Stream dilution and nuisance potential: In conventional stream sanitation design, a nuisance is likely to be experienced if the dilution factor (ratio of stream flow to wastewater flow) is less than which threshold?

Introduction / Context:
When untreated or partially treated wastewater is discharged to a receiving stream, adequate dilution is necessary to minimize odor, septic conditions, and visible pollution. The dilution factor is a simple indicator in preliminary assessments.

Given Data / Assumptions:

• Simple screening-level criterion for nuisance avoidance.
• Typical domestic wastewater without unusual industrial toxins.
• Moderate temperatures and normal re-aeration conditions.

Concept / Approach:

Nuisance conditions (odor, septic sludge deposits, anoxic zones) are more likely at low dilution ratios. Many classical references cite that if dilution falls below roughly 20:1, the chance of nuisance increases markedly unless treatment and reaeration are provided.

Step-by-Step Solution:

Verification / Alternative check:

Heuristic rules in older design manuals use ratios of 20:1 or higher to limit nuisance for raw or lightly treated wastes.

Why Other Options Are Wrong:

10 is too low; 40, 60, and 100 are more conservative than the commonly cited minimum threshold.

Common Pitfalls:

Using dilution alone without accounting for BOD decay, reaeration, and temperature; ignoring modern effluent standards that often require secondary treatment regardless of dilution.

Final Answer:

20