**3. Conclusions**

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

molecules to generate hydroxyl radicals by oxidizing donor molecules.

gen species. These electrons induce the redox reactions.

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

compounds. The illumination of the surface of the TiO<sup>2</sup>

TiO<sup>2</sup> + hv → h<sup>+</sup> + e<sup>−</sup>

recombination in metals occurs immediately [20].

semiconductor.

16 Photocatalysts - Applications and Attributes

*2.7.4. Chemical*

TiO<sup>2</sup>

• When the light energy in terms of photons falls on the surface of a semiconductor and if the energy of incident ray is equivalent or more than the bandgap energy of the semiconductor, the valence band electrons move to the conduction band of the

• The valance band of semiconductors is left with holes. These holes can react with water

• Superoxide ions are formed by reacting the conduction band electrons with dissolved oxy-

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**).

 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

tion: (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>

induces two types of carrier separa-

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 chlorinated residues can be eliminated by dechlorination.

In summary, the beneficial use of aquatic ecosystem protection may be compromised when chlorinated wastewater is discharged to receiving surface waters.

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 environment because of its corrosive nature.

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

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