Wastewater Story

Used water from homes, industries and hospitals is wastewater, and the liquid waste carried by drains is sewage. It contains organic impurities such as human faeces, urea, oil and food waste, inorganic impurities such as nitrates, phosphates and metals, and disease-causing bacteria and protozoa that spread cholera, typhoid and dysentery. A network of underground pipes called sewers, with manholes at intervals, forms the sewerage system that carries sewage to a wastewater treatment plant (WWTP). At the plant, bar screens remove large objects like rags and plastic, a grit and sand tank lets heavy particles settle, and in a large clarifier tank, solids settle as sludge while oil and grease are skimmed off. Air is then pumped into the clarified water so aerobic bacteria consume the remaining human waste. The cleaned water is released into rivers, sometimes after disinfection with chlorine or ozone, while sludge is decomposed by anaerobic bacteria to give biogas and manure. We can help by not pouring oil, fats, chemicals or solid waste into drains.

Detailed Worksheet: Wastewater Story Section A - Definitions (10 marks) 1. What is sewage? Name two sources of it. (2 marks) 2. What are sewers and manholes? (2 marks) 3. What is sludge? What is done with it? (2 marks) 4. What is activated sludge? (2 marks) 5. What is a contaminant? Give two examples found in sewage. (2 marks) Section B - Calculations and Applications (15 marks) 6. Classify the following contaminants of sewage as organic, inorganic, nutrients or microbes: urea, nitrates, faeces, phosphates, cholera bacteria, pesticides. (3 marks) 7. Arrange the steps of wastewater treatment in the correct order: aeration, grit and sand removal, bar screens, sludge removal in clarifier, release of clean water, disinfection. (3 marks) 8. Why should we not throw (i) cooking oil and fats (ii) used tea leaves and food remains (iii) medicines and paints into the drain? (3 marks) 9. A city produces 500 million litres of sewage daily but treats only 300 million litres. (i) What percentage is treated? (ii) How much goes untreated? (iii) What are the effects of untreated sewage? (3 marks) 10. Name three on-site sewage disposal methods for places without a sewerage system and state one advantage of a vermi-processing toilet. (3 marks) Section C - Diagrams (10 marks) 11. Draw a labelled flowchart of a wastewater treatment plant showing bar screens, grit and sand removal tank, clarifier, aeration tank and outlet. (4 marks) 12. Draw a labelled diagram of a septic tank. (3 marks) 13. Draw a simple filter made of sand, gravel and charcoal in a bottle to clean dirty water, with labels. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Open drains in a colony overflow during the rains. (i) What health problems can arise? (ii) Why do mosquitoes and flies breed there? (iii) What should the residents do? (iv) What is the role of the local body? (v) How does open defecation make it worse? (5 marks) 15. Eucalyptus trees are planted along sewage ponds. (i) Why are they chosen? (ii) How do they help? (iii) Is it a substitute for treatment? (iv) Suggest one other natural method to clean wastewater. (5 marks) 16. At a WWTP, sludge is put into a tank where anaerobic bacteria act on it. (i) What gas is produced? (ii) How is it used? (iii) What is the dried sludge used for? (iv) Why is this called resource recovery? (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Write the stages of treatment in the correct order and draw neat flowcharts.