Renaissance of the 7" Fine Bubble Disc

Few of us would trade our 4K TV for black and white, the internet for the yellow pages or Amazon for Woolworths, but sometimes things disappear from our lives that probably shouldn't.

One of these is the 7" fine bubble disc diffuser. Supplanted by 9" discs, tube diffusers, plates and strips, the 7" disc is buried in the old textbooks like a relic from another age, before we knew how to make 'better' sizes and shapes.

Did you know the 7" disc, at it's design airflow rate (a flux rate equal to the typical design airflux rate of a 9" disc), returns a 25% higher SOTE ?

Back in the 1980's there were 7" Norton* domes. Nopon's* porous plastics were also 7" and there were others like Hawker Siddeley* and Aercor.* Here is a typical plant in Illinois with 7" domes:

And another one in Ohio just like it:

A confluence of factors resulted in a shift primarily towards 9" discs and away from 7" discs. Here are some of them:

"There are too many parts and pieces."

"We can save money with a larger diffuser on labor and piping!"

"It needs to open bid so we need three equals."

"It leaks at the center bolt."

Today most of our industry design metrics come from the substantial data set we have on 9" discs.

We plug in 2%/ft, an airflow rate of 1.2 SCFM/ diffuser at the average daily flow rate, and so long as we are in the typical range of diffuser density and submergence, the design should be ok. We could be using 2.5%/ft as a plug-in number instead of 2.0%, if we shifted to modern 7" membrane discs. If that were the case, what would be the point of panels, plates, strips, sonic oscillators, or any other gizmos and contraptions that promise high efficiency with a caveat?

Panels, plates and strips have notoriously high back pressure and small pores that can be highly sensitive to fouling. They can be hard to maintain in the field and the membranes are sensitive to temperature and prone to stretching. Even if the products don't have the same set of problems, why do so many diffuser manufacturers name their products after a large bottles, guns and panels? It seems like they think it's a good thing for a diffuser to be large and panel-like.

Maybe they are headed in the wrong direction. 7" discs should be more reliable than 9" discs. There is less stress and consequently less strain on the membrane. The reduced buoyancy puts less load on the plastic diffuser housing and pipe connection. The bubble column goes straight up without significant coalescence or compression.

In our view at SSI, a 7" disc should be quick and easy to install, so that contractors are not forced to handle thousands of retaining rings, loose membranes, and grease. The ECD single-mold product makes a lot of sense here because it arrives on the jobsite ready for installation. Screw the ECD into a threaded QTS saddle or grommet, and you are done.

Here is how it looks under test:

If you are trying to maximize efficiency and reduce power consumption, we would recommend the following steps:

1. Select vendor with a small fine bubble disc diffuser that can operate at very low flux rates returning uniform air distribution. Considering that this technology has low headloss (DWP), high oxygen transfer efficiency, long expected life, and an attractive capital cost, it's going to be very hard to beat.
2. Ideally that product will connect to a piping system with a standard mount, such as a 3/4" threaded nipple. You are free to buy replacements from others.
3. Consider intermittent operation of the diffusers, where applicable for your process. It doesn't have to be an SBR for you to cycle air to one or more grids or zones on a regular basis.
4. Ask for your vendor's ISO-9001:2015 quality certification.

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