At ordinary temperatures, “fresh” domestic sewage begins to turn stale (become septic/odorous) after approximately how much detention time in a closed sewer without aeration?

Introduction / Context:
Characterizing sewage freshness is important for design of pumping stations, force mains, and retention structures to avoid septic conditions, odours, and corrosion. “Stale” sewage indicates onset of anaerobic conditions and production of hydrogen sulphide.

Given Data / Assumptions:

• Typical warm weather, domestic sewage.
• Minimal reaeration (closed conduit), appreciable biochemical activity.
• No chemical dosing (e.g., nitrate, oxygen, or iron salts).

Concept / Approach:

In the absence of oxygen replenishment, dissolved oxygen is quickly depleted by biochemical oxygen demand. Once DO is exhausted, septic anaerobic processes produce odorous gases. Empirical practice suggests fresh sewage can begin to turn stale in only a few hours, often 2–3 hours, depending on temperature and load.

Step-by-Step Solution:

Verification / Alternative check:

Utility SOPs often limit detention times in rising mains and wet wells to a few hours to prevent septicity and H2S formation, consistent with the 2–3 hour guideline.

Why Other Options Are Wrong:

(a) One hour is typically too short for significant septicity under average conditions. (c), (d), and (e) exceed typical onset period; by then septic conditions are established unless mitigated.

Common Pitfalls:

Ignoring temperature effect (warmer sewage turns septic faster); assuming open channels afford the same timeline as closed sewers; neglecting industrial contributions that can accelerate septicity.

Final Answer:

About 2 to 3 hours