow analysis, it is clear that huge amount of water will be lost as overflow, even in a dry year with a tank size of 10,000 L. This means, harvested rainwater can be used for other purposes that is potential uses of rainwater can be increased with a larger tank size. In regards to annual total rainwater savings, it is found that huge amounts of rainwater can be saved with the increase of tank size. For any rainwater tank it is expected that it will require some augmented supply in some longer dry period(s) of the year, unless the demand is very low with a considerably large tank. From the analysis of townwater used, it is found that augmentation from townwater supply can be made to nil under low demand with a tank size of 7000 L. However, for high demand scenario, it will require a larger tank size to achieve zero augmentation from townwater supply. In addition, this study has demonstrated that significant water savings (from town water supplies) can be attained from rainwater harvesting, even in dry years in Nigeria. These findings highlight the value that rainwater harvesting could have if fully implemented as a water management strategy in Nigeria. This study was based on a particular geographical area in Nigeria. The results would vary with geographical locations (i.e., with different climatic conditions or in general with different rainfall intensities and patterns). In reality, there will be numerous optimal solutions with different combinations of storage volumes, roof sizes and rainwater demand. A comprehensive decision support tool would be able to produce reliability, cumulative water savings, cumulative overflow losses and cumulative townwater supply used for any geographical location with any value of abovementioned variables.
Fig. 5. Total annual townwater used under different demand scenarios.
References sizes, under different demand scenarios. It is found that total rainwater savings can be increased significantly with an increase in tank size. However, for the current condition with limited demand, a very large tank is unnecessary. If the demand increases with other potential uses, then a bigger size tank can be considered and rainwater savings can be increased. Fig. 5 shows the total (annual) townwater used (to augment intended demand) varying with tank sizes, under different demand scenario. It is found that with the increase in tank size, required augmentation from town water supply can be reduced significantly. 5. Discussion and conclusions This paper investigated reliability of rainwater tanks in southwest Nigeria for a typical dry year under different scenarios (low demand and high demand) and varying tank sizes. Low demand is defined as rainwater use for toilet flushing only and the high demand is defined as rainwater use for both toilet flushing and laundry. The study considered recorded daily rainfall data instead of annual/monthly average rainfall data. Analysis using recorded rainfall data reveals more realistic scenarios instead of a hypothetical average scenario. Analysis was performed for a typical dry year to assess probable conditions under a worse case. As daily rainfall data is quite scarce for this part of the country, the year 1998 was selected because of data availability. Average roof (connected to the tank) size of 80 m2 was considered based on an earlier study. With the current analysis it was found that analysis using monthly average rainfall data overestimates the required tank size. The reliability was defined as the percentage of the total days in a year, when the rainwater tank was able to fulfil the intended
Adeboye OB, Alatise MO. Surface water potential of River Osun at Apoje Sub-Basin, Nigeria. Soil Water Research 2008;3(2):74–9. Aladenola OO, Adeboye OB. Assessing the potential or rainwater harvesting. Water Resources Management 2010;24:2129–37. Appan A. A dual-mode system for harnessing roof water for non-potable uses. Urban Water 2000;1(4):317–21. Coombes, P. Rainwater Research, www.rainwaterharvesting.com.au; 2007 (Viewed 13/03.10). Coombes P, Kuczera G. Analysis of the performance of rainwater tanks in Australian capital cities. In: 28th International hydrology and water resources symposium. Wollongong, NSW: The Institution of Engineers, Australia; 2003. Eroksuz E, Rahman A. Rainwater tanks in multi-unit buildings: a case study for three Australian cities. Resources, Conservation and Recycling 2010;54:1449–52. Fewkes A. The use of rainwater for WC flushing: the field testing of a collection system. Building and Environment 1999;34(6):765–72. Flower DJM, Mitchell VG, Codner GP. Urban water systems: drivers of climate change? In: Proceedings of the rainwater and urban design 2007, IRCSA XIII conference; 2007. 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. Handia L, Tembo JM, Mwiindwa C. Potential of rainwater harvesting in urban Zambia. Physical and Chemical Earth 2003;28(20–27):893–6. Hermann T, Schimda U. Rainwater utilization in Germany: efficiency, dimensioning, hydraulic and environmental aspects. Urban Water 1999;1(4):307–16. Hills S, Birks R, Mckenzie B. The millennium dome ‘water-cycle’ experiment: to evaluate water efficiency and customer perception at a recycling scheme for 6 million visitors. In: Proceedings of the IWA second world water congress; 2001. p. 15–9. Imteaz MA, Ahsan A, Naser J, Rahman A. Reliability analysis of rainwater tanks in Melbourne using daily water balance model. Resources, Conservation & Recycling 2011b;56:80–6. Imteaz MA, Shanableh A, Rahman A, Ahsan A. Optimisation of rainwater tank design from large roofs: a case study in Melbourne, Australia. Resources, Conservation & Recycling 2011a;55:1022–9.
