Dilution factor guidance: Under what condition (dilution factor value) is no treatment of sewage generally considered acceptable in dilution-based disposal practice?

Introduction / Context:
Historically, some outfalls relied on the natural assimilative capacity of receiving waters. The dilution factor—ratio of available clean water to sewage—served as a guide to the extent of required treatment. Modern practice emphasizes treatment regardless, but the classic guideline remains a teaching reference.

Given Data / Assumptions:

• Dilution factor = quantity of diluting water / quantity of sewage.
• High dilution implies rapid reduction of BOD and pathogens by natural processes.
• No unusual toxicity or temperature effects assumed.

Concept / Approach:

Older standards suggested that if dilution factor exceeded about 500, the receiving water could assimilate the load without artificial treatment, given adequate DO levels and dispersion. However, environmental regulations today typically require at least primary and often secondary treatment to protect ecosystems and human health.

Step-by-Step Solution:

Verification / Alternative check:

Textbook charts and design manuals cite increasing required treatment as DF decreases, with the least requirement at very high DF values.

Why Other Options Are Wrong:

DF values below or near 500 indicate insufficient natural assimilation; some level of treatment becomes necessary to protect water quality.

Common Pitfalls:

Applying dilution-only logic under modern discharge permits; ignoring cumulative impacts and sensitive habitats.

Final Answer:

More than 500