Biochemical Oxy­gen Demand (BOD) in Water | Biotechnology

In this article we will discuss about the principle, requirements and procedure for determination of Biochemical Oxy­gen Demand (BOD) in water.

Principle:

The sewage/water is incubated at 20°C in BOD incubator for 5 days and the amount of dissolved oxygen is calculated along with blanks and both initial and final volume of O₂ is calculated.

Requirements:

1. Effluent.

2. BOD bottles.

3. Aerated diluent (distilled water).

4. BOD incubator set at 20°C.

5. Water sample.

Reagents:

(i) MnSO₄.4H₂O solution

MnSO₄.4H₂O 480 g

Distilled water 1000 ml

This solution should not give colour with starch when added to an acidified solu­tion of KI.

(ii) Alkali-azide (Alkali-Iodide-azide) rea­gent

a. NaOH 500 g:

Sodium iodide 135 g

Distilled water 1000 ml

b. KOH 700 g:

KI 150g

Distilled water 1000 ml

c. Sodium azide 10 g (NaN₃):

Distilled water 40 ml

Mix a+b and then add c.

1. Stock solution of Na₂S₂O₃ (0.1 N)

Na₂S₂O₃.6H₂O 24.82 g

Distilled water 1000 ml

2. Standard Na₂S₂O₃ solution (0.025 N) 250 ml of stock solution in 750 ml of dis­tilled water or 6.205 g of Na₂S₂O₃ plus 1000 ml distilled water.

3. Concentrated H₂SO₄ analytical grade.

4. Starch solution 1%.

5. Volumetric and graduated pipettes.

6. Burettes.

Procedure:

1. Aerate distilled water for 2 to 3 hours.

2. Take four BOD bottles and label. Blank initial (BI), Blank final (BF), Sample initial (SI) and sample final (SF).

3. First fill Blank initial (BI) and blank final (BF) with aerated distilled water taking care to see that no bubbling takes place.

4. Next fill sample initial (SI) and sample final (SF) with sewage water diluted with aerated distilled water-no bubbling.

5. Stopper the bottles and water seal (no air space).

6. Place blank final (BF) and sample final (SF) bottles in BOD incubator at 20°C for five days.

7. Analyse the blank initial (BI) and sam­ple initial (SI) immediately for their dis­solved oxygen content.

Determination of Dissolved Oxygen (DO):

8. Add 2 ml of MnS0₄ into the bottles, tilt and mix thoroughly.

9. Add 2 ml of alkali azide iodide reagent, tilt and mix to precipitate the dissolved O₂.

10. Add 2 ml concentrated H₂SO₄ to dis­solve the precipitate.

11. Take 200 ml of sample containing liber­ated I₂ and titrate against 0.025(N) Na₂S₂O₃ using starch as indicator-end point dark blue to colourless.

12. Take initial and final (5th day) readings of blank and samples and calculate BOD using titre value.

mg BOD/litre = R X dilution factor,

where R= O₂ consumed,

i.e. (SI-SF) – (BI-BF)

i.e. initial and final sample readings mi­nus initial and final blank readings.

1.0 ml of standard Na₂S₂O₃ solution is 0. 025 (N).

e.g. BI = 6.5 BF = 4.2

SI = 6.6 SF = 2.3

BOD mg/lit = R X Dilution factor.

R = O₂ consumed (SI-SF) – (BI-BF)

1. e. (6.6-2.3) – (6.5 – 4.2) X 1000 = 2000 mg/lit.