Dissolved oxygen (DO) is the amount of oxygen present in water. We can easily measure the amount of DO in water with a DO probe and sensor. Measuring DO is important because oxygen (O2) plays a key role in many industries, like wastewater treatments and aquaculture systems because of the dramatic shifts that can occur in the chemical, biological, and ecological processes.
We can easily increase dissolved oxygen in water by bubbling it with air, however, decreasing or removing DO requires physical (mechanical) or chemical methods.
Physical techniques to remove DO from water:
Chemical techniques to remove DO from water:
Using nutrients & plants
Boiling at 1 atm:
When temperatures increase, the solubility of oxygen in the water is reduced, therefore we know that warmer water holds less DO than cold water. This is why boiling water is one way to reduce and remove DO from water.
During the thermal gassing process, water is placed into a conical flask and boiled for 30 minutes on a hot plate in the open air. The water in the flask is then slowly poured into a bottle (usually plastic). Before the cap is tightly screwed onto the bottle, any remaining air is squeezed out of the bottle. With the cap tightly screwed on the bottle, it is then cooled under running tap water. This method is the least effective way to remove DO from water.
Boiling under reduced pressure:
A hand vacuum pump allows water to be boiled under pressure. When water is boiled under reduced pressure the water requires less energy to change into a gas form and boiling can take place at a lower temperature making it more energy-efficient.
Water is placed inside a Büchner flask and attached to a hand vacuum pump. A water trap is usually placed between the flask and the vacuum pump to prevent the pump from becoming damaged from the flask water. The water is then boiled for 30 minutes to remove the DO from the water. After the water has boiled the flask is sealed and is placed under running water to cool it down.
Sonification under reduced pressure:
This technique is similar to boiling under pressure, except the Büchner flask is placed in a sonic bath and sonicated for 30 minutes and 1 hour periods.
For those that skipped physics class, sonication is related to Henry’s Law. When water is under pressure, it makes DO less soluble. Sonication and stirring solutions such as water under reduced pressure typically enhance the removal efficiency of DO.
Sonication uses sound energy to shake the oxygen particles in the water, which are then removed through pressure reduction inside the specialized vacuum chamber, also known as a vacuum degasser.
Another way to remove DO from water is to increase the nutrient concentration in the water. When nutrients increase in water, plant growth increases, and as plants utilize oxygen, DO levels in water decrease due to decomposition and respiration.
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