Published online 9 August 2006
Published in J Environ Qual 35:1812-1817 (2006)
DOI: 10.2134/jeq2005.0357
© 2006 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
TECHNICAL REPORTS
Surface Water Quality
Is Dew Water Potable? Chemical and Biological Analyses of Dew Water in Ajaccio (Corsica Island, France)
Marc Musellia,b,
Daniel Beysensb,c,*,
Emmanuel Soyeuxd and
Owen Clusa,b
a Université de Corse, UMR CNRS 6134, Route des Sanguinaires, 20000 Ajaccio, France
b International Organization for Dew Utilization (OPUR), 60, rue Emeriau, 75015 Paris, France
c Equipe du Supercritique pour l'Environnement, les Matériaux et l'Espace, CEA-Grenoble, France & Ecole Supérieure de Physique et Chimie Industrielles, 10, rue Vauquelin, 75231 Paris Cedex 05, Paris, France
d Veolia Environnement, Research Department, 38 avenue Kléber, 75 116 Paris, France
* Corresponding author (daniel.beysens{at}cea.fr)
Received for publication September 19, 2005.
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ABSTRACT
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To determine to what extent dew water is potable without further treatment, a thorough set of chemical and biological analyses were performed on 10 samples of dew water collected on a large scale radiative collector (29.83 m2) in Ajaccio (Corsica Island, France), between 21 May 2002 and 5 Mar. 2003. Samples were collected following four protocols according to the dew volume amount and 48 parameters (ions, minerals, and bacteria) were analyzed and compared to French and European Union legislation and also World Health Organization (WHO) recommendations. Aluminum and Fe were the main pollutants whose concentrations were significantly larger than recommended. Their presence is due to local deposition of aerosols coming from the Sahara (a characteristic of the Mediterranean basin). A large number of biologically cultivable microorganisms were found, together with bacteria typical of fecal contamination. For dew water to be potable with respect to present legislation at the Ajaccio site, it should be disinfected and treated for turbidity.
Abbreviations: WHO, World Health Organization
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INTRODUCTION
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IT IS THE OBJECT of this work to report on the potable quality of dew water. Dew water forms by radiative cooling of a surface, most often during nighttime where the radiative heat balance is negative. The maximum water yield is on the order of 0.5 L/m2 per night and corresponds to the radiative cooling energy (maximum: 100 W/m2) needed to transform water vapor into liquid water (the latent heat) (Monteith, 1957; Garratt and Segal, 1988; Nikolayev et al., 1996).
When compared to rain and fog, dew water quality has been the object of few investigations. Chemical and bacteriological investigations are scarce. Dew acidity studies have been described in Pierson et al. (1986), Mulawa et al. (1986), Okochi et al. (1996), Pierson et al. (1988), Pierson and Brachaczek (1990), and Sceller (2003). Chemical composition research has been reported by Chang et al. (1987), Foster et al. (1990), Ortiz et al. (2000), Jiries (2001), Muselli et al. (2002), and Rubio et al. (2003); evaporated dew in the smog photochemical process was studied by Rubio et al. (2002). A one-year study of dew chemistry and bacteria performed in Bordeaux (France) presented results on the dew quality when the condensation substrate was cleaned every day (Beysens et al., 2006).
In the present work, a thorough study of the chemical and bacteriological composition of dew water collected on a large open system is presented. Dew water was collected on a passive dew condenser, of about 30-m2 working area, in Ajaccio, Corsica Island (France). To determine whether dew water can be potable, 50 bacteriological and chemical parameters were analyzed. Below we describe the dew collection device and the protocols of dew collection. This is followed by data analyses concerning the chemical and bacteriological compositions of dew samples, and a discussion of the possible influence of the condensation substrate and the potential contribution of atmospheric pollution.
