3. bod& cod
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BOD
The oxygen content of water samples stored ina full, airtight bottle decrease with time
owing to the oxidation oforganic matter by microorganisms
Excretions of aquatic biota, water-solublehumus compounds and industrial,domestic and agricultural effluents
BOD is an empirical determination of theamount of oxygen required to oxidize theorganic matter in the sample
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UBOD
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BOD
BOD5 does not represent the total BOD,since the biological oxidation of organic
matter takes a lot longer than 5 days to
go to completion
95-99% of the reaction is complete after
20 days
The BOD test is widely used to
determine the pollution strength of
wastewater and the quality of receiving
surface waters
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BOD - METHODOLOGY
• The water sample is diluted with a suitable
volume of oxygen-saturated water
containing nutrients and incubated in thedark for 5 days at 20oC
• The DO concentration is determined
before and after incubation and BOD5 iscalculated by difference
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MATERIALS FOR BOD ANALYSIS
o All the reagents required for the DO testby the Winkler method
o BOD incubation bottles: 250 mL capacity
o Air incubator or water bath at 20 1oCplace in the dark
o Phosphate buffer solution
o Calcium chloride solution
o Magnesium sulfate solution
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MATERIALS FOR BOD ANALYSIS
Ferric chloride solution
Dilution water
HCl 1:1
NaOH 6M
pH meter and electrode
NOTE : DISCARD ANY OF THE
REAGENT SOLUTIONS IF THEY SHOW
SIGNS OF BACTERIAL GROWTH
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SAMPLING & STORAGE FOR
BOD ANALYSISCollect 0.5 – 1 L of sample
The test should be started as soon as
possible after sampling
Otherwise, store at 3-5oC in a refrigerator
for up to 24 h.
Bring the sample to 20oC before starting
the test
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BOD ANALYSIS
Measure the pH of the sample, and adjustto pH 7-8 using HCl or NaOH. If the pH is
between 6.5 & 8 do not adjust. The recommended dilution
§ raw sewage : 30-100 times dilution
§ purified effluent : 3-10 times dilution§ polluted river water : 1-5 times dilution
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BOD ANALYSIS
Completely fill three 250 mL BOD bottleswith the diluted solution, ensuring that no
air bubbles are trapped
Determine the DO in one bottleimmediately using the Winkler method
Stopper the other two bottles, water-seal,
place in the air incubator and incubate inthe dark for 5 days at 20oC. After 5 days,
determine the DO using the Winkler
method
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BOD ANALYSIS
Fill 3 bottles with dilution water in the same way.Determine DO in one bottle immediately and the
other after incubating for 5 days
This dilution water blank serves as a rough
check on the quality of the dilution water and itshould not exceed 0.2 mg/L
Calculate the BOD = F (Di – Df )
Di is the initial DO in the diluted sampleDf is the DO in diluted sample after 5 days of
incubation
F is the dilution factor
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COD (Chemical Oxygen Demand)
A measure of the oxygen equivalent of theorganic matter in the sample that is
susceptible to oxidation by a strongoxidizing agent.
BOD determines compounds that can bebiologically oxidized, COD measuresthose substances which can be chemicallyoxidized.
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METHODOLOGY-COD
o The sample is refluxed with excess
potassium dichromate in concentrated
sulfuric acid for 2 h.
o This oxidizes most of the organic matter in
the sample
o Silver sulfate is included as a catalyst to
speed up the oxidation process
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METHODOLOGY-COD
o After digestion, the unreacted dichromate
remaining in solution is titrated with ferrous
ammonium sulfate:
Cr 2O72- + 6Fe2+ + 14H+ 2Cr 3+ + 6Fe3+ +
7H2O
The amount of dichromate consumed is
calculated and the oxidisable organicmatter is reported in term of oxygen
equivalent
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COD analysis
COD (mg/L O2) = 8000 x M x (V1 – V2)/Vs
V1 is the volume of ferrous ammoniumsulfate titrant used to titrate the blank (mL)
V2 is the volume of ferrous ammoniumsulfate titrant used to titrate the sample(mL)
Vs is the volume of sample (mL)
M is the exact concentration of ferrousammonium sulfate titrant (M)