Reliability analysis of household rainwater harvesting tanks in Greece

Transcription

1 Water Utility Journal 16: 17-24, E.W. Publications Reliability analysis of household rainwater harvesting tanks in Greece P.A. Londra 1*, A.T. Theocharis 2, Ε. Baltas 3 and V.A. Tsihrintzis 4 1 Laboratory of Agricultural Hydraulics, Department of Natural Resources Management and Agricultural Engineering, School of Agricultural Production, Infrastructures and Environment, Agricultural University of Athens, 75 Iera Odos, Athens, Greece 2 General Directorate of Sustainable Plant Production, Hellenic Ministry of Reconstruction of Production, Environment and Energy, 2 Acharnon, 176 Athens, Greece 3 Department of Water Resources and Environmental Engineering, School of Civil Engineering, National Technical University of Athens, Zografou, Greece 4 Centre for the Assessment of Natural Hazards and Proactive Planning & Laboratory of Reclamation Works and Water Resources Management, Department of Infrastructure and Rural Development, School of Rural and Surveying Engineering, National Technical University of Athens, Zografou, Greece * v.londra@aua.gr Abstract: Key words: Rainwater harvesting is a worldwide spread sustainable practice to meet water needs for domestic use. In this study, a rainwater tank reliability analysis was conducted to meet %, % and % of daily water demand of a 4-member household in six regions of Greece with different rainfall regimes. The reliability was defined as percentage of days when the rainwater tank was able to supply a specific demand. The rainwater tank size was determined using a daily water balance method for rainwater collection areas of and m 2. The analysis of the results showed that reliability levels were strongly influenced by rainfall regime of the region and they increase as collection area increases. Reliability, daily water balance, rainwater harvesting tank sizing 1. INTRODUCTION Today, rainwater harvesting for domestic use has become increasingly important as a relative cheap and simple water saving technology, being one of the most promising alternative water supply options against the problem of increasing water scarcity and demand (Zhang et al., ; Campisano and Modica, 12). Moreover, it provides supply security and independence. In Greece, rainwater harvesting in tanks (cisterns) was a well known practice from the Minoan Period (3 BC). Some of these structures were constructed so well that worked for centuries and were used as the main source of water supply (Antoniou et al., 6). Nowadays, the Greek urban planning legislation imposes the construction of rainwater harvesting tanks in 27 islands of the Aegean (Official Gazette of the Hellenic Republic, Fourth Issue - No July 7, 1993; Official Gazette of the Hellenic Republic, Fourth Issue - No 2 - May 17, 2). Determination of the optimal rainwater tank capacity is one of the most important steps in designing a rainwater harvesting system. Various methodologies have been developed based either on daily water balance models (Fewkes, 1999; Fewkes and Butler, ; Villarreal and Dixon, 5; Ghisi and Ferreira, 7; Mitchell, 7; Zhou et al., ; Imteaz et al., 11; Palla et al., 11; Ward et al., 11; Campisano and Modica, 12; Tsihrintzis and Baltas, 13; Londra et al., 15a), or on stochastic rainfall generations (Lee et al., ; Tsubo et al., 5; Guo and Baetz, 7; Cowden et al., 8; Su et al., 9; Basinger et al., ; Chang et al., 11). However, the rainwater tank capacity cannot be formulated universally, because it is influenced by several local variables, such as local rainfall, collection surfaces, demand and number of served residents, socioeconomic factors, etc. (Aladenola and Adeboye, ; Eroksuz and Rahman, ; Ghisi, ; Palla et al., 12; Londra et al., 15a, b; Gwenzi and Nyamadzawo, 14; Valdez et al., 16). In this study, a daily water balance model was used to determine the rainwater tank size, taking into account the daily rainfall data records, the rainwater collection areas, the runoff coefficient, the

