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Building and Environment 42 (2007) 1731–1742 www.elsevier.com/locate/buildenv

Potential for potable water savings by combining the use of rainwater and greywater in houses in southern Brazil Enedir Ghisi, Sulayre Mengotti de Oliveira Laboratory of Energy Efficiency in Buildings, Department of Civil Engineering, Federal University of Santa Catarina, Floriano´polis-SC, 88040-900, Brazil Received 31 October 2005; received in revised form 14 November 2005; accepted 6 February 2006

Abstract Research on rainwater and greywater have been performed all over the world as a way of promoting potable water savings. The main objective of this article is to evaluate the potential for potable water savings by using rainwater and greywater in two houses in southern Brazil. An economic analysis is performed to evaluate the benefits of using rainwater and greywater either separately or together. Results indicate that the potential for potable water savings in both houses range from 33.8% (house B) to 36.6% (house A), considering that water for toilet flushing and washing machine does not need to be potable. By using rainwater, the potable water savings in house A would be 35.5% and in house B, 33.6%. When greywater is considered alone, potable water savings are lower, i.e., 30.4% in house A and 25.6% in house B. As for the use of rainwater and greywater combined, the potable water savings are 36.4% in house A and 33.8% in house B. The three systems that were investigated seem not to be cost effective as the payback periods were very high (above 17 years), but the greywater system was the most attractive one. The main conclusion that can be made from the research is that there needs to be government incentives in order to promote the use of rainwater or greywater in houses in southern Brazil. r 2006 Elsevier Ltd. All rights reserved. Keywords: Potable water savings; Water end-uses; Rainwater usage; Reuse of greywater

1. Introduction Rainwater has been acknowledged to promote potable water savings in different types of buildings and in different countries. This has been reported in Refs. [1–7], amongst others. It has also been reported by different researchers that the reuse of greywater also promotes potable water savings in buildings [8–11]. However, there have been just a few reports on the combination of rainwater and greywater to promote potable water savings [12,13]. As for Brazil, it has already been reported that the use of rainwater in the residential sector can promote potable water savings ranging from 48% in the southeast region to 100% in the north region [14]. When focusing on the use of rainwater in the state of Santa Catarina, southern Brazil, potential for potable water savings ranging from 34% to 92% were obtained [2]. When analysing the southeast region separately, the potential for potable water savings Corresponding author. Tel.: +48 33315185; fax: +48 33315191.

E-mail address: [email protected] (E. Ghisi). 0360-1323/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2006.02.001

ranged from 12% to 79% [15]. However, there have been no reports at all on investigations about using rainwater and greywater either separately or together in houses in southern Brazil. 2. Objective The main objective of this article is to estimate the potential for potable water savings by using rainwater and reusing greywater in two houses in the city of Palhoc- a, southern Brazil. An economic analysis is performed in order to evaluate the benefits of using rainwater or greywater separately and also by applying both strategies together. 3. Location and houses The city of Palhoc- a is located 15 km from Floriano´polis, which is the capital city of the state of Santa Catarina. It is located at the latitude 281380 south and longitude 481400 west. Fig. 1 shows a map of Brazil and Santa Catarina

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Fig. 1. Map of Brazil and state of Santa Catarina with location of the city of Palhoc- a.

Table 1 Number of residents and plumbing fixtures in both houses House

Number of residents

Plumbing fixtures

Male

Female

A

2

1

One One One One One One

B

1

1

Three toilets Two showers Three lavatories One bathtub One kitchen sink One washing machine One laundry trough

toilet shower lavatory kitchen sink washing machine laundry trough

indicating the location of Palhoc- a, whose land area is 323 km2. The total population in 2004 was 113,312 inhabitants [16]. The study was performed by analysing two single-storey houses. Number of residents and plumbing fixtures of both houses are shown in Table 1. 4. Methodology The methodology applied to accomplish the objective mentioned above is as follows. 4.1. Actual water consumption The actual water consumption of both houses was obtained by checking the water bills, which were supplied by the houses’ owners. The period September 2003 to April 2005 was taken into account. 4.2. Estimated water consumption The specific water consumption of each plumbing fixture was estimated by obtaining the frequency of use and the

water flow rate of each plumbing fixture. This was performed over a 28-day period, from 1st to 28th February 2005. 4.2.1. Frequency of use The frequency of use of each plumbing fixture was obtained by asking the residents to fill in a form every time they used any plumbing fixture. They were requested to inform the type of plumbing fixture that was used as well as the amount of time the water was used. To avoid mistakes, forms containing each resident’s name were left in the bathroom. As for the washing machine and kitchen sink, the frequency and time of use of water were given by the female resident of each house. 4.2.2. Water flow rate The water flow rate of taps and showers was estimated by measuring the time for a small container with a known capacity to fill with water. This was done by opening taps and showers according to each user’s needs. Three measurements were taken for each plumbing fixture and then an average was performed. The volume of water used in the washing machines was determined by measuring the amount of water used after the operation. As for the toilets, which are all with wall flushing valves, the water flow rate was assumed to be 1.7 litres per second as recommended by the Brazilian Standard NBR 5626 [17]. 4.2.3. Water consumption From the frequency, time of use and water flow rate obtained for each plumbing fixture, the water consumption of each plumbing fixture could be estimated by using Eq. (1). C d ¼ F  T  Q,

(1)

where Cd is the daily water consumption of each plumbing fixture (litres/day), F is the daily frequency of use of each plumbing fixture (times/day), T is the average time of use of each plumbing fixture (seconds/time of use), and Q is the water flow rate of each plumbing fixture (litres/ second).

