Parameters Influencing the Sizing of Rainwater Tanks for Use in Houses

Water Resour Manage(2010)24:2381–2403DOI10.1007/s11269-009-9557-4Parameters Influencing the Sizing of Rainwater Tanksfor Use in HousesEnedir GhisiReceived:19November2008/Accepted:6December2009/Published online:18December2009©Springer Science+Business Media B.V.2009Abstract Rainwater harvesting has been studied in different countries as a way of easing water availability problems and reducing potable water demand in buildings. The most important factor relating to the efficiency of a rainwater system is the cor-rect sizing of the rainwater tank.Therefore,the objective of this article is to assess the influence of rainfall,roof area,number of residents,potable water demand and rain-water demand on rainwater tank sizing.The analysis was performed by using com-puter simulation and by considering daily rainfall data for three cities located in the state of São Paulo,Brazil.The roof areas considered were50,100,200and400m2; the potable water demands were50,100,150,200,250and300l per capita per day; the rainwater demands were taken as a percentage of the potable water demand, i.e.,10%to100%at increments of10%;and the number of residents was two and four.Results indicated a wide variation of rainwater tank sizes for each city and also for each parameter.The main conclusion that can be made from the study is that rainwater tank sizing for houses must be performed for each specific situation,i.e., considering local rainfall,roof area,potable water demand,rainwater demand and number of residents.Therefore,sizing rainwater tanks according to local tradition is not recommended as it may incur low efficiency.Keywords Rainwater tank sizing·Computer simulations·Houses·Sustainable development1IntroductionRainwater harvesting,amongst other strategies,has been studied in different coun-tries as a way of easing water availability problems.It can be found in the literature E.Ghisi(B)Laboratory of Energy Efficiency in Buildings,Department of Civil Engineering,Federal University of Santa Catarina,Florianópolis,SC,88040-900,Brazile-mail:enedir@labeee.ufsc.br2382 E.Ghisi that rainwater is being used,for example,in hotels in China(Deng2003),schools in Taiwan(Cheng2003;Cheng and Hong2004;Wung et al.2006),houses and residential buildings in Germany(Herrmann and Schmida1999),houses in Australia (Coombes et al.1999)and in the UK(Fewkes1999a),amongst others.There have been studies on rainwater treatment as well(Kim et al.2005,2007;Nolde2007; Panigrahi et al.2007).However,there is little information on rainwater tank sizing (Fewkes1999b;Appan1999;Domínguez et al.2001;Ghisi et al.2007;Van Zyl et al.2008).The correct sizing is important in order to avoid extra costs when the tank is over sized and avoid low efficiency when it is under sized.The accurate sizing of rainwater tanks is a function of rainfall,roof area,potable water demand,rainwater demand,and number of residents.Thus,this study contributes to improving the knowledge about the influence of such parameters on rainwater tank sizing for houses.2ObjectivesThe main objective of this article is to assess the influence of parameters such as rainfall,roof area,number of residents,potable water demand and rainwater demand on rainwater tank sizing for houses.3MethodologyThe research was based on the estimation of rainwater tank capacities and the corresponding potential for potable water savings by using computer simulation.The methodology applied to accomplish the objectives of this research is as follows.3.1Study AreaThe analyses were performed by considering daily rainfall for three cities located in the state of São Paulo,that is,Itaquaquecetuba,Espírito Santo do Pinhal,and Santos. Their location can be seen on the map shown in Fig.1.These cities were selected due to the availability of long-run rainfall data.Any other cities around the world could have been selected for the analysis as the objective of this study is to evaluate the influence of some parameters on the rainwater tank sizing for use in houses.It is not intended to assess problems related to water availability,water resources and type of climate.3.2RainfallDaily rainfall data for the three cities were obtained from the Department of Water and Electricity of São Paulo(2004).The data for the three cities ranged from1936 to2000,but did not cover the same period;and for some short periods there were some missing data.Such missing data and also rainfall less than or equal to1mm a day were assumed to