# Applications of Photocatalytic Disinfection

Authors: Joanne Gamage; Zisheng Zhang
Journal: International Journal of Photoenergy
 (2010)

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## Abstract

Due to the superior ability of photocatalysis to inactivate a wide range of harmful microorganisms, it is being examined as a viable alternative to traditional disinfection methods such as chlorination, which can produce harmful byproducts. Photocatalysis is a versatile and effective process that can be adapted for use in many applications for disinfection in both air and water matrices. Additionally, photocatalytic surfaces are being developed and tested for use in the context of “self-disinfecting” materials. Studies on the photocatalytic technique for disinfection demonstrate this process to have potential for widespread applications in indoor air and environmental health, biological, and medical applications, laboratory and hospital applications, pharmaceutical and food industry, plant protection applications, wastewater and effluents treatment, and drinking water disinfection. Studies on photocatalytic disinfection using a variety of techniques and test organisms are reviewed, with an emphasis on the end-use application of developed technologies and methods.


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## Body

## 1. Introduction

Applications of photocatalytic processes are widely recognized as viable solutions to environmental problems [1–3]. Disinfection of bacteria is of particular importance, because traditional methods such as chlorination are chemical intensive and have many associated disadvantages. For example, in water treatment applications, chlorine used for disinfection can react with organic material to generate chloro-organic compounds that are highly carcinogenic [4, 5]. Furthermore, some pathogens such as viruses, certain bacteria such as Legionella, and protozoans such as Cryptosporidium and Giardia lamblia cysts have been known to be resistant to chlorine disinfection [6, 7]. Other treatment alternatives such as ozonation and irradiation using germicidal lamps (254 nm) have their own problems and limitations, such as the lack of residual effect [8] and generation of small colony variants [9] for the latter and production of toxic disinfection byproducts for the former [10].In comparison, theTiO2 semiconductor commonly used in photocatalytic processes is nontoxic, chemically stable, available at a reasonable cost, and capable of repeated use without substantial loss of catalytic ability [11]. Heterogeneous photocatalysis using titanium dioxide is a safe, nonhazardous, and ecofriendly process which does not produce any harmful byproducts. Extensive research in this field has been done in the area of photocatalytic removal of organic, inorganic, and microbial pollutants [12, 13].The mechanism of bactericidal action ofTiO2 photocatalysis, as reported by Sunada et al. is attributed to the combination of cell membrane damage and further oxidative attack of internal cellular components, ultimately resulting in cell death [14].Since the breakthrough work of Matsunaga et al. in 1985 reporting the application ofTiO2 photocatalysis for the destruction of Lactobacillus acidophilus, Saccharomyces cerevisiae,and Escherichia coli using platinum-loaded TiO2 [15], there has been much interest in the biological applications of this process. A very comprehensive review of the application of TiO2 photocatalysis for disinfection of water is given by Mccullagh et al. [16], with many others available in the literature [17–21].Research in the field of photocatalytic disinfection has been very diverse, with theTiO2/UV process being shown to successfully inactivate many microorganisms including bacteria such as E. coli[22–24], L. acidophilus[15],Serratia domonas stutzeri[25], Bacillus pumilus[26], Streptococcus mutans[1],yeasts such as S. cerevisiae[15], algae such as Chlorella vulgaris[15], and viruses such as phage MS2 [15, 27, 28], B. fragilis bacteriophage[15, 27], Poliovirus I[28], Cryptosporidium parvum[29],and Giardia intestinalis[30].Research efforts are being made to improve the efficiency of theTiO2 catalyst by means of doping with various metals [31–33] and nonmetals [34, 35]. Other parameters which can be varied in a photocatalytic process, such as the source of ultraviolet irradiation [18] and factors affecting process efficiency [36] have also been under investigation. Additionally, there are countless reactor designs and configurations [37, 38] used to exploit photocatalytic disinfection for a wide range of applications, as this process can be used in both water and air matrices [39]. The current review will focus on developments in photocatalytic disinfection for application in the following contexts: indoor air and environmental health, biological and medical applications, laboratory and hospital applications, pharmaceutical and food industry, plant protection applications, wastewater and effluents treatment, and finally, drinking water disinfection.


