Types of Diffused Aeration Systems

There are two basic types of diffused aeration systems:
Course Bubble and Fine bubble.

The factors of importance relating to the efficiency of a diffused aeration system are:
(1) Bubble surface area in contact with the water.
(2) Amount of water that the bubbles move by air lift effect.
(3) Contact time of the bubble as it passes up through the water column
(4) Speed at which the bubble rises up through the water

The greater the surface area the more efficient the transfer of oxygen from the air bubble to the water.
For example as bubble size is reduced by 1/2 the surface area is increased by a factor of 4.
Therefore if bubble size was reduced from 1/4 " (course bubble) to 1/16" (fine bubble), surface area is increased by a factor of 16.

Contact time of the bubble as it passes up through the water column:
As water depth is increased oxygen transfer is increased since the bubble will be in the water column for a longer period of time. However, of more importance is the speed at which the bubble rises up through the water.
 
With the fine bubble the vertical velocity of the bubble is very much slower than a course bubble, thus there is a greater period of time for the oxygen to dissolve into the water from the fine bubble.

Finally the reason for the fine bubbles slower vertical velocity is due to the greater surface area of the bubbles, and the increased friction there is between the water and the bubble surface.
 
Rising air bubbles do not push water vertically but drag the water. Thus with the increased friction with the small bubble very much more water will be
moved than in comparison to the same volume of air with course bubbles.

Course bubble aeration often looks very impressive because of the localized visible disturbance of the water surface.
However in actual fact most of the energy is being expended in blowing air into the atmosphere. The efficiency of fine bubble aeration is therefore in the order of 10 to 20 times greater than that of course bubble diffused aeration systems.
 
Fine bubble diffused aeration is much more efficient system than the course bubble process.
Visually, it is difficult to see the impact of fine bubble diffusers in an aquaculture installation, however on close inspection of the water a mass upward movement of slow moving water will be observed.

This water rises from the base of the system, progresses out over the surface and then submerges back down to the bottom providing excellent vertical mixing of the water.

Since very little kinetic energy is wasted with diffused aeration, and no energy is wasted in throwing water into the air, the efficiency of the fine bubble process tends to be very high in comparison with other systems.

Fine bubble diffused aeration produces a vertical rotating water motion.
This motion creates a momentum which results in the mass movement of a tremendous volume of water. It is estimate that the entire water volume in a system can be recalculated approximately every 10 minutes.
 
Water circulation increases as the depth increases .In aquaculture installations where water depth is in the order of 4 ft.,  every cubic foot of air will move between 300 and 1700 cubic feet of water per hour.

Fine bubble diffused aeration is a very efficient process, however equally important is that it induces excellent mixing and movement of water, reduces the potential development of low dissolved oxygen (anaerobic) therefore improveing water quality.

Surface Agitators
There are a wide variety of surface agitators such as paddle wheels, pumps which spray water into the air, and several other devises. 
The one aspect in common with all these systems is that they all expend a great deal of kinetic energy in throwing large quantities of water into the air.
Obviously if the systems are expending energy in this task, the energy is not being directly used to aerate or mix the water.
Surface agitators often look impressive, however their influence over the Dissolved oxygen levels is rather localized to the area surrounding the equipment.
This factor becomes very apparent in systems with a water depth of more than 5 feet.
If oxygen levels are measured on the bottom very low levels may be recorded. The low dissolved oxygen levels may lead to anaerobic sediment conditions and deterioration in water quality.

Injectors
Injectors all mix air and water together under a high localized pressure. This form of aeration is often very efficient, and as such the processes
can be used for injecting oxygen into the water.
However when the process is used with air, gas super-saturation with nitrogen gas can occur leading to possible gas bubble trauma or sub-lethal toxicological problems.
If an injection process is used for adding air to water it is important to also measure the total gas pressure of the water and the partial pressure of nitrogen gas.
Injectors produce a jet of water containing the air bubbles, this jet of water creates good horizontal movement of the water. The systems are, however, generally poor at vertically mixing the water column and at bringing oxygen enriched water to the bottom of a system.
Under these conditions it is likely that the bottom will be prone to becoming low in dissolved oxygen (anaerobic).

All of the different types of aeration system discussed will dissolve oxygen into the water, however, the maximum amount of oxygen that can be dissolved in the water using air will be in the order of 95% saturation.
Mechanical agitators and injectors all exert kinetic energy in physically moving the water in order to dissolve the gases into solution. 


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