It is the oxygen quantity demanded by the aerobic micro organisms (bacteria) to stabilize the organic matter. Since BOD is directly proportional to the organic matter concentration, value of BOD is taken as the strength of pollution.
In environmental engineering, this parameter is very important for designing treatment plant and assessing the efficiency of the plant.
Significance
Many biological treatment plant effluents contents sufficient number of nitrifying organisms to cause nitrification in BOD tests. Because oxidation of nitoginus compound can occur in such sample, in hibition of nitrification is recommended for sample of secondary effluent for sample seeded with secondary effluent and sample of polluted waters.
Principle
The method consists of filling a specially manufactured airtight BOD bottle with sample to overflowing, and incubating it at specified temperature conventionally at 20oC for 5 days and the BOD is computed from the difference between initial and final D.O.
Reagents
1. Dilution water saturated with air: Sufficient quantity of distilled water is aerated with fish pond aerator/air compressor for more than 8-10 hours, and the aerated water is kept at low temperature till use. At the time of experiment this water is used for preparation of different dilution of the sample.
2. Seed: Sewage (fresh) or settled for 24 hrs and the supernatant is used as seed. Quantity of seed should be chosen such that it won’t exert more than 20% depletion of DO 3.
Buffers and nutrients:
a) Phosphate buffer: Dissolved 8.5g KH2PO4 21.75g K2HPO4 33.4g Na2HPO4, 7H2O, and 1.7 g NH4Cl in 500Ml distilled water. Mix and make the final volume to 1L pH of the solution is 7.5.
b) MgSO4: Dissolved 22.5 g MgSO4, 7H2O in distilled water and make the volume to 1L.
c) CaCl2: Dissolve 27.5 anhydrous CaCl2 in distilled water and make the volume to 1L
d) FeCl36H20: Dissolve 0.25g of FeCl36H2O in distilled water and make the volume to 1L.
Procedure
a) Preparation of dilution water
1. Add 1ml each of phosphate buffer, magnesium sulfate, calcium chloride and mix well.
2. In case of wastes, which are not expected to have sufficient bacterial population as seed, it is necessary to add supernatant sewage to the dilution water as a source of bacteria. 2ml/L of settled sewage is sufficient as a seed.
3. For nitrification inhibition, add 10mg of 2 chloro 6-(trichloromethyl) pyridine (TCMP) to each 1ltr. of dilution water. Nitrification inhibition is essential for biologically treated samples.
b) Dilution of sample
1. Neutralize the sample to pH around 7.0 if it is highly alkaline or acidic. For acidifying do not use organic acid. Use only mineral acid.
2. The sample should be free from residual chlorine. If it contained residual chlorine remove it by using Na2S2O3 solution as follows:
Take 50 ml of the sample and acidify with addition of 10ml (1+1) acetic acid. Add about 1g Kl. Titrate withNa2S2O3 (0.025N) using starch indicator. Calculate the volume of Na2S2O4 required per ml of the sample and add accordingly to the sample to be tested for BOD.
3. Sample having high DO content i.e. DO 9 mg/L due to either algal growth or some other reason, reduce the DO content by aerating and agitating samples.
4. BOD is essentially the difference between 0 day DO (mg/L) and 5th day (mg/L). It is therefore necessary to have DO after the end of the incubation time (5day) is at 20oC. For diluting the sample, use following equation .
I) % Dilution required for BOD =2 x 100/60% COD
1000 ml of each dilution is sufficient for BOD bottle. Out of these 3 bottles one can be used for finding ‘0’ day DO (I OD) and duplicate bottle will be used for 5th day DO.
It is always advisable to keep minimum 3 dilution and for each dilution triplicate bottle. These bottles along with blank are kept in the BOD incubator set at 20oC for five days. Water seal must be maintained throughout the incubation period to prevent exchange of gases.
The entire bottles must be subjected to DO determination according to the procedure as discussed in DO at the end of 5th day.
Calculation
BOD, mg/L =
(D1-D2)-(B1-B2) X F X 1000 /ml of original sample taken
where,
D1 = DO of diluted sample on 0th day
D2 = DO of diluted sample on 5th day B1
= DO of the blank on 0th day
B2 = DO of the blank on 5th day
F = Actual normality of Na2S2O3
0.25 (Norm. of std. Na2S203)
DETERMINATION OF RATE CONSTANT FOR BOD
The rate of biochemical reaction or rate constant K, can be evaluated in a number of ways, such as ‘Least Square Method’, ‘Rapid Ratio Method’ and ‘Thomas Method’.
All are dependent upon determining correct BOD observations at two or more time intervals so as to establish the trajectory of the reactions.
Among all these methods, Thomas Method is the easiest method for determination of K values. In this case BOD is to be determined at equal time intervals for 10 days.
The initial DO of blank and sample are to be determined as per the procedure given for BOD5, 20oC.
To insure accurate demand values, the time intervals should be controlled carefully.
This is especially true of the early periods, when a few hours may produce large changes in demand. The DO of sample at different time intervals are to be determined by the same method given for 5 days BOD test.
The method required the observation of time, t and y, the BOD value on that particular day. 3t/y is plotted as ordinate against t as abscissa to the natural scale and the line of best fit is drawn. K is determined from the formula.
K = 2.61 b/a
Where ‘a’ is the intercept on the y-axis and ‘b’ is the slope of the line.
The ultimate BODL can be calculated as
BODL = 1/(2.3 Ka3)
Where ‘K’ is the calculated rate constant and ‘a’ is the intercept.