M.A. Imteaz et al. / Resources, Conservation and Recycling 62 (2012) 51–55 Jenkins G. Use of continuous simulation for the selection of an appropriate urban rainwater tank. Australian Journal of Water Resources 2007;11(2):231–46. Orebiyi EO, Awomeso JA, Oyedepo JA. Assessment of bacteria pollution of shallow well water in Abeokuta, Southwestern Nigeria. Life Sciences Journal 2008;5(1):59–65. Thomas TH, Martinson DB. Roofwater harvesting: a handbook for practitioners. Technical Paper No. 49, 2007, IRC International Water and Sanitation Center Delft, The Netherlands, 160 pp.
55
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. Zaizen M, Urakawa T, Matsumoto Y, Takai H. The collection of rainwater from dome stadiums in Japan. Urban Water 1999;1(4):335–59.
Contents lists available at SciVerse ScienceDirect
Resources, Conservation and Recycling journal homepage: www.elsevier.com/locate/resconrec
Rainwater harvesting potential for southwest Nigeria using daily water balance model Monzur Alam Imteaz a,∗ , Omotayo B. Adeboye b,c , Scott Rayburg a , Abdallah Shanableh d a
Faculty of Engineering and Industrial Sciences, Swinburne University of Technology, Melbourne, Australia Department of Agricultural Engineering, Obafemi Awolowo University, Ile-Ife, Osun State, Nigeria Department of Water Engineering (Land and Water Development), UNESCO-IHE Institute For Water Education, Delft, Netherlands d Department of Civil & Environmental Engineering, University of Sharjah, Sharjah, United Arab Emirates b c
a r t i c l e
i n f o
Article history: Received 11 October 2011 Received in revised form 7 February 2012 Accepted 9 February 2012 Keywords: Rainwater tank Water balance model Reliability Dry year and optimum tank size
a b s t r a c t For the performance analysis and design of rainwater tanks, a simple spreadsheet based daily water balance model was developed using daily rainfall data, contributing roof area, rainfall loss factor, available storage volume, tank overflow and rainwater demand. This water balance model was then used to design an optimum size of domestic rainwater tank to be used for southwest Nigeria. The optimisation criterion was set to provide uninterrupted intended demand from the selected rainwater tank during the critical (dry) months. For the tank water, two demand scenarios were assessed: (i) toilet flushing only; and (ii) toilet flushing and laundry use. Analysis was performed for a typical dry year (1998) in southwest Nigeria. Current analysis outcomes were compared with an earlier analysis using monthly average rainfall data. It is found that analysis using monthly average rainfall data overestimates the required rainwater tank size. In addition, the newly developed model was used to assess the reliability of domestic rainwater tanks in augmenting partial household water demand. This analysis showed that a reliability of 100% is possible to achieve with a tank size of 7000 L under low demand. However, with higher demand a bigger tank size (∼10,000 L) is required to achieve 100% reliability even though very high reliability could also be attained with a tank size of 7000 L. From overflow analysis, the results of this study showed that a large quantity of water is lost as overflow, even in a dry year with a tank size of 10,000 L. Thus, harvested rainwater could be used for other purposes if larger tanks are used as these would capture more of the excess rainwater which could then be tasked to other purposes without compromising the reliability of water availability for primary uses. © 2012 Elsevier B.V. All rights reserved.
