BOD [Biochemical oxygen demand]

Biochemical oxygen demand (BOD) is the amount of oxygen required for microbes to carry out the biological decomposition of the dissolved solids or the organic material in the sewage under the aerobic conditions at a given temperature.

More organic matter in sewage and polluted bodies of water, higher BOD; the higher the BOD, the lower the amount of dissolved oxygen (DO) available for higher animals such as fishes.

BOD is a reliable gauge of organic pollution of the body of water.

The main reasons for treating wastewater before its discharge into the water resource are to reduce its BOD, reducing the need for oxygen, thereby reducing its demand from the rivers, lakes, rivers, or inlets in which it is released.

It is widely used as an indication of water quality, especially in sewage treatment plants.

The goal of sewage treatment is to stabilize the organic material and transform it into a compound of acceptable purity for safe disposal.

How is BOD measured?

BOD is calculated by using the formula BOD = (D1 – D2)/P


D1 = initial dissolved oxygen concentration.

D2 = fifth day dissolved oxygen concentration.

P = volumetric fraction of wastewater.

BOD values range from 1 mg/L for natural waters to about 300mg/L for untreated domestic sewage waters.

If the BOD is 300mg/L and above, sewage is said to be weak, and if it is 100mg/L or below, it is said to be healthy.

BOD of wastewater is typically 110-440mg/L and must be reduced to 20mg/L for discharge.

More waste in the sewers will lead to more decomposing bacteria breaking down the organic matter, thus using a lot of oxygen.

So, the oxygen concentration in the water is less, which means higher BOD.

Oxygen demand for the oxidation of both organic and inorganic material in the sewage, the oxygen is demanded.

This demand for oxygen for inorganic matter in the wastewater is completed up to 97.50% in a period of 24 hours with an average temperature of 20°C.

The demand for oxygen for organic matter is called Biological oxygen demand (BOD). The bacteria in the water consume oxygen.

These micros get energy by decomposing the organic matter to convert it into the purest form CO2 and H20 [in the presence of oxygen], this requirement is called a bold.

Due to this consumption of 02 DO depletion takes place (the proportion of DO is reduced from 7.8 mg1 I up to 2 to 3 mg/L.

And due to this depiction of DO, the vegetation gets decomposed and the animals migrate, creating anoxic conditions to control this condition, it is necessary to supply oxygen to the water.

Biochemical oxygen demand


  • The primary method is to know about the biodegradability and robustness of sewage samples.
  • In addition, it is an important parameter for designing sewage treatment plants, i.e., the size of trickling filters and activated sludge units.
  • It is an important method for measuring the self-purification capacity of a stream.
  • Also, it is an indicator of the efficiency of the treatment plant, i.e., it is used to examine the difference between effectiveness as far as the BOD ratio is concerned.


The following procedure is followed for the determination of BOD:

  1. Take a sample of water for the pond.
  2. Without making air bubbles, fill the sample bottle.
  3. Add 2 ml of manganese sulfate to the sample bottle.
  4. Also, mix 2 ml of alkali-iodide-azide reagent in the sample bottle.
  5. Close the bottle and mix the sample until it turns brown [to indicate the presence of oxygen]
  6. Allow the sample to settle on the bottom of the bottle.
  7. Add 2 ml. H2SO4 (without creating air bubbles)
  8. Close the bottle and mix the solution to dissolve the precipitate.
  9. Keep the sample bottle in the BOD incubator for five days.
  10. After the incubation period, titrate 50 mL of sample water with 0.025 N sodium thiosulfate to turn yellow.
  11. Add 2 ml. of starch solution. (This will change the sample color to blue)
  12. Continue the titration until the sample is clear and then take readings.
Also read: Chemical Oxygen Demand (COD) & CBR Test ( California Bering Ratio Test)


The concentration of Dissolved Oxygen (D.O.) in the sample BOD is equivalent to the number of milliliters of titrant used.

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3 thoughts on “BOD [Biochemical oxygen demand]”

  1. “If the BOD is 300mg/l and above, sewage is said to be healthy, and if is 100mg/l or below it is said to be weak”. I doubt if this statement is right, it should have been if the BOD is 300mg/l and above, sewage is said to be weak, and if is 100mg/l or below it is said to be healthy

  2. We can also say if BOD is above 300 mg/L, it is a water of concern and not fit for directly discharging .
    If it is 100 mg/L or less, it is acceptable and can be discharged


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