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As for the water consumption of washing machines, it was estimated by using Eq. (2). C dwm ¼ F  Q,

(2)

where Cdwm is the daily water consumption of washing machines (litres/day), F is the daily frequency of use of washing machines (times/day), and Q is the volume of water consumed over each operation of washing machines (litres/ cycle) — 100 litres for house A and 80 litres for house B. 4.2.4. Water consumption measured daily In order to accomplish accurate results, daily water measurements were performed at both houses over the period 1–28 February 2005. This was performed by checking the water meter of each house.

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Table 2 Variation on frequency, amount of time of use and water flow rate for the sensitivity analysis of both houses Plumbing fixtures

Parameter

Range

Interval

Toilet

Water flow rate Frequency Time

70.5 l/s 73 times 74 s

0.1 l/s 1 time 1s

Shower

Frequency Time

71 time 72 min

0.5 time 1 min

Lavatory

Frequency Time

75 times 75 s

1 time 1s

Shaving

Frequency Time

71 time 72 min

0.2 time 0.5 min

4.3. Water end-uses By using the frequency and amount of time each plumbing fixture is used as well as the respective water flow rate, the specific water consumption of each plumbing fixture could be calculated as explained above. In order to check the accuracy of the specific water consumptions they were summed together and compared to the actual water consumption measured daily. The water end-uses were then estimated by dividing the specific water consumptions by the actual water consumption of each house. 4.4. Sensitivity analysis In order to account for discrepancies produced by inaccurate answers on frequency and amount of time the plumbing fixtures were used, a sensitivity analysis was performed. This was done on frequency and amount of time of use of showers, toilets, and lavatories (for tooth brushing, hands washing, and shaving). The water flow rate of toilets was also considered as it was not measured, and this might be a source of error. Table 2 shows the details adopted for such an analysis. The difference between the estimated and the measured water consumption was then assumed to be due to the most sensitive fixture. Corrected water end-uses were then calculated. 4.5. Dimensioning the rainwater system Based on the water end-uses determined as shown previously, the rainwater system could be dimensioned. It was assumed that rainwater could be used for toilet flushing and laundry. Therefore, the potential for potable water savings by using rainwater in both houses could be determined. 4.5.1. Rainfall data The analysis was performed considering daily rainfall data for a 34-year period, i.e., 1st January 1969–31st December 2002. Missing data over this period were assumed to be zero.

4.5.2. Roof area The roof area for each house was determined by analysing their projected area and performing on-site measurements. 4.5.3. Gutters and pipes Gutters and pipes were dimensioned according to the Brazilian standard NBR 10844 [18]. The use of filters preceding the rainwater tanks was also taken into account. 4.5.4. Rainwater tank size Rainwater tank capacity was estimated by using a computer programme called Neptune [19]. Input data are roof area, number of residents, average daily water demand, water end-uses in which potable water could be replaced by rainwater, runoff coefficient, and daily rainfall. Rainfall lower than 1 mm per day was not taken into account. By using Neptune, it is possible to estimate the potential potable water savings using different rainwater tank capacities. First, the daily rainwater demand in each house could be estimated by using Eq. (3). RD ¼ P  WD  n,

(3)

where RD is the rainwater demand in the house (litres), P is the percentage of the potable water demand that could be replaced by rainwater as obtained in the water end-uses (non-dimensional), WD is the potable water demand (litres/capita per day), and n is the number of people living in the house. Then, the daily volume of rainwater that could be collected in each house was determined by using Eq. (4). DVR ¼ DR  TRA  Rc ,

(4)

where DVR is the daily volume of rainwater that could be harvested in each house (litres/day per house), DR is the daily rainfall (mm/day ¼ litre/m2 per day), TRA is the total roof area of each house (m2), and Rc is the runoff coefficient (non-dimensional).

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To estimate the volume of rainwater available in the rainwater tank, the daily rainwater demand in each house was subtracted from the daily volume of rainwater that could be harvested in each house as shown in Eq. (5). VRT ¼ VRT pd þ DVR  RD,