## 2. Indoor Air and Environmental Health

The photocatalytic process is well recognized for the removal of organic pollutants in the gaseous phase such as volatile organic compounds (VOCs), having great potential applications to contaminant control in indoor environments such as residences, office buildings, factories, aircrafts, and spacecrafts [40, 41].To increase the scope of the photocatalytic process in application to indoor air, the disinfection capabilities of this technique are under investigation [39]. Disinfection is of importance in indoor air applications because of the risk of exposure to harmful airborne contaminants. Bioaerosols are a major contributor to indoor air pollution, and more than 60 bacteria, viruses, and fungi are documented as infectious airborne pathogens. Diseases transmitted via bioaerosols include tuberculosis, Legionaries, influenza, colds, mumps, measles, rubella, small pox, aspergillosis, pneumonia, meningitis, diphtheria, and scarlet fever [42]. Traditional technologies to clean indoor air include the use of activated charcoal filters, HEPA filters, ozonation, air ionization, and bioguard filters. None of these technologies is completely effective [20].In the pioneering work by Goswami et al. [43, 44] investigating the disinfection of indoor air by photocatalysis, a recirculating duct facility was developed to inactivate biological contaminants in air with photocatalytic techniques. Experiments using Serratia Marcescens in air achieved a 100% destruction of microorganisms in a recirculating loop in 600 minutes [43]. This time was reduced to less than 3 minutes in later experiments [45].Photocatalytic oxidation can also inactivate infectious microorganisms which can be airborne bioterrorism weapons, such asBacillus anthracis (Anthrax) [46–48]. A photocatalytic system was investigated by Knight in 2003 to reduce the spread of severe acute respiratory syndrome (SARS) on flights [49], following the outbreak of the disease. Similarly, in 2007 the avian influenza virus A/H5N2 was shown to be inactivated from the gaseous phase using a photocatalytic prototype system [39].Inactivation of various gram-positive and gram-negative bacteria using visible light and a doped catalyst [50] and fluorescent light irradiation similar to that used in indoor environments was studied [51] and shows great promise for widespread applications.It was also shown thatE. coli could be completely mineralized on a TiO2 coated surface in air [42]. Carbon mass balance and kinetic data for complete oxidation of E. coli, A. niger, Micrococcus luteus,and B. subtillus cells and spores were subsequently presented [52]. A comprehensive mechanism and detailed description of the TiO2 photokilling of E. coli on coated surfaces in air has been extensively studied in order understand to a considerable degree and in a quantitative way the kinetics of E. coli immobilization and abatement using photocatalysis, using FTIR, AFM, and CFU as a function of time and peroxidation of the membrane cell walls [53–57].Novel photoreactors and photo-assisted catalytic systems for air disinfection applications such as those using polyester supports for the catalyst [58], carbon nanotubes [59], combination with other disinfection systems [60], membrane systems [61], use of silver bactericidal agents in cotton textiles [62–64] for the abatement of E. coli in air, high surface area CuO catalysts [65], and structure silica surfaces [66]  have also been reported.In terms of environmental health, the antifungal capability ofTiO2 photocatalysis against mold fungi on coated wood boards used in buildings was confirmed [67] using A. nigeras a test microbe, and UVA irradiation.


## 3. Biological and Medical Applications

Due to the disinfection abilities of photocatalytic processes, they are being explored for use in medical applications. Studies have been done usingTiO2 coatings on bioimplants to implement photocatalysis for antibacterial purposes [47, 68, 69]. Shiraishi et al. explored the photocatalytic activity of S. aureus, a common pathogenic bacterium in implant-related infection, using TiO2 film on stainless steel and titanium substrates [70]. The bactericidal effect of the coating was confirmed upon UV irradiation, and the use of these coated photocatalytic substrates present a useful strategy for the control of such infections associated with biomedical implants.Photocatalysis is also able to kill animal cells, such as in the antitumor activity shown using subcutaneous titania injection onto skin tumours followed by 40 minutes of UV illumination [71]. This procedure produced a tenfold tumour volume reduction after three weeks, where the catalyst and light alone control runs showed tumor increases in volume by factors of 30–50. The use of photocatalysis for cancer cell treatment has also been documented elsewhere [1, 72].As previously alluded to in air-disinfection strategies, photocatalysis can be employed to remove harmful airborne biological threats such as Anthrax [48, 73]. In this sense, it can be an effective technique for combating bioterror and preventing the spread of airborne biological threats.


