A diffuser’s design has a significant affect on aeration and is just as important of a choice as is the aerator size. Every square inch of the diffuser’s surface area will experience PSI.
- The greater the surface area, the more resistance the aerator with experience.
- The smaller the bubble size, the greater the resistance the aerator will experience.
- The deeper the diffuser is placed, the greater the resistance the aerator will experience.
Based on these points, it would appear a smaller diffuser is the better option for deep ponds because it can be placed deeper and will work the aerator less. Technically, this is true. However, what is the goal?
A diffuser with minimal horizontal surface area or those with large holes for air discharge, will move less water. Less water movement means the potential for poor levels of dissolved oxygen in certain areas of the pond, including near the diffuser. This is where claims of a pump’s operating depth will come into play. It's true to state a ¼ hp piston pump aerator can operate at depths of 50’ – if the diffuser design doesn't move much water. A good comparison is a boat paddle. There's a reason for its size and design.
Also rarely mentioned, a system stating an operating depth means a total depth. If you add another diffuser, the combination of depth cannot exceed specified max depth. This includes diffuser setups that add multiple discs and sell as one unit. The same principle applies.
Moving Water with Bubbles
When it comes to bubbles, every bubble released by a diffuser creates what is called drag force. As the buoyancy of a bubble forces its way to the surface, it first displaces water and then creates ‘drag’, pulling water in the direction of the bubble, which is to the surface. Creating the most drag per volume of air is dependent on the size of bubble.
Below are two images of bubbles – one single bubble on the left and numerous bubbles on the right. The typical expectation would be for a single large bubble to drag more water than smaller bubbles, but that is far from the case. What is lost with that thought is the cavity of the bubble; the volume of air within the bubble that does not have surface to create drag.
Now, take a close look at the image on the right. The small bubbles image was laid over the single large bubble image from the left. Looking even closer at the outer edge of the small bubbles, the single row of outer edge of bubbles has created approximately the same surface area of the single bubble. Add all the remaining bubbles and we just dramatically increased the amount of surface area for drag, using approximately the same amount of air! This results in more water movement and at a greater force. Of course, what affects the size of bubbles? The holes made in the diffuser to allow for the release of pressure created by the pump.
Below are two images of diffusers in operation. Figure 1 is Aeration Central's diffuser producing mostly micro-bubbles and has a large surface area. Figure 2 is producing mostly larger bubbles and has minimal surface area. Because Figure 1 has a much larger surface area and is producing much smaller bubbles, it will do a substantially better job at aerating the entire pond. At the water surface, both will display turbulence and unfortunately this can be deceiving as both will seem to be doing the job. If you ever want to see how deceiving this can be, remove the diffuser and just throw the hose in the pond. You will see a lot of surface turbulence, but that will do next to nothing for aerating the entire pond.
Uniformity of the holes is critical too as air will travel to the point of least resistance. If a diffuser has a mix of hole sizes, holes that can produce the desired smaller bubbles will not be productive. Air will release through the larger holes.
Turning our focus to surface area and its relation to operating depth, Figure 2 is likely to operate in twice the depth as Figure 1, when all else is equal. The surface area of Figure 1 is substantially larger and will experience greater water pressure due to the larger surface area. Additionally, more force is required to produce smaller bubbles. This is where operating depth comes into play. Figure 1 is likely to have a maximum operating depth of 30’ with a ¼ HP piston pump whereas Figure 2 will likely have an operating depth of 50’. The thought of an aeration system performing at great depths gives the perception of a powerful system and thus must do an excellent job of aeration in comparison to other systems. As you have read, there is so much more to it.
Operating depth is useless if it only creates surface turbulence, not circulation. Most pond owners judge the performance of an aeration system by the surface action produced by a diffuser. It will not matter the type of diffuser used or if there’s just a hose laying at the bottom. Air coming from the bottom and raising to the surface is going to create turbulence at the surface. What matters is what happens before and after the turbulence.
Speaking from years of field experience, I have come across a wide variety of aeration systems. Some of them did an excellent job. Some, not so good. My appreciation for a quality and properly sized aeration system comes from understanding what happens throughout a pond. When I created my line of aeration systems, I did so knowing how the design must work. There are going to be times where things will seem as though it could not get any worse, but if you choose wisely with an aeration system and take additional steps such as adding beneficial bacteria, turning a tough situation around will happen much, much faster when the right pieces are in place.