(5)

where VRT is the daily volume of rainwater in the tank (litres/day), VRTpd is the volume of rainwater in the tank from the previous day (litres), DVR is the daily volume of rainwater that could be harvested in each house (litres/day per house), and RD is the rainwater demand in the house (litres/day per house). Note: 0 r VRT r Rainwater tank capacity The potential for potable water savings for each rainwater tank capacity was estimated by using Eq. (6). Pd i¼1 RC PWS ¼ 100 , (6) WDD  d where PWS is the potential for potable water savings over the period being analysed (%), RC is the rainwater that is actually consumed in the house, d is the number of days in the period being analysed, and WDD is the daily potable water demand per house (litres/day per house) — it represents the product between WD and n as shown in Eq. (3). Such a procedure was performed for incremental rainwater tank capacities ranging from 0 to 15,000 litres at increments of 1000 litres. In each house, there will be two rainwater tanks, one at ground level, which is dimensioned according to the procedure shown previously, and another one placed on the roof, which is dimensioned according to the daily rainwater demand. The ideal capacity of the ground-level tank, was assumed as the one in which the potential for potable water savings increased less than 1.0% by increasing the rainwater tank capacity in 1000 litres. 4.6. Dimensioning the greywater system The dimensioning of the greywater system was also based on the water end-uses. The greywater tank capacity is related to the volume of greywater produced in the houses. Thus, the volume of greywater produced daily in the houses was determined according to the water end-uses for showers, lavatories, and washing machines. The greywater will be used only for toilet flushing. In each house there will be two greywater tanks, one at ground level to store the greywater produced daily and another one placed on the roof to supply water to the toilet. 4.6.1. Treatment system Prior to being stored, greywater will be filtered and will go through a wetland system. The wetland was dimensioned by following recommendations shown in [20], i.e., 0.8 m2 per person. The depth adopted was 0.6 m.

4.7. The combination of rainwater and greywater When rainwater and greywater were analysed together, greywater was considered to supply the toilets, and rainwater was considered to supplement the demand of greywater for toilets and also to supply the washing machine. The dimensioning is the same as presented above. 4.8. Economic analysis In order to evaluate the economic benefits of using rainwater, greywater, or both at the same time, costs and benefits related to each system were estimated as indicated below. 4.8.1. Potable water costs The potable water costs were based on the water consumption measured in February 2005. Tariffs for the residential sector practiced by the local water utility on that date are shown in Table 3. The annual costs for potable water were estimated by using Eq. (7). C ¼ C m  T  12,

(7)

where C is the estimated potable water costs prior to any strategy for water savings (R$/year), Cm is the monthly water consumption (m3/month), and T is the tariff practiced by the local water utility (R$/m3). 4.8.2. Rainwater and greywater systems costs Costs of all material and equipment needed for installing rainwater and greywater systems either separately or together were obtained from local stores. 4.8.3. Electricity costs Electricity costs for pumping either rainwater or greywater to the upper tank were estimated by using Eq. (8). C el  C p  t  T  12,

(8)

where Cel is the annual electricity costs for water pumping (R$), Cp is the pump electricity demand (kW), t is the amount of time the pump works over the month (hour/ month), and T is the tariff practiced by the local electricity utility (R$/kWh). Tariffs for the residential sector practiced by the local electricity utility are shown in Table 4. Table 3 Tariff practiced by the local water utility in February 2005 Water consumption range (m3)

Tariff (R$/m3)

0–10 11–25 Above 26

1.7050 2.9750 4.0640

Note: R$ stands for Brazilian Real (On 22 September 2005 R$ 1 ¼ US $0.4396 ¼ £0.2456).

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250

Table 4 Tariff practiced by the local electricity utility in February 2005

Residential low income

Residential

Electricity consumption range (kWh)

Tariff (R$/kWh)

Up to 30 31–100 101–150 151–160 161–220

0.11805 0.20228 0.30348 0.35608 0.39567

Up to 150 Above 150

0.33722 0.39567

Rainfall (mm)

200

Classification

150 100 50 0 Jan

Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Month

Fig. 2. Monthly average rainfall for Palhoc- a over the period 1969–2002.

4.8.5. Economic analysis methods The methods used to evaluate the economic viability of using rainwater and greywater either separately or together were the simple payback and the present net value. Details on both methods can be found in [21,22]. 5. Results 5.1. Rainfall data Daily rainfall data were used to perform the simulations. Fig. 2 shows the monthly average rainfall obtained for Palhoc- a over the period 1969–2002. It can be observed that rainfall is abundant all over the year, but it is higher over the summer season. The average rainfall obtained for the whole period is 1706 mm/year.

10 5 0 Jul/04 Aug/04 Sep/04 Oct/04 Nov/04 Dec/04 Jan/05 Feb/05 Mar/05 Apr/05

where B is the monetary savings by using either rainwater or greywater (R$/year), C is the estimated potable water costs prior to any strategy for water savings as given by Eq. (7) (R$/year), and P is the percentage of potable water that could be replaced by rainwater or greywater (nondimensional).

House B

15

Oct/03 Nov/03 Dec/03 Jan/04 Feb/04 Mar/04 Apr/04 May/04 Jun/04

(9)

House A

20

Sep/03

B ¼ C  P,

25 Potable water consumption (m3)

4.8.4. Potable water savings Economic benefits due to the potable water savings by using rainwater or greywater were calculated by using Eq. (9).

Month/year

Fig. 3. Monthly potable water consumption in houses A and B.

number of people living in house A ranged from four to five; and from December 2004 there were three people. 5.3. Estimated water consumption From the forms filled in by the residents, frequency of use and amount of time each plumbing fixture was used were obtained. By using these data and water flow rate, the water consumption over the period 1–28 February 2005 could be estimated. 5.3.1. Frequency of use Table 5 shows the frequency and the average amount of time each plumbing fixture is used by each resident of houses A and B. Results for the washing machine and kitchen sink are shown in Table 6.