be zero.Such an assumption implies that the potential for potable water savings that can be obtained in real situations over the three cities can be slightly higher than thefigures shown in this article.Parameters Influencing the Sizing of Rainwater Tanks in Houses2383-10-20-3070605040Fig.1Map of Brazil with location of the three cities considered in the study3.3Roof AreaIn order to take different roof areas into account,four roof areas were considered, i.e.,50,100,200and400m2.3.4Potable Water DemandAverage potable water demand in Brazil ranges from85to232l per capita per day (SNIS2004).Therefore,potable water demands of50,100,150,200,250and300l per capita per day were considered in the research.The variation of water demand along the year is an important issue regarding the performance of the system.Thus, such a variation of water demands intends to cover the different water consumption regarding the season.3.5Rainwater DemandRainwater was taken into account as a percentage of the potable water demand, i.e.,10%to100%at increments of10%.Rainwater quality should be evaluated and it should go through proper treatment in case it is meant to be used for potable purposes,such as drinking,cooking,etc.However,this is not a concern in this study as it is a theoretical analysis only to evaluate the influence of some parameters on the rainwater tank sizing.Table1Summary on rainfall for the three citiesCity Period taken into Days Years Average rainfallaccount(mm/year) Itaquaquecetuba1943–200021,186581,380Espírito Santo do Pinhal1936–200023,521641,537Santos1937–199922,998633,3212384E.GhisiFig.2Daily rainfall inItaquaquecetuba for a 10-yearperiod50100150200250300136572910931457182121852549291332773641DaysD a i l y r a i n f a l l (m m )Fig.3Daily rainfall inEspírito Santo do Pinhal for a 10-yearperiod50100150200250300136572910931457182121852549291332773641DaysD a i l y r a i n f a l l (m m )Fig.4Daily rainfall in Santos for a 10-yearperiod50100150200250300136572910931457182121852549291332773641DaysD a i l y r a i n f a l l (m m )Parameters Influencing the Sizing of Rainwater Tanks in Houses 23853.6Number of ResidentsTo evaluate the influence that the number of residents has on the rainwater tank sizing,two and four people were considered.3.7Computer SimulationsThe rainwater tank sizing and potential for potable water savings were estimated by using the algorithm of the Neptune computer programme (Ghisi and Trés 2004).TheRainwater demand (% of potable water demand)51015202530Rainwater tank capacity (1000 litres)P o t e n t i a l f o r p o t a b l e w a t e r s a v i n g s (%)(a)51015202530Rainwater tank capacity (1000 litres)P o t e n t i a l f o r p o t a b l e w a t e r s a v i n g s (%)(b)Fig.5Potential for potable water savings as a function of rainwater tank capacity and rainwaterdemand in Itaquaquecetuba for roof area of 50m 2and two residents:a Potable water demand of 50l per capita per day,b Potable water demand of 300l per capita per day2386 E.GhisiFig.6Rainwater tank capacity as a function of rainwater demand,potable water demand,roof areaand number of residents in houses located in Itaquaquecetuba:a Roof area of50m2and two people,b Roof area of50m2and four people,c Roof area of100m2and two people,d Roof area of100m2and four people,e Roof area of200m2and two people,f Roof area of200m2and four people,g Roof area of400m2and two people,h Roof area of400m2and four peoplewater balance model used in Neptune is neither the yield after spillage(YAS)nor the yield before spillage(YBS)(Fewkes1999b).Neptune was developed to estimate the potential for potable water savings for different rainwater tank capacities when there is rainwater usage in residential buildings under the rainfall of Florianópolis,a city located in southern Brazil. Therefore,Neptune could not be used in this study.Alternatively,its algorithm was applied on spreadsheets where rainfall data for Florianópolis were replaced by the rainfall data for the three cities considered in the analysis.3.7.1Input DataInput data needed to run Neptune are daily rainfall data,daily potable water demand per capita,daily rainwater demand as a percentage of potable water demand,number of residents,roof area,rainwater tank capacity and run-off coefficient.Rainfall, roof area,potable water demand,rainwater demand and number of residents were investigated using the ranges explained above.The run-off coefficient accounts for the rainwater that is disposed of at the beginning of a rainfall in order to avoid debris into