Dilution (disposal to water bodies): In which situation is the dilution method of sewage disposal not preferred on public-health and water-supply grounds?

Introduction / Context:

Dilution refers to discharging treated (or sometimes preliminarily treated) wastewater into a natural water body, relying on mixing and assimilative capacity. While historically practiced, modern regulations require care to protect downstream uses, especially potable supply intakes.

Given Data / Assumptions:

• Sewage disposal near a source water intake poses pathogen and contaminant risks.
• Receiving water quality (DO levels and currents) affects dilution, but does not eliminate risk to water supplies.

Concept / Approach:

The dilution method is contraindicated where the water body serves as a drinking-water source near the outfall. Even with high DO and good currents, incomplete pathogen removal or shock loads can compromise public health. Buffer distances, advanced treatment, and outfall siting are critical if any mixing zone is contemplated.

Step-by-Step Solution:

Verification / Alternative check:

Guidelines typically require stringent treatment and setback distances from drinking-water intakes; many jurisdictions prohibit such discharges in source-water protection zones.

Why Other Options Are Wrong:

• Fresh sewage and high DO or adequate currents speak to assimilative capacity but do not outweigh the risk at drinking-water intakes.
• None of these is incorrect because one scenario is clearly unsuitable.

Common Pitfalls:

• Overreliance on dilution as a substitute for adequate treatment.
• Ignoring seasonal and flow variability that can reduce mixing and DO.

Final Answer:

When the receiving water is also used for municipal supply near the disposal point