Chapter 10: Wastewater Story

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Wastewater Story


Describe the steps involved in getting clarified water from wastewater.


The treatment of wastewater involves physical, chemical, and biological processes which remove physical, chemical, and biological matter that contaminates the wastewater. This process involves:

1. Screening – The wastewater is passed through bar screens. The bar screen removes large objects like rags, cans sticks, plastic bags, napkins, sanitary towels, etc. from wastewater.

2. Grit and Sand removal – After passing the bar screen, the wastewater is passed slowly through a grit and sand removal tank. The speed is decreased to allow the sand, grit, and pebbles to settle down at the bottom and are removed.

3. Sedimentation tank –The water is then allowed to settle in a large tank. The solids like faeces settle at the bottom and are removed with a scraper. This solid part is called sludge. A skimmer removes floatable materials like oil and grease. Water so cleared is called clarified water. Sludge is transferred to a separate tank where it is decomposed by anaerobic bacteria to produce biogas. Biogas is used to fuel or can be used to produce electricity.

4. Aeration tank – Clarified water is then passed through an aerator tank where the air is pumped into the water. It helps aerobic bacteria to grow which decompose organic matter like a human and animal waste. After several hours, the suspended microbes settle at the bottom of the tank as activated sludge. The dried, activated sludge is used as manure. The activated sludge is about 97% water. The water is then removed from the top.

5. Chlorination – The treated water has a low level of organic material and suspended matter. It is discharged into a sea, a river, or into the ground. Sometimes it is disinfected through chemicals like chlorine and ozone before discharging it into water bodies.

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