Chemical Oxygen Demand (COD) understanding: Select the correct comprehensive statement(s) about COD determination and its relation to BOD and potable water quality.

Introduction / Context:
Chemical Oxygen Demand (COD) is a rapid, reproducible test that estimates the amount of oxidisable substances—primarily organic matter—in water and wastewater. It is widely used for process control, industrial discharge monitoring, and correlation with Biochemical Oxygen Demand (BOD). This question checks your familiarity with COD definition, standard dichromate method, and typical values for clean water.

Given Data / Assumptions:

• Standard COD method uses potassium dichromate (K2Cr2O7) in strong acid with a catalyst and boiling reflux.
• COD often correlates with BOD but the ratio depends on waste composition.
• Potable waters exhibit very low COD values.

Concept / Approach:
In the dichromate COD test, organic compounds are chemically oxidised under reflux in concentrated sulphuric acid with dichromate as the oxidant (often with silver catalyst and mercury salt to mitigate chloride interference). The amount of dichromate consumed, determined by titration or colorimetry, indicates COD in mg O2/L. Because COD oxidises many substances that microbes may not, it is usually greater than BOD for the same sample. Treated drinking water has COD typically around 1–2 mg/L or even lower.

Step-by-Step Solution:
Assess option A: correct definition of COD.Assess option B: describes the classical dichromate reflux procedure.Assess option C: acknowledges variable COD/BOD relationship and low COD in potable water.Therefore option D (“all”) is correct.

Verification / Alternative check:
Standard Methods for the Examination of Water and Wastewater documents the dichromate method and typical ranges for drinking water.

Why Other Options Are Wrong:
Options A–C are each individually correct; choosing any one would be incomplete when an “all” option is provided.

Common Pitfalls:

• Assuming a fixed COD/BOD ratio; composition-specific factors vary the ratio widely.
• Ignoring chloride interference and the need for masking agents in saline waters.

Final Answer:
all (a), (b) and (c).