ARTICLE IN PRESS

Building and Environment 41 (2006) 1544–1550 www.elsevier.com/locate/buildenv

Potential for potable water savings by using rainwater in the residential sector of Brazil Enedir Ghisi Department of Civil Engineering, Laboratory of Energy Efficiency in Buildings, Federal University of Santa Catarina, Floriano´polis-SC, 88040-900, Brazil Received 19 August 2004; received in revised form 15 March 2005; accepted 16 March 2005

Abstract While water availability has been decreasing all over the world, rainwater usage has been suggested to promote potable water savings and ease water availability problems. This paper describes the water availability scenario in Brazil, shows the potential for potable water savings estimated for the residential sector and proposes a new water availability indicator that takes into account the benefits of using rainwater. It is demonstrated that average water availability in Brazil amounts to about 33,000 m3 per capita per year, but it is lower than 5000 m3 per capita per year in two out of the five geographic regions of Brazil. As for the potential for potable water savings by using rainwater, it is shown that it ranges from 48% to 100% depending on the geographic region. The new water availability indicator that is proposed shows that water availability may increase when rainwater is taken into account. r 2005 Elsevier Ltd. All rights reserved. Keywords: Potable water savings; Rainwater usage; Water availability indicator

1. Introduction As the population of many countries has increased rapidly, water availability and water supply have become a matter of increasing concern all over the world [1–3]. According to United Nations, the world population is currently growing at 77 million people per year [4], which means that by keeping this growth rate there will be about 9 billion people in the world in 2050. This represents a 50% increase on the world population. Water resources are limited, therefore there will be water availability problems in many countries and it will be a challenge for governments to ensure an adequate potable water supply to all the population. In order to ease water availability problems and decrease potable water demand, rainwater harvesting has been suggested by many researchers. It has been reported that rainwater promotes potable water savings Tel.: +55 48 3315185; fax: +55 48 3315191.

E-mail address: [email protected]. 0360-1323/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2005.03.018

in hotels in China [5], schools in Taiwan [6,7], houses and multi-storey residential buildings in Germany [8], houses in Australia [9], houses in the UK [10], multistorey residential buildings in southern Brazil [11], petrol stations in southern Brazil [12] and others. However, there have been no reports on any methodology to estimate potable water savings over large areas, such as a whole country, by using rainwater. Neither has a water availability indicator been developed that represents the benefits of using rainwater.

2. Objective The main objective of this paper is to evaluate the actual water availability and to estimate the potential for potable water savings over different geographic regions of Brazil by using rainwater. A water availability indicator that represents the benefits of using rainwater to decrease potable water demand is also discussed.

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situation in all areas of Brazil. When comparing the regions of Brazil, one can notice that in the northeast, southeast and south regions water availability is very low compared to the other two regions (Table 2). In the northeast and southeast, the water availability is significantly lower than the world average of 7000 m3

3. Water availability It is well known that water is abundant in Brazil; it accounts for 11% of the world water and for 50% of South American water [13]. Although abundant, water is not evenly distributed over the country. Fig. 1 shows, on the left-hand side, the percentage of land area, water availability and population over the five geographic regions of Brazil; on the right-hand side, the location of the five regions on a map of Brazil is shown. North region, which houses the Amazon Basin, comprises some 45% of the land area, 69% of the available water but houses only 8% of the population. In contrast the southeast region accommodates 43% of the population, but has only 6% of the available water in the country; similarly the northeast region has 28% of the population but only 3% of the available water. This indicates that the southeast and northeast regions are the most likely to face water availability problems in the near future. Some researchers have been trying to develop indicators to address the water problem [16]. However, the relation between water availability and population is still the indicator most widely used. United Nations Environment Programme (UNEP) adopts a classification as shown in Table 1 [4]. Average water availability in Brazil was over 328,000 m3 per capita per year in 1900 as shown in Table 2. Water availability in 1900 was very high for all the geographic regions of Brazil according to UNEP’s classification. A hundred and one years later, in 2000, water availability in Brazil decreased to about 33,000 m3 per capita per year, still very high according to UNEP. However this national average does not represent the

