Q4. What is sewage? In which way can sewage be harmful to us? (NCERT Exercise 7, page 159.)
Sewage is municipal waste water, whose major component is human excreta; it contains large amounts of organic matter and microbes, many of them pathogenic (NCERT page 153). Discharged untreated into rivers, it pollutes them and increases water-borne diseases (NCERT page 155).
Q5. Why is a small part of the activated sludge pumped back into the aeration tank?
To serve as the inoculum: it seeds the tank with the microbes that grow into flocs and consume the organic matter (NCERT page 154).
Q6. What does the BOD test measure, and what does BOD tell us indirectly?
It measures the rate of uptake of oxygen by micro-organisms in a water sample; indirectly, BOD is a measure of the organic matter present in the water (NCERT page 154).
Q7. A plant lowers the BOD of its sewage from 250 mg/L to 25 mg/L. By what percentage has the BOD fallen?
Fall = 250 − 25 = 225 mg/L; 225 ÷ 250 × 100 = 90 per cent (our own numbers; BOD as defined in NCERT page 154).
Q8. Which ministry started the Ganga and Yamuna Action Plans, and what do they propose?
The Ministry of Environment and Forests; they propose building a large number of sewage treatment plants so that only treated sewage is discharged into these rivers (NCERT page 155).