BOD [Biochemical oxygen demand]: Procedure & Significance

BOD [Biochemical oxygen demand]


Biochemical oxygen demand (BOD) is the amount of dissolved oxygen used by microorganisms in the biological process of metabolizing organic matter in water.

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

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

One of the main reasons for treating wastewater before its discharge into a water resource is to lower its BOD, reduce its need for oxygen and thereby lessen its demand from the streams, lakes, rivers, or estuaries into which it is released.

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

The goal of sewage treatment is to stabilize organic matter and convert it into an affluent of acceptable purity to dispose of it safely.

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 healthy, and if it is 100mg/L or below, it is said to be weak.

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 to breakdown the organic matter, thus using a lot of oxygen. So, the oxygen concentration in the water is less, which means higher BOD.

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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.

B.O.D. can be defined as “It 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.”

Biochemical oxygen demand

Importance of the BOD test:

  • It is a primary method to know about the biodegradability of the sewage sample and its strength.
  • Also, it is an important parameter to design the sewage treatment plants, i.e., the size of the trickling filters and activated sludge units.
  • It is an important method to measure the self-purification capacity of the stream.
  • Also, it is an indicator of the efficiency of the treatment plant, i.e., it is used to check the difference between the influent and the effluent as far as the B.O.D. proportion is concerned.

Biochemical oxygen demand test procedure:

The following procedure is followed for the determination of BOD

  1. Collect the water sample for the pond.
  2. Without making the air bubbles, fill the sample bottle.
  3. Add 2 ml of manganese sulfate, into the sample bottle.
  4. Also, add 2 ml of the alkali-iodide-azide reagent in the sample bottle.
  5. Close the bottle and mix the sample till it gets the brownish color [to indicate the presence of oxygen]
  6. Allow the sample to settle out to the bottom of the bottle.
  7. Add 2 ml of conc. H2SO4 (without forming 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 pale yellow color.
  11. Add 2 ml. of starch solution. (It would change the sample color to Blue)
  12. Continue the titration until the sample gets clear and then take the readings.

Frequently Asked Questions:

What BOD means?

BOD indicates the amount of decay-able organic material present in water.

What is significance BOD?

Biochemical oxygen demand provides an index to estimate the impact of wastage on the environment received.

What happens if BOD is high?

When BOD levels are high, the level of dissolved oxygen (DO) decreases as the oxygen available in the water is being consumed by the bacteria.

What is a good BOD level?

A BOD level of 1-2 ppm is considered very good.

Also, read 1. Slump Test 2. Rock quality designation 3. Chemical Oxygen Demand


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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