Experiment No. 1: Determination of Biochemical Oxygen Demand (BOD)

This lab report details Experiment No. 1, focusing on the Determination of Biochemical Oxygen Demand (BOD). This test is crucial in Sanitary Engineering for assessing water quality and organic pollution levels.

OBJECTIVE: Determination of Biochemical Oxygen Demand

To determine the concentration of Dissolved Oxygen (DO) in a given water sample using modified Winkler’s (Iodometric) method and estimate its Biochemical Oxygen Demand (BOD).

APPARATUS REQUIRED:

• BOD bottle (300 mL capacity)
• Burette
• Pipette
• Conical flask (100 or 250 mL)
• Measuring cylinder

REAGENTS REQUIRED:

• Manganese (II) sulfate solution (MnSO₄)
• Alkaline iodide-azide reagent (KOH + KI + NaN₃)
• Concentrated sulfuric acid (H₂SO₄)
• Standard sodium thiosulphate solution (Na₂S₂O₃)
• Starch indicator solution
• Distilled water

THEORY/PRINCIPLE: Determination of Biochemical Oxygen Demand

Dissolved oxygen (DO) refers to the level of free, non-compound oxygen present in water or other liquids. It is a critical parameter for assessing water quality because it is essential for the survival of aerobic aquatic organisms, such as fish and invertebrates.

Hence, analysis of the D.O. is a key test in water pollution control activities and waste treatment process control. The Determination of Biochemical Oxygen Demand often employs Winkler’s method, a titrimetric technique used to determine the concentration of dissolved oxygen (DO) in water. It involves a series of redox reactions where the dissolved oxygen is chemically fixed, converted into iodine, and then titrated using sodium thiosulphate.

Water is treated with manganese (II) sulfate (MnSO₄) and an alkaline potassium iodide (KOH + KI) solution.

The dissolved oxygen (DO) oxidizes Mn(OH)₂:

On adding concentrated H₂SO₄:

The iodine released is titrated with Na₂S₂O₃:

Each mole of iodine corresponds to a mole of dissolved oxygen.

The Biochemical Oxygen Demand (B.O.D.) of sewage or of polluted water is the amount of oxygen required for the biological decomposition of dissolved organic matter to occur under aerobic conditions and at the standardized time and temperature. Usually, the time is taken as 5 days and the temperature 20°C as per the global standard.

PROCEDURE:

1. A clean 300 mL glass-stoppered BOD bottle was completely filled with the sample, ensuring that no air bubbles were trapped.
2. The samples were prepared in two sets: one for initial BOD and another for incubation.

For Initial DO (DO_initial):

3. One milliliter of manganese sulfate (MnSO₄) solution was pipetted out and added by placing the pipette tip just below the liquid surface.
4. One milliliter of alkali-iodide-azide reagent was pipetted out and added in the same manner, avoiding any air entry.
5. The bottle was immediately stoppered and inverted several times to mix. The brown precipitate was allowed to form and settle.
6. One milliliter of concentrated sulfuric acid was added by allowing it to run gently down the inside wall of the bottle.
7. The bottle was stoppered again and mixed by inversion until the precipitate was completely dissolved and the solution became uniformly brownish-yellow.
8. Fifty milliliters of the treated sample were measured using a measuring cylinder and transferred into a clean conical flask.
9. The solution was titrated with standard N/40 sodium thiosulphate (Na₂S₂O₃) until the color became pale yellow.
10. One to two milliliters of starch indicator were added, and the solution turned blue-black.
11. The titration was continued dropwise with constant swirling until the blue color disappeared and the solution became colorless, indicating the endpoint.
12. The final burette reading was recorded, and the titration was repeated to obtain concordant values.

For DO₅:

13. The sample was incubated for five days, and steps 2 to 11 were repeated.

OBSERVATION:

Volume of sample taken for titration = 50 mL

S.N.	Day	Trial	Initial Reading (ml)	Final Reading (ml)	Hypo Consumed (ml)	Concurrent Reading (ml)
Sample I	Day 0	Trial 1	0	1.4	1.4	1.3
		Trial 2	2.8	4.1	1.3
		Trial 3	4.1	5.4	1.3
Sample II	Day 5	Trial 1	0	0.4	0.4	0.3
		Trial 2	0.4	0.7	0.3
		Trial 3	0.9	1.2	0.3

CALCULATIONS: Determination of BOD

Calculation for Initial DO (DO_i):

Calculation for DO after 5 days (DO₅):

Calculation for BOD:

RESULT

The 5-day Biochemical Oxygen Demand of the given wastewater sample is 1004 mg/L.

DISCUSSION

The measured BOD of 1004 mg/L indicates that the sample contains an extremely high concentration of organic matter. This value is significantly higher than typical domestic sewage (often 200–400 mg/L) and vastly exceeds the general sewage effluent guideline of Nepal (BOD < 50 mg/L). Such a high BOD suggests the sample is likely strong industrial effluent or highly concentrated raw sewage. It absolutely cannot be discharged into natural water bodies without extensive treatment. However, the residual dissolved oxygen after 5 days (DO₅ = 1.2 mg/L) satisfies the standard criteria (residual DO ≥ 1 mg/L), validating the accuracy of the dilution and test procedure.

ENGINEERING SIGNIFICANCE

Engineers compute BOD removal efficiency to evaluate the performance of units like:

• Aeration tanks
• Trickling filters
• Constructed wetlands, etc.

BOD is the primary parameter for sizing aeration tanks, calculating oxygen requirement, food-to-microorganism (F/M) ratio, and sludge production in activated sludge processes. It is also used to ensure compliance with standards.

PRECAUTIONS

• BOD bottles should be completely filled and stoppered without any air bubbles to avoid additional oxygen entry.
• A water seal should be provided, and bottles should be kept in the dark in a BOD incubator at 20 ± 1°C throughout the 5-day period.
• Dilution water must be oxygen-saturated and free of contaminants.
• Manganese sulphate and alkali-iodide-azide reagents should be added below the surface to prevent oxygen pickup.