## 4. Laboratory and Hospital Applications

Particularly in microbiological laboratories and in areas in intensive medical use, frequent and thorough disinfection of surfaces is needed in order to reduce the concentration of bacteria and to prevent bacterial transmission. Conventional methods of disinfection with wiping are not long-term effective, and are staff and time intensive. These methods also involve the use of harsh and aggressive chemicals. Disinfection with hard ultraviolet light (UVC) is usually unsatisfactory, since the depth of penetration is inadequate and there are occupational health risks [74].Photocatalytic oxidation on surfaces coated with titanium dioxide offers an alternative to traditional methods of surface disinfection. Research has examined the biocidal activity of thin films of titania anchored to solid surfaces [74–76]. The effectiveness of this process was demonstrated using bacteria relevant to hygiene such as E. coli, p. aeruginosa, S. aureus,and E. faecium[74]. The inactivation of E. coli (ATCC8739) cells deposited on membrane filters during irradiation with fluorescent light was also shown as an application of self-disinfecting surfaces [77].TiO2 thin films deposited on stainless steel using a novel flame-assisted CVD technique were also tested for antimicrobial activity on E. coli[69]. There is a wider range of applications for this self-disinfecting material because of the desirable mechanical properties and resistance to corrosion of stainless steel. Transparent TiO2 films on this substrate have also been shown to be effective for sterilization of B. pumilus [78]. In this study, the TiO2-coated stainless steel was shown to have a higher photocatalytic activity than the same coating on glass substrates.Titania photocatalysts doped with CuO were coated on surfaces and evaluated for biocidal activity [79]. This investigation also explored the synergistic effect of photocatalysis and toxicity of copper to inactivate bacteriophage T4 and E. coli.Enhanced photocatalysis using nitrogen-dopedTiO2 was also reported for its visible light-induced bactericidal activity against human pathogens [80]. It was proposed in this study that photocatalytic disinfection using visible light can offer a means of continuous disinfection for surfaces constantly in contact with humans, such as door handles and push buttons. Visible light-induced inactivation of E. coliwas also studied using titania codoped with nitrogen and sulfur [81–84]. This introduces new disinfectant opportunities in public environments, such as public toilets, schools, hospitals, stations, airports, hotels, or public transportation, which are ideal places for the transmission of pathogens [85, 86].Photocatalysis has also been investigated for the inactivation of prions, the infectious agents of a family of transmissible, fatal, neurodegenerative disorders affecting both humans and animals [87]. These prions may be transmitted via ingestion of contaminated food or during medical treatments with contaminated biological materials or surgical tools. The effectiveness of photocatalytic oxidation for inactivating these prions can help to reduce the risk of spread and demonstrates the practical applications of this technology for disinfection of contaminated surfaces and inanimate objects.Another application of photocatalysis in a hospital setting is for the control of Legionnaire’s disease, which is associated to hot water distribution systems containing bacteria of theLegionella species [88]. In laboratory scale studies, it was shown that photocatalytic oxidation using TiO2/UV was able to mineralize the cells of four strains of L. pneumophiliaserogroup 1 (strain 977, strain 1009, strain 1004, and ATCC 33153) upon prolonged treatment. This implies that the process used might be a viable alternative to the traditional disinfection processes used for the control of Legionellabacteria in hospital hot water systems, such as thermal eradication and hyperchlorination [89].


## 5. Pharmaceutical and Food Industry

Due to the antibacterial applications ofTiO2-mediated photooxidation, this process shows promise for the elimination of microorganisms in areas where the use of chemical cleaning agents or biocides is ineffective or is restricted by regulations, for example in the pharmaceutical and food industries [33]. TiO2 is nontoxic and has been approved by the American Food and Drug Administration for use in human food, drugs, cosmetics, and food contact materials [90].TiO2 powder-coated packaging film was shown to inactivate E. coli (ATCC 11775) in vitrowhen irradiated with UVA light [90]. Actual tests on cut lettuce stored in a TiO2-coated film bag under such irradiation also showed this method to be effective for the reduction of E. coli colonies, indicating that the TiO2 coated film could reduce microbial contamination on the surfaces of solid food products and hence reduce the risk of microbial growth in food packaging. TiO2 photocatalysis has also shown to be effective for the inactivation of other foodborne bacteria such as Salmonella chloraesuissubsp., Vibrio parahaemolyticus,and Listeria monocytogenes[69].Surface disinfection is also of importance to food processing, as foodborne infections can be caused by the proliferation and resistance to cleaning procedures of pathogenic germs on surfaces of the production equipment in such industries. Studies withE. colistrains (PHL 1273) [91] synthesizing curli, a type of appendage that allows the bacteria to stick to surfaces and form biofilms, were able to inactivate this organism using titania and various types of UV irradiation. In dark events studies, following the bacterial inactivation, no bacterial cultivability was recovered after 48 hours, indicating that the durability of the disinfection was adequate. Nitrogen doping of the titania photocatalyst was also reported in a separate study [92] with the use of visible light to inactivate E. coli and biofilm bacteria. Disinfection of E. coli using TiO2-containing paper and UV fluorescent irradiation has also been shown [93].


