Biological Oxygen Demand (BOD)
Definition
Water pollution may arise due to the biochemical degradation of organic matter by micro-organisms utilising the dissolved oxygen. The quanity of oxygen consumed by
the miçro-organisms to break down the organic matter in a water sample over a period of 5 days at 20°C is referred to as the Biochemical Oxygen Demand. It is usually expressed as mg/litre or ppm. The BOD may, therefore, be defined asthe number of parts by weight of oxygen required by the micro-organisms to break down the organic present in million parts by weight of water sample Over a period of 5 days at 20C.
Determination of BOD
Principle
The BOD of a water sample is determined by DO content of the sample before and measuring the DO after incubation for 5 days at 20°C. The depletion in DO content is the measure of BOD
Method
1. Preparation of dilution water
The diiution water is needed to dilute many waste water samples where the DO is less than 7 ppm. It is prepared by adding 1 ml each of phosphate bufter (pH =7.2), MgSO, soution (22.5g/ litre), CaCl,solution (27.5g / litre) and ferric chloride solution (0.25 g/ litre) to a desired volune of distilled water in a measuring flask. The solution is then made upto l litre.
2. Dilution technique
The waste water sample is properly diluted with dilution water. The extent of dilution depends on the nature of sample and'may vary fróm 1% to 100%. For example, L ml of industrial waste water is diluted to 100 ml with dịlution water whereas for polluted river
samples, 25-50 ml are usually diluted to 100ml.
3. Seeding
Microorganisms carry out oxidation of organic matter in waste water. Addịtion of microorganisıns (seeding) is necessary for waste water sanples which are deficient in microorganisms. Seeding is not required for domestic waste water or surface water in which enough microorganisms are already present.
4. Procedure
i) Aerate the waste water sampie in a glass container by. bubbling air for 5 minutes.
ii) Now, the water sample is diluted with dilution water if BOD is greater than DO level.
ii) Seed the diluted waste water sample with a little diluted domestic waste water. Measure the DO of the sample. (D).
iv) Fill a screw - topped incubation bottle to the brimn with the remaining diluted water sample. Seal the bottle and incubate in the dark for S days at 20°C, Measure DO of the sample (D,).
v) Similarly, measure DO of the dilution water before ańd after incubation. Let it be B, and B,
Calculation
BOD (mg/litre) =(D1 - D2 ) - (B1 -B2) x Dilution factor.
Chemical Oxygen Demand (COD)
Definition sample of water
The total amount of organic matter present in a is measured in terms of Chemical Oxygen Demand (COD)) It may be defined as the amount of oxygen (in mg) required by organic matter in a known volume (in litre) of wate for its oxidation by a strong oxidising agent such as acidified K2Cr2O7
Determination of COD
A known volume (say 250 ml) of the water sample is refluxed with a known excess of standard K2Cr2O7 and dil.H2SO4 in prescnce ,of a little Ag2SO4 and HgSO4 (catalysts) for 1% hours. The unreacted K2Cr2O7 is then titrated against standard ferrous amm.Sulphate (FAS) solution using ferroin as the indicator.
The difference between the dichromate added and the dichromate remnaining unreacted gives the amount of dichromateused up for oxidation. It is a measure of oxygen consumed by the organic matter present in the water sample.
Iml of IN K2Cr2O7= 0.008gm of oxygen
COD in ppm= Vx N x 0.008 x 10⁶/X
where
V= Volume of K2Cr207 consumed in ml
N= Normality of K2Cr2O7
X= Volume of water sample taken in ml
Water quality standards
The permissible or tolerance limits of various water pollutarts are prescribed by Word Health Organisation (WHO),Indian Council of Medical Research.(ICMR) and Indian Standards Institution (ISI)