The lack of access to safe drinking water and energy is one of the main challenges that the humanity is facing in the 21th century. Despite great efforts have been made, globally, at least 1.8 billion people use a drinking water source faecally contaminated, which is the cause of more than half-million diarrhoeal death each year; and 1.2 billion people still do not have access to electricity which is associated with approximately 3.5 million deaths anually. In particular, in developing countries heath risks are mainly associated with the consumption of water contaminated with faeces, which also prevents good sanitation and hygiene.
Technologies for the inactivation of pathogenic microorganisms in water are therefore of great of significance for human health and well-being. However, conventional technologies for water disinfection (chlorination, ozonation, UV lamps, etc.) are often chemically, energetically and operationally intensive. They require considerable investment of capital, which is difficult to face even in industrialized countries, and regardless of their contribution to protect public health, they are not always effective for the elimination of some pathogens, and in some cases these technologies form toxic disinfection by-products. Furthermore, their implementation in developing regions (the most vulnerable to waterborne diseases) is limited due to their dependence on chemicals (difficult or expensive to obtain), electricity access, the lack of trained operators and the rejection of water after treatment because its unfavourable taste and odour (especially in the case of chlorination). These shortcomings have led to the rapid research and development of advance alternative technologies for the field of water intended for human consumption. Solar disinfection (SODIS) has been found as one of the most appropriate methods for drinking water treatment in developing countries. However, systems that relay exclusively on natural UV sunlight for disinfection only use a 5 % of the total available energy, so their total efficiency is reduced. Therefore, the development of this type of solar water disinfection systems will only be feasible if the solar spectrum is used in the most efficient way possible, taking full advantage of the energy contained in each spectral band.
Natalia Pichel Mira, researcher at IMDEA Water, has addressed this problem in her doctoral thesis, directed by Dr. Marta Vivar, and defended on February 2, 2018 at the University of Alcalá. This thesis, entitled “Photovoltaic and photochemical hybrid system for water treatment”, was focused on the development and study of a new hybrid technology for simultaneous water disinfection and electricity generation (SOLWAT) that increase the total conversion efficiency of solar energy using each part of the spectrum for those mechanisms that are more efficient. It was combined with the assessment of conventional SODIS treatment (PET bottles) studying the contribution of the UV light and thermal components to the bacteria inactivation. Finally, it also included the analysis of the current state of the drinking water access and the associated health risks in the Saharawi refugee camps as a potential site for the SOLWAT implementation.
Schematic diagram of the SOLWAT system, where the integration of the PV module on the bottom and the water reactor on the top can be observed (including the different materials). It can also be observed the use of the solar spectrum in the new concept: UV and far infrared for water disinfection and visible and near infrared for electricity generation.
The results here obtained confirm that the SOLWAT system integrates the functions of solar water disinfection and renewable energy generation in a single and compact device, which showed higher disinfection efficiencies than conventional PET bottles for all the microbiological indicators analysed and regardless of the UV and temperature conditions tested. Electrical results showed that the total energy output was not affected by the water disinfection reactor located above the photovoltaic module during the water disinfection process (6 h).
SOLWAT, reference system, PET bottle and radiometers during SODIS tests at the rooftop facilities of IMDEA Water.
Finally, the study confirm the feasibility of this technology to contribute to palliate the lack of access to safe drinking water and electricity, which could be also used to treat wastewater from industrial and urban effluents, especially in tertiary treatments, through non-energy and non-chemical dependant technologies. Therefore, considering the microbiological contaminations risks detected in the drinking water supply of the Saharawi refugee camps and taking into account the irradiance conditions of the Sahara desert (located within the ‘sun belt’ zone) with high UV levels and ambient temperatures, the SOLWAT system could be an appropriate technology to be implemented at household and/or community level for water disinfection purposes.