Purchasers of fine bubble diffusers for their sewage treatment plant or industrial wastewater treatment plant have a lot of choices to make when selecting equipment for their project.
Diffusers are available in many shapes, including discs, tubes, squares, and rectangular panels, and in different materials, including elastomers like EPDM and porous media like Aluminum Oxide, Porcelain, or HDPE. Many books and papers have been written on the differences between these media.
However there are still many questions about the superiority or application of disc vs. tube vs. panel.
Many of the manufacturers in the business make multiple types of diffuser, though they tend to favor and promote one moreso than the others, typically for commercial reasons (they have a higher margin on one product) or for reasons of product differentiation (when specified by a client or engineer, it is difficult to find “equal” competitors).
Many tests of oxygen transfer efficiency have been carried out over the years on each type of diffuser (some of which are published in the ATV Handbuch), however product development is dynamic, and what was tested in the mid 1980’s may not apply today to modern discs, tubes and panels.
There are some common sense principles to follow, regardless of the development of the technologies.
In a retrievable system made of costly stainless steel, the diffuser which can handle the most air with the least stainless infrastructure is going to be an attractive choice. Typically tube diffusers are seen on retrievable systems for this reason.
In a fixed system where the pipes are bolted to the floor, and longevity and low maintenance are required, disc diffusers are more popular than tubes.
Where failure mode is important (i.e. catastrophic vs. slow) disc diffusers may be favored over tubes. Tube diffusers typically have a large air orifice, hence in case of a membrane rupture or clamp failure, a large volume of air can escape from that orifice, starving the rest of the system.
The deeper the tank, the less benefit panel diffusers offer in terms of efficiency. Panels are typically designed to produce very fine bubbles. In a shallow tank, this is a benefit, albeit a costly one since panel systems often carry a hefty price tag due to the amount of equipment required to purchase and install. However in a deeper tank, so much of the oxygen is transferred by the time the bubble has risen 15 ft that the bubble is said to be oxygen depleted, hence the benefit disappears. One must also be cognizant of the headloss of panel diffusers, since what is gained in terms of efficiency from small bubbles is often lost in extra energy needed to overcome the high back pressure of panel membranes.
Regardless of the type of diffuser, one should consider PTFE coated membrane diffusers, which may extend the lifetime of the membrane and will also reduce surface fouling as well.