Before discharging foul sewage to a natural water body, which stages of treatment are generally applied in municipal practice?

Introduction / Context:
Surface water quality protection requires that sewage be treated to remove floatables, settleable solids, organic load (BOD), pathogens, and stabilized sludge prior to discharge. Treatment trains combine several unit operations and processes sequentially.

Given Data / Assumptions:

• Municipal wastewater with typical solids and BOD.
• Compliance with discharge standards requiring multiple barriers.
• Conventional plant layout.

Concept / Approach:

Preliminary treatment (screening, grit removal) protects downstream units. Primary treatment (sedimentation) removes settleable solids and some BOD. Secondary treatment (oxidation via biological processes) reduces soluble and colloidal organics. Sludge digestion stabilizes solids for safe disposal, while disinfection reduces pathogens to acceptable levels before discharge or reuse.

Step-by-Step Solution:

Verification / Alternative check:

National standards typically mandate secondary treatment and, where required, tertiary disinfection—affirming the multi-stage approach.

Why Other Options Are Wrong:

Options a–d describe single stages and are insufficient alone to meet discharge norms.

Common Pitfalls:

Assuming primary treatment alone is adequate; overlooking pathogen removal needs.

Final Answer:

All the above (screening, sedimentation, oxidation, sludge digestion and disinfection)