Science: Class 7 — Chapter 13: Wastewater Story

• Wastewater: The dirty, lather-rich, oil-mixed, black-brown water that goes down the drains from sinks, showers, toilets, and laundries.
• Clean Water Scarcity: More than one billion people worldwide lack access to safe drinking water, leading to water-related diseases and deaths.
• Water for Life: The United Nations General Assembly proclaimed the period 2005–2015 as the International Decade for action on "Water for life" to halve the population without safe drinking water.
• Cleaning of Water: The process of removing pollutants before wastewater enters a water body or is reused, commonly known as Sewage Treatment.

Sewage is a complex liquid waste containing suspended solids, organic and inorganic impurities, nutrients, and disease-causing microbes:

• Organic Impurities: Human faeces, animal waste, oil, urea (urine), pesticides, herbicides, fruit and vegetable waste.
• Inorganic Impurities: Nitrates, phosphates, and metals.
• Nutrients: Phosphorus and Nitrogen.
• Bacteria: Pathogens such as Vibrio cholerae (causes cholera) and Salmonella paratyphi (causes typhoid).
• Other Microbes: Protozoans which cause dysentery.

• Sewers: Underground pipes that carry wastewater from its point of origin to the treatment plant.
• Sewerage: A network of interconnected sewers forming a transport system.
• Manholes: Access points located every 50 m to 60 m in the sewerage system, placed at junctions or points where there is a change in direction.

1. Bar Screens: Wastewater is passed through screens to remove large physical objects like rags, sticks, cans, plastic packets, and napkins.
2. Grit and Sand Removal: Water enters a tank where the flow speed is decreased, allowing sand, grit, and pebbles to settle down.
3. Clarifier (Sedimentation): Water settles in a large sloped tank. Solid waste like faeces settles at the bottom as sludge (removed by a scraper), while floatable solids like oil and grease are removed by a skimmer to produce clarified water.
4. Aeration: Air is pumped into the clarified water to help aerobic bacteria grow. These bacteria consume remaining organic matter, food waste, and soaps.
5. Disinfection: Before discharge, treated water may be disinfected using chemicals like chlorine or ozone.

• By-products: Sludge (decomposed by anaerobic bacteria to produce biogas or dried for manure) and biogas (used as fuel or to generate electricity).

• Oils and Fats: Should not be thrown down the drain as they harden and block pipes, or clog soil pores in open drains. Throw them in the dustbin.
• Chemicals: Paints, solvents, motor oil, and medicines should not be discarded in drains as they kill water-purifying microbes.
• Solid Wastes: Used tea leaves, solid food remains, cotton, and sanitary towels should be thrown in the dustbin to prevent choking drains and blocking oxygen flow.

• Diseases: Untreated human excreta pollutes surface and groundwater, leading to water-borne diseases like cholera, typhoid, polio, meningitis, hepatitis, and dysentery.
• Vermi-processing Toilet: A novel, low-water toilet design tested in India where human excreta is treated by earthworms and converted into nutrient-rich vermi cakes.
• On-site Sewage Disposal: Low-cost sanitation systems like septic tanks, chemical toilets, and composting pits are used in areas without sewerage systems.
• Swachh Bharat Mission: Initiated by the Government of India in 2016 to promote proper sewage disposal and provide toilets for all.