5.2. Actual water consumption Fig. 3 shows the monthly water consumption for both houses over the period September 2003 – April 2005. The monthly average water consumption in houses A and B are 14.70 m3 and 9.00 m3, respectively. In house B, if one considers only the water consumption after a water meter was installed (May 2004), the average water consumption is 8.33 m3/month. Prior to May 2004, the recorded consumption is 10 m3/month, which is the minimum water tariff charged by the water utility. Prior to November 2004 the

5.3.2. Water flow rate Water flow rates were determined as described in the methodology. Results are shown in Table 7. 5.4. Estimated versus measured water consumption Figs. 4 and 5 show the estimated and the measured water consumption in houses A and B, respectively, over the period 1–28 February. Estimated water consumption was obtained by using the data registered by the residents, and

ARTICLE IN PRESS Time (s)

545.22 8.04 6.04 260.87 1.09 3.22 3.26 0.87

Male

Frequency (times/day)

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measured water consumption was obtained by checking the water meter daily. It can be observed that the error between estimated and measured water consumption ranged from 9.4% to 1.0% in house A, and from 5.9% to 8.2% in house B. The average error was 3.8% in house A and 2.0% in house B, which can be considered a good estimate. In house B, the residents were away over five days in February 2005.

409.57 7.74 5.09 — 1.17 6.26 5.00 — 1.58 4.13 3.21 0.25 1.36 6.32 3.86 0.29 462.86 7.71 6.93 — 1.50 8.75 6.32 —

475.71 7.46 6.93 210.00

687.50 9.29 7.21 230.00

Time (s) Frequency (times/day) Frequency (times/day) Frequency (times/day) Time (s) Frequency (times/day)

Time (s) Male 1 Female

House A

By using the estimated water consumption derived from the residents’ answers, water end-uses were calculated. Results are shown in Table 8. It can be observed that in both houses, water used for shower represents the highest percentage of all, being 34.0% in house A and 45.4% in house B. Toilet flushing responds for the second largest use in house B (25.8%) and the third in house A (27.7%). In house A, water used in the kitchen sink represents the second largest use as most residents have their meals at home. 5.6. Sensitivity analysis By performing the sensitivity analysis, it was observed that toilet and shower were the most sensitive plumbing fixtures (Figs. 6 and 7). As the only variable not measured was the water flow rate for the toilet, the difference between measured and estimated potable water consumption in both houses was assumed to be due to the toilet flushing. Therefore, the water end-uses were recalculated assuming that the error was due to the toilet. 5.7. Corrected water end-uses Based on the sensitivity analysis, it was observed that the toilet was the plumbing fixture most sensitive to water consumption. Therefore, the water end-uses were corrected by changing the toilet water consumption in order to obtain a total water consumption of 16,122.0 litres in house A and 6804.0 litres in house B, which were the actual water consumptions registered by the water meter over the 28 days. Table 9 shows the corrected water end-uses for houses A and B. From the corrected water end-uses, it was possible to determine the volume of greywater produced daily, the volume of greywater needed for toilet flushing, and the volume of rainwater needed for toilet flushing and washing machine. These allowed for the dimensioning of the three systems, i.e., rainwater, greywater, and rainwater and greywater together.

Shower Lavatory Toilet Shaving

5.8. Dimensioning the rainwater system Plumbing fixture

Table 5 Frequency and amount of time for each plumbing fixture and resident of houses A and B

Male 2

Time (s)

Female

House B

5.5. Water end-uses

5.8.1. Rainwater demand The volume of rainwater demand in both houses is shown in Table 10 when only rainwater is considered to promote potable water savings. In a rainwater system to

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Table 6 Frequency and amount of time for washing machine and kitchen sink of houses A and B Plumbing fixture

House A

Washing machine Kitchen sink

House B

Frequency (times/day)

Time (s)

Frequency (times/day)

Time (s)

0.36 2.68

— 220.71

0.30 2.09

— 226.96

Table 7 Water flow rates of plumbing fixtures of houses A and B Plumbing fixture

House A (l/s)

House B (l/s)

Female Shower Lavatory Toilet Shaving Washing machine Kitchen sink

Male 1

0.09 0.07 1.70 —

0.08 0.07 1.70 0.05 100 litres/cycle 0.25

10

1000 Measured

Error

750

5

500

0

250

-5

0

Error (%)

Daily potable water consumption (litres)

Estimated

-10 1

4

7

10

13 16 Day

19

22

25

28

Fig. 4. Daily water consumption in house A over the period 1–28 February 2005.

10

500 Measured

Error

375

5

250

0

125

-5

0

Error (%)

Daily potable water consumption (litres)

Estimated

-10 1

4

7

10

13 16 Day

19

22

25

28

Fig. 5. Daily water consumption in house B over the period 1–28 February 2005.

supply both toilet and washing machine, there should be 210.5 litres of rainwater per day in house A and 95.1 litres/ day in house B.

