Photochemical oxidation processes, based on the generation of more reactive hydroxyl radical (•OH) using UV/TiO2 and UV/H2O2 methods, can lead to mineralization of most contaminants. Examples of photocatalysts are TiO2, ZnO etc.

There are two types of photocatalysis: Homogenous photocatalysis & heterogenous photocatalysis.
When the photocatalysts and the reactants are present in the same phase, such type of photocatalysis is known as Homogenous photocatalysis. The most widely used homogeneous photocatalysts are ozone and photo-Fenton systems (Fe+ and Fe+/H2O2).
In the process of heterogenous photo catalysis, the reactants and the photo-catalysts are present in the same phase. The use of a this photocatalyst is a method for waste water treatment which includes redox reactions from adsorbed waste water, oxygen molecules and hydroxyl anions, or other organic molecules.

Fig. photocatalysis

6. RENEWABLE ENERGY RESOURCES

Renewable energy is generated from natural processes that are continuously restored. Examples are solar energy, geothermal heat, wind energy, tidal energy, water, and various forms of biomass. This energy cannot be depleted and is constantly produced.
Renewable energy is clean energy and non-polluting. It is a sustainable energy source that can be relied on for the long-term. Renewable energy is cost-effective and efficient. The main limitations are that it is more expensive than