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A dual-mode system for harnessing roofwater for non-potable uses
Urban Water 1 (1999) 317±321
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A dual-mode system for harnessing roofwater for non-potable uses Adhityan Appan School of Civil and Structural Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore Received 26 November 1999; received in revised form 2 June 2000; accepted 26 July 2000
Abstract In a limited land area of about 630 km2 Singapore has an average annual rainfall of 2250 mm. As catchment area is limited and about 60% of water is being imported, there is a continuous search for new sources. One such source is roofwater from high-rise buildings in which almost 84% of the population lives. The main objectives of this paper are to study the feasibility of collecting the roofwater from the north spine of the Nanyang Technological University (NTU) campus and to utilise it for non-potable uses. A well-established simple input±output model incorporating a dual-mode system (DMS) of operation has been used so as to ensure that all the collected roofwater from an area of 38,700 m2 is used only for non-potable purposes. The design parameters were optimized leading to a rainwater storage tank linked to the existing water tanks with pump cut-in and cut-out electrodes placed at depths of 1 and 1.5 m above the base of the tank that will ensure that the supply of water will always be available for the non-potable uses. The overall system will realise a monthly saving in water consumption expenditure in the NTU complex by about S$18,500.00. Ó 2000 Published by Elsevier Science Ltd. Keywords: Roofwater; Collection; Singapore; Dual-mode system; Water quality; Economic bene®t
1. Introduction The annual rainfall in Singapore is about 2250 mm but, due to industrial development and increasing population, water demands are constantly increasing. The land area is only 630 km2 with only half the area being available as catchment because there are competing demands for land use. Consequently, almost 60% of the water requirements are being imported. Hence, the augmentation of water resources and investigation of potentially new areas are on-going issues in Singapore. Since 84% of the urban population lives in high-rise buildings, the roofs of such structures have good potential to act as catchments to collect roofwater at a highlevel and use it, preferably, for non-potable uses. The main objectives of this study are to review some computer models on roofwater collection, to select a simple model and using existing data to determine the optimum design parameters. The collected roofwater is to be used for the ¯ushing of toilets and the proposed system is to be integrated with an existing potable water system by incorporating a dual-mode facility which will ensure the perennial supply for non-potable and potable uses.
E-mail address: [email protected] (A. Appan). 1462-0758/00/$ - see front matter Ó 2000 Published by Elsevier Science Ltd. PII: S 1 4 6 2 - 0 7 5 8 ( 0 0 ) 0 0 0 2 5 - X
2. Use of computer models for roofwater collection systems 2.1. Simulation models Models have been developed which use rainfall, catchment area and yield to determine storage requirements. Such models have varied from simple deterministic types (Hoey & West 1982) to probabilistic (Kok, Fong, Murabayashi, & Lo, 1982) and stochastic models (Leung & Fok, 1982). In the United Kingdom a model was also developed (Fewkes & Ferris, 1982) where the rainwater was only used for toilet ¯ushing. In this model, rainfall was simulated using the Monte Carlo method and the percentage water saved per annum for a range of tank capacities, roof areas and family sizes were determined. Computerized methods in optimizing storage volumes have also been reviewed (Schiller & Latham, 1982), the storage volume being determined by dierent methods based primarily on the classical mass curve analysis (Rippl, 1883), yield after storage model (Jenkins, Pearson, Moore, Kim, & Valentine, 1978) or a statistical method (Ree, Wimberly, Guinn, & Lauritzeb, 1971). In another simulation model (Morris, AcevidoPimentel, & Ayala, 1984), the storage required to deliver
318
A. Appan / Urban Water 1 (1999) 317±321
a constant ¯ow rate at each rainfall station was determined. A special feature of this model was the dualmode of withdrawal with and without rationing. 2.2. Models developed at NTU A simple input/output simulation model was used (Appan, 1982) with a minor addition in the output side as follows: Qi Ari ÿ f Ei bi A Di g;
