Definition of Dissolved Oxygen

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

Importance of Dissolved Oxygen in Water

Aquatic Life:

• Respiration: Dissolved oxygen (DO) is crucial for the respiration of aquatic organisms, such as fish, invertebrates, and microorganisms, as they rely on extracting oxygen from the water to survive.

• Biodiversity: Adequate DO levels are essential to support a diverse range of aquatic species. Low DO levels can result in hypoxia, causing stress or death to aquatic life, and ultimately leading to a reduction in biodiversity.

Water Quality Monitoring Indicator:

• Pollution Detection: Dissolved oxygen (DO) levels can indicate the presence of pollutants. Low DO levels often suggest pollution from organic matter, which consumes oxygen during decomposition.

• Eutrophication: Excessive nutrients, such as nitrogen and phosphorus, can lead to algal blooms. When these algae die and decompose, they consume large amounts of oxygen, drastically reducing DO levels and harming aquatic life.

Biochemical Processes:

• Decomposition: Dissolved oxygen (DO) plays a crucial role in the aerobic decomposition of organic matter. Higher levels of DO promote the breakdown of organic wastes, which helps prevent the accumulation of harmful substances.

• Nutrient Cycling: DO has an impact on the cycling of nutrients like nitrogen and phosphorus. Environments with high oxygen levels facilitate the conversion of ammonia to nitrate, which is a less harmful form for aquatic life.

Industrial Processes:

• Wastewater Treatment: DO is crucial in wastewater treatment processes, particularly in aerobic biological treatment systems where microorganisms break down organic matter.

Aquaculture:

• Aquaculture: In aquaculture, maintaining appropriate DO levels is essential for the health and growth of farmed fish and shellfish.

Factors Affecting DO Levels

Temperature

Colder water has a higher capacity to hold dissolved oxygen compared to warmer water. This is because as water temperature rises, the ability of water to dissolve oxygen decreases, leading to lower levels of dissolved oxygen in warmer water.

Salinity

Freshwater contains more dissolved oxygen than saltwater due to the presence of salts, which decreases the water's oxygen-holding capacity. As a result, freshwater environments generally have higher levels of dissolved oxygen compared to saline environments.

Atmospheric Pressure

Atmospheric pressure affects the amount of oxygen that can dissolve in water. At higher altitudes, the reduced atmospheric pressure leads to lower DO levels.

Photosynthesis

During photosynthesis, aquatic plants and algae produce oxygen, which increases the levels of dissolved oxygen (DO) in the water. This process is essential for maintaining the oxygen balance in aquatic ecosystems.

Respiration and Decomposition

The respiration of aquatic organisms and the decomposition of organic matter consume oxygen, reducing DO levels.

Water Movement

Movement and turbulence in water bodies facilitate the mixing of atmospheric oxygen into the water, enhancing DO levels. This is why fast-moving streams and rivers usually have higher DO levels compared to stagnant waters.

Monitoring Dissolved Oxygen by the Sensor Method

Using sensors to monitor dissolved oxygen (DO) levels in water is a common and efficient method, particularly for real-time and continuous monitoring. There are two main types of DO sensors: electrochemical do sensors and optical do sensors.

Electrochemical DO sensors measure the concentration of oxygen by detecting the electrical current generated by the reduction of oxygen at an electrode. This kind of do sensor typically consist of the following components: Cathode, Anode, Electrolyte Solution, Membrane. Electrochemical sensors offer fast readings, making them ideal for real-time monitoring. They are often portable and suitable for use in field conditions. Additionally, they are capable of detecting low levels of dissolved oxygen, making them useful for a wide range of applications. But, regular calibration is necessary to maintain accuracy. The membrane and electrolyte require periodic replacement, which may be labor-intensive. Additionally, temperature changes can affect the readings, necessitating compensation.

Optical DO meters measure oxygen concentration based on the quenching of luminescence. A luminescent dye is excited by a light source, and the presence of oxygen affects the intensity and duration of the emitted light. Optical sensors offer precise and stable readings over extended periods without the need to replace membranes or electrolyte solutions, resulting in lower maintenance requirements. They are also less affected by temperature changes, making them more reliable in varying environmental conditions. In contrast to electrochemical sensors, optical sensors do not consume oxygen during measurement, allowing for more accurate readings in low dissolved oxygen environments.