the tank and pipes,and also for losses due to absorption and evaporation of rainwater by reaching the catchment surface.Run-off coefficient was assumed to be 80%in all simulations.As for the rainwater tank capacities,simulations were performed considering capacities ranging from1,000to35,000l at increments of1,000l.The ideal tank capacity was taken as the one in which the potential for potable water savings increased2%or less when increasing the tank capacity by1,000l.3.7.2The AlgorithmThe algorithm used in this research considers data on a daily basis.The rainwater that flows daily on the catchment surface is conducted to the rainwater tank.Prior to that, rainwater losses(run-off coefficient)are discarded.If the rainwater tank capacity is exceeded,rainwater demand is supplied and the overflow is discarded.If not,daily rainwater demand is supplied partly or totally by the water utility.Equations below help to understand the procedure.First,the volume of rainwater harvested is estimated by using Eq.1.H t=P t×A×C p(1) where H t is the volume of rainwater harvested at time t(litres/day per house),P t is the local rainfall at time t(mm/day=litres/m2per day),A is the roof area of each house(m2),C p is the run-off coefficient,which was assumed to be0.8to represent losses of20%(non-dimensional).Then,the volume of rainwater consumed daily and the volume of rainwater available in the rainwater tank are estimated at the same time by using Eqs.2and3, respectively.Thus,the volume of rainwater harvested daily is summed to the volumeParameters Influencing the Sizing of Rainwater Tanks in Houses2387Potable water demandRainwater demand (% of potable water demand)Rainwater demand (% of potable water demand)Rainwater demand (% of potable water demand)Rainwater demand (% of potable water demand)R a i n w a t e r t a n k c a p a c i t y (1000 l i t r e s )(a)Rainwater demand (% of potable water demand)Rainwater demand (% of potable water demand)Rainwater demand (% of potable water demand)Rainwater demand (% of potable water demand)R a i n w a t e r t a n k c a p a c i t y (1000 l i t r e s )(b)R a i n w a t e r t a n k c a p a c i t y (1000 l i t r e s )(c)R a i n w a t e r t a n k c a p a c i t y (1000 l i t r e s )(d)R a i n w a t e r t a n k c a p a c i t y (1000 l i t r e s )(e)R a i n w a t e r t a n k c a p a c i t y (1000 l i t r e s )(f)R a i n w a t e r t a n k c a p a c i t y (1000 l i t r e s )(g)R a i n w a t e r t a n k c a p a c i t y (1000 l i t r e s )(h)2388E.GhisiPotable water demandRainwater demand (% of potable water demand)R a i n w a t e r t a n k c a p a c i t y (1000 l i t r e s )(a)Rainwater demand (% of potable water demand)Rainwater demand (% of potable water demand)Rainwater demand (% of potable water demand)Rainwater demand (% of potable water demand)Rainwater demand (% of potable water demand)Rainwater demand (% of potable water demand)Rainwater demand (% of potable water demand)R a i n w a t e r t a n k c a p a c i t y (1000 l i t r e s )(b)R a i n w a t e r t a n k c a p a c i t y (1000 l i t r e s )(c)R a i n w a t e r t a n k c a p a c i t y (1000 l i t r e s )(d)R a i n w a t e r t a n k c a p a c i t y (1000 l i t r e s )(e)R a i n w a t e r t a n k c a p a c i t y (1000 l i t r e s )(f)R a i n w a t e r t a n k c a p a c i t y (1000 l i t r e s )(g)R a i n w a t e r t a n k c a p a c i t y (1000 l i t r e s )(h)Parameters Influencing the Sizing of Rainwater Tanks in Houses2389 Fig.7Rainwater tank capacity as a function of rainwater demand,potable water demand,roof area and number of residents in houses located in Santos:a Roof area of50m2and two people,b Roof area of50m2and four people,c Roof area of100m2and two people,d Roof area of100m2and four people,e Roof area of200m2and two people,f Roof area of200m2and four people,g Roof area of400m2and two people,h Roof area of400m2and four peopleof rainwater available in the tank;from such whole volume,the volume of rainwater consumed daily is subtracted.If such summing results in a volume greater than the rainwater tank capacity,the excess is discarded.C t=minD p×D×nVt−1+H t(2)where C t is the volume of rainwater consumed in each house at time t(litres/day per house),D p is the daily rainwater demand taken as a percentage of the potable water demand(non-dimensional),D is the daily potable water demand(litres per capita/day),n is the number of residents in the house,V t−1is the volume of rainwater available in the rainwater tank at time t-1(litres),and H t is the volume of rainwater harvested