1
where during any duration i (say, 15 min), Qi is the available quantity (in say, m3 ), ri the rainfall (m), Ei the evaporation (m), bi the absorption (m), Di the draw-o (m3 ) and A the catchment (m2 ) which is independent of i. Eq. (1) is applied to a known storage volume and the cistern will be empty or subject to spillage according as Ari is lesser or greater than f Ei bi A Di g. The main advantage of this model is that it can be adjusted to suit the durations in which data like rainfall, evaporation and absorption are available. This model has been used, for varying ®eld conditions, over a period of time. In a study on the collection of rainwater from a bus park cum interchange (Appan, Alsogo, & Tan, 1988), a simple computer program (NTURWCS.MK1) was written to calculate the optimum tank size required to meet the daily needs. In this program, as evaporation and absorption data were not readily available, suitable runo coecients were used as the runo was being collected from both roofs and paved areas. In another study undertaken (Chan & Heng, 1992), the model was further developed to accommodate varying discretised time intervals of rainfall. This model was later upgraded (NTURWCS.MK3) to determine both the eciency of an existing DMS in the Singapore Changi Airport and optimum reservoir size required to meet hourly demand for ®re ®ghting and toilet ¯ushing (Appan, Jeyaprakasham, & Punithan, 1995). In a more recent study (Appan, Ho, & Wong, 1997), rain water from all the high-rise buildings in a housing estate having a typical average height of 12 storeys has been collected and stored at roof level. Using a DMS and a separate piping system, the collected roofwater is used only for non-potable uses thus also eecting considerable savings in energy costs.
Fig. 1. Existing and proposed reticulation system for NTU campus.
readily available, is pumped to two high-level tanks, which cater for the water requirements of the whole campus. Water from one of these tanks (see Fig. 1) is gravitated to the north spine where it is stored in a 31.0 m ´ 15.5 m ´ 4 m deep distribution tank that consists of two inter-connected compartments. The water is then distributed by seven dierent pumps to constant head tanks located at the roofs of various buildings in the north spine. 3.2. Daily demand At the outlets in the distribution tank, eight sets of 24 hourly readings were taken and the consumption is as shown in Fig. 2. There is a wide variation in demand from 5 m3 at 2100±0500 h to a maximum of 65 m3 at 1200 h. The average consumption per day was 540 m3 .
3. Case study 3.1. Water supply and reticulation system Spread over an area of about 200 ha, the NTU campus lies in the south-western part of Singapore. In this area there are six schools, two administrative blocks, halls of residence and canteens which lie around the north and south spines. Potable water, which is
Fig. 2. Daily water demands.
A. Appan / Urban Water 1 (1999) 317±321
3.3. Rainfall quantity and its quality Continuous recording is done in a rainfall station in the NTU campus using a standard tipping bucket type rain gauge. The readings obtained covered a period of about two hydrologic years between 1989 and 1991. During this period, 38 water samples were collected at the roof level and tested immediately for relevant physico-chemical and bacteriological parameters. 3.4. Proposal to use the DMS in the rainwater tank This study is con®ned to the collection of rainwater from an area of about 38,700 m2 in the north spine. The approach in this study is to channel all rainwater into a new rainwater tank that shares a wall in common with the existing distribution tank. The distribution tank will be supplying water only for potable uses whereas the rainwater tank will meet all the demands for non-potable uses. The two water tanks will be separate units but they will be inter-connected by a simple DMS such that when the rainwater tank is empty, potable water will be pumped into it to a pre-determined level and the water will be used for non-potable uses. It is ensured that the in¯ow pipe from the distribution tank is at least 15 cm above the top water level of the rainwater tank so that there can be no back ¯ow. This separate system will ensure that there will be no intrusion of rainwater into the existing distribution tank besides which the dual mode of supply will be perennially operational.
