Typical BOD (biological oxygen demand) range for raw municipal sewage Select the most representative BOD concentration range for untreated domestic sewage, expressed in mg/L.

Introduction / Context:
Biological oxygen demand (BOD) indicates the amount of oxygen microorganisms need to oxidise biodegradable organic matter in water. Correctly estimating BOD for raw municipal sewage is essential for sizing aeration tanks, clarifiers, and for meeting discharge standards at wastewater treatment plants.

Given Data / Assumptions:

• Untreated domestic sewage without significant industrial contribution.
• Standard 5-day BOD at 20°C is implied by convention.
• Values are approximate ranges commonly cited in design practice.

Concept / Approach:
Raw municipal sewage typically exhibits moderate to high biodegradable organic load, with BOD commonly in the 150–300 mg/L range. Very low values (1–10 mg/L) are typical of surface waters, not sewage. Extremely high levels in the thousands of mg/L are characteristic of strong industrial wastes or concentrated slurries.

Step-by-Step Solution:

Verification / Alternative check:
Textbook design tables for municipal plants routinely list BOD in the above range, aligning with typical influent data logs.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing BOD with COD; COD is usually higher than BOD for the same sample due to inclusion of non-biodegradable organics.

Final Answer:
150-300