1. Introduction With increasing population and changing climate regime, water supply systems in many cities of the world are under stress. To tackle this problem, Water Corporations and River Basin Authorities are adopting several measures including demand management, and identifying alternative water sources, such as stormwater harvesting, greywater and wastewater reuse and desalination. Among all the alternative water sources, stormwater (or rainwater) harvesting perhaps has received the most attention. Rainwater harvesting is not a new concept in water resources management. Indeed, the concept has been practised for a long period of time, even before the advent of large scale public water systems. In modern times, rainwater harvesting is actively being encouraged and promoted in China, Brazil, Australia and India and is beginning to gain traction in other parts of the world as well. The value of
∗ Corresponding author. E-mail address: [email protected] (M.A. Imteaz). 0921-3449/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.resconrec.2012.02.007
rainwater harvesting as a means of supplementing or replacing town water supplies has been demonstrated by numerous researchers. For example, rainwater has been reported to promote significant potable water savings in buildings (Hermann and Schimda, 1999; Fewkes, 1999; Appan, 2000; Handia et al., 2003). As a consequence of these findings, many cities have instigated large scale rainwater use projects in public facilities including in Japan (Zaizen et al., 1999), London, England (Hills et al., 2001) and Melbourne in Australia (Imteaz et al., 2011a). In Australia, government authorities have been promoting stormwater harvesting through media campaigns, as well as offering incentives and grants to promote water saving ideas and innovations. For example, in Victoria, rebates of up to $500 are available for those who install new rainwater tanks on their properties. This move to include rainwater harvesting as a vital component of urban water management also has secondary benefits as the expanded use of rainwater harvesting and other simple innovative technologies has the potential to reduce greenhouse gas emissions from water storage reservoirs and water treatment processes which contribute to climate change (Flower et al., 2007).
52
M.A. Imteaz et al. / Resources, Conservation and Recycling 62 (2012) 51–55
Coombes and Kuczera (2003) found that for an individual building with a 150 m2 roof area and 1–5 kL tank in Sydney, Australia can yield 10–58% mains water savings (depending on the number of people using the building). According to Coombes and Kuczera (2003), depending on roof area and the number of occupants, rainwater tank use can result in annual mains water savings of 18–55 kL for 1 kL sized tanks and 25–144 kL for 10 kL sized tanks. In Sweden, Villarreal and Dixon (2005) investigated water savings potential of stormwater harvesting systems from roof areas. Villarreal and Dixon discovered that mains water saving of 30% can be achieved using a 40 m3 sized tank (toilet and washing machine use only). Ghisi et al. (2007, 2009) investigated the water savings potential from rainwater harvesting systems in Brazil (South America) and found that average potential for potable water savings of 12–79% per year for the cities analysed. Despite positive outcome from many studies, there remains a general community reluctance to adopt stormwater harvesting on a wider scale. Part of the reason for this reluctance can be attributed to lack of information about the effectiveness of a stormwater harvesting system and the optimum storage size required to satisfy the performance requirements under the specific site conditions (Imteaz et al., 2011a). A proper in-depth understanding of the effectiveness of any proposed on-site stormwater harvesting system is often lacking. Vaes and Berlamont (2001) developed a model to determine the effectiveness of rainwater tanks and stormwater runoff using long term historical rainfall data. Coombes (2007) conducted studies on the modelling of the rainwater tanks and the opportunities for effective retention storage using the PURRS (Probabilistic Urban Rainwater and Wastewater Reuse Simulator) water balance model. Jenkins (2007) developed a computer model for the continuous simulations of amount of rainwater stored in the tank, amount of rainwater used, amount overflowed and amount of mainwater used to top up the tank for household rainwater tanks. Jenkins (2007) concluded that the climate characteristics of the site have a significant influence on the effectiveness of the rainwater tank. Eroksuz and Rahman (2010) investigated the water savings potential of rainwater tanks in multi-storied residential buildings for three cities in eastern Australia. They have concluded that rainwater tank of appropriate size in a multi-storied building can provide significant water savings even in dry years. Imteaz et al. (2011b) have presented a range of reliability charts for residential rainwater tanks in Central Melbourne that serve to illustrate the benefits of rainwater harvesting even in a highly variable climatic regime like that experienced in Australia. The preponderance of data and modelling associated with rainwater harvesting is focussed on developed countries, where often, water scarcity is a matter of economics rather than basic human health. Consequently, there is a need to consider how rainwater harvesting might improve conditions in the developing world where access to fresh water is