Biological oxygen demand (BOD)

BIOLOGICAL OXYGEN DEMAND (BOD)

         BOD is defined as the amount of oxygen required by bacteria for stabilizing decomposable organic mater under aerobic conditions. BOD is the major criteria used in stream pollution control, where organic loading must be restricted to maintain desired dissolved oxygen levels. It is highly useful in studies of measuring the purification capacity of streams and serves regulatory authorities as a means of checking the quality of effluent discharged to such waters. It is the only test that gives a measure of the amount of biologically oxidisable organic matter.

Aim

         To determine the dissolved oxygen content of the given water sample.

Principle

         Measuring the oxygen consumed by bacterial and chemical action in a closed sample of water maintained at 200C for 5 days.

         A sample with a low BOD can be straight away used for the BOD determination. But a sample with high BOD must be diluted and pretreated before BOD determination.

Materials required

i)                    Phosphate buffer solution: Dissolve 8.5 g of KH2PO4, 21.75g of K2HPO4 and 1 g  NH4Cl in 500 mL distilled water. Dilute this solution to 1 litre

ii)                  Ferric chloride : Dissolve  0.25g FeCl3.6H2O in water and dilute the solution to 1 lit.

iii)                Calcium chloride : Dissolve 27.5g CaCl2 in 1 litre if distilled water

iv)                Magnesium sulphate solution : Dissolve 82.5g MgSO4.7H2O in 1 litre of distilled water and

v)                  Water for dilution:  Take 2 litres of distilled water in 3 litre bottle hake this partially filled bottle for about ten minutes so that the distilled water gets saturated with atmospheric oxygen. Then add to this 2 mL of phosphate buffer solution, 2 mL of MgSO4 solution, 2 mL CaCl2 solution and 2 mL of ferric chloride solution.

Procedure

BOD determination without dilution

If the sample is fairly clean, it can be used as such for BOD determination. Sample with very low bacterial content may not be stabilized in 5 days. These samples require seeding with a small known volume of domestic sewage water. When it is done, a seed correction in the measured BOD value is made. The value for correction is determined by measuring the BOD value of sewage water used for seeding.

·         Fill four BOD bottles upto the rim and after 15 minutes tightly stopper these and make sure that there are no air bubbles inside the bottles.

·         Use the samples in two bottled for initial dissolved oxygen measurement

·         Place the other two bottles in an incubator at 200C for 5 days.

·         After 5 days, determine the dissolved oxygen in these two bottles.

BOD determination after dilution

         This method is used when the BOD value exceeds 5.

Dilution of sample required for various ranges of expected BOD.

Expected BOD (me L-1) in sample

Volume of sample                       (mL L-1)

Dilution factor

0-6

1000

1

4-12

500

2

10-30

200

5

20-60

100

10

40-120

50

20

100-300

20

50

200-600

10

100

400-1200

5

200

1000-3000

2

500

2000-6000

1

1000

> 6000

0.5

2000

Determine the DO of the diluted sample using one of the bottles immediately. Incubate the other bottle for 5 days at 200C and determine the DO.

Calculation

BOD (mg L-1)           =

Where,

DOI                          = DO of diluted water sample before incubation

DOF                         = DO of diluted water sample after five days of incubation

BC                           = Blank correction (Initial DO – Final DO of the blank).