Male 2

Female

Male

0.08 0.08 1.70 0.04

0.12 0.10 1.70 —

0.13 0.11 1.70 0.05 80 litres/cycle 0.08

5.8.2. Rainwater tank size The potential for potable water savings was estimated for different rainwater tank capacities. Input data used to perform the computer simulations are shown in Table 11. Fig. 8 shows the results for houses A and B when only rainwater is considered to promote potable water savings for toilet flushing and washing machine. In order to supply all the rainwater demand in house A, a rainwater tank capacity of 13,000 litres would be needed; in house B, the capacity needed would be 5000 litres. However, in house A, the chosen capacity was 5000 litres, which gives a potential for potable water savings of 35.5%, and in house B, 3000 litres, with 33.6% potable water savings. As for the upper rainwater tank, the chosen capacity was 250 litres as it is the smallest tank that can be found in the local stores. Such a capacity provides for a volume larger than the daily demand for rainwater in houses A (210.5 litres) and B (95.1 litres). 5.9. Dimensioning the greywater system 5.9.1. Greywater demand Table 12 shows the volume of greywater available and needed in both houses when only greywater is considered to promote potable water savings. The volume of greywater available was estimated by considering the water end-uses for washing machine, shower, and lavatory (tooth brushing, hands and face washing, and shaving). That represents 239.8 l/day in house A, which means 41.7% of the water used in the house. As the volume of greywater needed for toilet flushing is 174.8 l/day, there is greywater enough for that use. The same was observed in house B,

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Table 8 Water end-uses for houses A and B based on estimated water consumption Plumbing fixture

Shower Lavatory Toilet Shaving Washing machine Kitchen sink Total

House A (litres)

House A (%)

Female

Male 1

Male 2

Total

1662.6 132.1 2092.7 — 333.3 1505.0

1528.9 92.8 1268.2 75.6 333.3 1505.0

2092.8 73.8 941.8 55.2 333.3 1505.0

5284.3 298.7 4302.7 130.8 1000.0 4515.0 15,531.5

Variation on potable water consumption (%)

House B (%)

Female

Male

Total

1339.2 112.1 997.9 — 280.0 460.8

1762.8 66.2 766.7 300.0 280.0 460.8

3102.0 178.3 1764.6 300.0 560.0 921.6 6826.5

45.4 2.6 25.8 4.4 8.2 13.5 100.0

5.10. Rainwater and greywater combined

10.0 House A 5.0 House B 0.0 -5.0 -10.0 1.3

1.5 1.7 1.9 Variation on toilet water flow rate (l/s)

2.1

Fig. 6. Sensitivity analysis on the toilet water flow rate.

Variation on potable water consumption (%)

34.0 1.9 27.7 0.8 6.4 29.1 100.0

House B (litres)

10.0 House A 5.0 House B 0.0 -5.0 -10.0 -2

-1 0 1 Variation on duration of shower (min)

2

Fig. 7. Sensitivity analysis on the duration of shower.

where there is 170.1 l/day of available greywater and the demand for toilet flushing is 62.2 l/day. 5.9.2. Greywater tank size The daily production of greywater in houses A and B is 239.8 and 170.1 litres, respectively. Therefore, a lower greywater tank of 250 litres would suffice. As for the daily greywater demand for toilet flushing, it is 174.8 litres in house A and 62.2 litres in house B. Such a demand is lower than the greywater production in both houses. Therefore, upper greywater tanks of 250 litres were adopted in both houses. 5.9.3. Treatment system The greywater treatment system adopted was a wetland. Table 13 shows the details on the wetlands for both houses.

5.10.1. Rainwater and greywater demand When rainwater and greywater are considered together as a way of promoting potable water savings, toilet and washing machine are the plumbing fixtures to be supplied. As seen previously, there is enough greywater to supply the toilet. Therefore, rainwater is considered to supply the washing machine only. Table 14 shows the volume of rainwater and greywater needed in both houses. 5.10.2. Rainwater tank capacity Input data for the computer programme Neptune are shown in Table 15. Fig. 9 shows the potential for potable savings for houses A and B when rainwater and greywater are considered together. The chosen rainwater tank capacity for both houses was 500 litres. This provides for potable water savings of 6.0% in house A and 8.2% in house B. As for the upper rainwater tank, 250-litres tanks will be adopted in both houses. Such a capacity is the smallest available in the local stores, but it is enough to store the daily rainwater demand in both houses, which is 35.7 litres in house A and 32.9 litres in house B. 5.10.3. Greywater tank capacity The production of greywater and its demand to supply toilets is the same as shown previously. Therefore, both lower and upper greywater tanks will have a 250-litres capacity each. 5.10.4. Greywater treatment system Dimensions of the wetland system will be the same as shown previously for both houses. 5.11. Economic analysis 5.11.1. Potable water costs Potable water costs were estimated for both houses considering the water tariff charged by the water utility in February 2005 as shown previously in Table 3. Table 16 shows the results obtained for both houses. The

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Table 9 Corrected water end-uses for houses A and B

