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Understanding the difference between COD&BOD
Release time:
2026-07-13
1. Concept
COD: Chemical oxygen Demand, amount of a specified oxidant that reacts with the sample under controlled conditions. The quantity of oxidant consumed is expressed in terms of its oxygen equivalence.
Key Points:
1. Chemical method (using oxidizing agent);
2. Fast results (few hours);
3. Measures the degree of pollution of the water by reductive substance;
4. Includes most of the organic compounds, and reducing inorganic salts (e.g., Nitrite).
BOD: Biochemical Oxygen Demand, measure of oxygen consumed by microorganisms under specific conditions.
Key Points:
1. Biological method (using microorganisms)
2. Slow process (5 days)
3. Measures organic pollution in water;
4. Includes biodegradable organic matter.
2. Formula
COD:
COD can be measured by K2Cr2O7, and the values can be calculated using this formula.
B = mL FAS used for sample,
A = mL FAS used for blank,
M = molarity of FAS, and
8000 = milliequivalent weight of oxygen _ 1000 mL/L.
BOD:
For each test bottle with at least 2.0 mg/L DO depletion and at least 1.0 mg/L residual DO—before seed correction, calculate BOD as follows:
D1 = DO of diluted sample immediately after preparation, mg/L,
D2 = DO of diluted sample after 5 d incubation at 20°C, mg/L,
S = oxygen uptake of seed [ΔDO/mL seed suspension added per bottle (5210B.6d) (S _ 0 if samples are unseeded)],
Vs = volume of seed in respective test bottle, mL,
P = decimal volumetric fraction of sample used; 1/P = dilution factor.
3. Key Differences
Parameter | COD | BOD |
Full form | Chemical Oxygen Demand | Biochemical Oxygen Demand |
Principle | Using oxidizing agent (e.g. K2Cr2O7) | Using microorganisms |
Time required | Few hours | 5 days （20℃ constant temperature） |
Measured pollutants | most of the organic compounds and some inorganics | Only biodegradable organic matter |
Reagents/Method | Chemical reagents | Biological techniques |
Result | Usually higher than BOD | Usually lower than COD |
Purpose | Measures overall pollution load | Measures biodegradable pollution load |
4. The effluent’s biodegradability
Before making a decision on the use of a biological treatment technology for wastewater, biodegradability needs to be assessed. The BOD/COD ratio is a useful tool for making such a decision.
BOD/COD≥0.4, suitable for biodegradation
BOD/COD = 0.2~0.4, biodegradable (with provision of much longer residence time of the wastewater in the biotreatment unit)
BOD/COD＜0.2, difficult to biodegrade
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