Journal of Photochemistry and Photobiology B: Biology
Disinfection of drinking water contaminated with Cryptosporidium parvum oocysts under natural sunlight and using the photocatalyst TiO2
Introduction
Cryptosporidium species are protozoan parasites that infect the microvillus border of the gastrointestinal and respiratory epithelium of a wide range of vertebrate hosts, including humans. Infected individuals show a wide spectrum of clinical presentations, but the pathogenicity of Cryptosporidium varies with the species involved and the type, age, and immune status of the host. Cryptosporidiosis by Cryptosporidium parvum and Cryptosporidium hominis is a frequent cause of diarrhoeal disease in humans and several groups of humans are particularly susceptible to infection. In developing countries, cryptosporidiosis occurs mostly in children younger than 5 years with peak occurrence of infection and diarrhoea in children younger than 2 years. In industrialized countries, epidemic cryptosporidiosis can occur in adults by the foodborne or waterborne routes [1]. The oocyst’s capability of surviving in the environment for long periods of time makes waterborne transmission of cryptosporidiosis a serious global issue in drinking water safety [2], [3]. Due to the robust oocyst structure, conventional water treatment may not be totally effective, and oocysts may be present and infective in treated water even if no treatment failure has occurred [4]. The parasite C. parvum is known to be a major cause of human and animal diarrhoea outbreaks as a result of contaminated water supplies. The largest outbreak occurred in 1993 in Milwaukee, WI, USA when an estimated 403,000 people contracted the disease following contamination of the municipal water supply [5], and approximately 2 years after the outbreak, the number of resulting deaths was 54, of whom 85% had acquired immunodeficiency syndrome (AIDS) [6]. The symptoms of the disease are more pronounced in these children leading to malnutrition and impaired physical and cognitive development [7], [8], [9]. It is estimated that contaminated drinking water and lack of sanitation leads to the death of approximately 4500 children each day [10].
SODIS involves storing contaminated drinking water in transparent containers that are placed in direct sunlight for periods of up to 8 h, before consumption [11], [12], [13], [14]. This technique is highly effective against a broad range of pathogens [14], [15], [16], [17], [18], [19], [20], [21]. Previous studies report reductions in incidence of diarrhoea for children who consumed water treated by SODIS compared with children that did not [13], [16], [22], [23]. The biocidal effect of sunlight is due to optical and thermal processes and a strong synergistic effect occurs between the two at temperatures exceeding 45 °C [24], [25], [26]. Batch solar inactivation of C. parvum oocysts using a solar simulator arrangement has been reported previously [27], [28].
The non-toxic photocatalyst TiO2 has been used in combination with batch-process and re-circulated continuous flow SODIS reactors to enhance and accelerate the inactivation rate of bacterial pathogens [29], [30], [31] in a process dubbed solar photocatalytic disinfection (SPCDIS). The use of TiO2 in suspension, as part of a routine intervention for improving the potability of water at the household level (point-of-use water treatment) is not feasible. The photocatalyst particles would have to be removed after solar exposure and before consumption [32]. An additional step such as this in the solar disinfection protocol is likely to have a negative impact on the probability of compliance within communities in developing countries. A more acceptable alternative would be to isolate the photocatalyst onto some form of coated flexible insert, which would reside permanently within the batch-process SODIS reactor. Such a flexible photocatalytic insert has been used previously to accelerate the inactivation kinetics of bacterial and fungal pathogens in drinking water [19].
The aims of the current study were:
- 1.
To study the inactivation kinetics of solar disinfection on drinking water contaminated with C. parvum oocysts under real sunlight conditions.
- 2.
To determine if photocatalytically enhanced batch-process solar disinfection (SPCDIS) could be used to improve the inactivation rate of oocysts of C. parvum.
Section snippets
Sample collection and preparation
Cryptosporidium oocysts were collected from naturally infected calves by rectal sampling and were stored at 4 °C in 0.085 mol l−1 potassium dichromate solutions for periods no longer than 1 month. Concentration and purification from faeces were performed using diphasic centrifugation with phosphate-buffered saline (PBS, pH 7.2)/ethyl ether and discontinuous Percoll® (Sigma, St. Louis, MO, USA) gradients, according to Lorenzo et al. [33]. These oocysts were identified as C. parvum using the
SODIS inactivation of C. parvum oocysts
Inactivation kinetics for oocysts of C. parvum exposed to natural sunlight within glass and transparent polypropylene containers are shown in Fig. 2. Oocyst viability at the start of the experiments was 98.3% (±0.3%). The viability of oocysts in the control samples that had been wrapped in opaque foil and floated in the water bath beside the SODIS test samples in full sun condition for 12 h, was found to be 97.7% (±0.66%).
For comparison purposes the previously reported inactivation curve for
Discussion
The real sunlight inactivation curves for oocysts contained in both glass and polypropylene containers are almost identical within experimental error (Fig. 2). This occurs despite the transmittance of the two materials being very different, mainly in the UV range. This can be explained by the fact that both containers receive the required energy to reach the final disinfection result. Similar behaviour has been recently reported assuming that a bacterial suspension is inactivated once it
Conclusions
We conclude that SODIS and SPCDIS are appropriate technologies for point of use water disinfection of C. parvum oocysts at household level. SPCDIS may enhance the reduction in viability of the parasite when the oocysts are exposed to sunlight under low radiation conditions. The results of this study, when considered in combination with results reported previously for other pathogens [19], [30] reinforces the conclusion that these technologies are appropriate for point of use water disinfection
Abbreviations
- DAPI
4′,6-diamidino-2-phenylindole
- PI
propidium iodide
- SODIS
solar disinfection
- SPCDIS
solar photocatalytic disinfection
Acknowledgements
This work was part-funded by the European Union under Contract No. FP6-2004-INCO-DEV-031650-SODISWATER, the European Science Foundation under the STSM programme of COST Action P9 and the Irish Health Research Board under Grant No. (NS/2003/07). F.M.H. would like to thank the Spanish Ministry of Education and Science (Programa de Acceso de Grandes Instalaciones Científicas Españolas GIC-05-17) for financial support.
References (44)
Cryptosporidium: a water-borne zoonotic parasite
Vet. Parasitol.
(2004)- et al.
Study of the combined influence of environmental factors on viability of Cryptosporidium parvum oocysts in water evaluated by fluorogenic vital dyes and excystation techniques
Vet. Parasitol.
(2000) - et al.
Sunlight as disinfectant
Lancet
(1989) - et al.
Disinfection of oral rehydration solutions by sunlight
Lancet
(1980) - et al.
Solar disinfection of drinking water and diarrhoea in Maasai children: a controlled field trial
Lancet
(1996) - et al.
Effect of agitation, turbidity, aluminium foil reflectors and container volume on the inactivation efficiency of batch-process solar disinfectors
Water Res.
(2001) - et al.
Solar and photocatalytic disinfection of protozoan, fungal and bacterial microbes in drinking water
Water Res.
(2005) - et al.
A novel TiO2-assisted solar photocatalytic batch-process disinfection reactor for the treatment of biological and chemical contaminants in domestic drinking water in developing countries
Solar Energy
(2004) - et al.
Photocatalytical inactivation of E. coli: effect of light intensity and of TiO2 concentration
Appl. Catal. B: Environ.
(2003) - et al.
Supported titanium dioxide as photocatalyst in water decontamination
Catal. Today
(1997)