319
that over¯ows and amount of potable water that will be needed when the stored rainwater runs low. The program was run for dierent storage volumes by varying the pump cut-in/cut-out electrode levels that represent the bottom and intermediate levels respectively in the rainwater tank.
4. Results and discussion A series of runs was carried out and some of the salient parameters arrived at are as follows. 4.1. Optimum length of rainwater tank The rainwater tank was to be placed adjacent to the existing distribution tank. Initially a series of runs was carried out varying the volume of the rainwater tank. This was done by retaining the breadth and depth of the distribution tank (due to space constraints on site) but progressively increasing the length thus leading to larger volumes for the storage of rainwater. The results obtained are as shown in Fig. 3. It can be seen that when the rain over¯ow was reduced to zero, thus indicating full utilisation (or 100%) of the collected roofwater, the optimum length of the rainwater tank is 41 m.
3.5. Choice of an appropriate model for determining design parameters Due to the availability of hourly data, the model chosen is NTURWCS.MK1 (Appan et al., 1988) in which the basic inputs are rainfall data, size of storage tank, evaporation, absorption and the hourly demand. The output will be the rainwater collected, the quantity
Fig. 3. Rainwater tank length analysis.
Table 1 Water quality ± rain, potable and hydraulic laboratory qualitya
a
Parameter
Rainwater
Potable waterb
Hydraulic lab
pH Colour Turbidity (NTU) TSS (mg/l) TDS (mg/l) Hardness as CaCO3 (mg/l) PO4 as P (mg/l) Total coliformc Faecal coliformc
4.1 8.7 4.6 9.1 19.5 0.1 0.1 92.0 6.7
7.0±7.5 <5 <5 240±400 ± 20±40 ± 0 ±
7.4 (0.1) 26.3 (32.1) 41.5 (16.3) 8.5 (6.0) 427 (119) 32.5 (2.1) 0.1 (0.1) 1.5 (2.1) 1.1 (0.1)
(0.4) (9.9) (5.7) (8.9) (12.5) (0.3) (0.6) (97.1) (8.9)
Note: Values are means and those in parentheses are standard deviation. Wong (1986). c MPN/100 ml. b
320
A. Appan / Urban Water 1 (1999) 317±321
However, if the arrangements on site only allow a shorter length, some adjustments can be made widthwise ensuring that the volume of the rainwater tank is unaltered. 4.2. Pump cut-in/cut-out electrode levels Though the optimum volume of the rainwater tank ensures that all the collected rainwater is utilised, there is the need to determine the quantity of potable water that has to be pumped into it whenever it is almost empty. This corresponds to the bottom water level (BWL) of 1 m which is actually the pump cut-in level of the electrode. With this in mind, a series of runs was carried out to determine the relevant volumes to be used in relation to the number of cut-ins. The most suitable level for minimizing the use of potable water was determined to be at a height of 1.5 m, which corresponds to the pump cut-out electrode, or intermediate level of the tank. The transfer of water from the existing distribution to the rainwater tank is done by installing a pump having a capacity greater than the maximum water demand rates for non-potable uses. Such a DMS will ensure that the non-potable demands are met perpetually and, at the same time, there will be no intermixing of potable and non-potable water in the existing distribution tank, which will be dedicated to the supply of potable water. 4.3. Rainwater quality The qualitative analyses of the collected water samples are presented in Table 1. Also presented are the prevalent potable water standards in Singapore and the quality of water, non-potable type, prevalent in the hydraulic laboratories in NTU wherein water is recycled. The collected rainwater has a generally high quality in comparison with potable water supply and the laboratory storage tanks except for pH and the total coliform counts. Treatment has to be carried out so that the pH is raised to the range of 7±7.5 besides which suitable disinfection has to be carried out, as it has been experienced in the past that such stored water has been the breeding ground for mosquitoes leading to dengue fever. 5. Summary and conclusion 1. A simple input/output model has been used to arrive at an optimum size of rainwater tank, which will ensure that all the collected roofwater will be used for non-potable uses. 2. The collected water has an acceptable quality for non-potable uses and it is recommended that pro-