## 6. Plant Protection Applications

Photocatalytic disinfection is potentially very important in the control and inactivation of pathogenic species present in the nutritive solution in circulating hydroponic agricultures [94]. Many plant pathogens can be transmitted by irrigation and recycled waters used in hydroponic agriculture. Conventional bactericidal methods often apply chemical pesticides to disinfect these pathogens, but these are often harmful to animals, humans, and the environment due to their residual toxicity [95]. Photocatalytic disinfection of these plant pathogens using TiO2 may be used as a new tool for plant protection and an alternative to the use of harsh chemicals.UsingTiO2 thin film on a glass substrate and UVA irradiation, Enterobacter cloacae SM1 and Erwinia carotovorasubsp. CaratovoraZL1, phytopathogenic enterobacteria that cause basal rot and soft rot in a variety of vegetable crops, were efficiently inactivated [95]. Subsequent studies investigated the effects of doping the titania catalyst with various photosensitive dyes using visible light irradiation [96]. It was shown that the disinfection of the phytopathogenic bacteria causing basal and soft rot could be efficiently carried out under visible light using these doped catalysts.Solar photocatalytic disinfection using batch process reactors and titania photocatalysts was also shown to be effective for the disinfection of five wild strains of theFusariumgenus (F. equiseti, F. oxysporum, F. anthophilum, F. verticilloides,and F. solani), a common plant pathogen [97]. In this case, natural solar radiation was used and the photocatalytic solar disinfection was compared to solar-only disinfection for these fungi. The photocatalytic process was found to be faster than the solar-only disinfection in all trials.The disinfecting ability of titania photocatalyst films was also tested on pathogens of mushroom diseases:Trichoderma harzianum, Cladobotryum varium, Spicellum roseum,and P. tolaasii.The disinfection of these species was confirmed by experiments conducted in mushroom growing rooms under black light irradiation, and subsequently, white light irradiation [98].


## 7. Wastewater and Effluents

The use of photocatalysis for water and wastewater treatment is a topic well documented in the literature, especially with respect to solar photocatalysis [17–21, 99–102]. Due to the ability of photocatalysis to mineralize many organic pollutants, it has been used for remediation of contaminated groundwaters through the use of parabolic solar concentrating type reactors. Photocatalysis has been used in engineering scale for solar photocatalytic treatment of industrial nonbiodegradable persistent chlorinated water contaminants [21], and in field scale for treatment of effluents from a resins factory [103]. This process has also shown to be effective for treatment of wastewaters from a 5-fluororacil (a cancer drug) manufacturing plant [104], distillery wastewater [105], pulp and paper mill wastewater [106], dyehouse wastewater [17], and oilfield produced water [35].However, the disinfection capabilities of photocatalytic processes have not thoroughly been exploited for treatment of wastewaters. Wastewater reclamation and reuse is of growing importance, especially in areas where the freshwater supply is limited, and so effective disinfection of wastewaters is necessary. Any technical means of sewage reuse is limited by persistent organic pollutants and microorganisms which are not removed by the conventional mechanical and biological treatment train [107]. Additional treatment is therefore necessary before any reuse can take place.Early work on photocatalytic disinfection of municipal secondary wastewater effluents showed an inactivation of coliform bacteria andPoliovirus I using suspensions of titanium dioxide and fluorescent and sunlight irradiation, respectively [28].Photocatalysis is also useful for disinfection of sewage containing organisms which are highly resistant to traditional disinfection methods, such asCryptosporidium parvum[108] and noroviruses [109].Municipal wastewater effluents from a sewage disposal plant in Hannover, Germany were treated in a slurryTiO2  reactor under UVA irradiation to simultaneously detoxify and disinfect the samples [110]. The photocatalytic treatment was able to diminish the concentration of dissolved organic pollutants (indicated by TOC and COD), and as well inactivate pathogenic microorganisms (indicated by E. coli). A similar result was obtained from studies monitoring Faecal streptococci and total coliforms using slurry TiO2 systems with UVA lamps and solar irradiation, respectively [111].The investigation of bacterial consortia ofE. coli and Enterococcus species present in real wastewaters from a biological wastewater treatment plant in Lausanne (Switzerland) [112] indicated that the Enterococcus species are less sensitive to photocatalytic treatment than coliforms and other gram-negative bacteria. Additionally, the effects of temperature, turbidity, and various other physical parameters of the samples on the photocatalytic inactivation of E. coli were investigated [113].Further research investigates enhanced photocatalysis to improve the efficiency of disinfection of wastewaters for reuse, for example, by the use of titania-activated carbon catalyst mixtures [114], and through the development of nanocrystalline photocatalytic TiO2 membranes [115]. The latter is of particular importance in aeronautical applications, as it combines membrane separation technologies with advanced oxidation technologies to create photocatalytic composite membranes designed for the treatment and reuse of water on long-duration space missions [116].An inexpensive approach to synthesizing a novel nitrogen-dopedTiO2 photocatalyst has also been developed [117], improving the efficiency of visible light-induced disinfection of wastewaters, and introducing a new generation of catalysts for this application.