Shower Lavatory Toilet Shaving Washing machine Kitchen sink Total

House A (litres)

House A (%)

Female

Male 1

Male 2

Total

1662.6 132.1 2289.6 — 333.3 1505.0

1528.9 92.8 1465.1 75.6 333.3 1505.0

2092.8 73.8 1138.6 55.2 333.3 1505.0

5284.3 298.7 4893.2 130.8 1000.0 4515.0 16,122.0

Table 10 Rainwater demand in houses A and B Plumbing fixture

Toilet Washing machine Total

House A

House B

litres/day

%

litres/day

%

174.8 35.7 210.5

30.4 6.2 36.6

62.2 32.9 95.1

25.6 8.2 33.8

32.8 1.9 30.4 0.8 6.2 28.0 100.0

Potential for potable water savings (%)

Plumbing fixture

House B (litres)

House B (%)

Female

Male

Total

1339.2 112.1 986.7 — 280.0 460.8

1762.8 66.2 755.4 300.0 280.0 460.8

3102.0 178.3 1742.1 300.0 560.0 921.6 6804.0

40 35 30 25 20 15 10 5 0

45.6 2.6 25.6 4.4 8.2 13.5 100.0

House A House B 0

2500

5000 7500 10000 Rainwater tank capacity (litres)

12500

15000

Fig. 8. Potential for potable water savings in houses A and B. Table 11 Input data for simulations using the computer programme Neptune when only rainwater is used Input data

House A

House B

Potable water consumption (l/capita per day) Number of residents Roof area (m2) Runoff coefficient Rainwater demand (% of potable water demand)

202.2 3 203.8 0.8 36.6

147.9 2 212.4 0.8 33.8

5.11.3. Electricity costs As there will be pumps to conduct water (rainwater and greywater) to the upper tanks, the electricity costs involved were estimated. Table 18 shows the results, considering the tariff for the residential sector as shown in Table 4.

potable water consumption considered in the calculations was 16,122 litres for house A and 6804 litres for house B.

5.11.4. Potable water savings Table 19 shows the potable water savings that can be obtained in both houses for each one of the three systems. It can be observed that the lowest savings are obtained when greywater is considered alone and the highest when rainwater and greywater are considered together.

5.11.2. Costs of the systems Considering all materials and labour involved, costs were estimated for each one of the three systems (rainwater only, greywater only, and rainwater and greywater together). Table 17 shows the results. Costs in house B were higher as the house is larger than house A. When rainwater and greywater are considered together, four tanks are considered in each house (a lower and an upper tank for both rainwater and greywater), besides the potable water tank, which is not considered in the calculations.

5.11.5. Payback period Finally, by considering the total costs and benefits of each system, the payback period was estimated. Table 20 shows the results. It can be noticed that the payback periods are very high, but the more attractive investment is the system that considered greywater alone, followed by the one that considered rainwater alone. For house B, the calculations were performed the same way as for house A. However, there will be no benefits for the potable water savings as the potable water consumption is lower than 10 m3/month. The local water utility (and probably most

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Table 12 Volume of greywater available and needed in houses A and B Plumbing fixture

Shower Lavatory Toilet Shaving Washing machine Total

House A

House B

Greywater available

Greywater needed

litres/day

%

litres/day

188.7 10.7

32.8 1.9

4.7 35.7 239.8

0.8 6.2 41.7

174.8

174.8

Table 13 Dimensions and other details on the wetlands Parameter

House A

House B

Specific area (m2/ person) Area (m2) Depth (m) Length (m) Width (m) Plant species

0.8

0.8

2.4 0.6 2.4 1.0 Zizaniopsis bonariensis 0.5

1.6 0.6 1.8 0.9 Zizaniopsis bonariensis 0.5

Base tilt (%)

water utilities in Brazil) charges a minimum tariff equal to 10 m3 of water per month. By considering the present net value method, corrected paybacks were calculated for interest rates of 1%, 5%, and 10% a year. Table 21 shows the results. As expected, payback periods are higher, but more realistic. 6. Conclusions The potential for potable water savings by using rainwater and reusing greywater either separately or together in two houses in Palhoc- a, southern Brazil has been assessed. Such an assessment was performed by estimating the potable water end-uses and making an economic analysis. When rainwater alone is considered, its use is for the washing machine and toilet flushing; when greywater alone is considered, its use is for toilet flushing only; and when both strategies are considered together, rainwater is considered for the washing machine and greywater for toilet flushing. Results for the water end-uses show that the potential for potable water savings by using rainwater in house A is 36.6% and by reusing greywater is 30.4%. As for house B, the potential for potable water savings by using rainwater

