Ultraviolet (UV) disinfection

Alternative disinfection technologies include ultraviolet (UV) disinfection, ozone disinfection, silver for water disinfection and photo catalytic disinfection by nanomaterial. Ultraviolet (UV) light wavelength ranges between 100 nm to 400 nm (shorter than visible light but longer than X-rays) and this range is conventionally subdivided into four sub-regions, namely vacuum UV (from 100 to 200 nm), UV-C (from 200 to 280 nm), UV-B (from 280 to 315 nm), and UV-A (from 315 to 400 nm). UV has been used in treatment of water since the 1970s92 and is effective to disinfect water by killing microorganisms as it has the advantage that it does not produce chemical by-products that can affect health.93 UV light penetrates the microbe cell wall and damages the genetic information contained in DNA and RNA, thus stopping it from reproducing.

UV light is commonly produced in low-pressure mercury lamps, which emit in the 200–300 nm wavelength range and are introduced into specific UV reactors along the water flux. To properly design and operate a UV reactor, the designer must consider several operating parameters, optimizing the lamp-to-wall distance and radiation dosage depending on water quality. UV dosage, that is the ratio between total incident radiation intensity (averaged on all directions and all wavelengths) and exposure time, is a fundamental parameter.94 As the dosage is increased, its effect ranges from causing cell inactivation due to DNA/RNA damage up to causing cell wall damage due to protein absorption, with microorganism death.95 The effective surface area and distribution of the microbes is another important factor,96 as UV inactivation is significantly different for different microbes.97 Therefore, UV disinfection effectiveness is a process that is heavily dependent on the type of microorganisms, their concentration, and the quality of the water.95

Disinfection by UV light hinges on physical degradation on the microbes instead of the action of chemicals, starting from the seminal work in ref. 98 on the performance of UV irradiation for the treatment of Cryptosporidium and Giardia. This proved that UV treatment had higher efficiency than chemical processes99 investigated the degradation of protozoan cysts under UV radiation, which showed that UV radiation impairs DNA and RNA replication and transcription, inhibiting microbe or virus reproduction. Microbes and virus reproduction was inhibited independently of physio-chemical parameters such as temperature, pH and reactive organic matter, parameters, which contrary strongly affect chemical disinfection.100 The factors affecting disinfection efficacy have been studied by ref. 93 Here the physiological state of the microorganisms was correlated with the optical reflection, adsorption, and refraction of UV light through the water, as the lamp intensity was varied. It was shown that microbe sensitivity to UV is mainly related to the inactivation rate, as more sensitive organisms have higher rate constant and the effect of optical parameters such as reflection, refraction, and adsorption of UV light is lower.

Disinfection by UV light carries several advantages, being a faster method devoid of harmful or odorous by-products and volatile organic compounds (VOC) or toxic gas emission, and preserving water minerals, however it also has its limits. It is unsuitable for turbid water containing high percentage of solid suspended matter or soluble organic matter, as the light cannot penetrate in depth and disinfection is less efficient. It is also difficult to determine the performance of UV disinfection for changing water quality.101 A UV system requires regular inspections to determine residual microbe activity in the case of drinking water, increasing maintenance costs, as a periodic disinfection assessment is required by performing a Heterotrophic Plate Count (HPC) test.102

An interesting work have been done103 to study a new way of pre-treatment disinfection step prior to RO desalination by utilizing medium pressure ultraviolet (MP-UV) treatment. The study was conducted for four months at a brackish water reverse osmosis (BWRO) desalination plant. It was reported that MP-UV prolonged the performance between cleaning in the desalination plant also affecting the characteristics of the microorganisms and creatures on RO membranes asextracellular polymeric substances (EPS) were found to be significantly reduced. In another work,104 H 2 O 2 with MP-UV was found to reduce the amount of heterotrophic counts biofilm cells and EPS on the RO membranes105 studied the influence of suspended Nano-filler (TiO 2 ) as a photocatalyst and found that total organic carbon TOC increased probably due to larger number of organic by-products formed that was also corroborated in another work.106

Ozone disinfection

The triatomic form of gaseous oxygen is called ozone (O 3 ), and it has powerful oxidizing properties due to its strong oxidation-reduction potential (\({\rm E}_{\rm O}^{\rm H}\)) of 2.07 V107 as it undergoes the spontaneous transition back to oxygen, it forms a monoatomic oxygen radical that is extremely reactive, whereas having a short lifespan of a few milliseconds at ambient conditions. Ozone can be easily generated by feeding an electrical discharge across a flux of dry air and pure oxygen and then directing the gas into a down-flow contact chamber containing the contaminated water for disinfection. The initial ozone flux is quickly absorbed by the water-present matter and salts, and then the disinfection process happens as the ozone directly oxidizes the organic matter.108 Ozone also undergoes instant decomposition due to a complicated reaction resulting in the production of free hydroxyl radicals (OH•), which further contribute to increase the disinfection efficacy,109 though with a strong dependence on water type.

