Oxygen saturation describes the maximum concentration of molecular oxygen (O₂) that can dissolve in water under defined conditions (temperature, pressure, salt content). It is expressed as a saturation concentration in milligrams per liter (mg/L) or as a percentage saturation (% saturation). The term is often used to assess the efficiency of oxygen transfer in water and wastewater treatment processes and is an important indicator of water quality.

Significance of oxygen saturation

1. Biological processes:

   • In water and wastewater treatment, oxygen is essential for the aerobic aerobic biological processesespecially in the activated sludge process. Microorganisms use oxygen to oxidize and break down organic substances.

2. Ecological relevance:

   • Oxygen saturation directly affects the living conditions of aquatic organisms such as fish and invertebrates. Low oxygen saturation can lead to oxygen deficiency (hypoxia), while oversaturation can be harmful to organisms.

3. Efficiency assessment of ventilation systems:

   • Oxygen saturation is a key parameter for assessing the performance of aeration systems in wastewater treatment plants or industrial water treatment processes.

4. Corrosion behavior in water circuits:

   • Oxygen content and saturation influence the corrosion potential in cooling water systems, pipelines and boiler systems.

Technical background to oxygen saturation

1. solubility of oxygen

The amount of oxygen that can be dissolved in water depends on the following physico-chemical factors:

• Temperature:

  • Oxygen solubility decreases with increasing temperature. Cold water can absorb more oxygen than warm water.
  • Example: At 10 °C the saturation concentration is about 11.3 mg/L, at 30 °C only about 7.5 mg/L (at normal pressure and pure water).

• Print:

  • A higher ambient pressure increases oxygen solubility. This explains the use of pressurized aeration systems in certain processes.
  • Example: In pressurized water reactors, oxygen transfer is optimized by increasing the pressure.

• Salt content:

  • As the salt content (salinity) increases, the oxygen solubility decreases. Seawater has a lower oxygen saturation than fresh water.

2. relative oxygen saturation

The relative oxygen saturation indicates how close the current oxygen content is to the maximum saturation concentration.

• Values below 100 % indicate under-aeration or biological oxygen consumption.
• Values above 100 % indicate supersaturation, which can be caused by intensive aeration or photosynthesis.

Measurement of oxygen saturation

Oxygen saturation is determined using modern measurement technology. Common methods include:

1. Electrochemical sensors (Clark electrode):

   • Measure the oxygen concentration by electrochemical reaction on a membrane.
   • Advantages: Precise and widely used.
   • Disadvantages: Regular calibration required.

2. Optical oxygen sensors:

   • Use fluorescence-based technologies in which the attenuation of the signal by oxygen is measured.
   • Advantages: Low maintenance and robust.
   • Disadvantages: Higher acquisition costs.

3. Winkler titration (laboratory method):

   • Classic method in which oxygen is chemically bound and quantified by titration.
   • Advantages: High accuracy.
   • Disadvantages: Time-consuming, not suitable for real-time measurement.

Applications of oxygen saturation in practice

1. wastewater treatment

• Activated sludge process:

  • Aerobic microorganisms require oxygen for the decomposition of organic substances. Oxygen saturation is controlled by aeration systems such as diffusers or surface aerators.
  • Optimum range: 2-3 mg/L dissolved oxygen in the aeration tank.
  • Nitrification:
    • The breakdown of ammonium (NH₄⁺) to nitrate (NO₃-) requires higher oxygen levels (4-6 mg/L), as nitrifying bacteria react more sensitively to a lack of oxygen.