Q.Explain the process of sewage water treatment before it can be discharged into natural bodies. Why is this treatment essential?

Step 1. Preliminary treatment. Sewage first passes through screening, where screens or nets trap large suspended and floating debris, followed by a grit chamber, where large stones and brick ballast let coarse particulate matter settle out by gravity.

Step 2. Primary (physical) treatment. The screened sewage is pumped into a primary sedimentation tank, where about 50-70% of suspended solids settle as primary sludge (removed mechanically), and coliform bacterial counts fall by roughly 30-40% simply through this settling; the remaining supernatant effluent still carries dissolved organic matter and micro-organisms.

Step 3. Secondary (biological) treatment. The primary effluent is passed into aeration tanks, constantly agitated with pumped-in air; aerobic bacteria grow vigorously here and form flocs (bacteria held together by slime and fungal hyphae) that consume most of the remaining organic matter, sharply lowering the effluent's Biochemical Oxygen Demand (BOD).

Step 4. Tertiary treatment. The water then moves to a settling tank, where the bacterial flocs settle as activated sludge; part of this is recycled to the aeration tank, and the rest is pumped into anaerobic sludge digesters, where anaerobic bacteria further digest the sludge, releasing gases such as methane, hydrogen sulphide and CO2. The digester's effluent is chlorinated (killing remaining pathogenic bacteria) before release into rivers and streams, while the digested sludge cake is disposed of separately.

Step 5. Why treatment is essential. Raw sewage carries pathogenic bacteria, viruses and protozoa — including the agents of dysentery, cholera, typhoid, polio and infectious hepatitis — plus a heavy dissolved organic load that would deplete the oxygen (raise the BOD) of any natural water body it entered untreated. Treating sewage in these four stages therefore protects both public health and the ecological health of the rivers, lakes or seas the treated water is finally released into.

Walking through the four sewage-treatment stages in sequence, then explaining the public-health/pollution rationale for treatment

• Merging preliminary and primary treatment into one step, when the chapter treats screening/grit-removal (preliminary) and sedimentation (primary) as distinct stages.
• Omitting chlorination as the specific step that kills remaining pathogens just before the treated water is released.
• Not separately addressing 'why treatment is essential', which the question explicitly asks for in addition to describing the process.