## 8. Drinking Water Disinfection

Titania photocatalysis has been proven to be effective in the removal of chemical compounds and microbiological pathogens from water. A thorough review by Mccullagh et al. [16] of the application of photocatalysis for the removal of biological species in this context examines studies on the disinfection effect of TiO2 suspensions, effect of additives and pH, respectively, on the photocatalytic abilities and disinfection effect of TiO2 thin films, and the effect of electrochemically applied potential on the photobactericidal effect of TiO2 thin films. The current discussion will focus on the various applications of photocatalytic drinking water disinfection.


### 8.1. Drinking Water Production in Developing Countries

In 2004, it was estimated that about 15% of the world’s population, mostly living in the less-favored regions of the planet, did not have access to enough fresh water to satisfy their daily needs, and this number was expected to double by 2015 [118]. This represents a serious public health issue since waterborne, water-washed, and water-based diseases are associated with lack of improvement in domestic water supply and adequate sanitation [119]. Development of low cost-effective methods for removal of pollutants from water supplies can help alleviate this problem. Especially in rural communities, water disinfection must have sufficiently low operational costs. Alternative technologies to traditional chlorination are now being considered for household use [120].Solar disinfection (SODIS) is a simple technology that is capable of inactivating many waterborne pathogenic bacteria using the combined effect of solar UVA radiation and temperature [121–124]. This method is low cost and does not produce toxic byproducts, however, limits the volume of water subject to treatment (typically 2L per exposed water bottle) and has a disadvantageous long time of process (typically 2 day exposure for complete inactivation) [119].The combination of sunlight and photocatalyst is a promising option for water treatment in areas with insufficient infrastructure but high yearly sunshine. The use of compound parabolic reactors as an efficient technology to collect and focus diffuse and direct solar radiation onto a transparent pipe containing contaminated water has demonstrated feasibility to disinfect water usingTiO2 suspensions [125–127].The European Union International Cooperation program (INCO) has sponsored initiatives for developing a solar photocatalysis-based cost-effective technology for water decontamination and disinfection in rural areas of developing countries, the SOLWATER and AQUACAT projects, respectively [94]. These projects are aimed at developing a solar reactor to decontaminate and disinfect small volumes of water, and field tests with the final prototypes were carried out to validate operation under real conditions [127].The final SOLWATER prototype was composed of two tubes containing Alstrohm paper impregnated with titanium dioxide, and two tubes containing a supported photosensitizer [94]. These tubes are placed on a compound parabolic concentrating collector and run in series, where the electricity is provided by a solar panel (Figure 1).Figure 1
Final SOLWATER and AQUACAT water (solar photocatalytic) disinfection system installed at Ecole Superieure de Technologie de Fez, Morroco [94].Field tests using the SOLWATER prototype placed the reactor in the yard of a shanty house in Los Pereya, Tucuman, Argentina and studied the removal of bacterial contamination during three months of testing using natural water contaminated with coliforms,E. Faecalis, and P. aeruginosa, as well as high levels of natural organic matter and variable inorganic pollutants [127]. The SOLWATER reactor was shown to be effective for this application. Similar tests have been performed in photoreactors installed in various geographic regions, including Egypt, France, Greece, Mexico, Morocco, Peru, Spain, Switzerland, and Tunisia [94].Other research in the field of potable water production in developing countries includes the development of affordable and efficient technology in the form of batch borosilicate glass and PET plastic SODIS reactors fitter with flexible plastic inserts coated withTiO2 powder [128]. These were shown to be 20 and 25% more effective, respectively, than SODIS alone for the inactivation of E. coli K12. This novel system was also able to reduce the concentration of Cryptosporidium parvum oocysts present [129]. It should be also noted that there has also been significant research done in the advance of solar disinfection of this highly resistant organism using SODIS alone [123, 130, 131].