2 18 P.A. Londra et al. number of residents served and rainwater water needs. The model was applied in six regions of Greece, characterized by different rainfall regimes, and the rainwater tank size reliability was investigated. 2. METHODOLOGY A daily water balance model (Tsihrintzis and Baltas, 13) was used for sizing the rainwater harvesting tank: S t = S t 1 + R t D t S t 1 V tank (1) where S t is the stored volume at the end of t th day (m 3 ), S t-1 the stored volume at the beginning of t th day (m 3 ), R t the harvested rainwater volume during the t th day (m 3 ), D t the daily water demand during the t th day (m 3 ), and V tank the capacity of the rainwater storage tank (m 3 ). Taking into account that the daily harvested rainwater volume, R t (m 3 ), from a collection (roof) area is calculated as R t = C A P (2) eff, t and the daily water demand, D t, of a household is calculated as t cap ( ) D = N q p (3) then the daily rainwater stored volume can be calculated as ( ) St = St 1 + C A Peff, t Ncap q p (4) where C is the runoff coefficient of the collection area (-), A is the size of the collection area (m 2 ), P eff,t is the daily effective rainfall depth during the t th day (m), N cap is the number of capita (residents) served by the storage tank, q is the daily water consumption per capita (m 3 ), and p is the percentage of total water demand satisfied by the harvested rainwater in the storage tank. In this study, the runoff coefficient was assumed equal to.9, and the daily effective rainfall equal to the daily rainfall P minus a first flush depth equal to.33 mm (Yaziz et al., 1989). The first flush is the first part of rainwater water fallen on the collection surface, which is of poorer quality (because it contains dust, leaves and bird droppings, among other impurities, accumulated on the collection area between rainfall events), and thus, it is diverted outside the storage tank for maintaining a better quality of harvested rainwater in the tank (Gikas and Tsihrintzis, 12, 17; Taffere et al., 16). Assuming a capacity of rainwater tank, the daily stored water in the tank was calculated using heuristic algorithms iteratively, allowing the excess water to overflow. The procedure is described in detail by Tsihrintzis and Baltas (13) and Londra et al. (15a), and has also been used in determining rainwater harvesting tank size for livestock use (Londra et al., 17). The reliability coefficient, R e, of a rainwater harvesting tank, i.e., the ability of rainwater tank to satisfy the household water demand, D t, is given by the following equation: Ns Re = % (5) N t where Ν s is the number of days where the household water demand is fully met by harvested rainwater, and N t is the total number of days simulated (i.e., the number of days of the rainfall record).

3 Water Utility Journal 16 (17) RAINFALL DATA Six rainfall stations from different regions of Greece, characterized by different rainfall regimes, were selected to apply the procedure described above (Table 1). The daily rainfall data used were obtained from the Ministry of the Environment, Energy and Climate Change ( for the time period Rainfall data were selected based, among others, on availability and completeness of the record. Time series duration exceeds the demands of DIN (2) German standard for rainwater harvesting tank sizing using daily water balance. Table 1. Mean annual rainfall values ( P ), mean values of the longest annual dry periods ( N ) and the corresponding dd standard deviations ( σ ) and (σ Ndd ) for the rainfall time series of stations studied Rainfall station P σ P P N σ N dd dd Rainfall time series (mm) (mm) (days) (days) (years) Agnanta (Arta) Meskla (Chania) Kimmeria (Xanthi) Keramia (Lesvos) Drama (Drama) Faneromeni (Naxos) The daily water balance method was applied using the daily rainfall records of the abovementioned stations taking into account tank sizes between 2 and m 3, and rainfall collection areas between and m 2 for meeting %, % and % of daily total water demand of a 4-member household. More specifically, the range of tank sizes from 2 to m 3 used represent both usual and statutorily imposed sizes (Londra et al., 15a), and collection areas between and m 2 represent achievable overall collection areas within suburban and rural regions. A 4-member household was selected as a typical size of a Greek family. Its water demand for non-potable use was determined, assuming daily water use per capita from rainwater storage of 1 L/cap/day, i.e., total L/day (Official Gazette of the Hellenic Republic, Second Issue - No 174 March 26, 1991), and three percentages p (Eq. 3) of total water demand to be fulfilled by harvested rainwater: % (i.e., 1 L/day), % (i.e., 2 L/day) and % (i.e., L/day). These percentages roughly correspond to water use for toilet flushing (~%), bathroom-shower (%-%), or/and cloth and dish washing (~15%) (Londra et al., 15a). 4. RESULTS AND DISCUSSION Table 1 presents the mean annual rainfall values and mean values of the longest annual dry periods, as they came from the daily rainfall time series for each station studied. Dry period was defined as the rainless period or the period with daily effective rainfall values less than 1 mm. The values in Table 1 reveal that there is a great deviation both in rainfall ( mm) and dry period ( days). The dry period represents a simple expression of rainfall occurrence, which means that in stations with low dry period values, rainfall occurs often, even low, marking continental climate, while in stations with high dry period values, the annual rainfall is concentrated in the winter period, leaving a long rainless summer period, typical of the Mediterranean climate. Stations with continental climate are located in central and north continental Greece (Agnanta, Drama, Kimmeria), while stations with Mediterranean climate are located in Greek islands (Meskla, Keramia, Faneromeni). Reliability curves of rainwater harvesting systems are presented in Figures 1-6. They refer to tank sizes of 2- m 3, connected to a or m 2 collection area, to meet a 1 (%), 2 (%) and L/day (%) demand of a 4-member household. Figure 1 presents reliability curves of rainwater harvesting system in relation to tank sizes to meet the demand of % for a 4-member household (1 L/day) with collection area of m 2 in

