Question

Describe various steps of cleaning wastewater in a wastewater treatment plant.

Answer 1

Wastewater treatment is the process of removing contaminants from wastewater to get clean, drinking water. It involves the removal of physical, chemical and biological contaminants through a series of steps like filtration, sedimentation and microbial degradation. The various steps involved in this method are:

(i) Preliminary treatment: First of all, wastewater is passed through bar screens which remove large solid materials like plastics, cans, and rags. Wastewater is then passed through a grit and sand removal tank, where the speed of wastewater is reduced to allow sand, silt and gravel to settle down.

(ii) Primary treatment: Wastewater is then made to pass through sedimentation tanks where organic matter like faeces settle down at the bottom. This is removed with the help of a scraper. Floatable solids like oil and grease are removed with a skimmer. The water so obtained is clarified water.

(iii) Secondary treatment: This is a biological process, where the organic matter is broken down by the action of natural decomposers like bacteria to release biogas, which can be used as fuel or to produce electricity. This is referred to as an activated sludge system. Bacteria, protozoans and worms feed on organic matter, which tends to form clumps of hummus-like material that settle at the bottom.

(iv) Final cleansing and disinfection: Water is removed from the top. Compressed air is passed through it and disinfectants such as chlorine and ozone gas are added to kill the microorganisms. This treated water is finally released into natural water bodies like lakes and rivers.