in each house at time t(litres/day per house).The product D p×D×n represents the daily rainwater demand(in litres)and should be less than or equal to the volume of rainwater available in the tank.V t=maxminV t−1+H t−C tV−C t(3)where V t is the volume of rainwater available in the rainwater tank at time t(litres), V t−1is the volume of rainwater available in the rainwater tank at time t-1(litres),H t is the volume of rainwater harvested in each house at time t(litres/day per house),C t is the volume of rainwater consumed in each house at time t(litres/day per house), and V is the rainwater tank capacity(litres).The potential for potable water savings was estimated by dividing the total volume of rainwater consumed over the period to the total potable water demand over the same period,as shown in Eq.4.E=100dt=1C tD×n×d(4)where E is the potential for potable water savings by using rainwater(%),d is the total number of days considered in the analysis,D is the daily potable water demand (litres per capita/day),n is the number of residents in the house,C t is the volume of rainwater consumed in each house at time t(litres/day per house).3.7.3Parametric AnalysisBy using the algorithm described above,simulations were performed for all com-binations of roof area,potable water demand,rainwater demand and number of residents.For each city,480results of rainwater tank capacity and corresponding potential for potable water savings were obtained.2390E.GhisiPo table water demandRainwater demand (% of potable water demand)Rainwater demand (% of potable water demand)Rainwater demand (% of potable water demand)Rainwater demand (% of potable water demand)Rainwater demand (% of potable water demand)Rainwater demand (% of potable water demand)Rainwater demand (% of potable water demand)Rainwater demand (% of potable water demand)P o t a b l e w a t e r s a v i n g s (%)(a)P o t a b l e w a t e r s a v i n g s (%)(b)P o t a b l e w a t e r s a v i n g s (%)(c)P o t a b l e w a t e r s a v i n g s (%)(d)P o t a b l e w a t e r s a v i n g s (%)(e)P o t a b l e w a t e r s a v i n g s (%)(f)P o t a b l e w a t e r s a v i n g s (%)(g)P o t a b l e w a t e r s a v i n g s (%)(h)Parameters Influencing the Sizing of Rainwater Tanks in Houses2391 Fig.8Potential for potable water savings as a function of rainwater demand,potable water demand, roof area and number of residents in houses in Itaquaquecetuba:a Roof area of50m2and two people,b Roof area of50m2and four people,c Roof area of100m2and two people,d Roof area of100m2and four people,e Roof area of200m2and two people,f Roof area of200m2and four people,g Roof area of400m2and two people,h Roof area of400m2and four people3.8CorrelationsIn order to try to obtain conclusive results and provide some guidance concerning rainwater tank sizing,some correlations were investigated:1.between rainwater tank capacity and potable water demand,2.between potential for potable water savings and rainwater tank capacity,3.between rainwater demand per roof area and rainwater tank capacity,4.between rainwater demand per roof area and potable water savings,and5.between rainwater tank capacity and the ratio between rainwater demand andrainwater availability,which is a factor obtained by using Eq.5.Such a factor represents the ratio between rainwater demand and rainwater availabil-ity on an annual basis.Therefore,such a factor comprises all parameters considered in this study,i.e.,rainfall,roof area,number of residents,potable water demand and rainwater demand.F=D p×D×n×365A×P a(5)where F is the factor that represents the ratio between rainwater demand and rainwater availability on an annual basis(non-dimensional),D p is the daily rainwater demand taken as a percentage of the potable water demand(non-dimensional),D is the daily potable water demand(litres per capita/day),n is the number of residents in the house,A is the roof area(m2),P a is the local average annual rainfall(m/year). 