4 P.A. Londra et al. the six regions of Greece. It is observed that reliability, as expected, increases as tank size increases but with a different rate for each station and starting from a different level, depending on rainfall regime. More specifically, in high rainfall-low dry period station (Agnanta), the reliability starting point is close to % for an only 2 m 3 tank size and it reaches % for tank sizes of about 11 m 3 and larger. On the other hand, in low rainfall-high dry period station (Faneromeni) the starting point is % and the % percentage is never reached even with a m 3 tank size. Moreover, reliability is always higher in stations with low dry period (Agnanta, Kimmeria, Drama), regardless of the annual rainfall, indicating the important role of the dry period. 1 L/day, m Figure 1. Reliability curves of rainwater harvesting system in relation to tank sizes to meet the demand of % for a 4- member household (1 L/day) with collection area of m 2 in six regions of Greece In Figure 2, the case of 2 L/day demand with a m 2 collection area is presented. It is shown that the reliability starting point gets lower for all stations and the % percentage is never reached for half of them. Moreover, despite the demand increase, the dry period remains more decisive compared to rainfall, in most cases, except for high tank sizes (more than m 3 ), where low rainfall dominates. 2 L/day, m Agnanta Meskla Kimmeria Figure 2. Reliability curves of rainwater harvesting system in relation to tank sizes to meet the demand of % for a 4-member household (2 L/day) with collection area of m 2 in six regions of Greece

5 Water Utility Journal 16 (17) 21 In Figure 3, the case of L/day demand with a m 2 collection area is presented. As anticipated, since rainfall water demand is increased, the reliability values get even lower and the % is never reached for most of them, except for the high rainfall-low dry period station (Agnanta). Also, high reliability values (>%) are achieved only with large tank sizes (> m 3 ) and only for medium/high rainfall stations (Meskla, Kimmeria). Moreover, low rainfall becomes decisive instead of dry period and puts a limit on reliability values, almost regardless of tank size (Drama, Faneromeni). L/day, m Figure 3. Reliability curves of rainwater harvesting system in relation to tank sizes to meet the demand of % for a 4- member household ( L/day) with collection area of m 2 in six regions of Greece. In Figure 4, the case of 1 L/day demand with a m 2 collection area is presented. Comparison between Figure 4 and Figure 1 shows that the increase of collection area results, as expected, in increased reliability values for all stations studied. Still, low rainfall-high dry period station (Faneromeni) never reaches % reliability. Also, the domination of dry period becomes more obvious and the aforementioned climate division of stations is clearly depicted. 1 L/day, m Figure 4. Reliability curves of rainwater harvesting system in relation to tank sizes to meet the demand of % for a 4- member household (1 L/day) with collection area of m 2 in six regions of Greece

6 22 P.A. Londra et al. In Figure 5, the case of 2 L/day demand with a m 2 collection area is presented. It is shown that the demand increase results in lower reliability values, but the main characteristics remain the same as in Figure 4, i.e., the dry period dominates and % reliability is reached easily only in medium/high rainfall-low dry period stations (Agnanta, Kimmeria), while only a high rainfall-high dry period station (Meskla) achieves this for large tank sizes (>m 3 ). 2 L/day, m Figure 5. Reliability curves of rainwater harvesting system in relation to tank sizes to meet the demand of % for a 4- member household (2 L/day) with collection area of m 2 in six regions of Greece In Figure 6, the case of L/day demand with a m 2 collection area is presented. It is shown that all reliability values became lower for all tank sizes studied and the effect of low rainfall becomes obvious in large tank sizes. Also, despite the increased demand, reliability values remain above %, even with a m 3 tank, due to the increased collection area. L/day, m Figure 6. Reliability curves of rainwater harvesting system in relation to tank sizes to meet the demand of % for a 4- member household ( L/day) with collection area of m 2 in six regions of Greece

