Dissolved Oxygen (DO)

Definition

Dissolved Oxygen (DO) refers to the amount of oxygen that is present in water. It is a crucial parameter for assessing water quality, as it directly impacts the health of aquatic ecosystems.

Importance

• Essential for the respiration of aquatic organisms such as fish, invertebrates, and aerobic bacteria.
• High DO levels typically indicate good water quality and a healthy ecosystem.
• Low DO levels can lead to hypoxia (oxygen deficiency), which can cause stress or death to aquatic life.

Factors Affecting DO

1. Temperature: Colder water can hold more oxygen. As temperature increases, DO decreases.
2. Salinity: Saltier water holds less oxygen. Higher salinity reduces DO.
3. Photosynthesis: Aquatic plants and algae produce oxygen during photosynthesis, increasing DO levels.
4. Respiration and Decomposition: Organisms consume oxygen for respiration. Decomposition of organic matter by bacteria also consumes oxygen, decreasing DO levels.
5. Water Movement: Flowing or turbulent water, such as in rivers or streams, usually has higher DO levels due to increased aeration compared to stagnant water.

Measurement

• DO is measured in milligrams per liter (mg/L) using a DO meter or a chemical titration method such as the Winkler method.

Biochemical Oxygen Demand (BOD)

Definition

Biochemical Oxygen Demand (BOD) is the amount of oxygen required by aerobic microorganisms to decompose organic matter in water over a specific period, usually 5 days at 20°C (BOD5).

Importance

• Indicates the level of organic pollution in water.
• High BOD values suggest high levels of biodegradable organic matter, which can lead to oxygen depletion.
• Used to gauge the effectiveness of wastewater treatment processes.

Factors Affecting BOD

1. Organic Matter: The more organic waste present in the water, the higher the BOD.
2. Microbial Population: A higher concentration of microorganisms can increase BOD due to more active decomposition.
3. Temperature: Higher temperatures can accelerate microbial activity, increasing BOD.
4. Presence of Toxic Substances: Toxic substances can inhibit microbial activity, leading to lower BOD values.

Measurement

• Water samples are incubated in the dark at 20°C for 5 days.
• The initial DO is measured, and the DO after 5 days is measured. The BOD is the difference between these two measurements.

Chemical Oxygen Demand (COD)

Definition

Chemical Oxygen Demand (COD) measures the total quantity of oxygen required to oxidize both organic and inorganic substances in water using a strong chemical oxidant.

Importance

• Provides a quick measure of the organic pollutant load in water.
• Unlike BOD, COD includes both biodegradable and non-biodegradable substances, giving a more comprehensive assessment of water quality.
• Useful for monitoring and controlling water pollution, especially in industrial effluents.

Factors Affecting COD

1. Concentration of Organic and Inorganic Matter: Higher concentrations increase the COD.
2. Type of Organic Matter: Certain types of organic compounds are more resistant to oxidation, affecting COD values.
3. Presence of Oxidizable Inorganics: Inorganic substances that can be oxidized also contribute to COD.

Measurement:

• A water sample is treated with a strong oxidizing agent, usually potassium dichromate in an acidic solution.
• The sample is heated, and the amount of oxidant consumed is measured, often using a colorimetric or titrimetric method.
• The result is expressed in milligrams of oxygen per liter (mg/L).

Comparison of DO, BOD, and COD