4ResultsRainwater tank capacities and the corresponding potential for potable water savings were obtained from the computer simulations.The following analyses are based on such results and also on correlations between them and the parameters used as input data for the computer simulations.4.1RainfallDaily rainfall used in the simulations comprised periods ranging from21,186to 23,521days,i.e.,58to64years.Table1shows a summary on rainfall data for the three cities.The three cities were selected for the analysis not only on the basis of long-run data series,but also because of their variation in rainfall amounts.Itaquaquecetuba and Espírito Santo do Pinhal have a similar rainfall pattern,while Santos has a much higher rainfall.Figures2,3and4show daily rainfall for the three cities for a10-year period.A fairly short dry season is observed in the three cities.2392E.GhisiPotable water demandP o t a b l e w a t e r s a v i n g s (%)(a)Rainwater demand (% of potable water demand)P o t a b l e w a t e r s a v i n g s (%)(b)Rainwater demand (% of potable water demand)P o t a b l e w a t e r s a v i n g s (%)(c)Rainwater demand (% of potable water demand)P o t a b l e w a t e r s a v i n g s (%)(d)Rainwater demand (% of potable water demand)P o t a b l e w a t e r s a v i n g s (%)(e)Rainwater demand (% of potable water demand)P o t a b l e w a t e r s a v i n g s (%)(f)Rainwater demand (% of potable water demand)Rainwater demand (% of potable water demand)P o t a b l e w a t e r s a v i n g s (%)(g)Rainwater demand (% of potable water demand)P o t a b l e w a t e r s a v i n g s (%)(h)Parameters Influencing the Sizing of Rainwater Tanks in Houses 2393Fig.9Potential for potable water savings as a function of rainwater demand,potable water demand,roof area and number of residents in houses in Santos:a Roof area of 50m 2and two people,b Roof area of 50m 2and four people,c Roof area of 100m 2and two people,d Roof area of 100m 2and four people,e Roof area of 200m 2and two people,f Roof area of 200m 2and four people,g Roof area of 400m 2and two people,h Roof area of 400m 2and four people4.2Computer SimulationsFigure 5shows some results obtained from the simulations.As an example,results are shown for the city of Itaquaquecetuba,roof area of 50m 2,two residents,rainwater demand ranging from 10%to 100%of the potable water demand,and potable water demand of 50l per capita per day (Fig.5a)and 300l per capita per day (Fig.5b).It can be observed that for potable water demand of 50l per capita per day,the potential for potable water savings tends to be similar to the rainwater demand.If the rainwater demand is,for example,50%of the potable water demand,the potential for potable water savings is also 50%when the tank capacity is greater than 5,000l.However,when the potable water demand is 300l per capita per day (Fig.5b),the maximum potential for potable water savings that can be obtained is approximately 25%for any rainwater demand greater than 20%of the potable water demand.This is due to high rainwater demand and small roof area.From results like those shown in Fig.5,it was possible to obtain the ideal rainwater tank capacity for each case.In Fig.5a,for example,when the rainwater demand is 50%of the potable water demand,the tank capacity considered ideal was 3,000l (giving a potable water saving of 48.17%),because the potable water saving obtained with a tank of 4,000l is 49.07%(a 1.87%difference,which is less than the limit of 2%as discussed in the methodology).With a tank of 2,000l,the potable watersaving012345678910110200400600800100012001400Potable water demand (litres per house per day)R a i n w a t e r t a n k c a p a c i t y (1000 l i t r e s )Fig.10Correlation between ideal rainwater tank capacity and potable water demand for the three cities together2394 E.Ghisiis 46.53%,i.e.,3.52%lower than the saving obtained with a tank of 3,000l;as the difference is greater than 2%,the 2,000-l tank was not taken as the ideal one.Figure 5is given as an example of the results generated from the computer simu-lations.In all other simulations,the ideal rainwater tank capacities were obtained directly from the calculations on spreadsheets.Tank capacities were changed at 1000-litre increment and the corresponding potential for potable water saving was observed;if such potential was less than or equal to 2%,the previous tank capacity was taken as ideal.4.2.1Rainwater Tank CapacityFigures 6and 7show the rainwater tank capacities obtained for the cities of Itaquaquecetuba,and Santos,respectively.Results are shown as a function of potable water demand,rainwater demand,roof area and number of residents.Coefficients of determination (R 2)for the curves ranged from 0.7569to 0.9972.Results for Espírito Santo do Pinhal are not shown as they are similar to those obtained for Itaquaquecetuba.It can be observed that there is not a clear trend for the variation of tank capacity as a function of potable water demand,rainwater demand,roof area and number of residents.In general,for roof area greater than 100m 2,rainwater tank capacity tends to be larger as the