WHAT IS BOD AND COD? EXPLAIN IN BRIEF ABOUT IT.

• The term BOD and COD are stands for Biological oxygen demand and Chemical oxygen demand respectively. They are used in case of self purification of water from natural sources and diluted waste water. Mostly for polluted water sources having organic impurities and contaminates.

      BOD test :

• The amount of oxygen required by micro-organism to oxidize organic waste aerobically is called biological oxygen demand. 
• The BOD test is used to measure waste loads to treatment plants, determine plant efficiency (in terms of BOD removal), and control plant processes.  It is also used to determine the effects of discharges on receiving waters.  A major disadvantage of the BOD test is the amount of time (5 days) required to obtain the results.

• A sample is putting into a BOD bottle containing aerated dilution water.  At initiation, DO content is determined and recorded and the bottle is kept in the dark for five days at 20°C.  At the end of five days, the final DO content is determined and the difference between the final DOf reading and the initial DOi  reading is calculated. The decrease in DO is corrected for sample dilution, and represents the biochemical oxygen demand of the sample.
• The BOD of dilluted water sample is calculated by,                              BOD5 = DOi - DOf                                                                                    P
• Where, DOi = Dissolved oxygen in dilluted water at initial                         stage                                                                        DOf  = Dissoved oxygen in dilluted water at final                            stage                                                                        P  =  The dillution factor                                                           = Volume of undilluted sample                                              Volume of dilluted sample

            COD test :

• The amount of oxygen required for oxidize all the organics (biodegradable or non-biodegradable) in a waste water or polluted water.
• The COD test only takes a few hours to complete, giving it a major advantage over the 5-day BOD test. Wastewater treatment system personnel can use COD as an almost real-time operational adjustment parameter.
• COD can test wastewater that is too toxic for the BOD test.
• The COD test uses a chemicals (potassium dichromate in a 50% sulfuric acid solution) that oxidizes both biodegradable and non-biodegradable substances in a wastewater sample, which results in a higher COD concentration than BOD concentration for the same wastewater sample since only organic compounds are consumed during BOD testing.

• The difference between BOD and COD.                            BOD
• It is stands for biological oxygen demand.
• Demanded oxygen is supplied by the water itself.
• This method is used for biodegradable waste only.
• The value of BOD is always less than the value of COD.
• The time duration take by the BOD is 5 days.
• It is less scientific.                                                                 COD
• It is stands for chemical oxygen demand.
• Demanded oxygen is supplied by the solution of chemicals (pottasium chromate in 50% solution of sulphuric acid)
• This is used for both biodegradable and non-biodegradable waste.
• The value of COD is always higher than the value of BOD.
• The time duration take by the COD is nearly about 2 hours.
• It is more scientific.