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National Technical University of Athens (NTUA) 2014

Sizing of rainwater harvesting tank for domestic use in Greece

Londra, Paraskevi A. ; Λόντρα, Παρασκευή Α.

Titre : Sizing of rainwater harvesting tank for domestic use in Greece

Διαστασιολόγηση δεξαμενών όμβριων υδάτων στην Ελλάδα για αστική χρήση

Auteur : Londra, Paraskevi A. ; Λόντρα, Παρασκευή Α.

Etablissement de soutenance : National Technical University of Athens (NTUA)

Grade : Master thesis 2014

Résumé
The aim of this postgraduate thesis is the sizing of rainwater harvesting tanks in Greece for domestic use. To determine the optimal size of rainwater harvesting tanks two methods, the daily water balance method and the dry period demand method, are used in 75 regions of Greece to meet non-potable water demand of a household 3 to 5 residents and their results are considered. A daily water balance model was developed, introducing heuristic algorithms which allowed excess water to overflow and adjusted public water supply to zero. Daily rainfall data within the period 1980-1996 from 75 stations distributed across the 14 Water Districts of Greece were obtained with mean annual rainfall values ranging from 323 to 1405.2 mm and maximum values of the longest annual dry periods ranging from 46 to 217 days. In this study, the water demand for non-potable use of a household with number of capita Νcap=3 to 5 was determined, assuming q=150 l/cap/day and p ranged from 30% to 50%, i.e. 45 to 75 l/cap/day, respectively. These percentages correspond to water use for toilet ( 30%), bathroom-shower (20%-30%) or/and cloths and dishes washing ( 15%). Firstly, the minimum required rainwater collection area to satisfy a certain percentage of total water demand for certain residents was calculated, assuming that the mean annual harvested rainwater volume is equal to mean annual demand. Secondly, a daily water balance model was applied and the required rainwater tank volumes for the above mentioned minimum areas for the given water demand were calculated. Finally, in order to size as smaller tanks as possible, the reduction of tank volume by increasing the collection area from the minimum required one was investigated. The results demonstrated that in the majority of 75 rainfall stations studied, tank sizes up to 50 m3 can meet a 135 to 225 l/d demand (30% of total demand of 3 to 5 residents), with roof areas not exceeding 200 m2, while a 240 l/d demand (40% of 4 residents) can be met with the same tank size by just increasing roof area to 300 m2. More than 50 m3 tank size is needed in order to meet demands of 300 l/d (40% of 5, 50% of 4 residents) or 375 l/d (50% of 5 residents). Only a few exceptions exist, regarding either stations with high annual rainfall and low dry period (e.g Agnanta, Arta), which can meet a 375 l/d (50% of 5 residents) demand with tank size up to 50 m3, or stations with low annual rainfall and high dry period (Faneromeni, Naxos) which can meet only a 135 l/d demand (30% of 3 residents) with tank size of 50 m3 if the roof area reaches 300 m2. The smallest tanks are needed in Epirus mountain areas, where a 135 l/d demand (30% of 3 residents) can be met with a 10 m3 tank, while the largest tanks are needed in Attica-Cyclades area where a 135 l/d demand needs a 50 m3 tank. Tank size is strongly affected by dry period value, since comparison among stations with large difference in dry period, regardless their difference in annual rainfall, showed a large difference in tank sizes. As a result of the dry period’s dominant role in tank sizing, it was concluded that small dry period leads to small tanks, with the exception of low rainfall-high demand (300-375 l/d) case, where low rainfall increases sizes, having the dominant role. In low rainfall areas regardless dry period value, roof areas of more than 300 m2 are needed in order to meet high demands (300-375 l/d). Comparison among dry period demand and daily water balance method showed that in all cases, dry period demand method calculates smaller tanks, with the exception of areas with medium-high rainfall and high dry period on one hand, and low-medium demand (135-225 l

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