Lab Report: Biochemical Oxygen Demand (BOD)

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4.4.1. Biochemical oxygen demand (BOD)
Bacteria placed in contact with organic material will utilize it as a food source. In this utilization the organic material will eventually be oxidized to stable end products such as CO2 and water. The amount of oxygen used in this process is called biochemical oxygen demand (BOD) and is considered to be a measure of the organic content of the waste. All aerobic organisms require oxygen for respiration. The determination of BOD, which is a major parameter in water pollution control, requires the determination of dissolved oxygen.
Dissolved oxygen (DO) is essential to all normal forms of aquatic life. 5 mg/l is widely considered to be the minimum. DO require to support a balanced population of desirable
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The main effects of pollutants on DO in surface waters arise from discharge of:
• organic material
• fats, oil, and greases
• a large volume of hot water.

BOD determination
The BOD determination has been standardised and measures the amount of oxygen utilised by micro-organisms in the stabilisation of wastewater during 5 days at 20°C.
In a simple test, DO is determined at time zero, (DO0); another sample of the same wastewater is placed in a sealed dark bottle and incubated at 20°C for five days. During the incubation, the DO decreases and DO5 (the level of oxygen after 5 days) is determined. The 5-day BOD at 20°C is equal to the difference in the DO values:

The typical BOD time curve is shown in Figure 2.
Equation 3, along with Equation 2, makes it possible to convert test results from different time periods and temperatures to the standard 5 days at 20°C test. Values of the 5-days 20°C BOD of municipal wastewater varies from about 100 to 500 mg/L.

Figure 2: Definition sketch of BOD time curve
(Source: Linsley
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The BOD test is widely employed, particularly in wastewater treatment and water quality control. It will be discussed in detail in the second part of the unit.

Tests for colour, turbidity, taste and odour are widely used to establish the aesthetic quality, or potability, of water. Physical tests do not attempt to specify the chemical or group of chemicals responsible for a particular effect. The methods of analysis for colour and turbidity are relatively simple and are widely used in routine analysis and plant control. The assessment of taste and odour in water is subjective and a panel of ‘testers’ is needed to carry out the analysis.

The identification and enumeration of micro-organisms is generally a tedious task involving a series of tests, although for a significant number of micro-organisms, relatively rapid procedures have been developed.

Enumeration techniques must effectively identify the micro-organisms of interest. To this group of techniques belongs:
• Most Probable Number Technique (MPN); and
• Membrane Filter Technique

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