Table 1 Classification of water availability by UNEP [4] Water availability (m3 per capita/year)

Classification

Higher than 20,000 10,000–20,000 5000–10,000 2000–5000 1000–2000 Lower than 1000

Very high High Medium Low Very low Catastrophically low

Table 2 Water availability in Brazil Region

Water availability (km3 /year)

North Northeast Southeast South Central–West Brazil

3968 186 334 365 879 5733

(m3 per capita/ year)

5,708,864 27,587 42,715 203,396 2,353,814 328,745

307,603 3900 4615 14,553 75,511 33,762

Fortaleza Fortaleza

Belém Belém

69

Land area (%) Water availability (%) Population (%)

70 60 Percentage (%)

Year 2000

(m3 per capita/ year)

Source: Based on IBGE and ANA [14,15].

80

50

Year 1900

São Luis

NORTH

Natal Recife Maceió Maceió

45

NORTHEAST

43

CENTRAL-WEST CENTRALWEST Brasília

Salvador Salvador

40 28 SOUTHEAST

30 18 20

11

8 10

15

3

6

7 6

19 15 7

Northeast

Curitiba Curi ba

Rio de Janeiro

SOUTH

0 North

Vitória

São Paulo

Southeast South Geographic region

Central-West

Fig. 1. Proportion of land area, water availability and population over the five geographic regions of Brazil. Source: Based on IBGE and ANA [14,15].

Porto Alegre

Florianópolis

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400

50

North

North Northeast Southeast South Central-West

Predicted population (million)

Population (million)

75

25

0 1900

350

Northeast Southeast

300

South

250

Central-West 200 150 100 50 0 2000

1920

1940

1960

1980

2020

2040

2000

2060

2080

2100

Year

Year

Fig. 3. Predicted population over the period 2000–2100.

Fig. 2. Brazilian population per geographic region. Source: Based on IBGE [14].

per capita per year and can be classified as having low water availability [4]. Such decreasing in water availability in Brazil can be explained by the population increase observed over the period as shown in Fig. 2.

4. Prediction of water availability Considering the average growth rate over the period 1991–2000, for each geographic region, the predicted population and also water availability were estimated up to the year 2100 as shown in Figs. 3 and 4, respectively. It can be observed in Fig. 4 that from 2050 onwards, the northeast and southeast regions will have water availability lower than 2000 m3 per capita per year, which is considered very low by UNEP. In the southeast region, water availability will be lower than 1000 m3 per capita per year from 2094; in the northeast region that will happen from 2100 onwards. From 2075 onwards, water availability in the south region will decrease to figures below 5000 m3 per capita per year, which is a low water availability. Therefore, action must be taken in order to avoid water scarcity mainly in the southeast and northeast regions of Brazil.

5. Rainwater harvesting Unless there is a decrease in water demand or a population increase with a lower growth rate, some geographic regions of Brazil will face water scarcity problems by the end of the 21st century. To avoid scarcity of water Brazil should implement programmes to promote rainwater harvesting. Average rainfall in the world amounts to 760 mm per year [17] while in Brazil it reaches about 1443 mm a year (Table 3). However, rainfall in Brazil is not evenly distributed across the

Predicted water availability (m3 per capita/year)

1000000

100000

10000

1000 North Southeast Central-West 100 2000

2020

2040

Northeast South 2060

2080

2100

Year Fig. 4. Predicted water availability over the period 2000–2100.