### 8.2. Surface Water Treatment

While the majority of photocatalytic disinfection studies reported are carried out with distilled water or buffer solutions [16], there have been attempts to quantify the effects of the chemical constituents of natural surface waters on TiO2 photocatalysis [132, 133]. It has been shown, using surface water samples, that the presence of inorganic ions and humic acids decrease the photocatalytic disinfection rate of E. coli[133].Other efforts have been made to evaluate photocatalysis applications using real waters [134–138]. For example, the integration of TiO2 photocatalysis into traditional water treatment processes for the removal of organic matter, which has variable levels during the year, was studied in the UK using three surface water samples [136].Natural water samples from the Cauca River in Cali, Columbia showed drasticE. coli culturable cell concentration increase 24 hours after stopping irradiation [135]. This was not observed for the control experiment using an E. coli suspension in distilled water. It was concluded that caution should be taken when making predictions based on simple models as they are not necessarily representative of natural crude water samples.The effect of pH, inorganic ions, organic matter, andH2O2 on E. coli photocatalytic inactivation by TiO2 was studied by simulating natural and environmental conditions of these parameters using distilled and tap water samples [132]. The results of this study and others [133] confirmed that laboratory results using ultrapure water samples are not representative of the real application in natural waters.In studies done on surface water samples by Ireland et al. [134], it was concluded that inorganic-radical scavengers can have a major negative impact on the efficacy of the photocatalytic process, and the presence of organic matter in the water samples also degrades the E. coliinactivation kinetics.Using a field-scale compound parabolic collector at the Swiss Federal Institute of Technology (EPFL), in Lausanne, natural water from the Leman Lake was used to suspendE. coli in the presence of TiO2 and irradiation under solar conditions [126]. From studies on the postirradiation period, the effective disinfection time (EDT) was defined as the time necessary to avoid bacterial regrowth after 24 h (or 48 h) in the dark after stopping phototreatment. It was suggested that the EDT necessary be used as an indicator of the impact of the solar photocatalytic process on bacteria instead of the UV dose required to achieve a certain level of disinfection.


### 8.3. Eutrophic Water Treatment

Another application of photocatalytic disinfection is in the treatment of eutrophic water. Control of algal blooms in eutrophic water is important because toxic cyanobacterial blooms in drinking water supplies may cause human health problems [137]. Copper-based algaecides can be used to control these blooms, however this method introduces secondary environmental problems [138].Photocatalytic inactivation of three species of algae:Anabaena, microcystis,and Melosira, was studied using TiO2 coated glass beads and UV-light irradiation [138]. Complete photocatalytic inactivation of Anabaena, microcystis, and Melosira was obtained in about 30 minutes, while the inactivation efficiency for Melosira was somewhat lower due to the inorganic siliceous wall surrounding the cells.The floatingTiO2-coated hollow glass beads were introduced into a mesocosm installed at the Nakdong River, Kimhae, Korea [138]. This mesocosm was a 25 m2 and 2 m deep semipermeable membrane. The concentrations of chlorophyll-a were measured for one month, and it was shown that more than 50% of the chlorophyll-a concentration could be reduced using photocatalysts and natural solar radiation. A picture of the experimental mesocosm is depicted in Figure 2.Figure 2
Experimental mesocosm used in Nakdong River, Korea [138].


### 8.4. Groundwater Treatment

The ability of photocatalysis to break down and detoxify harmful organic chemicals has been exploited for groundwater treatment, as shown by engineering scale demonstrations using solar photocatalysis to remediate groundwater contaminated from leaking underground storage tanks [139].The disinfecting abilities of photocatalytic processes for application to treating groundwater contaminated with microorganisms such asF. Solani[140] was also investigated and shown to be effective for the removal of such microorganisms. Natural well water containing the F. Solani species and solar illumination and employing CPCs was also explored as a process configuration for this application [141].