Ozone was first employed for water disinfection in 1886 by De Meritens, leading to its popularity as a replacement for chlorination, as ozonation does not produce trihalomethanes (TMH) and organochlorine,110 also leading to a better water disinfection.111 The ozonation process is also affected by parameters such as pH, temperature, and quality of water, though the pH effect was found to be negligible,112 whereas higher temperatures reduce water solubility and stability of ozone,113 and disinfection rate was shown to be temperature-independent.114

Ozone is also effective against protozoan cysts in water115 and to inactivate bacteria and viruses116 without having significant regrowth processes.117 The reactivity of ozone though, has corrosive effects, requiring-resistant materials such as high grade stainless steel, bringing the cost higher with respect to UV disinfection and chlorination. As it reacts with natural organic matter, ozone produces by-products like carboxylic acids, aldehydes, and ketoacids,118 which can be harmful in high concentrations. The presence of bromide ions brings about brominated by-products, which can result in brominated organ halogen compounds such as halobenzoquinones, which have been mentioned as one cause for bladder cancer.119

Silver for water disinfection

Silver (Ag) has been known to have antibacterial properties as Roman times and in the modern era it has been used extensively for water disinfections, including potable water.120 The remarkable antimicrobial property of silver is mainly attributed to its strong binding with disulfide (S–S) and sulfhydryl (–SH) groups found in the proteins of microbial cell walls, which disrupts the normal metabolic processes leading to cell death.121 Silver is considered as an alternative to the harmful chlorine-based water disinfection processes and used widely across the world, even extending its use up to the space shuttles.122 Application of nanoparticulate and ionic forms of silver are reported for water disinfection demonstrating their antimicrobial effect against many different types of microorganisms, including bacteria, viruses, and protozoa.117 Silver’s antimicrobial and antiviral mechanisms are summarized in a study carried out by ref. 121.

Mechanisms to remove bacteria includes: release of silver into the system;123 oxidative destruction catalyzed by silver;123 Targeting of Na+-translocating NADH: ubiquinone oxidoreductase (NQR) at low concentration of Ag+;124 Targeting of membrane proteins;124 Inhibition of oxidative metabolism required by the cells;125 Inhibition of uptake of nutrients;123 Metabolite leakage.123 Mechanisms to remove viruses includes: Site-specific Fenton mechanism;126 Immobilization of the virus to a surface;126 Inactivation of the nucleic acid within the viral capsid;126 Destruction of blockade of host-cell receptors.126 Finally, mechanisms to remove both bacteria and viruses are: Affinity for sulfhydryl groups,117,125 binding to DNA.126

Apart from drinking water, silver together with copper has been used in hospitals as an effective disinfectant, which does not produce toxic by-products.127 Silver is also effective against biofilm formation and used successfully in diverse applications for the prevention of biofouling.128

Nano photocatalytic disinfection

Photocatalysis has recently emerged as a promising avenue for disinfection of water, based on a nanostructured light-activated catalyst, which causes degradation of the organic and inorganic compounds and microorganisms that pollute the water. Photocatalysis can be described as “change in the rate of a chemical reaction or its initiation under the action of ultraviolet, visible, or infrared radiation in the presence of a substance—the photo catalyst—that absorbs the light and is involved in the chemical transformation of the reaction partners”.129 Photo catalysis is a complex process based on a five-step mechanism:130 (i) reactant diffusion, (ii) catalyst surface adsorption of reactants, (iii) catalyst surface reaction, (iv) catalyst surface by-products desorption, and (v) by-products diffusion.

The catalyst is typically based on metal oxide-semiconductor (MOS) nanostructures containing zinc oxide (ZnO), titania (TiO 2 ), tungsten oxide (WO 3 ), zinc stannate (Zn 2 SnO 4 ) etc. creating an interesting alternative for water disinfection because of their ability to degrade both chemical and biological pollutants.131 When hit by optical radiation, electron–hole (e–h) pairs are generated in the photocatalysts, which cause oxidation and reduction processes resulting in the formation of radicals with highly reactive properties, like super oxides (O 2 ●‒) and hydroxyl radicals (OH●), as shown in Fig. 6. The diffusion of these highly reactive radicals then causes the degradation and removal of organic/inorganic pollutants from the contaminated water, and results in the death of microbes as the radicals destroy their cell walls132,133 reported a possible course for photocatalytic degradation of organic pollutants (OP) in water. A FLV-MoS 2 disinfection schematic is proposed by ref. 134.

