Measurement of Dissolved Oxygen (DO) in Water

Dissolved oxygen is a measure of the amount of gaseous oxygen contained in water.

Dissolved Oxygen (DO) Enters Water by:

  • Direct absorption from the atmosphere.

  • Rapid movement from winds, waves, currents or mechanical aeration.

  • Aquatic plant life photosynthesis as a by-product of the process.


Measuring dissolved oxygen in water, and its treatment to maintain proper dissolved oxygen levels, are crucial functions to support life and treatment processes, it can also be detrimental, causing oxidation that damages equipment and compromises product.


Dissolved oxygen (DO) affects:


· Quality– Minimum level of DO is required to prevent water turning foul and unhealthy affecting the quality of the environment & drinking water.

· Regulatory Compliance– It requires certain level of DO to be present in waste water before it can discharged into a stream, lake or river.

· Aquatic Life :- DO is required in waters to support aquatic life.

· Energy Consumption Cost–Effectively monitoring DO can reduce costs by making processes more efficient.

· Process Control–DO levels are critical to control biological treatment of wastewater as well as the biofiltration phase of drinking water production.


Factors that Contribute to the Concentration of Dissolved Oxygen (DO) in Water:

  • Atmospheric pressure: Higher atmospheric pressure allows the water to hold more oxygen atoms & vice versa.

  • Temperature: At low temperature water contains more dissolved oxygen because oxygen atoms have less movement. Higher temperature allows oxygen atoms to escape out of the water into the air.

  • Depth of the water: The shallower the water, the higher the concentration of DO because wind creating waves on the surface increases DO and also aquatic plants living in shallower light filled waters creates DO as a by-product of photosynthesis.

  • Salinity: Lower salinity contributes to a higher concentration of DO because salts affect the solubility of gases essentially driving the oxygen atoms out of the water.

  • Bioactivity: Lower bioactivity of microorganisms in water leads to a higher concentration of DO because microorganisms feeding on organics and decaying matter use oxygen in their respiration.


As can be seen from above continuous measurement of DO is required as both lower or higher concentrations have their own adverse effects.

Broadly there are three types of principle involved in measuring DO in water

· Electrochemical oxygen sensors

In an electrochemical DO sensor, dissolved oxygen diffuses from the sample across an oxygen permeable membrane and into the sensor where oxygen undergoes a chemical reduction reaction, thereby producing an electrical signal proportional to oxygen concentration present in water.

· Polarographic dissolved oxygen sensors


A polarographic DO sensor requires a constant voltage to be applied to it. The dissolved oxygen is reduced at the surface of noble metal electrode acting as cathode.


· Optical dissolved oxygen sensors


The sensor cap contains a luminescent dye, which glows red when exposed to blue light. Oxygen interferes with the luminescent properties of the dye, an effect called “quenching.” A photodiode compares the “quenched” luminescence to a reference reading, allowing the calculation of dissolved oxygen concentration in water.


 

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