Definition of BOD of water and practical examples of determining BOD of water
BOD (Biochemical Oxygen Demand) of Water
The BOD (Biochemical Oxygen Demand) value is widely used to evaluate and compare the concentration of biodegradable organic pollutants present in surface water samples.
Definition: The amount of dissolved oxygen (DO) consumed from a water sample by aerobic microorganisms or bacteria to completely degrade and oxidize biodegradable organic pollutants at a specific temperature (usually 20°C) over a period of five days is defined as the BOD of that water sample. This biological oxidation process naturally leads to a depletion of the sample’s DO value.
Oxidation Reaction of Organic Wastes
Biodegradable organic waste in water primarily consists of elements such as carbon (C), hydrogen (H), sulfur (S), nitrogen (N), and oxygen (O). Aerobic bacteria utilize dissolved oxygen to systematically oxidize these elements as follows:
Theoretically, for every 1 milligram (1 mg) of carbon present in biodegradable organic waste, 2.67 milligrams (2.67 mg) of O2 is consumed during bacterial oxidation. Furthermore, additional dissolved oxygen is required to completely oxidize other constituent elements like hydrogen (H), sulfur (S), and nitrogen (N).
BOD Standards and Water Quality Guidelines (WHO)
| BOD Value (mg/L or ppm) | Water Quality Status |
|---|---|
| 1.0 – 2.0 mg/L | Excellent (Pure Water) |
| 3.0 mg/L | Moderately Good |
| ≤ 6.0 mg/L | Permissible upper limit safe for fish and aquatic life (WHO) |
| > 10.0 mg/L | Highly Polluted / Critical Condition |
Practical Example of Determining Water BOD
Step 1: In the laboratory, an oxygen sensor probe or electrode (DO meter) is submerged into the collected water sample to measure its initial dissolved oxygen level, recorded as Initial DO. Let us assume the Initial DO is 8.5 ppm.
Step 2: Next, a dedicated BOD bottle is completely filled with this water sample, airtightly sealed with a ground-glass stopper, and incubated in total darkness at a constant temperature of 20°C for exactly 5 days. This duration is called the Incubation Period. Keeping it in darkness prevents photosynthetic algae from producing new oxygen. During this period, aerobic bacteria decompose the organic pollutants, drawing down the available DO.
Step 3: After 5 days, the oxygen sensor probe is submerged again to measure the remaining dissolved oxygen level, recorded as Final DO. Let us assume the Final DO has dropped to 5.5 ppm.
Therefore, the BOD of the sample water = (8.5 – 5.5) ppm = 3.0 ppm (or 3.0 mg/L).
Conceptual Interpretation: To state that a water sample has a BOD value of 5 mg/L (or 5 ppm) implies that for every single liter of that water, 5 milligrams of dissolved oxygen will be consumed from its native DO pool by aerobic bacteria to fully stabilize and break down the organic wastes present within a 5-day period at 20°C.
Laboratory Procedure for BOD Measurement via Dilution Method
For heavily polluted wastewater or industrial effluents, direct BOD measurement is impossible because the biological oxygen demand surpasses the total available DO in the water. Consequently, the sample must be diluted using oxygen-saturated nutrient water before undergoing a 5-day incubation. The systematic laboratory steps are detailed below:
- Collect the raw wastewater samples from various target environmental or industrial effluent sites.
- Prepare the dilution water by adding the following nutrient solutions into a flask containing 50 mL of distilled water to sustain bacterial populations:
- (i) 1 mL phosphate buffer solution (pH = 7.2)
- (ii) 1 mL magnesium sulfate (MgSO4) solution (22.5 g/L)
- (iii) 1 mL calcium chloride (CaCl2) solution (27.5 g/L)
- (iv) 1 mL ferric chloride (FeCl3) solution (25 g/L)
- Dilute the source wastewater based on its suspected pollution level. Generally, use a dilution factor of 0.1% – 1% for industrial wastes, 1% – 1.5% for domestic sewage, and 25% – 100% for polluted river water.
- Aerate the prepared dilution water mixture for 5 to 10 minutes using a tube or pump to saturate its dissolved oxygen (DO) level to at least 7 ppm. If the expected BOD is higher than the DO, always use a higher volume of dilution water.
- Divide the diluted sample into two portions. Measure the dissolved oxygen level of the first portion immediately. This value is recorded as D1.
- Pour the remaining half of the diluted sample into a stoppered BOD bottle, ensuring no air bubbles are trapped, and incubate it at 20°C in darkness for 5 days. After 5 days, determine its final DO value, recorded as D2.
- In parallel, process a blank control sample containing only the pure aerated dilution water (without the wastewater sample). Measure its initial DO level as B1 and its post-incubation DO level after 5 days as B2.
Final Mathematical Equation for BOD Calculation
The true biochemical oxygen demand of the original undiluted wastewater sample is computed via the international standard formula:
| BOD = | (D1 – D2) – (B1 – B2)f | mg/L (or ppm) |
| p |
D1: Initial DO of the diluted wastewater sample immediately after preparation
D2: Final DO of the diluted wastewater sample after 5 days of incubation
B1: Initial DO of the blank control seed/dilution water before incubation
B2: Final DO of the blank control seed/dilution water after 5 days of incubation
p: Decimal volumetric fraction of the raw waste sample used (Volume of sample / Total volume)
f: Seed ratio factor representing the ratio of dilution water in the sample to that in the blank control
