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SODIS is not a universal solution to the problem of access to safe drinking water. However, solar disinfection is the lowest cost household-based intervention against waterborne disease when compared with other alternatives. ASDIS arises as an improved method that additionally employs a photocatalyst to accelerate the inactivation of waterborne pathogens in drinking water. Thus, its use is safer than SODIS, as it ensures a better disinfection. Besides, it may also degrade other organic pollutants present in the water.
ASDIS makes use of a novel photocatalyst (patent pending at the Spanish Patent and Trademark Office in Madrid, Application No. P202031314 with priority date 29 December 2020, and at the Ecuador’s National Service of Intellectual Rights in Quito, Application No. SENADI-2021-16937 with priority date 10 March 2021), which produces hydroxyl radicals (HO•) upon solar radiation. This radical is highly oxidizing and extremely reactive, being able to inactivate virtually all the pathogens present in the water, as well as degrade any other organic pollutants.
Dry clay and semiconductor powders in the desired mass ratio are mixed thoroughly to a homogeneous mixture. Then, water is added while gently stirring until reaching the adequate plasticity. This mixture is extruded to produce elongated rolls, which are fired at relatively low temperature.
These photocatalysts have been characterized by X-ray fluorescence and diffraction, thermogravimetric analysis, and scanning electron microscopy. Their specific surface area has been analyzed by N2 adsorption, the band gap measured by diffuse reflectance, and several other studies have been also performed.
Such an economic, easy to use and recover, and stable photocatalyst can be employed for household water disinfection, with the only aid of a plastic bottle and sunlight, by users from developing countries after receiving just a fast training.
ASDIS and SODIS methods have pretty similar operating modes. In both cases transparent water or soda bottles, preferably made of PET, are filled with water from contaminated sources. Highly turbid water (turbidity higher than 30 NTU) must be filtered prior to exposure to the sunlight. These filled bottles are placed horizontally on a roof or rack and then exposed to the sun. The treatment efficiency can be improved if the plastic bottles are placed on sunlight reflecting surfaces, such as corrugated aluminium or zinc roofs, and the bottles must not be placed so that they are in shade for part of the day.
SODIS requires the following exposition times:
After treatment, the water can be consumed. The risk of recontamination can be minimized if water is stored in the same bottles. Thus, water should be consumed directly from the bottle or poured into clean drinking cups.
These exposition times could be notably shortened by using ASDIS; but employing the same recommended periods, this method ensures a much better disinfection. The following graphs show the inactivation of four different bacteria by only solar radiation and with ASDIS photocatalyst, measured under the same conditions.
ASDIS achieves a 7-log reduction of four different bacteria (E. coli, E. faecalis, K. pneumoniae, S. aureus) in less than 45 minutes, revealing that it is much more effective than SODIS method for water disinfection. Furthermore, other non-pathogenic organisms, such as algae, may grow in the conditions created in a SODIS bottle, while ASDIS has been also proved to inactivate sunlight-resistant cyanobacteria, like Anabaena spp.
ASDIS is a green solar-driven method which uses discarded plastic bottles (preferably made of polyethylene terephthalate or PET) and a cheap and durable photocatalyst. The estimated cost of ASDIS photocatalyst is US $0.30-0.50 per bar, which can be reused countless times. Distribution, education, and community motivation can also add some expenses. Still, this corresponds to a very low cost per litre treated.
Given the water requirements of consumption and basic personal hygiene, a few water bottles treated with ASDIS method may not always meet the needs of households or even small communities. In this case, a large supply of intact, clean, suitable plastic bottles would be required. Unlike SODIS method, ASDIS can be easily scaled up, by designing appropriate bigger-size containers for water and the photocatalyst. Efforts are also being made in this direction.
The most favourable geographical regions for solar disinfection are located between latitudes 15° N and 35° N (as well as 15° S and 35° S). The majority of developing countries, as well as most of the countries with the largest number of refugees and displaced people, lie in or close to this global region (between latitudes 35° N and 35° S).
Benefits
Drawbacks
ASDIS transfers its technology free of charge to cooperation agencies, NGOs, foundations, international bodies, and other non-profit institutions, in order to contribute towards the fulfilment of the Sustainable Development Goal 6 proposed by the United Nations, Ensure availability and sustainable management of water and sanitation for all.
ASDIS employs a photocatalyst that produces highly oxidizing hydroxyl radicals. Besides shortening the disinfection process, these radicals may also degrade other chemicals present in the treated water. All tested pollutants (an aromatic compound, a dye, an ionic liquid and two pesticides are shown) were effectively degraded by 20 g/L of the photocatalyst under sunlight:
Since several clays and semiconductors can be used for manufacturing the composite, ASDIS photocatalyst may present different compositions. Following graphs show the removal of phenol:
Semiconductors
Clays and other materials
ASDIS can be used to disinfect and degrade organic pollutants in several aqueous matrices, including seawater:
Aqueous matrices
ASDIS photocatalytic bar can be reused countless times, without any appreciable activity loss:
Reusability
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