## 8.1. Drinking Water Production in Developing Countries

In 2004, it was estimated that about 15% of the world’s population, mostly living in the less-favored regions of the planet, did not have access to enough fresh water to satisfy their daily needs, and this number was expected to double by 2015 [118]. This represents a serious public health issue since waterborne, water-washed, and water-based diseases are associated with lack of improvement in domestic water supply and adequate sanitation [119]. Development of low cost-effective methods for removal of pollutants from water supplies can help alleviate this problem. Especially in rural communities, water disinfection must have sufficiently low operational costs. Alternative technologies to traditional chlorination are now being considered for household use [120].Solar disinfection (SODIS) is a simple technology that is capable of inactivating many waterborne pathogenic bacteria using the combined effect of solar UVA radiation and temperature [121–124]. This method is low cost and does not produce toxic byproducts, however, limits the volume of water subject to treatment (typically 2L per exposed water bottle) and has a disadvantageous long time of process (typically 2 day exposure for complete inactivation) [119].The combination of sunlight and photocatalyst is a promising option for water treatment in areas with insufficient infrastructure but high yearly sunshine. The use of compound parabolic reactors as an efficient technology to collect and focus diffuse and direct solar radiation onto a transparent pipe containing contaminated water has demonstrated feasibility to disinfect water usingTiO2 suspensions [125–127].The European Union International Cooperation program (INCO) has sponsored initiatives for developing a solar photocatalysis-based cost-effective technology for water decontamination and disinfection in rural areas of developing countries, the SOLWATER and AQUACAT projects, respectively [94]. These projects are aimed at developing a solar reactor to decontaminate and disinfect small volumes of water, and field tests with the final prototypes were carried out to validate operation under real conditions [127].The final SOLWATER prototype was composed of two tubes containing Alstrohm paper impregnated with titanium dioxide, and two tubes containing a supported photosensitizer [94]. These tubes are placed on a compound parabolic concentrating collector and run in series, where the electricity is provided by a solar panel (Figure 1).Figure 1
Final SOLWATER and AQUACAT water (solar photocatalytic) disinfection system installed at Ecole Superieure de Technologie de Fez, Morroco [94].Field tests using the SOLWATER prototype placed the reactor in the yard of a shanty house in Los Pereya, Tucuman, Argentina and studied the removal of bacterial contamination during three months of testing using natural water contaminated with coliforms,E. Faecalis, and P. aeruginosa, as well as high levels of natural organic matter and variable inorganic pollutants [127]. The SOLWATER reactor was shown to be effective for this application. Similar tests have been performed in photoreactors installed in various geographic regions, including Egypt, France, Greece, Mexico, Morocco, Peru, Spain, Switzerland, and Tunisia [94].Other research in the field of potable water production in developing countries includes the development of affordable and efficient technology in the form of batch borosilicate glass and PET plastic SODIS reactors fitter with flexible plastic inserts coated withTiO2 powder [128]. These were shown to be 20 and 25% more effective, respectively, than SODIS alone for the inactivation of E. coli K12. This novel system was also able to reduce the concentration of Cryptosporidium parvum oocysts present [129]. It should be also noted that there has also been significant research done in the advance of solar disinfection of this highly resistant organism using SODIS alone [123, 130, 131].


## 8.2. Surface Water Treatment

While the majority of photocatalytic disinfection studies reported are carried out with distilled water or buffer solutions [16], there have been attempts to quantify the effects of the chemical constituents of natural surface waters on TiO2 photocatalysis [132, 133]. It has been shown, using surface water samples, that the presence of inorganic ions and humic acids decrease the photocatalytic disinfection rate of E. coli[133].Other efforts have been made to evaluate photocatalysis applications using real waters [134–138]. For example, the integration of TiO2 photocatalysis into traditional water treatment processes for the removal of organic matter, which has variable levels during the year, was studied in the UK using three surface water samples [136].Natural water samples from the Cauca River in Cali, Columbia showed drasticE. coli culturable cell concentration increase 24 hours after stopping irradiation [135]. This was not observed for the control experiment using an E. coli suspension in distilled water. It was concluded that caution should be taken when making predictions based on simple models as they are not necessarily representative of natural crude water samples.The effect of pH, inorganic ions, organic matter, andH2O2 on E. coli photocatalytic inactivation by TiO2 was studied by simulating natural and environmental conditions of these parameters using distilled and tap water samples [132]. The results of this study and others [133] confirmed that laboratory results using ultrapure water samples are not representative of the real application in natural waters.In studies done on surface water samples by Ireland et al. [134], it was concluded that inorganic-radical scavengers can have a major negative impact on the efficacy of the photocatalytic process, and the presence of organic matter in the water samples also degrades the E. coliinactivation kinetics.Using a field-scale compound parabolic collector at the Swiss Federal Institute of Technology (EPFL), in Lausanne, natural water from the Leman Lake was used to suspendE. coli in the presence of TiO2 and irradiation under solar conditions [126]. From studies on the postirradiation period, the effective disinfection time (EDT) was defined as the time necessary to avoid bacterial regrowth after 24 h (or 48 h) in the dark after stopping phototreatment. It was suggested that the EDT necessary be used as an indicator of the impact of the solar photocatalytic process on bacteria instead of the UV dose required to achieve a certain level of disinfection.