%

Greywater available

Greywater needed

litres/day

%

litres/day

%

110.8 6.4

44.6 2.6 62.2

25.6

20.0 32.9 170.1

4.4 8.2 60.8

62.2

25.6

30.4

30.4

is 33.8% and by reusing greywater is 25.6%. Such a potential is not higher because the water for showers, lavatories, and kitchen sinks was assumed to be potable and supplied by the water utility. For the system that takes into account rainwater alone, the lower rainwater tank capacity for house A was 5000 litres, which gives a potential for potable water savings of 35.5%. For house B, a tank of 3000 litres gives a potential for potable water savings of 33.6%. As for the upper rainwater tank, a volume of 250 litres was considered in each house, as it is the minimum volume available in the local market and can store the daily rainwater demand. When only greywater is taken into account, two tanks of 250 litres are considered in each house. These are enough to store the daily greywater production and to promote a potential for potable water savings of 30.4% in house A and 25.6% in house B. As for the system that takes into account rainwater and greywater together, in both houses, two lower tanks are considered, i.e., 500 litres for rainwater and 250 litres for greywater; and two 250-litres upper tanks are used. The potential for potable water savings in house A was 36.4% and in house B, 33.8%. By performing the economic analysis, high payback periods were obtained for each of the three systems, but the most attractive one was observed for the greywater system. This is attributable to the low water tariffs charged in Brazil. Another important issue relates to the minimum tariff charged by the water utility, which equates to a charge for 10 m3 of water per month even if the water consumption is lower than that figure. In house B, for example, there would be no payback period as the water consumption in the house is lower than 10 m3/month, which means that any potable water savings obtained by the residents will not reflect on the water bill. The benefits would be for the water utility only. Therefore, in order to promote potable water savings in houses in Brazil, legislation should be reviewed.

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Table 14 Volume of rainwater and greywater needed in houses A and B Plumbing fixture

House A

House B

Greywater needed

Toilet Washing machine Total

Rainwater needed

litres/day

%

174.8

30.4

174.8

30.4

Greywater needed

litres/day

%

35.7 35.7

6.2 6.2

Rainwater needed

litres/day

%

62.2

25.6

62.2

25.6

litres/day

%

32.9 32.9

8.2 8.2

Table 15 Input data for simulations using the computer programme Neptune when both rainwater and greywater are used

Table 18 Electricity costs involved in the pumping of water in houses (a) A and (b) B

Input data

House A

House B

Parameter

Rainwater

Greywater

Rainwater and greywatera

Potable water consumption (l/capita per day) Number of residents Roof area (m2) Runoff coefficient Rainwater demand (% of potable water demand)

202.2 3 203.8 0.8 6.2

147.9 2 212.4 0.8 8.2

(a) Pump electricity demand (HP) Pump water flow rate (l/ hour) Pump electricity consumption (kWh) Water demand (l/day) Pump working time (min) Electricity costs (R$/year)

1/4

1/4

1/4

1/4

2700

2700

2700

2700

0.5

0.5

0.5

0.5

210.5 4.68 4.73

174.8 3.88 3.93

35.7b 0.79 0.80

174.8c 3.88 3.93

(b) Pump electricity demand (HP) Pump water flow rate (l/ hour) Pump electricity consumption (kWh) Water demand (l/day) Pump working time (min) Electricity costs (R$/year)

1/4

1/4

1/4

1/4

2700

2700

2700

2700

0.5

0.5

0.5

0.5

95.1 2.11 2.13

62.2 1.38 1.40

32.9b 0.73 0.74

62.2c 1.38 1.40

Potential for potable water savings (%)

10 8 6 4 House A 2

House B

0 0

1000

2000 3000 4000 Rainwater tank capacity (litres)

5000

Fig. 9. Potential for potable water savings in houses A and B.

Table 19 Potable water savings in houses A and B

Table 16 Potable water costs for houses A and B Period

Monthly Yearly

a Two pumps are considered (one for rainwater and another for greywater). b Rainwater demand when rainwater and greywater are combined. c Greywater demand when rainwater and greywater are combined.

System

Potable water savings

Potable water costs (R$)

House A

House A

House B

35.26 423.16

11.60 139.21

Rainwater Greywater Rainwater and greywater

System

Rainwater system Greywater system Rainwater and greywater systems combined

R$/year

%

R$/year

%

117.10 100.05 119.84

35.5 30.4 36.4

46.75 35.60 47.01

33.6 25.6 33.8

Table 20 Simple payback period for houses A and B

Table 17 Costs of rainwater and greywater systems for houses A and B System

House B

Payback

Costs (R$) House A

House B

2530.96 1716.45 3377.64

3154.50 2185.68 4371.47

Rainwater Greywater Rainwater and greywater

House A

House B

21 years and 5 months 17 years and 8 months 28 years and 2 months

67 years and 4 months 61 years and 3 months 92 years and 8 months

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Table 21 Corrected payback period for houses A and B System

Rainwater Greywater Rainwater and greywater

House A (years)

House B (years)

1% a year

5% a year

10% a year

1% a year

5% a year

10% a year

25 19 37

4 250 40 4 250

4 250 4 250 4 250

116 99 4 250

4 250 4 250 4 250

4 250 4 250 4 250

Acknowledgements Dr. E. Ghisi would like to thank CAPES — Fundac- a˜o Coordenac- a˜o de Aperfeic- oamento de Pessoal de Nı´vel Superior, an agency of the Brazilian Government for post-graduate education, for the scholarship (ProDoc) that allowed him to supervise this research.

