Chemistry of water -Part 20-

Biochemical Oxygen Demand (BOD):

The amount of dissolved oxygen (DO) required by aerobic biological organisms to decompose organic material contained in a water sample is known as the biochemical oxygen demand (BOD).
The amount of oxygen consumed is not due to the process of oxidation of organic compounds only, but also to the oxidation of mineral compounds, such as nitrite and ammonium salts, in addition to the need for oxygen by living cells for the reproduction process, but despite this, BOD remains a prevailing measurement and expressive of the degree of organic pollution of water.

BOD test bottles

At 20 ° C, the amount of oxygen consumed is determined for a sample of water that has been diluted with distilled oxygen-rich water. The following salts are added to distilled water to prepare for the dilution process (1ml of each of the solutions):

• Ammonium chloride (concentration 2g/l).
• Iron chloride (concentration 1.5 g/l).
• Calcium chloride (concentration 25 g/l).
• Magnesium chloride (concentration 20 g/l).

The mentioned quantities are dissolved in a liter of distilled water, which is then allowed to air at 20 °C until the dissolved oxygen content reaches 8 mg/L. Then, for the purpose of culturing bacteria in the aqueous media, 5 ml of filtered wastewater is added, cultivation of bacteria is very important, especially for natural waters because they may be free of bacteria.

During dilution of water samples, a moderate medium should be maintained (PH between 6 and 8), and the samples are placed in special glass flasks, the dissolved oxygen is calibrated in each flask, and then it is closed tightly so that atmospheric air cannot leak into the bottle. In order to get BOD5 (the five-day biochemical oxygen demand), the samples are placed for five days in a dark place, at a temperature of 20 °C. The value of BOD5 is calculated by the following relationship:

BOD5= F (T0 - T5) - (F - 1) (D0 - D5)

D0: Oxygen concentration (mg/L) in the diluted water at the beginning of the experiment.
D5: Oxygen concentration (mg/L) in the diluted water after 5 days.
F: Expansion Coefficient.
T0: The concentration of oxygen (mg/L) in the sample water at the beginning of the experiment.
T5: The concentration of oxygen (mg/L) in the sample water after 5 days.

References:

• [AQUAPROX- Livre: Traitement des eaux de refroidissement. Imprimé en France par EMD S.A.S- 53110 Lassay-les-Chateaux. N° d'imprimeur: 15566- Dépot légal: juin 2006. N° 842- Cyclus print 90°]

• [Introduction to Water Chemistry (Pollution- Treatment- Analysis). Dr. Nasser Al-Hayek. Publication of the Higher Institute for Applied Sciences and Technology (HIAST). Syrian Arab Republic, 2017.]

• "Chapter A2. Selection of Equipment for Water Sampling" (PDF). National Field Manual for the Collection of Water-Quality Data (Report). Techniques of Water-Resources Investigations, Book 9. Reston, VA: U.S. Geological Survey. March 2003.

• Folco Laverdière, Anja Holstein, Laurent Thiebaut, Robert Mallee, Guillaume Gravejat, Benjamin Des clozeaux: Les principales methodes d’analyse,1999, p5.