Chapter 29, Problem 1SA

Describe how the BOD of a water sample is determined.

Summary Introduction

To review:

The method of determination of BOD (biological oxygen demand) of a water sample.

Introduction:

The oxygen required by the microorganisms for decomposing the organic waste is known as biological oxygen demand. This procedure is utilized during the secondary treatment of wastewater. This property of the water determines its microbial content.

Explanation of Solution

A high BOD indicates the presence of a high amount of organic waste in the water sample. While a low BOD indicates a water sample with low organic waste. The higher the organic waste, more time will be utilizedby the microorganisms to degrade the organic waste. In this process, they utilize oxygen, which is measured as BOD. When the organic waste is degraded completely, the BOD of a wastewater decline.

Every water body contains naturally dissolved oxygen (DO), which gets depleted in the presence of organic material. This property is utilized for calculating the BOD of a water sample. The initial DO is calculated from the fresh water sample. The container should be left undisturbed for 5 days at standard temperature (20 degrees Celsius) in dark.

The final oxygen is then measured after 5 days of incubation. A difference in the initial DO and final DO mark the BOD of the water sample. In the case of a highly polluted water body, the sample should be diluted to avoid the depletion of oxygen before the incubation period.

Conclusion

Therefore, it can be concluded that the BOD of a water sample is determined by measuring the initial and final dissolved oxygen from the fresh, aerated sample and sample after 5 days of incubation, respectively. The difference of the DO gives the BOD of the water sample.