Fig. 6 Schematic representation depicting the photo catalysis process on the surface of a nanostructured metal oxide semiconductor (e.g. ZnO, TiO 2 etc.) photocatalyst Full size image

A nanostructured photocatalyst is more effective than bulk, as it offers a higher surface-to-volume ratio, which increases the density of photo-electrons just where the surface reactions happen. An effective photocatalyst of a wide semiconductor bandgap (bandgap between 2 and 4 eV) produces electron–hole pairs that have enough energy to kick off the secondary reactions, but have a low recombination probability of the generated charges in their migration to the surface. Ideally, an efficient photocatalyst should be: (i) highly photoactive, (ii) biologically and chemically inert, (iii) highly photo stable, (iv) non-toxic, and (v) cost-effective.135

Photocatalyst-based systems are in wide use for the disinfection of contaminated water, as harmful organic matter is broken down into harmless by-products, carbon dioxide, and water, being effective on such diverse substances as alcohols, carboxylic acids, phenolic derivatives, and chlorinated aromatic contaminants.135 Photo catalytic compounds such as ZnO, TiO 2 etc., are very effective to degrade organic dyes present in water.136 It has also been used to successfully degrade natural organic matters or humic substances.137 In this regard, in ref. 138 TiO 2 nanoparticles were used to photo catalytically degrade humic acids in potable water. Photocatalysis is also effective against inorganic contaminants, like halide ions, cyanide, thiocyanate, ammonia, nitrates, and nitrites.139 An antimicrobial effect is also the property of many photocatalysts, which inhibit the growth of microbes in water by diffusing highly reactive radicals that destroy the walls of the microbial cell. This process has proved to be effective against such microorganisms as Streptococcus mutans, Streptococcus natuss, Streptococcus cricetus, Escherichia coli, Scaccharomyces cerevisisas, Lactobacillus acidophilus etc.140 Antimicrobial effect of ZnO nanorods have been reported by ref. 141 showing almost 99% microbial removal from water, which was used to develop portable water purification system. Similarly,142 have reviewed the heterogeneous photocatalytic inactivation of water borne microorganisms, including plausible mechanisms of microbial degradation as well as a kinetic model of the inactivation process and analyzed various factors involved in the photo catalytic process, such as light intensity, pH and water quality. The photocatalytic properties of ZnO nano rods have also been investigated against marine micro and macro fouling organisms in laboratory,143 mesocosm144, and field experiments.145

Photocatalytic compounds such as TiO 2 , ZnO can be nanostructured at low cost and the results they have shown for water purification are promising, leading to prospective applications to disinfect water in homes, as well as in small and large industries. However, the bandgap of these semiconductors pushes their light absorption range in the UV light band, so they can only be activated by a high-energy UV source, whereas the power distribution of the solar spectrum is split between 46% visible light, 47% infrared radiation, and only 7% UV light. For this reason, a research effort is underway to discover photocatalysts that can use the sizable visible light power available from the sun. Wide bandgap semiconductor catalysts have been modified in the attempt to harvest visible light and disinfect water, using a variety of techniques: (i) Transition-metal semiconductor doping with manganese, copper, nickel, cobalt etc.;146 (ii) Non-metal doping with nitrogen, sulfur, boron, halogens etc.;147 (iii) Coupling with narrow bandgap semiconductors;148 (iv) Introduction of organic dyes and polymers sensitive to visible light on the nanostructured catalyst surface;104 (v) Introduction of defect states in the mid-bandgap region of the semiconductor;149 (vi) Addition on plasmonic metal nanoparticles for visible-range surface-plasmon resonance enhanced photo catalysis.150

Reference. 105 studied the influence as photocatalysis of suspended Nano-filler (TiO 2 ) and thin film mode, using solar energy by continuously re-circulated sea water. The research concluded that the total organic carbon (TOC) increased probably because of increasing organic by-products due to complexity of sea water. Suspended TiO 2 showed better degradation results than thin film mode.106 By using a tubular photo catalytic reactor in the presence of TiO 2 thin film coating mode, the same results as in ref. 105 were obtained, with an appreciable decrease in total inorganic carbon.

The free and abundant nature of solar radiation gives a big advantage to photocatalytic methods using sunlight, and is also well suited for outdoor applications, like wastewater treatment processes.