EXPERIMENT NO.

01
AIM: - TO DETERMINE BIOCHEMICAL OXYGEN DEMAND (BOD) OF GIVEN
WATER/WASTE WATER SAMPLE.

Introduction:
The biochemical oxygen demand determination is a chemical procedure for determining the
amount of dissolved oxygen needed by aerobic organisms in a water body to break the organic
materials present in the given water sample at certain temperature over a specific period of
time.
BOD of water or polluted water is the amount of oxygen required for the biological
decomposition of dissolved organic matter to occur under standard condition at a standardized
time and temperature. Usually, the time is taken as 5 days and the temperature is 20°C.
The test measures the molecular oxygen utilized during a specified incubation period for the
biochemical degradation of organic material (carbonaceous demand) and the oxygen used to
oxidize inorganic material such as sulfides and ferrous ion. It also may measure the amount of
oxygen used to oxidize reduced forms of nitrogen (nitrogenous demand).
Environmental significance:
BOD is the principle test to give an idea of the biodegradability of any sample and strength of
the waste. Hence the amount of pollution can be easily measured by it. Efficiency of any
treatment plant can be judged by considering influent BOD and the effluent BOD and so also
the organic loading on the unit.
Application of the test to organic waste discharges allows calculation of the effect of the
discharges on the oxygen resources of the receiving water. Data from BOD tests are used for
the development of engineering criteria for the design of wastewater treatment plants.
Ordinary domestic sewage may have a BOD of 200 mg/L. Any effluent to be discharged into
natural bodies of water should have BOD less than 30 mg/L. This is important parameter to
assess the pollution of surface waters and ground waters where contamination occurred due to
disposal of domestic and industrial effluents. Drinking water usually has a BOD of less than 1
mg/L. But, when BOD value reaches 5 mg/L, the water is doubtful in purity. The
determination of BOD is used in studies to measure the self-purification capacity of streams
and serves regulatory authorities as a means of checking on the quality of effluents discharged
to stream waters.
The determination of the BOD of wastes is useful in the design of treatment facilities. It is the
only parameter, to give an idea of the biodegradability of any sample and self-purification
capacity of rivers and streams. The BOD test is among the most important method in sanitary
analysis to determine the polluting power, or strength of sewage, industrial wastes or polluted
water. It serves as a measure of the amount of clean diluting water required for the successful
disposal of sewage by dilution.
Guideline:
According to Bangladesh Environment Conservation Rules (1997), drinking water standard
for biochemical oxygen demand (BOD) is 0.2 mg/L (at 20°C). For wastewater effluent
allowable concentration of BOD varies from 50- 250 mg/L depending on discharge point of
the effluent (e.g., inland water, irrigation land, public sewer etc.)
Principle:
The sample is filled in an airtight bottle and incubated at specific temperature for 5 days. The
dissolved oxygen (DO) content of the sample is determined before and after five days of
incubation at 20°C and the BOD is calculated from the difference between initial and final
DO. The initial DO is determined shortly after the dilution is made; all oxygen uptake
occurring after this measurement is included in the BOD measurement.
Since the oxygen demand of typical waste is sever hundred milligrams per liter, and since the
saturated value of DO for water at 20uC is only 9.1 mg/L, it is usually necessary to dilute the
sample to keep final DO above zero. If during the five days of experiment, the DO drops to
zero, then the test is invalid since more oxygen would have been removed had more been
available.
The five-day BOD of a diluted sample is given by,
BOD5 = [DOi- DOf] × D.F. ………………………….(1)
Here,
Dilution factor (D.F.) =
In some cases, it becomes necessary to seed the dilution water with microorganisms to ensure
that there is an adequate bacterial population to carry out the biodegradation. In such cases,
two sets of BOD bottles must be prepared, one for just the seeded dilution water (called the
"blank") and the other for the mixture of wastewater and dilution wader. The changes in DO
in both are measured. The oxygen demand of waste water (BODw) is then determined from
the following relationship:
BODm× Vm= BODw × Vw + BODd × Vd ………………………(2)
Where, BODm, is the BOD of the mixture of wastewater and dilution water
BODd is the BOD of the dilution water alone;
Vw and Vd are the volumes of wastewater and dilution water respectively in the mixture
and
Vm= Vw+ Vd.
Sample handling and preservation:
Preservation of sample is not practical. Because biological activity will continue after a
sample has been taken, changes may occur during handling and storage.
If Analysis is to be carried out within two hours of collection, cool storage is not necessary. If
analysis cannot be started with in the two hours of sample collection to reduce the change in
sample, keep all samples at 4° C.
Do not allow samples to freeze. Do not open sample bottle before analysis. Begin analysis
within six hours of sample collection.
Apparatus:
1. BOD bottle
2. Beaker (250 ml)
3. Measuring cylinder
4. Dropper
5. Stirrer
Reagents:
1. Manganous sulfate solution
2. Alkaline potassium iodide solution
3. 0.025N sodium thiosulfate
4. Starch solution (indicator)
5. Concentrated sulfuric acid
Procedure:
Fill two BOD bottles with sample (or diluted sample); the bottles should be completely filled.
Determine initial DO (DOi) in one bottle immediately after filling with sample (or diluted
sample). Keep the other bottle in dark at 20°C and after particular days (usually 5-days)
determine DO (DOf) in the sample (or diluted sample). Dissolved oxygen (DO) is determined
according to the following procedure:
1. Add 1 mL of manganous sulfate solution to the BOD bottle by means of pipette, dipping in
end of the pipette just below the surface of the water.
2. Add 1 mL of alkaline potassium iodide solution to the BOD bottle in a similar manner.
3. Insert the stopper and mix by inverting the bottle several times.
4. Allow the "precipitates" to settle halfway and mix again.
5. Again allow the "precipitates" to settle halfway.
6. Add 1 mL of concentrated sulfuric acid. Immediately insert the stopper and mix as before.
7. Allow the solution to stand at least 5 minutes.
8. Withdraw 100 mL of solution into an Erlenmeyer flask and immediately add 0.025N
sodium thiosulfate drop by drop from a burette until the yellow color almost disappears.
9. Add about 1 mL of starch solution and continue the addition of the thiosulfate solution until
the blue color just disappears. Record the ml. of thiosulfate solution used (disregard any
return of the blue color)
Calculation:
Dissolved oxygen, DO (mg/L)
= mL of 0.025N sodium thiosulfate added x Multiplying Factor (M.F.)
Calculate BOD of the sample according to Eq. – 1 or Eq. – 2
DATA SHEET :

Table Source of Temperature BOD (mg/L)