Friday, January 22, 2016

Determination of B.O.D.

BIOCHEMICAL OXYGEN DEMAND (B.O.D.) TEST


AIM:- To determine the Biochemical oxygen demand of the sewage sample.

APPARATUS:-

i)                    Glassware
ii)                  B.O.D. bottle or incubation bottle
iii)                Incubator.

THEORY:-
           
            The B.O.D. of sewage (domestic or industrial) or any polluted waste water is defined as the amount of oxygen required for the biochemical decomposition of dissolve organic solids to occur under aerobic conditions & under standardized time & temperature. The laboratory test for B.O.D is an empirical test conduced at a temperature of 200C & an incubation period of 5 days (sometimes 7 days). This test is based on the principle of determination of dissolved oxygen before & often incubation for a specified period (5 days) at standard temperature (200C). the difference in D.O value  gives oxygen utilized by the bacteria for the oxidation of organic matter, which is expressed as mg/lit. the rate of oxidation depends upon the amount & type of organic matter, dissolved oxygen, temperature, types of organisms & environmental conditions. Under natural conditions, complete oxidation of organic matter takes about 2-3 months but with in 10 days nearly 90% biological demand is satisfied after which the rate of oxygen consumption becomes very slow. In the laboratory, 5-days BOD is tested which gives 60-70% of total demand.

            The decomposition or decay of organic substance take place by the help of enzymes, which are produced by the secretion of protein molecules from the living organisms. The organic matter & some of the complex mineral portions of those substances are utilized as a source of energy by a series of living organisms. A series of biochemical reaction is thereby set in motion, & polluted waters are returned to a normal state or purity.

RELEVANCE:-

            The BOD has tremendous applications in the field of sanitary engineering. Some of these applications are as follows.

i)                    To measure the strength of sewage & wastes.
ii)                  To determine the amount of pollution in a stream.
iii)                In the design of water treatment facilities.
iv)                To determine the efficiency of the treatment units.
v)                  In the establishment of stream & effluent standards for stream pollution regulatory agencies.
THE NORMAL VALUES OF B.O.D FOR RAW & TREATED SEWAGE (DOMESTI) ARE GIVEN BELOW.

Nature of sewage
 BOD5 at 200C in mg/l

1, Strong sewage

2. Average sewage

3. Weak sewage

4. Std. filter sewage effluent

5. Very good filter sewage effluent



                  450 – 500

                     350

                     250

                      20

                    5 – 10   



REAGENTS FOR DILUTION WATER:-

i)                    Distilled water for dilution.
ii)                  Phosphate buffer solution consisting of 8.5g KH2 PO4, 21.75g K2 HPO4, 33.4g NA2HPO4.7H2O and 1.7g of NH4CL dissolved in 1 lit. PH of this solution is 7.2.
iii)                Magnesium sulphate solution consisting of 22.5g MgSo4 7H2O dissolved in distilled water & diluted water diluted to 1 lit.
iv)                Calcium chloride solution consisting of 27.5g cacl2 dissolved in distilled water & diluted to 1 lit.
v)                  Ferric chloride solution, consisting of 0.25g Fecl3. 6H2o dissolved in distilled water & diluted to 1 lit.

REAGENT FOR D.O MEASUREMENT: (By AZIDE MODIFICATION METHOD)

i)                    Manganese sulphate solution: Dissolve 480g mnso4.4H2O in distilled water and dilute it to 1 lit.
ii)                  Alkali – Iodide – Azide reagent: Dissolve 500g Na oH and 150g KI in distilled water and dilute it to 1 lit. add 10g sodium Azide(NaN3) dissolved in 40ml distilled water
iii)                Concentrated H2So4.
iv)                Starch:- Dissolve 5g starch in 1 lit boiling distilled water.
v)                  Sodium tiosulphate titrant.:- Dissolve 6.205g Na2 S2 O3.5H2O in distilled water and dilute it to 1 lit. Standardize it using standard potassium dichromate solution.
vi)                Standard potassium dichromate solution (0.025N):- Dissolve 1.22g K2CR2 O7 in distilled water and dilute it to 1 lit.

