Difference between BOD and COD – With Explanation (With PDF)

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Today, I am going to discuss the difference between BOD and COD in a detailed manner with its definitions and explanation.

Before I start, I want to kindly inform you that, don’t forget to take a PDF of this whole article at the end. First, Let’s start with the bod and cod definition first.

So, Here we go…

difference between BOD and COD

Chemical Oxygen Demand (COD) and Biological Oxygen Demand (BOD) are the two main characteristic properties that indicate the strength of sewage.

Both parameters test the oxygen-demanding strength of the wastewater.

Biological Oxygen Demand (BOD)-

BOD is defined as ”The amount of oxygen demanded by the microorganisms in the sewage for the decomposition of bio-degradable matter under aerobic conditions”.

This is the most commonly used parameter to determine the strength of the municipal or organic quality of the water.

The standard BOD test determines the amount of oxygen required by the microorganisms for the decomposition of the bio-degradable matter present in the wastewater.

BOD is determined for designing treatment facilities. It is used to determine the size of activated sludge units and tricking filters.

It is used to determine the efficiency of various treatment units. 

Here are some important points of Biological Oxygen Demand (BOD)-

  • BOD is a measurement of the amount of oxygen that requires for the bacteria to degrade the organic components present in water/wastewater.
  • The term also refers to a chemical procedure for determining this amount. This is not a precise quantitative test although. It is widely used as an indication of the organic quality of water.
  • The BOD value is most commonly expressed in milligrams of oxygen consumed per liter of the sample during five days of incubation at 20°C.
  • BOD values are thus useful in the evaluation of the self-purification capacity of a water body and for possible control measures of pollution.
  • The quantity of oxygen in water (dissolved oxygen -DO) along with BOD is indicated by the kind of organisms present in water. Thus fish become rare at a DO(dissolved oxygen) value of 4 to 5 ppm of water. Further decrease in DO value may lead to an increase in anaerobic bacteria.

Chemical Oxygen Demand (COD)-

The amount of oxygen that is required for the chemical oxidation of the organic and inorganic chemicals present in the wastewater by utilizing oxidizing agents like potassium permanganate potassium dichromate etc. (KMNO4) and (K2Cr2O7) is called chemical oxygen demand (COD)”.

The presence of COD facilitates rapid chemical oxidation of organic matter without any additional equipment. This is the only method that enables determining the organic load in heavy toxic sewage.

To determine COD, the sample water is taken in a closed container and incubated with a strong oxidant like potassium dichromate with H₂SO4 under standard conditions of temperature and time.

COD is the oxygen demand that is compounded by both inorganic and organic matter present in the wasted water sample.

The chemical oxygen demand is expressed as the mass of oxygen consumed over the volume of the solution.

Its SI units is milligrams per liter (mg/l) COD or chemical oxygen demand is the total measurement of all chemical (organics and inorganics) in the water/wastewater.

Here are some important points of Chemical Oxygen Demand (COD)-

  • COD (chemical oxygen demand) is the amount of oxygen required to degenerate all in a chemical way.
  • It is expressed in milligrams per liter (mg/l) also referred to as ppm (parts per million) which indicates the mass of oxygen consumed per liter of solution.
  • Measures oxygen equivalent of organic matter provided the latter is susceptible to oxidation by (K2Cr2O7) potassium dichromate.
  • Oxidation is brought about under acidic conditions by H₂SO4 a reagent at high temperatures.
  • Not a good measure for biodegradable organic matter and is not capable of oxidizing all the organic matter.

Now, Let’s move to the difference section after knowing BOD and COD definitions and their important points.

Difference between BOD and COD-

Here are some major differences between Chemical Oxygen Demand and Biological Oxygen Demand-

S.NO.Biological Oxygen Demand (BOD)Chemical Oxygen Demand (COD)
01.BOD is the amount of oxygen consumed by bacteria while decomposing organic matter under aerobic conditions.COD is the amount of oxygen required for the oxidation of total organic matter in water.
02.Biological oxidation processChemical oxidation process
03.Determined by incubating scaled water under a specific temperature sample for five days and measuring the loss of oxygen from the beginning of the test.Determined by incubating a closed water sample with a strong oxidant like potassium dichromate (K2Cr4) in combination with boiling H2SO4 under a specific temperature for a specified period of time.
04.Five days are taken for the determination.COD measurement can be taken within a few days.
05.The permissible limit is 30 mg/L or ppm.The permissible limit is 250 to 500 ppm.
06.Value is lower than the COD value.Always greater than the BOD value.
07.Capable of oxidizing natural organic detritus and the organic waste in the water.Capable of degrading industrial sewage but does hot measure the oxygen consumption of acetate.
08.Accuracy ±10%Accuracy ±2%.


Wrapping Up-

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