potable water demandincreases.100200300400500600700800900100001234567891011Rainwater tank capacity (1000 litres)P o t a b l e w a t e r s a v i n g s (l i t r e s /d a y )(a)100200300400500600700800900100001234567891011Rainwater tank capacity (1000 litres)P o t a b l e w a t e r s a v i n g s (l i t r e s /d a y )(b)0100200300400500600700800900100001234567891011Rainwater tank capacity (1000 litres)P o t a b l e w a t e r s a v i n g s (l i t r e s /d a y )(c)0100200300400500600700800900100001234567891011Rainwater tank capacity (1000 litres)P o t a b l e w a t e r s a v i n g s (l i t r e s /d a y )(d)Fig.11Correlation between potable water savings per house and ideal rainwater tank capacity:a The three cities together,b Itaquaquecetuba,c Espírito Santo do Pinhal,d SantosParameters Influencing the Sizing of Rainwater Tanks in Houses 2395However,for small roof area and lower rainfall (such as in Itaquaquecetuba),tank capacity decreases as potable water demand and rainwater demand increase.This shows that there is no need to increase the tank capacity because,as the roof area is low and demand is high,it is not possible to supply all the rainwater demand.This was identified for the cities of Itaquaquecetuba (Fig.6a,b)and Espírito Santo do Pinhal,and Santos (Fig.7b).4.2.2Potential for Potable Water SavingsThe potential for potable water savings are shown in Figs.8and 9for Itaquaque-cetuba and Santos,respectively.Such potentials correspond to each tank capacity shown in Figs.6and 7.Results for Espírito Santo do Pinhal are not shown as they are somewhat similar to those obtained for Itaquaquecetuba.In general it can be noticed that:•for larger roof areas and low number of residents,the potential for potable water savings is reached more easily.For example,in Fig.8a,for a roof area of 50m 2and two residents,when the rainwater demand is 50%of the potable water demand,one can reach a 50%saving if the potable water demand is 50l per capita per day.On the other hand,when the potable water demand is 300lper0100200300400500600700800900100001234567891011Rainwater tank capacity (1000 litres)P o t a b l e w a t e r s a v i n g s (l i t r e s /d a y )(a)0100200300400500600700800900100001234567891011Rainwater tank capacity (1000 litres)P o t a b l e w a t e r s a v i n g s (l i t r e s /d a y )(b)1234567891011Rainwater tank capacity (1000 litres)P o t a b l e w a t e r s a v i n g s (l i t r e s /d a y )(c)0100200300400500600700800900100001234567891011Rainwater tank capacity (1000 litres)P o t a b l e w a t e r s a v i n g s (l i t r e s /d a y )(d)Fig.12Correlation between potable water savings per house and ideal rainwater tank capacity for the four roof areas of Itaquaquecetuba:a Roof area of 50m 2,b Roof area of 100m 2,c Roof area of 200m 2,d Roof area of 400m 22396 E.Ghisicapita per day,savings are less than 25%,which means that there is not enough rainwater to supply the rainwater demand.By increasing the roof area to 400m 2(Fig.8g),the saving increases to approximately 45%when the rainwater demand is 50%of the potable water demand (300l per capita per day).•the lower the potable water demand,the higher the potable water savings by using rainwater;such savings increase as the roof area increases and the number of residents decreases.•the potential for potable water savings increases as the rainfall increases.Such potentials are higher in Santos than in the other two cities (for rainfall figures see Figs.2,3and 4).4.3CorrelationsIn order to try to obtain conclusive results from the simulations and provide some guidance on rainwater tank sizing,some correlations were investigated.4.3.1Rainwater Tank Capacity and Potable Water DemandCorrelation between ideal rainwater tank capacity and total potable water demand per house,when considering all simulation results together,was inadequate asshown012345678910110510152025Daily rainwater demand (litres per house/m 2)R a i n w a t e r t a n k c a p a c i t y (1000 l i t r e s )(a)012345678910110510152025Daily rainwater demand (litres per house/m 2)R a i n w a t e r t a n k c a p a c i t y (1000 l i t r e s )(b)510152025Daily rainwater demand (litres per house/m 2)R a i n w a t e r t a n k c a p a c i t y (1000 l i t r e s )(c)0510152025Daily rainwater demand (litres per house/m 2)R a i n w a t e r t a n k c a p a c i t y (1000 l i t r e s )(d)Fig.13Correlation between rainwater demand (per house and roof area)and ideal rainwater tank capacity:a The three cities together,b Itaquaquecetuba,c Espírito Santo do Pinhal,d Santos。