Biochemical Oxygen Demand (BOD):

It is the amount of Oxygen is taken up by micro­organism that decomposes organic waste matter in water. It is therefore used as the measure of the amount of certain types of organic pollutants in water. BOD is calculated by taking a sample of water containing a known amount of oxygen for 5 days at 20°C in a BOD incubator.

Chemical Oxygen Demand (COD):

A parameter of water quality which measures the amount of oxygen in parts per million required to oxidize organic and oxidizable inorganic compounds in the water sample. COD is preferable to the BOD because it is rapidly measurable parameter for river, streams and industrial waste studies and control of water treatment plants.

In short,

1. COD or Chemical Oxygen Demand is the total measurement of all chemicals in the water that can be oxidized. Whereas, Biochemical Oxygen Demand is supposed to measure the amount of food (or organic carbons) that bacteria can oxidize.
2. COD is usually measured and the test is simple and easy to perform with the right equipment and can be done in 2 hours. BOD usually takes 5 days.
3. Permissible limit of COD =250 to 500 ppm.

Permissible limit for BOD = 30 mg/L.

Chemical Oxygen Demand (COD)

• COD is a measure of the oxygen equivalent of the organic matter in a water sample that is susceptible to oxidation by a strong chemical oxidant.
• COD is widely used as a measure of the susceptibility to oxidation of the organic and inorganic materials present in water bodies and in the municipal and industrial wastes.
• The COD test of natural water yields the total quantity of oxygen that is required for oxidation of a waste to carbon dioxide and water. In a BOD test, only biologically reactive carbon is oxidized while in a COD test, all organic matter is converted to carbon dioxide.
• The test for COD does not identify the oxidisable material or differentiate between the organic material and inorganic material present. Similarly, it does not indicate the total organic carbon present.
• Consequently, the COD values are higher compared to BOD.
• Nevertheless, COD is a useful variable that can be rapidly measured; the COD test can be performed in 3 hours against 5 days required for a BOD5 test.

Biochemical Oxygen Demand (BOD)

• BOD, also called biological oxygen demand, is the amount of dissolved oxygen needed (i.e., demanded) by aerobic biological organisms to break down organic material present in a given water sample at certain temperature over a specific time period.
• The BOD value is most commonly expressed in milligrams of oxygen consumed per liter of sample during 5 days of incubation at 20 °C and is often used as a surrogate of the degree of organic pollution of water.

Significance of COD & BOD

• COD & BOD are parameters used to measure the health of the water body (such as river or pond). By “health” we mean the amount of dissolved oxygen present in the water which is necessary to support aquatic life such as fishes. Healthy water contains about 7 to 9 ppm of dissolved oxygen.

• When the water body is polluted by decaying organic matter like food waste, dead animals, meat waste or fallen leaves, bacteria start growing because these are food for the bacteria. During bacterial growth, it uses up dissolved oxygen from the water for respiration. Therefore the amount of dissolved oxygen in the water decreases. More the amount of pollution, more bacteria will grow and therefore decrease in dissolved oxygen will be higher. If dissolved oxygen in the water falls below a certain level, fishes and other aquatic organisms start dying.

• BOD estimates the decrease in oxygen content of water due to bacterial growth.

But all types of organic substances are not digested by bacteria. For example, many organic chemicals such as dyes and petroleum wastes are sometimes discharged into water bodies by industries and cause pollution. Bacteria do not grow in them; so BOD cannot estimate pollution due to such compounds.

• All types of organic matter can be oxidised using strong oxidising agents like potassium dichromate in the laboratory. Therefore water samples can be subjected to such chemical oxidation in the lab, and the amount of oxygen required to oxidise all the organic matter present in the water (both biodegradable and non-biodegradable) can be calculated. This is COD. Therefore COD values are usually higher than BOD values for the same sample.

Thus both BOD and COD are indicators of the amount of organic pollution in a water body.