Diffused Aeration...

There are two basic types of diffused aeration systems ; (1) course bubble and (2) fine bubble. The factors of importance relating to the efficiency of a diffused aeration system are the bubble surface area in contact with the water, and the amount of water that the bubbles move by air lift effect. 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 is also of importance, thus 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 localised very visible disturbance of the water. 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 therefore seen to be a very different and much more efficient system than the course bubble process.

"Oxygen Transfer Chart of the Coarse Bubble Diffusers"...

Visually, it is difficult to see the impact of fine bubble diffusers in an aquaculture installation, however on close inspection of the pond a mass upward movement of slow moving water will be observed. This water rises from the base of the pond, progresses out over the surface and then submerges back down to the sediment providing excellent vertical mixing of the water as depicted in the diagram below. 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. We estimate that the entire water volume in a pond can be recirculated approximately every 10 minutes. Water circulation increases as the depth increases, for example in thermal destratification in lakes with a diffuser placed at 30 metres water depth, every cubic metre of air passed through the diffuser will move in the order of 200 to 300 cubic metres of water per hour. In aquaculture installations where water depth is in the order of 1 to 3 metres, every cubic metres of air will move between 10 and 50 cubic meters of water per hour.

"Diffuser Installation - 1"... "Diffuser Installation - 2"... "Diffuser Installation - 3"...

Fine bubble diffused aeration is seen to be a very efficient process, however equally important is that it induces excellent mixing and movement of water, reduces the potential development of anaerobic sediment, improves water quality and stabilises the environment.

Blowers...

Type	Picture	Air flowrate
Diaphragm air blowers		1 to 8 m3 / hr
Rotary vane air blowers		10 to 40 3 / hr
Roots air blowers		80 to 500 3 / hr

Diaphragm air blowers : For small aeration systems our diapgram air blowers are the ideal option.  The units are rated for continuous operation and are very quiet running.  An added advantage is that they are simple to maintain requiring only a replacement low cost diapragm once every year.

Rotary vane air blowers : For intermediate air flowrates from approximately 10 to 40 cubic metres of air per hour, rotary carbon vane units are the best choice. If more than 40 cubm/hr of air is required, then two or more of these unist may be connected to a common manifold.  The units require the carbon vanes to be replaced at approximately 7,000 hours of continuous operation. Replacement of the the vanes is simple and can be accomplished in a few minutes.

Roots Blowers : The roots air blowers are the main work horse of our aeration systems,   the units are heavy well engineered systems designed for high air flowrates and continuous operation. Servicing is again very simple,  the main requirement is to change the oil in the gear box at 1 year intervals.