In municipal wastewater engineering (Indian conditions), what is the typical average temperature of domestic sewage used for design and process calculations?

Introduction / Context:
Sewage temperature directly affects biochemical reaction rates, viscosity, oxygen transfer, and gas generation. For Indian design problems and exam standards, a representative average temperature is used when site-specific data are unavailable. This question checks that reference value.

Given Data / Assumptions:

• Domestic sewage in tropical/subtropical Indian climate.
• Average, not peak summer or winter extremes.
• Steady-state assumptions for textbook calculations (e.g., BOD rate constants, oxygen solubility).

Concept / Approach:

Design manuals tabulate typical sewage temperatures by climate. For India, many standard texts take about 20°C as a reasonable annual-average for calculations. This anchors temperature correction factors for reaction rates (e.g., k_T = k_20 * θ^(T-20)).

Step-by-Step Solution:

Verification / Alternative check:

At 20°C, oxygen solubility and viscosity values match common tables used for aeration and hydraulics, confirming consistency with design norms.

Why Other Options Are Wrong:

10–15°C are too low for most Indian cities; 25–30°C represent warm-season conditions but are not used as a single average baseline.

Common Pitfalls:

Confusing wastewater temperature with air temperature; using peak temperatures for all-year kinetic design without correction factors.

Final Answer:

20°C