vision be made to raise the pH and disinfect the water. 3. The collected rainwater is directed to a separate rainwater tank that is linked to the existing distribution tank which only supplies potable water. 4. The operation of both these tanks will ensure that potable and non-potable requirements are fully satis®ed and, at the same time, the potable water will not come in contact with rainwater. This is eected by a DMS which will ensure that when the rainwater tank is considered to be empty at the BWL of 1 m, a pump is activated and potable water from the distribution tank is transferred to the rainwater tank to the intermediate level of 1.5 m. 5. The utilisation of roofwater will realize savings of about S$18,400/month which is about 12.4% of the current monthly expenditure for water used in the north spine. References Appan, A. (1982). Some aspects of roofwater collection. In Proceedings of the conference on rain water cistern systems (RWCS) (pp. 220±226). University of Hawaii, Honolulu, USA, June. Appan, A., Alsogo, F., & Tan, K. L. (1988). A feasibility study on the utilization of surface runo from a small paved catchment as a supplementary source for non-potable use. In Proceedings of the 6th IWRA world congress on water resources, Vol. 4 (pp. 260±271). Ottawa, Canada. Appan, A., Ho, H. C., & Wong, H. J. (1997). Alternate dual-mode working systems for the collection and use of rainwater in high-rise buildings for non-potable uses. In Proceedings of the 8th International conference on rainwater catchment systems (pp. 3±9). Teheran, 25±29 April. Appan, A., Jeyaprakasham, T., & Punithan, S. (1995). A total approach towards the design of RWCS in airports subjected to tidal eects. In Proceedings of the 7th International conference on rainwater catchment systems (pp. 7-51±60), Beijing, China. Chan, S. L., & Heng, H. L. C. (1992). Investigation and design of a rainwater cistern system in Nanyang Technological University (unpublished) academic report. School of Civil & Structural Engineering, Nanyang Technological University, Singapore. Fewkes, A., & Ferris, S. A. (1982). Rain and waste reuse for toilet ¯ushing: a simulation model. In Proceedings of the international conference on RWCS (pp. 77±389). Honolulu, USA, June. Kok, Y. S., Fong, R. H. L., Murabayashi, E. T., & Lo, A. (1982). Deterministic and probabilistic processes of weekly rainfall. In Proceedings of the international conference on RWCS (pp. 83±91). Honolulu, USA, June. Hoey, P. J., & West, S. F. (1982). Recent initiation in rainwater supply systems for South Australia. In Proceedings of the international conference on RWCS (pp. 284±293). Honolulu, USA. Jenkins, D., Pearson, F., Moore, E., Kim, S. J., & Valentine, R. (1978). Feasibility of rainwater collection systems in California. Contribution No. 173, California Water Resources Center, University of California, Davis, USA. Leung, P., & Fok, Y. S. (1982). Determining desirable storage volume of rain catchment cistern system: a stochastic assessment. In Proceedings of the international conference on RWCS (pp. 128±134). Honolulu, USA. Morris, G. L., Acevido-Pimentel, R., & Ayala, G. (1984). Yield and cost of water supplies from rained cistern: Puerto Rico. In
A. Appan / Urban Water 1 (1999) 317±321 Proceedings of the 2nd international conference on RWCS (pp. FI1±13). Virgin Islands, USA, June. Ree, W. O., Wimberly, F. L., Guinn, W. R., & Lauritzen, C. W. (1971). Rainwater harvesting system. ARS41-184, Agricultural Research Service, US Department of Agriculture, p. 12. Rippl, W. (1883). The capacity of storage reservoirs for water supply. In Proceedings of the institution of civil engineers, Vol. 71 (pp. 270± 278). London.