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MATERIALS AND METHODS
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Dew Collection
Site and Condenser
According to initial studies (Muselli et al., 2002, 2006; Beysens et al., 2003), a 3- x 10-m (29.83-m2 working area) plane condenser was set up (Fig. 1
) near the ground, tilted at 30° with respect to horizontal. The study site is situated in the Ajaccio Gulf (Corsica Island, France; 41°55' N, 8°48' E), 400 m from the sea at 70 m above mean sea level. The wind regime is characterized by a nocturnal wind with a NE dominant direction (1.8 m/s average) and two directions (NW, SW) for the diurnal dominant wind, a characteristic of a Mediterranean island climate. The condensation substrate is made of TiO2 and BaSO4 microspheres embedded in a foil (0.39 mm thick) of polyethylene. This material is discussed in Nilsson et al. (1994) and Nilsson (1996) where it is explained why this foil exhibits improved emitting properties in the near infra-red (to provide radiative cooling of room temperature surfaces) and efficiently reflects the visible (sun) radiation. The foil is thermally insulated from the ground by 30-mm-thick polystyrene foam and is fixed by lateral and vertical cables. Water is gathered by gutters into a 25-L polyethylene tank. The hollow part of the device faces the direction of the dominant nocturnal wind (NE, Fig. 1). The condensing foil thus faces SW (220°) and remain shaded in the morning. It is precisely in the early morning that the air temperature is the lowest, thus closest to the dew point temperature favoring dew condensation. With such a design, dew can form even in daytime, as long as the sun does not directly irradiate the foil. Dew yield remains large for windspeed lower than 3 to 4 m/s (Muselli et al., 2006). Stronger winds increase heat losses, thus canceling the effect of radiative cooling.
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Fig. 1. Radiative dew condenser. The letter "C" denotes the collecting tank, while "NE" denotes the nocturnal dominant wind.
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Dew water is collected in two stages: (i) by gravity flow overnight and (ii) by scraping the foil in the morning to obtain the final volume of dew. Dew is filtered using a classical nylon filter with mesh of about 300-µm spacing. Yields are in the range 0 to 15 L/night.
Dew is collected in the morning (0800 h local time) and, according to the water volume, Protocol 1, 2, 3, or 4 (see below) is chosen. The different bottles are placed in an icebox and, the same day, sent by courier air mail to the Centre d'Analyses Environnementales of Veolia Environnement, Saint-Maurice, near Paris (France). Bacteriological analyses are performed immediately.
Protocol
Figure 2
presents the dew volume distribution for the period 10 Dec. 2001–11 Dec. 2003 (two years). The 333 dew days recorded corresponds to 45.6% of days during the study period. According to the amount of collected dew water, one of four protocols was selected (Table 1):
- Protocol 1: 3 to 4 L of dew (i.e., 12.6% of the distribution dew events).
- Protocol 2: 4 to 5 L of dew (i.e., 10.8% of the distribution dew events).
- Protocol 3: 5 to 7 L of dew (i.e., 21.3% of the dew distribution events).
- Protocol 4: >7 L of dew (i.e., 4.8% of the distribution dew events).
The higher the dew amount collected, the greater the number of analyses that could be performed. Ten samples of dew were analyzed during this study; the repartition of these samples according to the protocols is the following:- Protocol 1: two samples (21 May 2002 and 24 Feb. 2003).
- Protocol 2: none.
- Protocol 3: four samples (20 Oct. 2002, 19 Dec. 2002, and 4 and 5 Mar. 2003).
- Protocol 4: four samples (15 and 28 Oct. 2002, and 12 and 28 Nov. 2002).
These procedures correspond to selective chemical and bacteriological analyses according to Table 1. A minimum amount of 6.75 L of dew was necessary to perform all the analyses. Taking into account the water volume needed to perform preliminary analyses (0.25 L for pH, water temperature, and conductivity), 7 L of dew are required for the complete Protocol 4. The total number of analyzed components was 47 (plus one—the saturation index—by calculation) by developing the categories presented in Table 1.
