Biochemical Oxygen Demand And Chemical Oxygen Demand

Biochemical Oxygen Demand And Chemical Oxygen Demand


  • required for oxidation of inorganic and organic matter.
  • essential for the livelihood of micro organisms.
  • can be measured by
    • BOD – Biochemical oxygen demand
    •  COD – Chemical oxygen demand

      Biochemical Oxygen Demand And Chemical Oxygen Demand
      Biochemical Oxygen Demand And Chemical Oxygen Demand

Biochemical Oxygen Demand


  • measures the quantity of oxygen consumed by microorganisms during the decomposition of organic matter.
  • indirect measure of biodegradable organic compounds in water.


  • determining degree of H2O pollution.
  • Important measurement in operation of sewage treatment plant.
  • Comparing BOD of incoming sewage & effluent- efficiency, effectiveness of treatment is judged.
  • For example, in a typical residential city raw sewage has a BOD value of around 300 mg/L. If the effluent from the sewage treatment plant has a BOD. of about 30 mg/L, the plant has removed 90 percent of the BOD

Dilution Method

  • DO Is measured prior to incubation.
  • allowed to stand for five days at a controlled temperature of 20 °C (68 °F).
  • At the end of the five-day period, the remaining dissolved oxygen is measured.
  • BODt = (DOi – DOf) × D.F.


  • BODt = Biochemical oxygen demand at t days.
  • DOi = initial dissolved oxygen before incubation.
  • DOf = final dissolved oxygen.
  • D.F.= dilution factor= volume of the bottle/volume of the sample.



  • Dilution is required.
  • Pretreatment if toxic wastes.
  • Long period of time.
  • Seeding for industrial water.

Chemical Oxygen Demand


  • Measure of oxygen equivalent of the organic matter content of the sample that is susceptible to oxidation by a strong chemical oxidant (acid + heat).
  • COD test results are used for monitoring and control of discharges, and for assessing treatment plant performance.
  • Expressed in mg/l or ppm.


  • Potassium dichromate, a strong oxidizing agent is used under acidic conditions to find the amount of organic compound in waste water sample.
  • Acidity is usually achieved by the addition of sulphuric acid.
  • The amount of Cr3+ is determined after oxidization is complete, and is used as an indirect measure of the organic contents of the water sample.
  • To do so, the excess potassium dichromate is titrated with ferrous ammonium sulphate (FAS) until all of the excess oxidizing agent has been reduced to Cr3+.

Accuracy Of The Test

  • It is important that no outside organic material be accidentally added to the sample to be measured.
  • To control for this, blank sample is used created by adding all reagents (e.g. acid and oxidizing agent) to a volume of distilled water. COD is measured for both and the two are compared i.e. COD of blank – COD of sample.


  • Chemical Oxidant is not specific to oxygen-consuming chemicals that are organic or inorganic, both of these sources of oxygen demand are measured in a COD assay.
  • It does not distinguish between Biodegradable and Non-Biodegradable organic matter.
  • The test does not measure the oxygen demand caused. by the oxidation of ammonia into nitrate


  • Faster process control.
  • COD and BOD do not necessarily measure the same types of oxygen consumption.
  • COD is always greater than BOD measurements.
  • COD is a more stable measurement method.
  • How can a correlation be determined?
  • Collect empirical data
  • COD and BOD data for the same water sample collected over the same period of   time.
  • Graph data
  • Graph COD and BOD data to determine  whether or not a correlation exists.



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