Method

Description

Thermal enhancement

Since there is a high synergy between optical and thermal inactivation at temperatures above 45˚C, many strategies are developed to improve thermal inactivation [1] . In general, the thermal enhancement solutions of the SODIS reactor are [70] : blackening the bottle, circulating water on a black surface as a solar energy absorber, and using a collector and solar reflector. Blackened bottles have the lowest efficiency in converting solar energy into heat [1] . Unlike blackened surfaces, reflectors reflect UVA rays even on cloudy days. Thus, even on cloudy days, optical inactivation and an increase in water temperature for disinfection occur [70] . Using solar mirrors as reflectors reduces disinfection time to 3 - 4 h [82] . Combining the SODIS process with concentrating parabolic collectors in less than 6 h results in the complete disinfection of water [83] [84] .

Photocatalysis

Photocatalysis is one of the most effective technologies for mineralizing resistant organic compounds and inactivating water pathogens among advanced oxidation processes (AOPs) [85] [86] . Photocatalysts are divided into two categories: heterogeneous (semiconductor catalysts for water treatment) and homogeneous (photo-Fenton process) [87] . In the heterogeneous photocatalysis process, stubborn organic matter is degraded by the combined action of a semiconductor photocatalyst, an energy source, and high ROSs [88] . Among semiconductor photocatalysts, TiO₂ is one of the most widely used photocatalysts in water treatment applications [70] . For example, solar photocatalytic processes with TiO₂ photocatalysts are used to disinfect water, in which UVA-resistant microorganisms are inactivated by the TiO₂ photocatalyst [89] [90] .

The photo-Fenton process is the most familiar homogeneous solar photocatalytic process in water treatment [91] . The Fenton oxidation process is an AOP process, which produces ^•OH radicals by the catalytic reaction of H₂O₂ with iron ions (see Figure 15). The photo-Fenton process (Fe²⁺/H₂O₂/UV vis) has a higher oxidation rate, lower iron consumption, and less sludge production than the Fenton reaction [47] [92] . In this process, owing to light radiation in the range of near UV to visible and up to 600 nm wavelength, free radicals are formed and cause water disinfection [93] . The non-selectivity of this process has made it possible to remove a wide range of antimicrobial-resistant microorganisms [94] [95] . Parameters affecting the efficiency of this process are pH, temperature, the concentration of hydrogen peroxide (H₂O₂), iron and their ratio, and the intensity and wavelength of radiant light [96] . The type of light source has been shown to affect the disinfection of hydrogen peroxide significantly. As a result, the higher intensity of light radiation leads to a higher disinfection rate [1] [97] .