often lacking with significant implications for quality of life. Like many developing countries, Nigeria cannot satisfy its domestic water needs and in 2007 only 47% of the total population had access to water from improved sources (Aladenola and Adeboye, 2010). The country is already facing water supply shortages in both urban and rural areas despite the abundant land and water resources that are available in various climatic zones (Adeboye and Alatise, 2008). Rising population and urbanisation coupled with climate change may further increase the scarcity of water. If the abundant rainfall in the country particularly in southwest Nigeria is well harnessed, however, it might help to solve the perennial water scarcity being experienced in the entire country. In Nigeria, data on available rainwater harvesting for domestic use are very scanty and not well documented thereby making it difficult for objective assessment despite the fact that rainwater constitutes more than 90% of surface water. Similarly, the use
of rainwater for water closet (WC) flushing and laundry has not been estimated for any city or region in Nigeria. This paper presents analyses of household rainwater harvesting potential in Abeokuta (Nigeria) using a daily water balance model. Findings of the current model were compared with an earlier study conducted by Aladenola and Adeboye (2010) using monthly average rainfall data. Eventually, the model was used to prepare several rainwater tank reliability charts under different scenarios. These data provide an estimate of the benefits to be derived from rainwater harvesting in the developing world and can help to provide justification for rainwater harvesting in water management plans for these areas. 2. Study area Abeokuta is located in the humid tropical rainforest zone of the Southwestern part of Nigeria between Latitude 7◦ 5 N to 7◦ 20 N and Longitude 3◦ 17 E to 3◦ 20 E. The mean daily temperature is about 28 ◦ C. The city is drained mainly by the River Ogun which is dendritic in pattern. It covers a geographic area of 1256 km2 and has a population of 449,088 inhabitants. The water supply from Ogun State Water Corporation (OSWC) is inadequate to meet the daily domestic and industrial demands of the growing population; hence, there is a need to develop other water sources. Alternate water sources used in the city include borehole and shallow wells; however, these are prone to contamination (Orebiyi et al., 2008) and so are not of sufficient quality to meet the growing demand. However, rainwater could complement available water sources in Abeokuta and other parts of Nigeria if effectively harnessed. 3. Methodology A spreadsheet based on a daily water balance model was developed considering daily rainfall, contributing catchment (roof) area, losses due to leakage, spillage and evaporation, storage (tank) volume and water uses. The daily runoff volume is calculated from daily rainfall amount by multiplying the rainfall amount with the contributing roof area and deducting the losses. Based on a guideline produced by Thomas and Martinson (2007), a 15% deduction from the produced runoff was applied to account for losses (including leakage, spilling and evaporation) for hard roof in humid tropics. Generated runoff is then diverted to the connected available storage tank. Available storage capacity is compared with the accumulated daily runoff. If the accumulated runoff is higher than available storage volume, excess water (overflow) is deducted from the accumulated runoff. Then, the amount of water use(s) is deducted from the daily accumulated/stored runoff amount (if sufficient water is available in the storage). When sufficient water is not available in the storage, the model assumes that the remaining water demand is supplied from the town water supply. Thus, the model calculates daily stormwater use, daily water storage in the tank, daily overflow and daily town water use. In addition, the model calculates accumulated annual stormwater use, accumulated annual overflow and accumulated annual town water use. The overall process can be mathematically described as follows: Cumulative water storage equation, St = Vt + St−1 − D
(1)
St = 0, for St < 0
(2)
St = C, for St > C
(3)
where St is the cumulative water stored in the rainwater tank (L) after the end of tth day, Vt is the harvested rainwater (L) on the tth day, St−1 is the storage in the tank (L) at the beginning of tth day, D
M.A. Imteaz et al. / Resources, Conservation and Recycling 62 (2012) 51–55
is the daily rainwater demand (L), and, C is the capacity of rainwater tank (L), Townwater use equation, TW = D − St , for St < D
(4)
where TW is the townwater use on tth day (L). Overflow equation, OF = St − C, for St > C
(5)
53
Table 1 Required tank size: a comparison of analyses using monthly average data and daily recorded data. Demand (m3 ) per month
High = 2.45 Low = 1.80
Required rainwater tank size (L) Analysis with monthly average data
Analysis with recorded daily data
9720 5510
6000 4500
where OF is the overflow on tth day (L). Reliability is calculated with the equation, Re =
N−U × 100 N
(6)
