Question

Which of the following is a wrong statement? (a) The value of dissolved oxygen in polluted water is \(<4\) ppm. (b) COD gives the measure of organic pollution of water. (c) COD value determination is based on chemical oxidation of organic substances. (d) If BOD of a river is high, it means the river is not polluted.

Short answer

Option (d) is incorrect.

Step-by-step solution

- Understand the Options

Review each statement given in the exercise and understand its meaning with respect to water pollution and quality.

- Analyze Option (a)

Dissolved oxygen is a critical parameter for water quality. Generally, polluted water has a dissolved oxygen content of less than 4 ppm, which is correct.

- Analyze Option (b)

COD (Chemical Oxygen Demand) measures the amount of organic pollutants in the water by quantifying the amount of oxygen required to oxidize these pollutants. This statement is correct.

- Analyze Option (c)

COD value determination involves chemical oxidation processes to measure the amount of organic materials in water. Thus, this statement is correct.

- Analyze Option (d)

BOD (Biochemical Oxygen Demand) indicates the amount of dissolved oxygen needed by aerobic biological organisms to break down organic material in water. A high BOD indicates high pollution because more oxygen is required for decomposition of organic wastage. Therefore, this statement is incorrect.

Key concepts

Dissolved Oxygen (DO)

Dissolved Oxygen (DO) is the amount of oxygen available in water for aquatic organisms to breathe. It is a crucial indicator of water quality because aquatic life depends on sufficient oxygen levels to survive. Generally, a DO level of less than 4 ppm (parts per million) is considered low and indicates that the water is polluted. Low DO levels can lead to dead zones where aquatic life cannot sustain itself. Factors affecting DO include water temperature, salinity, and the presence of organic pollutants. Keep in mind:

• DO levels decrease as water temperature rises.
• High organic pollution can consume more oxygen, reducing DO levels.
• DO is essential for the survival of fish and other aquatic organisms.

Chemical Oxygen Demand (COD)

Chemical Oxygen Demand (COD) measures the amount of oxygen required to chemically oxidize organic and inorganic matter in water. It provides a quick estimation of the water's organic pollution levels. The COD test involves oxidizing the organic substances using a strong chemical oxidant, such as potassium dichromate, in an acidic environment. The amount of oxidant consumed is proportional to the level of organic pollutants. Main points to remember include:

• High COD values indicate high levels of organic pollutants.
• The test is faster than BOD (Biochemical Oxygen Demand) tests.
• COD does not differentiate between biologically available and inert organic matter.

Biochemical Oxygen Demand (BOD)

Biochemical Oxygen Demand (BOD) is the amount of oxygen that aerobic microorganisms require to break down organic material in water. BOD is used to gauge the level of organic pollution in water bodies. A higher BOD indicates more organic waste, which means more oxygen is consumed by bacteria for decomposition. This can result in oxygen depletion, harming aquatic life. Important points to note:

• BOD tests typically last for 5 days to measure oxygen consumption.
• High BOD levels are a sign of significant organic pollution.
• BOD is crucial for water treatment processes to ensure water quality.

Water Quality

Water quality refers to the condition of water with respect to its chemical, physical, and biological characteristics. It is essential for the health of ecosystems, human consumption, and recreational activities. Water quality indicators include DO, COD, BOD, and additional factors like pH, turbidity, and the presence of contaminants. Key considerations for water quality include:

• High water quality is vital for the survival of aquatic life and overall ecosystem health.
• Pollutants from industrial discharge, agriculture runoff, and wastewater can deteriorate water quality.
• Regular monitoring and adherence to water quality standards help protect public health and maintain environmental balance.