In the field of wastewater treatment, two crucial indicators are used to assess the organic pollution load in sewage: Biological Oxygen Demand (BOD) and Chemical Oxygen Demand (COD). These parameters help determine the oxygen-demanding strength of wastewater, giving engineers and operators a clear understanding of how polluted the water is and how much treatment is needed. This article explores what BOD and COD mean, how they are measured, and what sets them apart.
What is Biological Oxygen Demand (BOD)?
Biological Oxygen Demand (BOD) refers to the amount of dissolved oxygen that aerobic microorganisms need to decompose the organic matter present in wastewater over a specific period, typically 5 days at 20°C. It gives an estimate of the organic pollution that can be broken down naturally by bacteria and other microbes.
Why BOD Matters:
BOD is commonly used to gauge the effectiveness of sewage treatment plants, especially municipal ones, as it reflects the amount of biodegradable material in the water. A higher BOD indicates a higher level of pollution, which means more oxygen will be required to stabilize the waste.
BOD Testing Method:
The standard BOD test involves sealing a sample of wastewater in a bottle and incubating it at 20°C for 5 days. The oxygen consumed during this period is measured and reported in milligrams per liter (mg/L). This test simulates the natural biological decomposition process.
What is Chemical Oxygen Demand (COD)?
Chemical Oxygen Demand (COD) represents the total quantity of oxygen required to chemically oxidize both organic and inorganic substances in wastewater using strong oxidizing agents like potassium dichromate under acidic conditions.
Importance of COD:
Unlike BOD, which only captures the oxygen required by biodegradable components, COD accounts for both biodegradable and non-biodegradable substances. This makes COD a broader, more comprehensive indicator of water pollution—especially useful in industrial wastewater where many synthetic compounds may be present.
COD Testing Method:
COD is measured by mixing the wastewater sample with a powerful oxidizing agent (usually potassium dichromate in the presence of sulfuric acid) and heating it in a sealed container for two hours. The amount of oxygen consumed is calculated and expressed in mg/L.
Key Differences Between BOD and COD
| Parameter | BOD | COD |
|---|---|---|
| Basis | Measures oxygen needed by microorganisms to decompose organic matter | Measures oxygen needed to chemically oxidize both organic and inorganic matter |
| Test Duration | 5 days at 20°C | 2–3 hours |
| Agents Used | Aerobic bacteria | Chemical oxidants like potassium dichromate |
| Range | Lower than COD | Higher than BOD |
| Time Efficiency | Time-consuming | Fast and efficient |
| Application | Suitable for municipal sewage analysis | Ideal for industrial wastewater analysis |
| Environmental Impact | Indicates natural biodegradability | Indicates total oxygen demand including synthetic compounds |
BOD and COD Values in Domestic Wastewater
The concentration of BOD and COD in domestic wastewater can help classify the sewage based on pollution strength:
| Classification | BOD (mg/L) | COD (mg/L) |
|---|---|---|
| Weak | < 200 | < 400 |
| Medium | 350 | 700 |
| Strong | 500 | 1000 |
| Very Strong | > 750 | > 1500 |
Higher values of BOD and COD require more advanced treatment processes to meet environmental discharge standards.
You may like this also: BOD Level in STP
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