BOD Practical = Blank,Sample and Calculation explained

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Biological Oxygen Demand (BOD) Practical = Blank and Sample
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0:00 Introduction to BOD ​
0:48 Preparing Dilution Water ​
2:25 Sample Preparation ​
6:01 Blank Preparation ​
9:41 Starch Indicator & Color Change ​
10:08 BOD Calculation Formula

Here is a step-by-step example of how to calculate the Biochemical Oxygen Demand (BOD) using hypothetical lab data.
Example Scenario (Unseeded Sample)
Suppose you are testing a wastewater sample. You prepared a 2% dilution of the sample using aerated dilution water. You measured the Dissolved Oxygen (DO) on Day 0 and Day 5 for both the diluted sample and a blank (just the dilution water).
1. Lab Data Collected:
Dilution (P): 2% (which means 2 mL of sample in 100 mL total volume). As a decimal fraction, P = \frac{2}{100} = 0.02.
D_0 (Initial DO of diluted sample): 8.2 mg/L
D_5 (Final DO of diluted sample after 5 days): 3.5 mg/L
B_0 (Initial DO of blank): 8.5 mg/L
B_5 (Final DO of blank after 5 days): 8.3 mg/L
2. The Formula:
3. Step-by-Step Calculation:
Step A: Calculate the DO depletion of the sample
This is the total oxygen consumed in the sample bottle.
Sample Depletion = (D_0 - D_5)
Sample Depletion = 8.2 - 3.5
Sample Depletion = 4.7 mg/L
Step B: Calculate the DO depletion of the blank
This is the oxygen consumed by the dilution water itself (it should ideally be very small, usually less than 0.2 mg/L).
Blank Depletion = (B_0 - B_5)
Blank Depletion = 8.5 - 8.3
Blank Depletion = 0.2 mg/L
Step C: Correct the sample depletion by subtracting the blank depletion
This gives us the oxygen consumed only by the organic matter in your wastewater.
Corrected Depletion = Sample Depletion - Blank Depletion
Corrected Depletion = 4.7 - 0.2
Corrected Depletion = 4.5 mg/L
Step D: Divide by the dilution fraction (P)
Since the sample was diluted, we must multiply by the dilution factor to find the BOD of the original, undiluted wastewater. Dividing by the decimal fraction 0.02 is the same as multiplying by 50.
* Result: The Biochemical Oxygen Demand of the original wastewater sample is 225 mg/L.
Alternative: Seeded Sample Calculation
Sometimes, a wastewater sample (like industrial effluent) doesn't have enough natural bacteria to break down the waste. In this case, you add a "seed" (a known bacteria population, often settled sewage) to the dilution water. The calculation changes slightly to account for the oxygen the seed bacteria consume:
Where:
(S_0 - S_5) = DO depletion of the seed control (a separate bottle containing only dilution water and seed).
f = Ratio of seed volume in the diluted sample to the seed volume in the seed control. (e.g., if your sample bottle has 1 mL of seed and your seed control bottle has 3 mL of seed, f = \frac{1}{3} = 0.33).
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