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RESULTS AND DISCUSSION
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Dew Water Quality
To estimate the quality of dew samples, the results were compared to the threshold limits of the World Health Organization (2004), the European Union Council Directive 98/83/CE (3 Nov. 1998) on the quality of water intended for human consumption, and French Legislation (Code de la Santé Publique). In the Council Directive and in the French legislation which transposed the directive, values are given for microbiological and chemical parameters (noted "P") as well as values for indicator parameters (noted "I"). Set mainly for monitoring purposes, these indicator parameters provide a reference for proper management of water production and distribution.
Chemical Composition
Table 2 summarizes the chemical analyses of the 10 dew samples. The SO42–, Cl–, K+, and Ca2+ concentrations of the 10 samples are in the same range as found in an earlier study on dew chemistry in Ajaccio over 17 months (16 Oct. 1999–17 Mar. 2001) (Muselli et al., 2002). By comparing the data (Table 3) with the French legislation requirements, there were 63 non-conformities (20 for P parameters and 43 for I parameters) corresponding to nine chemicals. For the European Directive, 41 non-conformities were obtained for seven chemicals (10 for parameters and 31 for indicator parameters). Lastly, five non-conformities were found for the WHO water limits corresponding to one chemical (lead). The parameters involved are the following: aluminium, antimony, lead, iron, ammonium, turbidity, permanganate demand, conductivity, and saturation index (corrosive water). The presence of most of these constituents can be explained:
- Aluminium and iron: intense Saharan dust transport events are responsible for the highest concentrations of Al and Fe (Bergametti, 1987; Dulac et al., 1992). This deposition occurs currently in the entire Mediterranean basin and provides a characteristic composition to the dew water.
- Ammonium: probably of organic origin.
- Antimony: origin unknown.
- Lead: its presence in dew water was found in 5 samples out of 10 at concentrations around the threshold of 10 µg/L. The origin of this contamination is unknown because no element of the condenser structure contains the constituent Pb. According to Dulac et al. (1992), The Pb to Cd ratio gives an idea of the origin of Pb. High values of Pb to Cd ratio correspond to urban centers' Pb pollution. Pollution from industrial activities gives low values of Pb to Cd ratio. Here Pb to Cd ratio was >1 in all samples and is thus a signature of urban activities (the nearby city of Ajaccio).
- Turbidity and permanganate index: these non-conformities are presumably due to the presence of high iron concentrations.
- Conductivity and saturation index: low conductivity and saturation index indicating that the water is corrosive are related to the low mineralization of dew.
Concerning dissolved salts, the concentrations in mM/L, of Na+, Cl–, and Mg2+ ions are plotted in Fig. 3
. The correlations between cations and anions of each couple are evident and give [Na+]/[Cl–] = 0.7 ± 0.1, as compared with 1.16 ± 0.02 over one year in Bordeaux (Beysens et al., 2006), and [Mg2+]/[Cl–] = 8.8 ± 0.7 as compared with 11.4 ± 0.3 over one year in Bordeaux (Beysens et al., 2006). The presence of NaCl and MgCl2 salts is due to the vicinity of the Mediterranean sea (about 400 m from the dew collecting point). One notes that the ratio [Mg2+]/[Na+] = 0.17 ± 0.11 is comparable to what is found in sea water (0.113) (concentrations in mg/L).
Biological Composition
Table 4 summarizes the biological analyses of the 10 dew samples. Many culturable bacteria were found, coming from atmospheric deposition. This is unavoidable as dew is collected in the open. Such a result is similar to Beysens et al. (2006) in Bordeaux.
When dew is harvested with care on a small dew collector wiped each day, as reported by Beysens et al. (2006), little contamination by these bacteria was found. Here, significant biological pollution can be measured, however. Indeed, such a large surface is nearly impossible to maintain clean at the bacteriological level. Humans, by manipulation, and animals (small mammals, birds, insects, flies, ants), by drinking, can easily contaminate dew water. The presence of microorganisms was found, especially indicators of fecal contamination (enterococcus and coliforms) listed in the French and EU legislations and in WHO recommendations for drinking water. This means that, to remain potable, dew water from large collectors must be disinfected and treated for turbidity. This can be done by simple means such as a combination of filtration and chlorination. The variability of the dew water quality does not represent a treatment challenge.