Table 3 Average rainfall in Brazil per geographic region Region

Average rainfall (mm/year)

Number of cities

North Northeast Southeast South Central–West Brazil

2182 1146 1362 1615 1540 1443

27 75 55 26 23 206

Source: Based on Normais Climatolo´gicas [18].

country. Table 3 shows the average rainfall for the five geographic regions of Brazil. It ranges from 1146 mm per year in the northeast to 2182 mm per year in the north region. The averages were calculated over a number of cities as indicated on the right-hand column of Table 3. In order to estimate the potential for potable water savings by using rainwater over the five regions, the

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specific roof area per person as well as the potable water demand were determined. 5.1. Catchment area In multi-storey residential buildings the specific roof area per person is low. Therefore, to estimate an accurate average roof area per person over the five regions, the percentage of houses and flats in multistorey residential buildings was surveyed. Results are shown in Fig. 5, which shows that southeast and south are the regions with the highest percentage of flats in multi-storey residential buildings. Such percentages are likely to change along the years, but as there is no official information on the growth rate, they were assumed steady as shown in Fig. 5. A survey [19] performed over 12 out of 26 states in Brazil established the percentage of dwellings according to their floor plan area as shown in Fig. 6. Performing a weighted average using the figures shown in Fig. 6, one obtains 81 m2 . Such a figure was assumed to represent an average roof area to be considered for rainwater harvesting in houses. As for multi-storey residential

Percentage (%)

100

2.4

5.3

13.3

10.1

6.9

97 .6

94 .7

86 .7

89 .9

93 .1

80 60 40 20 0 North

Northeast Southeast South Geographic region Houses

Central-West

Flats

50

Percentage

buildings, due to the lack of official information, an average roof area of 15 m2 per flat was assumed to be adequate. The average roof area per person living either in houses or flats was estimated by using Eq. (1). Ai ¼

A , Pi

(1)

where Ai is the average roof area per person living either in houses or flats in each region i (m2 /person), A is the roof area for houses (81 m2 ) or flats (15 m2 ), and Pi is the average number of people per dwelling in each region i (person). Table 4 shows the average number of people per dwelling for each region in the year 2000 [14]. As there is no official information, it was assumed that the percentage of people living in houses is the same as the percentage of houses; and the same for people living in flats. Then, a weighted average roof area per person was calculated considering the percentage of houses and flats in each geographic region by using Eq. (2). HAHi þ FAFi , (2) 100 where Ai is the weighted average roof area per person in each region i (m2 /person), H is the percentage of houses in each region i (%), AHi is the average roof area per person living in houses in each region i (m2 /person), F is the percentage of flats in each region i ð%Þ, and AFi is the average roof area per person living in flats in each region i (m2 /person). Table 5 shows the specific roof area per person for each region as determined by using Eqs. (1) and (2). Ai ¼

Table 4 Average number of people per dwelling per geographic region

Fig. 5. Percentage of houses and flats in multi-storey residential buildings in the year 2000. Source: Based on IBGE [14].

38

40

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Region

Average number of people per dwelling in 2000

North Northeast Southeast South Central–West

4.51 4.14 3.52 3.42 3.61

26

30 21

Table 5 Specific roof area per geographic region

20 10 10

3

Region

0 200

Fig. 6. Percentage of dwellings according to the range of floor plan area. Source: Based on Eletrobra´s [19].

North Northeast Southeast South Central–West

Houses

Flats

Weighted average

18.0 19.6 23.0 23.6 22.4

3.3 3.6 4.3 4.4 4.2

17.6 18.7 20.5 21.7 21.2

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Table 6 Specific volume of rainwater, potable water demand and potential for potable water savings per geographic region Region

Specific volume of rainwater (m3 per capita/year)

North Northeast Southeast South Central–West

38.419 21.457 27.953 35.000 32.608

Potable water demand (litres per capita/day)

(m3 per capita/year)

88 97 158 117 120

32.120 35.405 57.670 42.705 43.800

5.2. Potential for potable water savings Having obtained the weighted average roof area per person and knowing the average rainwater in each region, then the volume of rainwater per person that could be collected annually over each region can be calculated by using Eq. (3). Ri Ai , (3) 1000 where V i is the specific volume of rainwater per person per year in each region i (m3 per capita/year), Ri is the average rainfall per year in each region i (mm), and Ai is the weighted average roof area per person in each region i ðm2 =personÞ. Then, the potential for potable water savings by using rainwater was calculated by using Eq. (4). Vi ¼

S i ¼ 100

Vi , Di

Potential for potable water savings (%)

