Chemical Oxygen Demand
Chemical Oxygen Demand (COD) is the amount of oxygen required to chemically oxidise organic and oxidisable inorganic substances present in a water or wastewater sample.
It is commonly used as an indicator of water pollution and the amount of oxidisable matter in water.
Why is COD important?
- Higher COD → higher pollution load generally.
- It indicates the amount of oxygen that would be consumed if oxidisable substances were chemically broken down.
- High COD in rivers and lakes can contribute to oxygen depletion, potentially harming aquatic organisms.
COD vs BOD
| Feature | COD | BOD |
|---|---|---|
| Full form | Chemical Oxygen Demand | Biochemical Oxygen Demand |
| Oxidation | Chemical oxidation | Biological/microbial oxidation |
| Time | Relatively rapid | Traditionally measured over 5 days (BOD₅) |
| Measures | Chemically oxidisable substances | Biodegradable organic matter |
| Typical value | Usually higher than BOD | Usually lower than COD |
| Unit | mg/L of O₂ | mg/L of O₂ |
Example
Think of wastewater containing organic pollutants.
COD asks:
“How much oxygen would be needed to chemically oxidise the oxidisable substances in this water?”
Thus, COD is a useful parameter for assessing the strength of wastewater and the efficiency of wastewater treatment plants.
Exam Point
COD is expressed in mg/L of O₂. A higher COD generally indicates a greater concentration of oxidisable pollutants in water.
Memory trick:
COD → Chemical oxidation → Oxygen demand.