Material Component Influences
The condensing foil contains TiO2 and BaSO4 microspheres embedded in polyethylene. The concentrations of Ti is in conformity with the French regulations, the European Directive, and the WHO limits.
For barium, all analyzed samples present concentrations inferior to the water quality limits. Concerning the polyethylene matrix, the concentrations in total organic carbon do not indicate that polyethylene dissolves in dew water. The maximum value of this parameter, obtained on 4 Mar. 2003, was due to the presence of pollen in water.
We can thus conclude that the condensation material is indeed inert to water under the conditions of this experiment.
Atmospheric Pollution
Several parameters were specially considered to study some possible atmospheric or anthropogenic pollution in dew water:
- Polycyclic aromatic hydrocarbon: four samples contained these chemical constituents. In Samples 2, 5, and 6, the six polycyclic aromatic hydrocarbons studied, namely benzo(a) pyrene, benzo(b)fluoranthene, benzo(g,h,i)perylene, benzo(k)fluoranthene, fluoranthene, and indeno(1,2,3-cd)pyrene, were not detected. However, one sample (Sample 3) had lower quantities of benzo(b)fluoranthene (0.002 µg/L) and fluoranthene (0.006 µg/L) in relation to the limit of 0.1 µg/L.
- Hydrocarbon index: this index was studied in eight collected samples. In all cases, the values of this index were lower than the detection limit of 0.1 mg/L.
- Phenol index: the 10 samples had values lower that the detection limit (10 µg/L).
Variability
The variability of the chemical composition is important as outlined in Table 2 where, in addition to the mean values, minimum and maximum values are reported. The difference between the minimum and the maximum values often exceeds a factor of 10 (with the noticeable exception of Ba). While the minimum values are rather randomly distributed, most of the maximum values are found for the 28 Nov. 2002 data. The reason is simply that the concentration of gas around the condenser and the mass of deposited aerosols depend on the wind importance and direction as well as the season.
Concerning the variability of the bacteriological composition, the same reasons can be invoked. In addition, the influence of animal and human contamination has to be accounted for, as well as seasonal variation for the vegetation. All these reasons lead to a chemical composition variation even larger than for the chemical composition.
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CONCLUSIONS
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The composition of dew water exhibits a large variability, even more than rain water. Its potability depends obviously on the criteria that are used. On the 39 parameters that were analyzed, only three parameters were found at concentrations exceeding the French and European Union standards for drinking water: antimony, lead, and turbidity. Among these three parameters, only lead is included in the WHO recommendations and was found at concentrations fluctuating around the threshold of 10 µg/L. Six other parameters, considered as indicators in the French and European Union legislation, were found at concentrations exceeding the recommended values: aluminium, iron, ammonium, permanganate demand, conductivity, and saturation index.
Many cultivable bacteria were present in the seven analyses. Nevertheless, bacteria indicators of fecal contamination were also present (coliforms and enterococcus). Their origin can be either human or by animals living in the vicinity of the condenser. The presence of such bacteria requires disinfection, which would increase operation costs of the dew collection system.
The moderate level of contamination of water by Pb is mainly due to urban activities in the Ajaccio Gulf and should not be present in places further upwind from the city.
For Al and Fe, high concentrations from dust particles of Saharan origin can be overcome using a better filter to prevent dew water contamination. This technique should also decrease the water turbidity to an acceptable level.
Note that, due to its low mineralization, dew water is corrosive and this should be taken into account when choosing pipe materials.
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ACKNOWLEDGMENTS
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We thank S. Berkowicz for a critical reading of the manuscript.
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REFERENCES
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TOP
ABSTRACT
INTRODUCTION
