1,000
mg/L
10
mL
2.5
meq
20
mg
1,000
mg/L
10
mL
2.5
meq
20
mg
The Chemical Oxygen Demand (COD) Calculator determines the amount of oxygen required to chemically oxidize all organic and inorganic substances in water using a strong chemical oxidant (typically potassium dichromate). COD provides a measure of the total organic load in water, including non-biodegradable compounds that BOD does not capture.
COD analysis is faster than BOD (2 hours vs 5 days) and is widely used in industrial wastewater monitoring, treatment plant operations, and regulatory compliance. The COD value is always equal to or greater than the BOD value for the same sample.
COD is calculated from the dichromate titrimetric method:
COD (mg/L) = ((B - S) × N × 8000) / V
Where:
Inputs
Results
COD = (25-15) x 0.25 x 8000 / 20 = 100 mg/L. This moderate COD is typical of partially treated industrial effluent.
Inputs
Results
COD = (25-5) x 0.25 x 8000 / 10 = 400 mg/L. Raw sewage typically has COD values of 250-800 mg/L.
BOD measures oxygen consumed by biological decomposition of organic matter (over 5 days), while COD measures the total oxygen required for complete chemical oxidation (in 2 hours using dichromate). COD includes both biodegradable and non-biodegradable organic matter, so COD is always greater than or equal to BOD.
The 8000 factor combines two conversions: the milliequivalent weight of oxygen (8 g/equivalent) and the conversion from milliequivalents to mg/L (times 1000). This converts the titrant volume difference and normality into oxygen demand in mg/L.
For domestic sewage, the COD/BOD ratio is typically 1.5-2.5. A ratio below 2 indicates highly biodegradable waste. Ratios above 3-4 suggest the presence of significant non-biodegradable organic compounds, common in industrial wastewaters from chemical, pharmaceutical, or petrochemical industries.
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