7 Water Utility Journal 16 (17) CONCLUSIONS A water budget model was developed and used to determine the size of a rainwater harvesting tank. The model was applied in testing the reliability of such a system in covering a percentage of the total water use in a four-person household, using the rainfall record from six regions of Greece, which differed in terms of the annual rainfall depth and the length of the dry period. Three water demand percentages and two collection area surfaces were used. For a given rainwater collection area, the reliability was found to decrease as water demand increased, while it increased as rainwater collection area increased. The greatest reliabilities were observed in areas with mean to high annual rainfall depth and low dry period for all water demand levels studied. Low dry period was found to be the key parameter in achieving high reliability values, except in areas of very low rainfall, where rainfall depth becomes the limiting factor. ACKNOWLEDGEMENTS An initial shorter version of the paper has been presented in Greek at the 3 rd Common Conference (13 th of the Hellenic Hydrotechnical Association, 9 th of the Hellenic Committee on Water Resources Management and 1 st of the Hellenic Water Association) Integrated Water Resources Management in the New Era, Athens, Greece, December -12, 15. REFERENCES Aladenola, O. O., Adeboye, O. B.,. Assessing the potential for rainwater harvesting. Water Resources Management; 24(): Antoniou, G., Xarchakou, R., Angelakis, A. N., 6. Water cistern systems in Greece from Minoan to Hellenistic Period. IWA 1 st International Symposium on Water and Wastewater Technologies in Ancient Civilizations, 28- October 6, Iraklio, Greece. Basinger, M., Montalto, F. Lall, U.,. A rainwater harvesting system reliability model based on nonparametric stochastic rainfall generator. Journal of Hydrology; 392: Campisano, A., Modica, C., 12. Optimal sizing of storage tanks for domestic rainwater harvesting in Sicily. Resources Conservation and Recycling; 63: Chang, N., Rivera, B. J., Wanielista, M. P., 11. Optimal design for water conservation and energy savings using green roofs in a green building under mixed uncertainties. Journal of Cleaner Production; 19: Cowden, J. R., Watkins, D. W. Jr., Mihelcic, J. R., 8. Stochastic rainfall modeling in West Africa: Parsimonious approaches for domestic rainwater harvesting assessment. Journal of Hydrology; 361: DIN , 2. Rainwater Harvesting Systems - Part 1: Planning, Installation, Operation and Maintenance, German Institute for Standardisation, Berlin. Eroksuz, E., Rahman, A.,. Rainwater tanks in multi-unit buildings: a case study for three Australian cities. Resources Conservation and Recycling; 54: Fewkes, A., Butler, D.,. Simulating the performance of rainwater collection systems using behavioural models? Building Services Engineering Research and Technology; 21(2): Fewkes, A., Modelling the performance of rainwater collection systems: towards a generalized approach. Urban Water; 1: Ghisi, E., Ferreira, D. F., 7. Potential for potable water savings by using rainwater and greywater in a multi-story residential building in southern Brazil. Building and Environment; 42(4): Ghisi, E.,. Parameters influencing the sizing of rainwater tanks for use in houses. Water Resources Management; 24: Gikas, G. D., Tsihrintzis, V. A., 12. Assessment of water quality of first-flush roof runoff and harvested rainwater, Journal of Hydrology; : , DOI:.16/j.jhydrol Gikas, G. D., Tsihrintzis, V. A., 17. Effect of first-flush device, roofing material, and antecedent dry days on water quality of harvested rainwater, Environmental Science and Pollution Research; 24(27): , DOI:.7/s Guo, Y., Baetz, B., 7. Sizing of rainwater storage units for Green Building applications. Journal of Hydrologic Engineering; 12(2): Gwenzi, W., Nyamadzawo, G., 14. Hydrological impacts of urbanization and urban roof water harvesting in water-limited catchments: a review. Environmental Processes; 1(4): , DOI:.7/s Imteaz, M. A., Shanableh, A., Rahman, A., Ahsan, A., 11. Optimization of rainwater tank design from large roofs: a case study in Melbourne, Australia. Resources Conservation and Recycling; 55: Lee, K. T., Lee, C. D., Yang, M. S., Yu, C. C.,. Probabilistic design of storage capacity for rainwater cistern systems. Journal of Agricultural Engineering and Research; 77(3):

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