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Schiller, E. J., & Latham, B. (1982). Computerized methods in optimizing rain water catchment systems. In Proceedings of the international conference on RWCS (pp. 92±101). Honolulu, USA, June. Wong, K. W. (1986). Use of ozone in the treatment of water for potable purposes. Journal of Water Science and Technology, 18(3), 180±184.
www.elsevier.com/locate/urbwat
A dual-mode system for harnessing roofwater for non-potable uses Adhityan Appan School of Civil and Structural Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore Received 26 November 1999; received in revised form 2 June 2000; accepted 26 July 2000
Abstract In a limited land area of about 630 km2 Singapore has an average annual rainfall of 2250 mm. As catchment area is limited and about 60% of water is being imported, there is a continuous search for new sources. One such source is roofwater from high-rise buildings in which almost 84% of the population lives. The main objectives of this paper are to study the feasibility of collecting the roofwater from the north spine of the Nanyang Technological University (NTU) campus and to utilise it for non-potable uses. A well-established simple input±output model incorporating a dual-mode system (DMS) of operation has been used so as to ensure that all the collected roofwater from an area of 38,700 m2 is used only for non-potable purposes. The design parameters were optimized leading to a rainwater storage tank linked to the existing water tanks with pump cut-in and cut-out electrodes placed at depths of 1 and 1.5 m above the base of the tank that will ensure that the supply of water will always be available for the non-potable uses. The overall system will realise a monthly saving in water consumption expenditure in the NTU complex by about S$18,500.00. Ó 2000 Published by Elsevier Science Ltd. Keywords: Roofwater; Collection; Singapore; Dual-mode system; Water quality; Economic bene®t
1. Introduction The annual rainfall in Singapore is about 2250 mm but, due to industrial development and increasing population, water demands are constantly increasing. The land area is only 630 km2 with only half the area being available as catchment because there are competing demands for land use. Consequently, almost 60% of the water requirements are being imported. Hence, the augmentation of water resources and investigation of potentially new areas are on-going issues in Singapore. Since 84% of the urban population lives in high-rise buildings, the roofs of such structures have good potential to act as catchments to collect roofwater at a highlevel and use it, preferably, for non-potable uses. The main objectives of this study are to review some computer models on roofwater collection, to select a simple model and using existing data to determine the optimum design parameters. The collected roofwater is to be used for the ¯ushing of toilets and the proposed system is to be integrated with an existing potable water system by incorporating a dual-mode facility which will ensure the perennial supply for non-potable and potable uses.
E-mail address: [email protected] (A. Appan). 1462-0758/00/$ - see front matter Ó 2000 Published by Elsevier Science Ltd. PII: S 1 4 6 2 - 0 7 5 8 ( 0 0 ) 0 0 0 2 5 - X
2. Use of computer models for roofwater collection systems 2.1. Simulation models Models have been developed which use rainfall, catchment area and yield to determine storage requirements. Such models have varied from simple deterministic types (Hoey & West 1982) to probabilistic (Kok, Fong, Murabayashi, & Lo, 1982) and stochastic models (Leung & Fok, 1982). In the United Kingdom a model was also developed (Fewkes & Ferris, 1982) where the rainwater was only used for toilet ¯ushing. In this model, rainfall was simulated using the Monte Carlo method and the percentage water saved per annum for a range of tank capacities, roof areas and family sizes were determined. Computerized methods in optimizing storage volumes have also been reviewed (Schiller & Latham, 1982), the storage volume being determined by dierent methods based primarily on the classical mass curve analysis (Rippl, 1883), yield after storage model (Jenkins, Pearson, Moore, Kim, & Valentine, 1978) or a statistical method (Ree, Wimberly, Guinn, & Lauritzeb, 1971). In another simulation model (Morris, AcevidoPimentel, & Ayala, 1984), the storage required to deliver
318
A. Appan / Urban Water 1 (1999) 317±321
