Chemical Oxygen Demand of Water (exp:11) - assignment - Q-A

Determination of Chemical Oxygen Demand of Water
(1) Q: What are the principal advantages and disadvantages of the COD test over the BOD test.
Ans: The major advantage of COD test is the short time required for evaluation. The determination can be made in about 3 hours rather than the 5 days required for the measurement of BOD . for this reason, it is used as a substitute fir the BOD test in many instances.
One of the chief limitations of COD test is its inability to differentiate between biologically oxidizable and biologically inert organic matter. In addition, it does not provide any evidence of the rate at which the biologically active material would be stabilized under conditions that exist in nature.
(2) Q: Explain why COD values are usually greater than BOD values.
Ans During the determination of COD, organic matter is converted to carbon dioxide and water regardless of the biological assimilability of the substance. For example, glucose and lignin are both oxidized completely. As a result, XOD values are greater than BOD values, especially when biologically resistant organic matter (e.g., lignin) is present.
(3) Q: What could be inferred from the following analytical results concerning the relative case of biodegradability of each waste?
Waste 5-day BOD (mg/L) 5-day COD (mg/L)
A 240 300
B 100 500
C 120 240
Ans:
Waste 5-day BOD (mg/L) 5-day COD (mg/L) Ratio of BOD&COD Comment
A 240 300 0.8 Highly Biodegradable
B 100 500 0.2 Slightly Biodegradable
C 120
240 0.5 Moderately Biodegradable

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