Water is consumed by both domestic and industrial sectors. With the exponential increase in the population and industrialization, consumption of water has increased tremendously, thereby producing more wastewater. Household water drains consist of grey water and black water. The grey water is permitted to be used for toilet flushing and watering of plants.

Purification and reuse of wastewater help us conserve the precious and fast-depleting freshwater resources. Wastewater purification involves a primary and a secondary treatment.

The primary treatment removes large floating objects like wooden sticks and leaves. The heavier and smaller objects like stone and sand are also removed in the primary treatment. The water still contains inorganic matter like dissolved calcium and magnesium salts and organic matter along with suspended solids. The suspended solids are removed by sedimentation. In this process, the suspended particles gradually sink to the bottom of a tank. The sediment thus obtained can be treated further and used as a bio-fertilizer or taken for a landfill operation. The output water obtained is taken for secondary treatment.

Secondary treatment removes about 85% of all the organic matter using aerobic bacteria with aeration along with bacteria. This decomposes organic compounds to harmless products. The water at this stage still contains odor and some harmful bacteria. Chlorination of this water removes the odor and kills the harmful bacteria. Chlorine oxidizes the organic compounds. It also chlorinates some organic compounds in the process. One drawback of this method is that accidental release of poisonous chlorine gas can be hazardous.

Ozone is a better alternative for oxidation purposes. Ozone, being unstable, has to be produced onsite. Final purification is then carried out using activated carbon filters to remove the organic compounds by adsorption, followed by reverse osmosis. Thus, water from various sources can be purified by suitable measures for reuse.