a constant ¯ow rate at each rainfall station was determined. A special feature of this model was the dualmode of withdrawal with and without rationing. 2.2. Models developed at NTU A simple input/output simulation model was used (Appan, 1982) with a minor addition in the output side as follows: Qi Ari ÿ f Ei bi A Di g;
1
where during any duration i (say, 15 min), Qi is the available quantity (in say, m3 ), ri the rainfall (m), Ei the evaporation (m), bi the absorption (m), Di the draw-o (m3 ) and A the catchment (m2 ) which is independent of i. Eq. (1) is applied to a known storage volume and the cistern will be empty or subject to spillage according as Ari is lesser or greater than f Ei bi A Di g. The main advantage of this model is that it can be adjusted to suit the durations in which data like rainfall, evaporation and absorption are available. This model has been used, for varying ®eld conditions, over a period of time. In a study on the collection of rainwater from a bus park cum interchange (Appan, Alsogo, & Tan, 1988), a simple computer program (NTURWCS.MK1) was written to calculate the optimum tank size required to meet the daily needs. In this program, as evaporation and absorption data were not readily available, suitable runo coecients were used as the runo was being collected from both roofs and paved areas. In another study undertaken (Chan & Heng, 1992), the model was further developed to accommodate varying discretised time intervals of rainfall. This model was later upgraded (NTURWCS.MK3) to determine both the eciency of an existing DMS in the Singapore Changi Airport and optimum reservoir size required to meet hourly demand for ®re ®ghting and toilet ¯ushing (Appan, Jeyaprakasham, & Punithan, 1995). In a more recent study (Appan, Ho, & Wong, 1997), rain water from all the high-rise buildings in a housing estate having a typical average height of 12 storeys has been collected and stored at roof level. Using a DMS and a separate piping system, the collected roofwater is used only for non-potable uses thus also eecting considerable savings in energy costs.
Fig. 1. Existing and proposed reticulation system for NTU campus.
readily available, is pumped to two high-level tanks, which cater for the water requirements of the whole campus. Water from one of these tanks (see Fig. 1) is gravitated to the north spine where it is stored in a 31.0 m ´ 15.5 m ´ 4 m deep distribution tank that consists of two inter-connected compartments. The water is then distributed by seven dierent pumps to constant head tanks located at the roofs of various buildings in the north spine. 3.2. Daily demand At the outlets in the distribution tank, eight sets of 24 hourly readings were taken and the consumption is as shown in Fig. 2. There is a wide variation in demand from 5 m3 at 2100±0500 h to a maximum of 65 m3 at 1200 h. The average consumption per day was 540 m3 .
3. Case study 3.1. Water supply and reticulation system Spread over an area of about 200 ha, the NTU campus lies in the south-western part of Singapore. In this area there are six schools, two administrative blocks, halls of residence and canteens which lie around the north and south spines. Potable water, which is
Fig. 2. Daily water demands.
A. Appan / Urban Water 1 (1999) 317±321
3.3. Rainfall quantity and its quality Continuous recording is done in a rainfall station in the NTU campus using a standard tipping bucket type rain gauge. The readings obtained covered a period of about two hydrologic years between 1989 and 1991. During this period, 38 water samples were collected at the roof level and tested immediately for relevant physico-chemical and bacteriological parameters. 3.4. Proposal to use the DMS in the rainwater tank This study is con®ned to the collection of rainwater from an area of about 38,700 m2 in the north spine. The approach in this study is to channel all rainwater into a new rainwater tank that shares a wall in common with the existing distribution tank. The distribution tank will be supplying water only for potable uses whereas the rainwater tank will meet all the demands for non-potable uses. The two water tanks will be separate units but they will be inter-connected by a simple DMS such that when the rainwater tank is empty, potable water will be pumped into it to a pre-determined level and the water will be used for non-potable uses. It is ensured that the in¯ow pipe from the distribution tank is at least 15 cm above the top water level of the rainwater tank so that there can be no back ¯ow. This separate system will ensure that there will be no intrusion of rainwater into the existing distribution tank besides which the dual mode of supply will be perennially operational.