where Re is the reliability of the tank to be able to supply intended demand (%), U is the number of days in a year the tank was unable to meet the demand, and N is the total number of days in a particular year. 4. Analysis and results Current analysis is based on a daily water balance (i.e., using daily rainfall and water uses data). However, daily rainfall data for different regions of Nigeria is quite scarce. For the current study, daily rainfall data was collected from Abeokuta city for a typical dry year, 1998. It is to be noted that the long-term annual average rainfall for the city is 1156 mm, whereas in 1998 the total annual rainfall was 567 mm. Fig. 1 shows the rainfall time series for the year 1998, starting from 1st January. As the preliminary objective was to compare the outcomes of the daily water balance model with typical average monthly data, other relevant data were taken from the study conducted by Aladenola and Adeboye (2010) who used monthly average rainfall data for their analysis. An average household roof area for the area was taken as 80 m2 . To avoid rainwater quality issues, harvested rainwater was not assumed to be used for drinking and/or cooking. Based on estimation of Aladenola and Adeboye (2010), two typical demand scenarios were selected, namely: (a) low demand; for toilet flushing only, which is 1.8 m3 in a month per household; and (b) high demand; for combined toilet flushing and laundry, which is 2.45 m3 in a month per household. As average monthly rainfalls during March to October is well above the above-mentioned demand, the critical months (November to February) were considered for current analysis. With the selected year’s daily rainfall data, the daily water balance model was simulated starting from March to February in the following year. The model was simulated from March, to allow for the reality that before the onset of the dry period (November), the tank may have some residual water from the previous months. The model was simulated to achieve an uninterrupted supply (i.e., rainwater tank size is sufficient to supply the intended demand) of
Fig. 1. Daily rainfall data for the year 1998 from Abeokuta city.
Fig. 2. Reliability curves under different demand scenarios.
rainwater. Table 1 shows the comparisons of the findings of the daily water balance model with the analysis considering average monthly rainfall. It is found that analysis with the average monthly data overestimate the tank sizes under both scenarios. It is to be noted that the daily water balance model is expected to produce more realistic outcomes. With the same dry year’s rainfall data, several analyses were performed using a whole year’s rainfall data to provide detail insights of performances for different tank sizes. Fig. 2 shows the reliability curves varying with tank size, under different demand scenarios. It is found that a 100% reliability can be achieved for a tank size of 7000 L for the low demand scenario and for a tank size of 10,000 L for the high demand scenario. Fig. 3 shows the overflow loss (annual) curves varying with tank sizes, under different demand scenarios. It is found that overflow losses can be reduced significantly with an increase in tank size. It is also found that even in a high demand scenario, with a tank size of 10,000 L, there will be an annual overflow loss of about 6000 L in a typical dry year. This loss would be more than 10,000 L under a low demand scenario. Fig. 4 shows the total (annual) rainwater savings varying with tank
Fig. 3. Cumulative overflow loss curves under different demand scenarios.
54
M.A. Imteaz et al. / Resources, Conservation and Recycling 62 (2012) 51–55
Fig. 4. Total annual rainwater savings under different demand scenarios.
demand. From presented reliability chart it is clear that a reliability of 100% is possible to achieve with a tank size of 7000 L under low demand. However, with higher demand a larger tank size (∼10,000 L) is required to achieve a 100% reliability. Nonetheless, under high demand ∼100% reliability is achievable even with a tank size of 7000 L. From overflow analysis, it is clear that huge amount of water will be lost as overflow, even in a dry year with a tank size of 10,000 L. This means, harvested rainwater can be used for other purposes that is potential uses of rainwater can be increased with a larger tank size. In regards to annual total rainwater savings, it is found that huge amounts of rainwater can be saved with the increase of tank size. For any rainwater tank it is expected that it will require some augmented supply in some longer dry period(s) of the year, unless the demand is very low with a considerably large tank. From the analysis of townwater used, it is found that augmentation from townwater supply can be made to nil under low demand with a tank size of 7000 L. However, for high demand scenario, it will require a larger tank size to achieve zero augmentation from townwater supply. In addition, this study has demonstrated that significant water savings (from town water supplies) can be attained from rainwater harvesting, even in dry years in Nigeria. These findings highlight the value that rainwater harvesting could have if fully implemented as a water management strategy in Nigeria. This study was based on a particular geographical area in Nigeria. The results would vary with geographical locations (i.e., with different climatic conditions or in general with different rainfall intensities and patterns). In reality, there will be numerous optimal solutions with different combinations of storage volumes, roof sizes and rainwater demand. A comprehensive decision support tool would be able to produce reliability, cumulative water savings, cumulative overflow losses and cumulative townwater supply used for any geographical location with any value of abovementioned variables.
Fig. 5. Total annual townwater used under different demand scenarios.
