*2.7.2. Limitations*

20% intensity in the first 100 h of operation. Properly calibrated UV detectors help the owner

The UV-treated water must be monitored regularly for the presence of heterotrophic bacteria and coliform bacteria monthly (first 6 months of device's use). The lamp's intensity should be

UV light can treat water without producing any major chemical or physical changes in the water. No negative effects have been noticed in utilizing UV-treated water. Fewer chances are there for the formation of DBPs as no new substance is added in this process. No change in the taste and color occurs. The dosage and frequency used for the disinfection do not produce any harmful substance. Even the overdosing of UV light does not lead to the formation of harmful products. To avoid exposure, protective clothing should be used by the operator [7]

The acceleration of a photoreaction in the presence of a catalyst is referred as photocatalysis. In catalyzed photolysis, adsorbed substrate is used to absorb light. In photogenerated catalysis, electron–hole pairs are created by the photocatalytic activity (PCA) generating free radicals (e.g., hydroxyl radicals: •OH) that have the ability to undergo secondary reactions. Its practical application was made possible by the discovering the electrolysis of water by

• Photocatalysis uses capacity for renewable and pollution-free solar energy, thus it is a good

in alerting when the light intensity falls below a certain level.

The followings are the advantages of photocatalytic disinfection

replacement for the energy-intensive conventional treatment methods.

checked if such organisms are noticed [16].

*2.6.5. Chemical*

**Figure 5.** Disinfection by UV.

14 Photocatalysts - Applications and Attributes

(**Figure 5**).

**2.7. Photocatalytic disinfection**

using of titanium dioxide.

*2.7.1. Advantages*

For the effective TiO<sup>2</sup> application in water treatment, the mass transfer limitation has to be minimized since photocatalytic degradation mainly occurs on the surface of TiO<sup>2</sup> . TiO<sup>2</sup> has poor affinity toward organic pollutants (more specifically the hydrophobic organic pollutants) so the adsorption of organic pollutants on the surface of TiO<sup>2</sup> is low that results in slow photocatalytic degradation rates. Therefore, targeting pollutants around the TiO<sup>2</sup> nanoparticles to enhance photocatalytic efficiency require consideration. Besides this, the TiO<sup>2</sup> nanoparticles may undergo aggregation due to the instability of the nanosized particle, which may hamper the light incidence on the active centers and consequently reduction in the catalytic activity occur. However, it should be noted that it may well happen that small particles show higher scattering, which can reduce their photocatalytic activity compared to larger ones. Furthermore, for the slurry system, one main practical challenge to overcome is to recover the nanosized TiO<sup>2</sup> particles from the treated water in regards to both the economic concern and safety concern.

To overcome those limitations of TiO<sup>2</sup> -based photocatalysis, the following countermeasures have been adopted in previous studies:


The purpose of these modifications and developments is to improve photocatalytic efficiency, complete degradation of organic pollutants, improve visible light absorption, improve stability and reproducibility, and to improve recycle and reuse abilities of TiO<sup>2</sup> [18].

## *2.7.3. Process*

Photocatalytic reaction depends mainly on light (photon) energy or wavelength and the catalyst. Generally, semiconductors are used as catalysts. These materials function as sensitizers for the irradiation of light-stimulated redox process because of their electronic structure. They have a filled valence band and a vacant conduction band.

**3. Conclusions**

Water can be affected by environmental factors. Both human and environmental risks are taken into account, which may be tangible and/or intangible. Chlorination can lead to the formation of by-products or toxic chemicals that are hazardous to aquatic life. High chlorine residues may range from avoidance to death of aquatic organisms. The threshold tolerance limit of some aquatic species to chlorine is 0.002 mg/l in freshwater and 0.01 mg/l in saline water. The by-products can also accumulate in the aquatic environment. The toxicity of the

Disinfection Methods

17

http://dx.doi.org/10.5772/intechopen.80999

In summary, the beneficial use of aquatic ecosystem protection may be compromised when

Chlorination might not be a risk to the environment if the treated wastewater is reused beneficially rather than discharging into receiving surface waters. An acceptable method for disinfecting wastewater reuse is chlorination. Chlorination is the best method for reuse applications when a residual is residual is required for microbial re-growth. However, there is a limitation of 1 mg/l of chlorine at the point of application of reclaimed water. These limits mostly do not harm the plant life. However, some sensitive crops may be damaged at a level of chlorine lower than 1 mg/l and users should consider the sensitivity of any crops that may be irrigated with chlorine disinfected reclaimed water. However, little environmental risks are associated with the direct use of chlorine. However, the manufacture, storage, and transportation of chlorine products still pose a risk to the environment. Toxic by-products are formed by the oxidation of ozone. Ozone gas might harm the environ-

Microfiltration only poses a risk to the environment if there is a spill of cleaning agents or the contaminated backwash waste is disposed of incorrectly. UV light poses less risk as compared to other disinfection methods, but it may pose a risk regarding photo-reactivation and mutation of the microbial population present in the discharge. No reuse option is available for UV

A major environmental risk associated with lagoon-based disinfection is the excessive growth of undesirable organisms, such as blue-green algae. Humans are at high risk as bluegreen algal blooms produces toxins. Environment is also at risk as the levels of SS and BOD increases. In terms of potential environmental cost, it would appear that UV, lagoons, and microfiltration have the least potential to impact adversely upon the environment, followed by ozonation and then chlorination. This ranking is based on the formation of by-products

The authors thank Ms. Aqdas Zoreen & Jamshaid Khan (Department of Microbiology & Biotechnology) for their sincere help in writing the chapter. The authors also acknowledge the guidance of other faculty members of the department for their guidance and

lamps. Controlling the natural systems like detention lagoons is difficult.

and the level of toxicity of the discharge to the receiving environment.

chlorinated residues can be eliminated by dechlorination.

ment because of its corrosive nature.

**Acknowledgements**

suggestions.

chlorinated wastewater is discharged to receiving surface waters.

**Figure 6.** Schematic representation of semiconductor photocatalytic mechanism.

The fundamental steps in the process of semiconductor photocatalysis are as follows:


These electrons and holes might undergo successive redox reactions with many species to form necessary products by absorbing on the surface of the semiconductor [19] (**Figure 6**).

### *2.7.4. Chemical*

TiO<sup>2</sup> is a semiconductive material that acts as a strong oxidizing agent during illumination by lowering the activation energy required for the decomposition of organic and inorganic compounds. The illumination of the surface of the TiO<sup>2</sup> induces two types of carrier separation: (1) an electron (e−) and (2) a hole (h+). For the production of these two carriers, sufficient amount of energy must be supplied by a photon to move an electron (e−) from the valence band to the conduction band, thus leaving a hole (h+) in the valence band. In comparison to the conducting materials, the recombination of holes and electrons is relatively slow in TiO<sup>2</sup> recombination in metals occurs immediately [20].

$$\text{TiO}\_2 + \text{hv} \rightarrow \text{h}^+ + \text{e}^-$$