319
that over¯ows and amount of potable water that will be needed when the stored rainwater runs low. The program was run for dierent storage volumes by varying the pump cut-in/cut-out electrode levels that represent the bottom and intermediate levels respectively in the rainwater tank.
4. Results and discussion A series of runs was carried out and some of the salient parameters arrived at are as follows. 4.1. Optimum length of rainwater tank The rainwater tank was to be placed adjacent to the existing distribution tank. Initially a series of runs was carried out varying the volume of the rainwater tank. This was done by retaining the breadth and depth of the distribution tank (due to space constraints on site) but progressively increasing the length thus leading to larger volumes for the storage of rainwater. The results obtained are as shown in Fig. 3. It can be seen that when the rain over¯ow was reduced to zero, thus indicating full utilisation (or 100%) of the collected roofwater, the optimum length of the rainwater tank is 41 m.
3.5. Choice of an appropriate model for determining design parameters Due to the availability of hourly data, the model chosen is NTURWCS.MK1 (Appan et al., 1988) in which the basic inputs are rainfall data, size of storage tank, evaporation, absorption and the hourly demand. The output will be the rainwater collected, the quantity
Fig. 3. Rainwater tank length analysis.
Table 1 Water quality ± rain, potable and hydraulic laboratory qualitya
a
Parameter
Rainwater
Potable waterb
Hydraulic lab
pH Colour Turbidity (NTU) TSS (mg/l) TDS (mg/l) Hardness as CaCO3 (mg/l) PO4 as P (mg/l) Total coliformc Faecal coliformc
4.1 8.7 4.6 9.1 19.5 0.1 0.1 92.0 6.7
7.0±7.5 <5 <5 240±400 ± 20±40 ± 0 ±
7.4 (0.1) 26.3 (32.1) 41.5 (16.3) 8.5 (6.0) 427 (119) 32.5 (2.1) 0.1 (0.1) 1.5 (2.1) 1.1 (0.1)
(0.4) (9.9) (5.7) (8.9) (12.5) (0.3) (0.6) (97.1) (8.9)
Note: Values are means and those in parentheses are standard deviation. Wong (1986). c MPN/100 ml. b
320
A. Appan / Urban Water 1 (1999) 317±321
However, if the arrangements on site only allow a shorter length, some adjustments can be made widthwise ensuring that the volume of the rainwater tank is unaltered. 4.2. Pump cut-in/cut-out electrode levels Though the optimum volume of the rainwater tank ensures that all the collected rainwater is utilised, there is the need to determine the quantity of potable water that has to be pumped into it whenever it is almost empty. This corresponds to the bottom water level (BWL) of 1 m which is actually the pump cut-in level of the electrode. With this in mind, a series of runs was carried out to determine the relevant volumes to be used in relation to the number of cut-ins. The most suitable level for minimizing the use of potable water was determined to be at a height of 1.5 m, which corresponds to the pump cut-out electrode, or intermediate level of the tank. The transfer of water from the existing distribution to the rainwater tank is done by installing a pump having a capacity greater than the maximum water demand rates for non-potable uses. Such a DMS will ensure that the non-potable demands are met perpetually and, at the same time, there will be no intermixing of potable and non-potable water in the existing distribution tank, which will be dedicated to the supply of potable water. 4.3. Rainwater quality The qualitative analyses of the collected water samples are presented in Table 1. Also presented are the prevalent potable water standards in Singapore and the quality of water, non-potable type, prevalent in the hydraulic laboratories in NTU wherein water is recycled. The collected rainwater has a generally high quality in comparison with potable water supply and the laboratory storage tanks except for pH and the total coliform counts. Treatment has to be carried out so that the pH is raised to the range of 7±7.5 besides which suitable disinfection has to be carried out, as it has been experienced in the past that such stored water has been the breeding ground for mosquitoes leading to dengue fever. 5. Summary and conclusion 1. A simple input/output model has been used to arrive at an optimum size of rainwater tank, which will ensure that all the collected roofwater will be used for non-potable uses. 2. The collected water has an acceptable quality for non-potable uses and it is recommended that pro-
