A practical demonstration of water disinfection using TiO₂ films and sunlight

Abstract

The scope of this study is the assessment of the efficiency of solar disinfection by heterogeneous photocatalysis with sol–gel immobilized (titanium dioxide) TiO₂ films over glass cylinders. The solar disinfection process known as SODIS was considered as a reference. Spring water naturally polluted with coliform bacteria was exposed to sunlight in plastic bottles with and without TiO₂ over simple solar collectors and the disinfection effectiveness was measured. Total and fecal coliforms quantification was performed by means of the chromogenic substrate method in order to obtain the efficiency of each disinfection treatment. The disinfection with TiO₂ was more efficient than the SODIS process, inactivating total coliforms as well as fecal coliforms. On a sunny day (more than 1000 W m⁻² irradiance), it took the disinfection with immobilized TiO₂ 15 min of irradiation to inactivate the fecal coliforms to make them undetectable. For inactivation of total coliforms, 30 min was required, so that in less than half the time it takes SODIS, the treated water complies with the microbial standards for drinking water in Mexico. Another important part of this study has been to determine the bacterial regrowth in water after the disinfection processes were tested. After SODIS, bacterial regrowth of coliforms was observed. In contrast, when using the TiO₂ catalyst, coliforms regrowth was not detected, neither for total nor for fecal coliforms. The disinfection process using TiO₂ kept treated water free of coliforms at least for seven days after sun irradiation. This demonstration opens the possibility of application of this simple method in rural areas of developing countries.

Introduction

The supply of safe drinking water is an issue that raises concerns, particularly in developing countries. Although several successful initiatives have been launched to supply safe drinking water to urban populations, efforts still fall short of the required targets for sustainable development. In developing countries, water delivery systems are plagued by leakages, illegal connections and vandalism, while precious water resources are squandered by greed and mismanagement. In Africa, Asia, Latin America and the Caribbean, nearly one billion people in rural areas have no access to improved water supplies. During the 1990s, a reduction in per capita water supply was recorded for Africa (2.8 times), Asia (2 times), and Latin America and the Caribbean (1.7 times) (WHO/UNICEF, 2000).

The lack of access to safe drinking water is directly related to poverty and in many cases to the inability of governments to finance satisfactory water and sanitation systems. The direct and indirect human costs of these failings are huge, including widespread health problems, heavy labor (particularly for women), and severe limitations for economic development (Gleick, 2000).

It is estimated that polluted water affects the health of more than 1.2 billion people worldwide, and contributes to the death of 15 million children every year. In 1994, WHO estimated that 1.3 billion people had no access to clean drinking water. By 2000, nearly 1.2 billion people lacked access to clean water (WHO/UNICEF, 2000). Mexico is not the exception and the lack of safe drinking water affects both urban and rural areas.

Human pathogens causing waterborne illness are adapted to living in human intestines, where they find a dark, humid environment and temperatures ranging between 36 and 37 °C. Once pathogens are discharged into the environment, they are very sensitive to the harsh conditions outside the human body. Therefore, temperature and UV radiation can be used to inactivate the pathogens present in polluted water (EAWAG and SANDEC, 2002).

Solar water disinfection (SODIS) is a simple, environmentally sustainable and low cost point of use treatment for drinking water. SODIS bases on the bacteriostatic effect of the UV-A solar radiation (wavelength 320–400 nm) as well as in the presence of dissolved oxygen, as it plays an important role in killing the pathogens: sunlight produces highly reactive forms of oxygen (oxygen free radicals and hydrogen peroxides) in the water. These reactive molecules contribute in the destruction process of the microorganisms.

SODIS disinfection uses UV-transparent polyethylene terephthalate (PET) bottles of volumes ranging from 0.3 to 2 l that are filled to $\frac{3}{4}$ of its capacity with water and agitated to increase the dissolved oxygen. The bottles are exposed to sunlight over a period of 6 h. According to EAWAG and SANDEC (2000), the best bacteria inactivation effect is reached on sunny days when heat and UV radiation combine synergistically. However, in a cloudy day (more than 50% covered sky) it is necessary to expose the water to sun irradiation for longer periods, two days or even more to disinfect water.

McGuigan et al., (1998), showed that thermal inactivation of Escherichia coli is important only when water reaches temperatures over 45 °C, when a strong synergy with the effect of radiation is observed. They concluded that in places with high heatstroke, disinfection by means of solar energy is a low cost and effective method to improve the microbiological quality of water. However, bacterial regrowth after short storage (24 h) of SODIS treated water has been currently observed (Martín et al., 2000).

Looking for an improvement of the SODIS, to diminish the irradiation time and to avoid bacteria regrowth we decided to test the use of titanium dioxide, TiO₂. Yu et al. (2002); Sunada et al. (2003) and Huang et al. (2000) have recently published papers reporting that titanium dioxide can promote cellular destruction in more than one way, leading to cell death under UV radiation. According to this, the use of TiO₂ offers an attractive complement to SODIS, accelerating bacteria death and destroying undesirable organic matter, at the same time.

Titanium dioxide is a semiconductor excitable under UV radiation (380 nm), generating an electron–hole pair. The hole reacts with water creating hydroxyl radicals (OH) while the electron reacts with dissolved oxygen forming peroxide, superoxide and other highly oxidant species.

Previous jobs have used TiO₂ to control different kinds of microorganisms, such as bacteria, virus and fungi. Lonnen et al. (2005) reduced the viability of diverse microorganisms utilizing Duran^® glass bottles and UV artificial light to simulate the SODIS process and to study a UV/TiO₂ system. The microorganisms tested were protozoa (the trophozoite stage of Acanthamoeba polyphaga), fungi (Candida albicans and Fusarium solani) and bacteria (Pseudomonas aeruginosa and E. coli). In all cases treatments with TiO₂ surpassed the performance of SODIS.

In photocatalytic chemical applications the catalyst is widely used as a powder to generate aqueous slurry. Such slurries have some disadvantages: difficult separation of the catalyst after photoreaction; aggregation of particles at high concentration and inefficiency of the process by light scattering away from the photoreactor are frequently mentioned. These disadvantages explain the gradual preference and today's popularity of the use of immobilized forms of TiO₂ (Sunada et al., 2003).

In the present job, a form of immobilized TiO₂ was tested for water disinfection with excellent results, promising an alternative to water disinfection in rural areas of developing countries, where other disinfection practices can hardly be applied.