## 8.3. Eutrophic Water Treatment

Another application of photocatalytic disinfection is in the treatment of eutrophic water. Control of algal blooms in eutrophic water is important because toxic cyanobacterial blooms in drinking water supplies may cause human health problems [137]. Copper-based algaecides can be used to control these blooms, however this method introduces secondary environmental problems [138].Photocatalytic inactivation of three species of algae:Anabaena, microcystis,and Melosira, was studied using TiO2 coated glass beads and UV-light irradiation [138]. Complete photocatalytic inactivation of Anabaena, microcystis, and Melosira was obtained in about 30 minutes, while the inactivation efficiency for Melosira was somewhat lower due to the inorganic siliceous wall surrounding the cells.The floatingTiO2-coated hollow glass beads were introduced into a mesocosm installed at the Nakdong River, Kimhae, Korea [138]. This mesocosm was a 25 m2 and 2 m deep semipermeable membrane. The concentrations of chlorophyll-a were measured for one month, and it was shown that more than 50% of the chlorophyll-a concentration could be reduced using photocatalysts and natural solar radiation. A picture of the experimental mesocosm is depicted in Figure 2.Figure 2
Experimental mesocosm used in Nakdong River, Korea [138].


## 8.4. Groundwater Treatment

The ability of photocatalysis to break down and detoxify harmful organic chemicals has been exploited for groundwater treatment, as shown by engineering scale demonstrations using solar photocatalysis to remediate groundwater contaminated from leaking underground storage tanks [139].The disinfecting abilities of photocatalytic processes for application to treating groundwater contaminated with microorganisms such asF. Solani[140] was also investigated and shown to be effective for the removal of such microorganisms. Natural well water containing the F. Solani species and solar illumination and employing CPCs was also explored as a process configuration for this application [141].


## 9. Conclusion

The photocatalytic technique is a versatile and efficient disinfection process capable of inactivating a wide range of harmful microorganisms in various media. It is a safe, nontoxic, and relatively inexpensive disinfection method whose adaptability allows it to be used for many purposes. Research in the field of photocatalytic disinfection is very diverse, covering a broad range of applications.Particularly, the use of photocatalysis was shown to be effective for various air-cleaning applications to inactivate harmful airborne microbial pathogens, or to combat airborne bioterror threats, such as Anthrax. Photocatalytic thin films on various substrates were also shown to have potential application for “self-disinfecting” surfaces and materials, which can be used for medical implants, “self-disinfecting” surgical tools and surfaces in laboratory and hospital settings, and equipment in the pharmaceutical and food industries. Photocatalytic food packaging was shown to be a potential way to reduce the risk of foodborne illnesses in cut lettuce and other packaged foods. In terms of plant protection, photocatalysis is being investigated for use in hydroponic agricultures as an alternative to harsh pesticides. For water treatment applications, photocatalytic disinfection has been studied and implemented for drinking water production using novel reactors and solar irradiation. Eutrophic waters containing algal blooms were also shown to be effectively treated usingTiO2-coated hollow beads and solar irradiation.The effectiveness of photocatalytic disinfection for inactivating microorganisms of concern for each of these applications was presented, highlighting key studies and research efforts conducted. While the performance of this technology is still to be optimized for the specific applications, based on the literature presented, it is abundantly evident that photocatalysis should be considered as a viable alternative to traditional disinfection methods in some cases.In a move towards a more environmentally friendly world, traditional solutions to classic problems, such as the production of safe drinking water, must shift towards more sustainable alternatives. Photocatalytic disinfection is not only a replacement technology for traditional methods in traditional applications, but also a novel approach for solving other disinfection problems, such as the control of bioterror threats. In this sense, the strength of photocatalytic disinfection lies in its versatility for use in many different applications.



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*Source: 764870-2010-10-11.xml*