vision be made to raise the pH and disinfect the water. 3. The collected rainwater is directed to a separate rainwater tank that is linked to the existing distribution tank which only supplies potable water. 4. The operation of both these tanks will ensure that potable and non-potable requirements are fully satis®ed and, at the same time, the potable water will not come in contact with rainwater. This is eected by a DMS which will ensure that when the rainwater tank is considered to be empty at the BWL of 1 m, a pump is activated and potable water from the distribution tank is transferred to the rainwater tank to the intermediate level of 1.5 m. 5. The utilisation of roofwater will realize savings of about S$18,400/month which is about 12.4% of the current monthly expenditure for water used in the north spine. References Appan, A. (1982). Some aspects of roofwater collection. In Proceedings of the conference on rain water cistern systems (RWCS) (pp. 220±226). University of Hawaii, Honolulu, USA, June. Appan, A., Alsogo, F., & Tan, K. L. (1988). A feasibility study on the utilization of surface runo from a small paved catchment as a supplementary source for non-potable use. In Proceedings of the 6th IWRA world congress on water resources, Vol. 4 (pp. 260±271). Ottawa, Canada. Appan, A., Ho, H. C., & Wong, H. J. (1997). Alternate dual-mode working systems for the collection and use of rainwater in high-rise buildings for non-potable uses. In Proceedings of the 8th International conference on rainwater catchment systems (pp. 3±9). Teheran, 25±29 April. Appan, A., Jeyaprakasham, T., & Punithan, S. (1995). A total approach towards the design of RWCS in airports subjected to tidal eects. In Proceedings of the 7th International conference on rainwater catchment systems (pp. 7-51±60), Beijing, China. Chan, S. L., & Heng, H. L. C. (1992). Investigation and design of a rainwater cistern system in Nanyang Technological University (unpublished) academic report. School of Civil & Structural Engineering, Nanyang Technological University, Singapore. Fewkes, A., & Ferris, S. A. (1982). Rain and waste reuse for toilet ¯ushing: a simulation model. In Proceedings of the international conference on RWCS (pp. 77±389). Honolulu, USA, June. Kok, Y. S., Fong, R. H. L., Murabayashi, E. T., & Lo, A. (1982). Deterministic and probabilistic processes of weekly rainfall. In Proceedings of the international conference on RWCS (pp. 83±91). Honolulu, USA, June. Hoey, P. J., & West, S. F. (1982). Recent initiation in rainwater supply systems for South Australia. In Proceedings of the international conference on RWCS (pp. 284±293). Honolulu, USA. Jenkins, D., Pearson, F., Moore, E., Kim, S. J., & Valentine, R. (1978). Feasibility of rainwater collection systems in California. Contribution No. 173, California Water Resources Center, University of California, Davis, USA. Leung, P., & Fok, Y. S. (1982). Determining desirable storage volume of rain catchment cistern system: a stochastic assessment. In Proceedings of the international conference on RWCS (pp. 128±134). Honolulu, USA. Morris, G. L., Acevido-Pimentel, R., & Ayala, G. (1984). Yield and cost of water supplies from rained cistern: Puerto Rico. In
A. Appan / Urban Water 1 (1999) 317±321 Proceedings of the 2nd international conference on RWCS (pp. FI1±13). Virgin Islands, USA, June. Ree, W. O., Wimberly, F. L., Guinn, W. R., & Lauritzen, C. W. (1971). Rainwater harvesting system. ARS41-184, Agricultural Research Service, US Department of Agriculture, p. 12. Rippl, W. (1883). The capacity of storage reservoirs for water supply. In Proceedings of the institution of civil engineers, Vol. 71 (pp. 270± 278). London.
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