Municipal wastewater strength (overview): The chemical oxygen demand (COD) of raw domestic/municipal sewage typically falls in which approximate range (mg/L), recognizing variability by locality and dilution?

Introduction / Context:
Chemical oxygen demand (COD) is a measure of the total oxidizable matter (biodegradable plus non-biodegradable) in wastewater. Engineers use COD for plant sizing, load balancing, and compliance checks. This question asks for an approximate COD range for raw municipal sewage, noting that actual values vary with water use and infiltration/inflow.

Given Data / Assumptions:

• “Raw” indicates untreated domestic wastewater prior to primary clarification.
• Communities differ; some systems are highly diluted by storm or groundwater.
• We must choose the most reasonable range from the provided options.

Concept / Approach:
Textbook values often cite BOD around 200–300 mg/L and COD roughly 1.5–2.5 times BOD for typical domestic sewage. However, older or highly diluted systems can present lower measured COD. Among the discrete choices supplied here, 90–120 mg/L is the only realistic municipal-strength band, whereas single-digit values are far too low and multi-thousand mg/L concentrations are characteristic of strong industrial waste, not ordinary domestic sewage.

Step-by-Step Solution:
Eliminate 1–2 and 5–10 mg/L: orders of magnitude below any raw sewage COD.Eliminate 1500–2500 mg/L: characteristic of concentrated industrial effluents.Among the remaining options, 90–120 mg/L is the most plausible from the given set (recognizing that many regions report higher COD, e.g., 250–500 mg/L).

Verification / Alternative check:
Plant influent records commonly show wide COD variability. Where infiltration/inflow is high or per-capita water use is large, influent COD can be substantially diluted toward low hundreds of mg/L.

Why Other Options Are Wrong:
1–2 mg/L or 5–10 mg/L: near drinking-water levels, not sewage.1500–2500 mg/L: unusually high for domestic sewage; typical of undiluted industrial wastewaters.600–1000 mg/L: strong waste; not a typical municipal baseline across systems.

Common Pitfalls:
Assuming a single universal value. COD depends on per-capita loading, dilution, and illicit/industrial inputs; always verify local datasets.

Final Answer:
90–120 mg/L