References sizes, under different demand scenarios. It is found that total rainwater savings can be increased significantly with an increase in tank size. However, for the current condition with limited demand, a very large tank is unnecessary. If the demand increases with other potential uses, then a bigger size tank can be considered and rainwater savings can be increased. Fig. 5 shows the total (annual) townwater used (to augment intended demand) varying with tank sizes, under different demand scenario. It is found that with the increase in tank size, required augmentation from town water supply can be reduced significantly. 5. Discussion and conclusions This paper investigated reliability of rainwater tanks in southwest Nigeria for a typical dry year under different scenarios (low demand and high demand) and varying tank sizes. Low demand is defined as rainwater use for toilet flushing only and the high demand is defined as rainwater use for both toilet flushing and laundry. The study considered recorded daily rainfall data instead of annual/monthly average rainfall data. Analysis using recorded rainfall data reveals more realistic scenarios instead of a hypothetical average scenario. Analysis was performed for a typical dry year to assess probable conditions under a worse case. As daily rainfall data is quite scarce for this part of the country, the year 1998 was selected because of data availability. Average roof (connected to the tank) size of 80 m2 was considered based on an earlier study. With the current analysis it was found that analysis using monthly average rainfall data overestimates the required tank size. The reliability was defined as the percentage of the total days in a year, when the rainwater tank was able to fulfil the intended
Adeboye OB, Alatise MO. Surface water potential of River Osun at Apoje Sub-Basin, Nigeria. Soil Water Research 2008;3(2):74–9. Aladenola OO, Adeboye OB. Assessing the potential or rainwater harvesting. Water Resources Management 2010;24:2129–37. Appan A. A dual-mode system for harnessing roof water for non-potable uses. Urban Water 2000;1(4):317–21. Coombes, P. Rainwater Research, www.rainwaterharvesting.com.au; 2007 (Viewed 13/03.10). Coombes P, Kuczera G. Analysis of the performance of rainwater tanks in Australian capital cities. In: 28th International hydrology and water resources symposium. Wollongong, NSW: The Institution of Engineers, Australia; 2003. Eroksuz E, Rahman A. Rainwater tanks in multi-unit buildings: a case study for three Australian cities. Resources, Conservation and Recycling 2010;54:1449–52. Fewkes A. The use of rainwater for WC flushing: the field testing of a collection system. Building and Environment 1999;34(6):765–72. Flower DJM, Mitchell VG, Codner GP. Urban water systems: drivers of climate change? In: Proceedings of the rainwater and urban design 2007, IRCSA XIII conference; 2007. 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. Handia L, Tembo JM, Mwiindwa C. Potential of rainwater harvesting in urban Zambia. Physical and Chemical Earth 2003;28(20–27):893–6. Hermann T, Schimda U. Rainwater utilization in Germany: efficiency, dimensioning, hydraulic and environmental aspects. Urban Water 1999;1(4):307–16. Hills S, Birks R, Mckenzie B. The millennium dome ‘water-cycle’ experiment: to evaluate water efficiency and customer perception at a recycling scheme for 6 million visitors. In: Proceedings of the IWA second world water congress; 2001. p. 15–9. Imteaz MA, Ahsan A, Naser J, Rahman A. Reliability analysis of rainwater tanks in Melbourne using daily water balance model. Resources, Conservation & Recycling 2011b;56:80–6. Imteaz MA, Shanableh A, Rahman A, Ahsan A. Optimisation of rainwater tank design from large roofs: a case study in Melbourne, Australia. Resources, Conservation & Recycling 2011a;55:1022–9.
M.A. Imteaz et al. / Resources, Conservation and Recycling 62 (2012) 51–55 Jenkins G. Use of continuous simulation for the selection of an appropriate urban rainwater tank. Australian Journal of Water Resources 2007;11(2):231–46. Orebiyi EO, Awomeso JA, Oyedepo JA. Assessment of bacteria pollution of shallow well water in Abeokuta, Southwestern Nigeria. Life Sciences Journal 2008;5(1):59–65. Thomas TH, Martinson DB. Roofwater harvesting: a handbook for practitioners. Technical Paper No. 49, 2007, IRC International Water and Sanitation Center Delft, The Netherlands, 160 pp.
55
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. Zaizen M, Urakawa T, Matsumoto Y, Takai H. The collection of rainwater from dome stadiums in Japan. Urban Water 1999;1(4):335–59.