Biochemical Oxygen Demand (BOD)

References: - 1) Standard Methods for the examination of water & wastewater.
                           by A.P.H.A. ( 16th Edition - 1985.)
                       2) Chemistry for Environmental Engineering.
                           By Sawyer, Mccarty & Parkin (4th Edition – 1988.)

Aim: - To find BOD of given wastewater sample.

Apparatus: - BOD incubator, BOD Bottles.

Reagents :-

1) Distilled Water

2) Phosphate Buffer Solution

3) Magnesium Sulphate Solution

4) Calcium Chloride Solution

5) Ferric Chloride Solution

6) Manganese Sulphate Solution.

7) Alkali – Iodide – Azide solution.

8) Standard Sodium Thiosulphate Solution (0.025N)

9) Starch Indicator.

10) Concentrated Sulphuric Acid.

Significance: -

1) To determine pollutional strength of wastewater.

2) To control stream pollution.

3) To evaluate the performance of treatment units & in designing the treatment units.

Standards of BOD₅ at 20⁰C

1) Effluents discharged into intend surface water 30mg/lit.

2) Effluents to be discharged on land 250 mg/lit.

Theory :-

         BOD is defined as the amount of dissolved oxygen required by bacteria to oxidize the decomposable organic matter, present in wastewater under aerobic condition.

             A known volume of a sample of wastewater is incubated at 20⁰ c for 5 days. DO depletion in the test bottles is a measure of amount of biodegradable organic matter present in the sample.

Significance of 5 days & 20⁰C :

1) Theoretically, time required for 100% exertion of BOD is infinite. BOD exertion varies with time, temperature. During first 5 days 2/3 (60%) of total BOD is exerted which gives fair idea about biodegradable organic matter present in wastewater.

2) During BOD reaction, in first 8 days, carbonaceous organic matter is degraded by bacteria into CO2, H2O, NH3 and new bacteria cells. After 8 days nitrifying bacteria start oxidizing NH3 to nitrites and Nitrates. Thus additional BOD is exerted after 8 days. Hence to get fair idea of pollutional strength without interference of nitrification it is customary to determine 5 days BOD.
                                                                             
3) Temperature is maintained constant at 20⁰C to eliminate variations in biological activity. This is the temperature of natural water bodies and development of bacteria that stabilize carbonaceous matter is satisfactory. Also with 20⁰C temperature growth of nitrifying bacteria is inhibited during the period of the test.

Necessity of Dilution: -

High strength wastewater is diluted to ensure that dissolved oxygen will be present through out the period of test. Dilution factor is selected such that final DO of diluted and incubated sample should be at least 2 Mg/lit.

Necessity of Seeding :-

For sewage samples seeding is not required as microorganisms are already present in it. But in case of industrial wastes, microorganisms are not present; in this case seeding is necessary to provide microorganisms in BOD test. Old settled sludge is used as seeding.

Necessity of Blank: -

The added seed may contain organic matter. Thus due to seeding there will be increase in the amount of organic matter. Therefore correction is applied by deducting the DO depletion in blank.

Conditions of BOD test: -

1) Temperature should be constant through out the test.

2) Sufficient nutrients should be present for the growth of microorganisms.

3) pH should be maintained in the range of 6.5 to 8.5.

4) Sufficient quantity of bacteria should be present in sample under incubation.

Procedure: -

(A)Preparation of dilution Water: -

1) Aerate one liter distilled water to get DO above 7 mg/lit.

2) Add 1ml each of phosphate buffer (to maintain pH), magnesium sulphate, calcium chloride, ferric chloride (to provide micronutrients and maintain osmotic condition for favourable growth of micro organisms)

3) Add 2ml settled sludge (for seeding)

(B) BOD Test: -

1) Fill two BOD bottles with sample diluted with dilution water.

2) Keep one bottle for incubation and find initial DO of another bottle (D₁)

3) Fill two bottles with dilution water.

4) Keep one bottle for incubation and find initial DO of another bottle (B₁)

5) Find DO of incubated bottles (D₂ and B₂) after 5 days incubation.

Observation Table:-

Sr. No.	Sample	Dilution Factor	DO of Sample   D1            D2	DO of Blank  B1             B2	B.O.D. mg/l

Calculation :-

Normality of  Na₂S₂O₃   =

BOD₅  mg/lit. = ( (D₁ – D₂) – (B₁ – B₂) x F x 1000 ) /  ml of original sample taken

                               

D₁  =  D.O of diluted sample on 0^th day

D₂  =  D.O of diluted sample at the end of 5^th day.

B₁  =  D.O of the dilution water on 0^th day.

B₂  =  D.O of dilution water at the end of 5^th day.

Result:

 

Assignment:-

1) Define BOD of wastewater?

2) Why dilution of sample is necessary?

3) Why addition of seed is carried out during BOD test?

1) Why addition of micronutrients is carried out during BOD test?

2) What is the permissible BOD value of disposal of treated wastewater in river and on land?

3) What is the formula for dilution factor?

4) Why aeration of dilution water is done?

5) What is the principle of Determination of BOD?

6) What is the necessity of taking blank during COD determination?



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