References [1] Villarreal EL, Dixon A. Analysis of a rainwater collection system for domestic water supply in Ringdansen, Norrko¨ping, Sweden. Building and Environment 2005;40(9):1174–84. [2] Ghisi E, Montibeller A, Schmidt RW. Potential for potable water savings by using rainwater: an analysis over 62 cities in southern Brazil. Building and Environment 2006;41(2):204–10. [3] Herrmann T, Schmida U. Rainwater utilisation in Germany: efficiency, dimensioning, hydraulic and environmental aspects. Urban Water 1999;1(4):307–16. [4] Fewkes A. The use of rainwater for WC flushing: the field testing of a collection system. Building and Environment 1999;34(6):765–72. [5] Marinoski DL, Ghisi E, Go´mez LA. Aproveitamento de a´gua pluvial e dimensionamento de reservato´rio para fins na˜o pota´veis: estudo de caso em um conjunto residencial localizado em Floriano´polis-SC [Rainwater harvesting and rainwater tank sizing: a case study in a multi-storey residential building located in Floriano´polis, Southern Brazil]. CLACS04-I Confereˆncia Latino-Americana de Construc- a˜o Sustenta´vel e ENTAC04-101 Encontro Nacional de Tecnologia do Ambiente Construı´ do, Sa˜o Paulo-SP, CD Rom; 2004 [in Portuguese]. [6] Simioni WI, Ghisi E, Go´mez LA. Potencial de economia de a´gua tratada atrave´s do aproveitamento de a´guas pluviais em postos de combustı´ veis: estudos de caso [Potential for potable water savings by using rainwater in petrol stations: case studies in Southern Brazil]. CLACS04-I Confereˆncia Latino-Americana de Construc- a˜o Sustenta´vel e ENTAC04-101 Encontro Nacional de Tecnologia do Ambiente Construı´ do, Sa˜o Paulo-SP, CD Rom; 2004 [in Portuguese]. [7] Netto JMA. Aproveitamento de a´guas de chuva para abastecimento [Rainwater harvesting]. BIO 1991;3(2):44–8 [in Portuguese]. [8] March JG, Gual M, Orozco F. Experiences on greywater re-use for toilet flushing in a hotel (Mallorca Island, Spain). Desalination 2004;164(3):241–7. [9] Al-Jayyousi OR. Greywater reuse: towards sustainable water management. Desalination 2003;156(1–3):181–92.

[10] Nolde E. Greywater reuse for toilet flushing in multi-storey buildings — over ten years experience in Berlin. Urban Water 1999;1(4): 275–84. [11] Christova-Boal D, Eden RE, McFarlane S. As investigation into greywater reuse for urban residential properties. Desalination 1996; 106(1–3):391–7. [12] Dixon A. Computer simulation of domestic water re-use systems: greywater and rainwater in Combination. PhD thesis, Imperial College, University of London; 2000. [13] Dixon A, Butler D, Fewkes A. Water saving potential of domestic water recycling systems using greywater and rainwater in combination. Water Science and Technology 1999;39(5):25–32. [14] Ghisi E. Potential for potable water savings by using rainwater in the residential sector of Brazil. Building and Environment, Corrected Proof, Available online 12 July 2005; in press. [15] 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, 16 August 2005; accepted for publication, doi:10.1016/j.buildenv. 2006.02.007. [16] IBGE Instituto Brasileiro de Geografia e Estatı´ stica [Brazilian Institute for Geography and Statistics]. Obtained from: ohttp:// www.ibge.gov.br/4. Accessed in April 2005. [17] ABNT Associac- a˜o Brasileira de Normas Te´cnicas. NBR 5626 — Instalac- a˜o predial de a´gua fria. Rio de Janeiro; 1998 [Brazilian Association for Standards, NBR 5626 — cold water building installation] [in Portuguese]. [18] ABNT Associac- a˜o Brasileira de Normas Te´cnicas. NBR 10844 — Instalac- o˜es prediais de a´gua pluvial. Rio de Janeiro; 1989 [Brazilian Association for Standards, NBR 10844 — Drainage of roofs and paved areas, code of practice, procedure] [in Portuguese]. [19] Ghisi E, Tre´s ACR, Kotani M. Netuno — aproveitamento de a´guas pluviais no setor residencial; 2004 [Neptune — a computer programme to evaluate potable water savings and rainwater tank capacity in the residential sector] [in Portuguese]. [20] Sezerino PH, Philippi LS. Utilizac- a˜o de um Sistema Experimental por Meio de ‘‘Wetland’’ Construı´ do no Tratamento de Esgotos Dome´sticos Po´s Tanque Se´ptico [Utilization of an experimental wetland system to treat septic tank effluent]. IX Simpo´sio LusoBrasileiro de Engenharia Sanita´ria e Ambiental. Anais. Porto SeguroBA, 688-97; 2000 [in Portuguese]. [21] Bierman H, Smidt S. The capital budgeting decision, 4th ed. New York: Macmillan Publishing; 1975. [22] Bussey LE. The economic analysis of industrial projects. In: Fabrycky WJ, Mize JH, editors. Prentice-Hall international series in industrial and systems engineering; 1978.