PROCEDURE:-

Standardization of sodium thiosulphate titrant:-

a)      Take 100ml distilled water in a flask and add 10ml KI solution. Add 10ml 1+9 H2SO4.
b)      Add 20ml K2CR2O7 solution and store in dark place for five minute.
c)      Titrate with Na2S2O3 titrant, adding starch towards the end when a pale straw colour develops. Continue titration to the colourless end point. Note down the volume of titrant as V2 ml.
d)     Determine the normality of the titrant ‘N2’ using the relation N1V1 = N2V2 where
N1 = normality K2 CR2 O7 solution = 0.02
V1 = volume of K2 CR2 O7 = 20ml



Preparation of dilution water and placing it in BOD bottle

a)      Aerate the distilled water saturation with D.O. Take 1 lit of this dilution water and 1ml of each of phosphate buffer, MgSo4, CaCl2 and FeCl2 solutions. Stir to mix the contents.
b)      Preparing samples of different dilution. The following dilutions are recommended depending upon the expected BOD of the waste water sample.
                                   25%, 40%, 60%, 75% and 100%

                        To make dilution of say 25%, take 1 lit measuring cylinder and fill it about half with dilution water by siphoning. Add 250ml sample and make it 1 lit with dilution water stir well to mix properly. Transfer this by siphon into two BOD bottles.

c)      Repeat the procedure for other dilutions.
d)     Place one bottle in the incubator and test the second bottle immediately for D.O. using Azide – Modification method discussed later. Let this value be “D1”.
e)      Incubate the sample for 5 days at 200C and test for D.O. again. Let this value be “D2”.

Alternate method of diluting the sample:-
            In this method, the sample sewage is diluted to the required extent by direct pipetting a given volume of sewage into a 300ml BOD bottle and filling the remaining volume of bottle with dilution water and calculating the dilution factor. For example, if 5ml of waste water is placed in the BOD bottle and mixed with dilution water to make 300ml diluted sample, the dilution ratio will be equal to 300/5=60. The following table gives the range of BOD that can be measured with various dilutions expressed either percentage mixtures or as ml directly pipetted into 300ml BOD bottles.



    By using % mixture
By direct pipetting into 300ml BOD bottle
% mixing
Range of BOD in    mg/lit
        ml
Range of BOD in mg/lit
0.01

0.02

0.05

0.10

0.20

0.50

1.0

2.0

5.0

10.0

20.0

50.0

100.0
20000-70000

10000-35000

4000-14000

2000-7000

1000-3500

400-1400

200-700

100-35

40-140

20-70

10-35

4-14

0-7
0.03

0.05

0.10

0.20

0.50

1.0

2.0

5.0

10.0

20.0

50.0

100.0

300.0
30000-105000

12000-42000

6000-21000

3000-10500

1200-4200

600-2100

300-1050

120-240

60-210

30-105

12-42

6-21

0-7

Estimation of D.O.:-

a)      To the sample in the BOD bottle, add 2ml manganese sulphate solution, 2ml Alkali – iodide – Azide reagent, well below the surface.
b)      Stopper and mix by inverting bottle at least 15 times. Let the brown precipitate settle for 5 minutes.
c)      Add 2ml conc. H2SO4 and stopper. Gently invert to dissolve the precipitate totally.
d)     Take 203ml of bottle water in a flask and titrate with 0.025N sodium thiosulphate solution.
e)      Add 1 to 2ml starch and continue titration till the first disappearance of pale blue colour.
f)       Note down the volume of titrant used upto colourless end point.

Note:- 200ml of sample have to be used in the titration. The volume corresponding to
200ml of original sample after correction for a total of 4ml (2ml manganese solution and 2ml alkali-azide reagent) in a 300 ml bottle is

                200*300 = 202.7ml = 203ml.
                 (300-4)

OBSERVATIONS:-

S.No
Burette Reading
Vol. of Titrant used
Initial
Final












CALCULATIONS:

Initial D.O. of dilute sample (before incubation)
In mg/l = D1 = ml of 0.025N sodium thiosulphate used.

Final D.O. of dilute sample (after incubation)
In mg/l = D2 = ml of 0.025N sodium thiosulphate used.

BOD of the sample in mg/l = D1 – D2 (if dilution is expressed in percentage)
                                                      P
Where P = Decimal fraction of sample used.

BOD of the sample in mg/l = (D1 – D2)* dilution ratio (if sample is directly pipetted into BOD bottle)

Where dilution ratio =        Volume of diluted sample       
                                     Volume of undiluted sewage sample

RESULTS:-


INTERPRETATION OF RESULTS:


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