(4)

where Si is the potential for potable water savings in each region i (%), V i is the specific volume of rainwater per person per year in each region i (m3 per capita/year), Di is the potable water demand (see Table 6) in each region i (m3 per capita/year). Table 6 shows the specific volume of rainwater per person that could be harvested in each one of the five regions. Such volume ranges from about 21 m3 per capita per year in the northeast to some 38 m3 per capita per year in the north. Based on data available at SNIS [20], potable water demand was calculated for each region. It can be observed that the potential for potable water savings by using rainwater is lower in the southeast, but it is still very significant, as there would be enough rainwater to supply almost half of the potable water demand.

6. Towards a new indicator Rainwater harvesting has been implemented in different countries as a way of easing water availability problems. Therefore, it is suggested in this paper that

100 61 48 82 74

the water availability indicator, as used by UNEP and other researchers, be modified when rainwater is used to contribute to decrease potable water demand. When there is a constant rainwater usage, water resources are preserved cumulatively along the years. Therefore, what is proposed to reflect such preservation of water resources is that the specific volume of rainwater used over the year be accumulated over the previous years and summed to the water availability as shown by Eq. (5). W mod ¼ Wn þ n

n X

V i,

(5)

i¼1

is the modified water availability indicator where W mod n over the year n (m3 per capita/year), W n is the water availability indicator over the year n (m3 per capita/ year), and V i is the accumulated specific volume of rainwater from year i ¼ 1 to n (m3 per capita/year). Thus, it is possible to show the contribution of rainwater usage to the water availability. Figs. 7 and 8 show, as an example, the predicted water availability over the period 2000–2100 for the northeast and southeast regions of Brazil, respectively. It can be observed that the inclusion of the rainwater in the water availability indicator promotes an increase on water availability.

7. Conclusions The water availability problem and the potential for potable water savings over the five geographic regions of Brazil have been assessed. Results for the water availability analysis show that the northeast and southeast regions will have water availability lower than 2000 m3 per capita per year from 2050 onwards. From about 2100, both these regions will have water availability lower than 1000 m3 per capita per year, which is considered catastrophically low by UNEP. This indicates that both northeast and southeast regions may face serious water availability problems in the near future

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Predicted water availability (m3 per capita/year)

5000 4000 3000 2000 1000 With no rainwater 0 2000

2020

2040

With rainwater 2060

2080

2100

Year Fig. 7. Predicted water availability in the northeast region.

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water availability. In the northeast and southeast regions of Brazil, for example, water availability is predicted to be lower than 1000 m3 per capita per year from about 2100. Thus, if rainwater were to be used in the residential sector of Brazil, the water availability in these two regions would not be lower than 3000 m3 per capita per year. Such an indicator would be even higher if rainwater usage were also considered in commercial, public and industrial buildings. The methodology presented in this paper to assess water availability, potential for potable water savings and the water availability indicator that represents the benefits of using rainwater can be applied to any country around the world.

Acknowledgements Predicted water availability (m3 per capita/year)

5000

The author 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 financial support to undertake this project.

4000 3000 2000

References

1000 With no rainwater 0 2000

2020

With rainwater

2040

2060

2080

2100

Year Fig. 8. Predicted water availability in the southeast region.

unless there are government programmes to promote water conservation. As for the potential for potable water savings by rainwater usage, it ranged from 48 to 100% over the five regions. It was demonstrated that in the north region the rainwater potential is higher than the water demand, which is as low as 88 litres per capita per day. In the southeast region, a potential for potable water savings of 48% was obtained. This indicates that the collected rainwater could be used for non-potable uses such as toilet flushing, garden watering, floor cleaning, car and clothes washing, which usually account for about 50% of the water consumption in a household. As for the other regions, whose potential for potable water savings surpasses 50%, rainwater should go through adequate treatment in order to be used for potable purposes. In polluted areas, the rainwater quality should be evaluated to avoid health problems. A new indicator that represents the benefits of rainwater usage on water availability was also assessed. It was shown that considering the collected rainwater along the years would promote an increasing of the

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