Porous air diffusers are commonly used for wastewater treatment around the world especially rubber made models. Why do so many people use porous air diffusers even though they inevitably clog and must be cleaned and replaced periodically?
The reason is most people expect the OTR (Oxygen Transfer Rate) of porous air diffusers to be high because so many fine bubbles are generated. But is this expectation realistic? Is the OTR really that high?

Verification 1: Result of Porous vs. OHR fine bubble generation test.
Verification 2: Real-world examples of comparing Porous vs. OHR at two wastewater treatment plants. What is the Alpha Value?

Using two transparent acrylic tanks filled with water, a disk type rubber porous air diffuser was installed in one tank and the OHR AERATOR AE-50N model was installed in the other tank. The same amount of air was sent to each tank at the same time.

The OHR AERATOR is the world’s only air diffuser with a powerful mixing function. Therefore, a lot of micron-size fine bubbles can be generated.

The OTR, which is one indicator of an air diffuser’s performance, is taken in clean water (not actual wastewater). Rubber porous air diffuser manufacturers advertise very high OTR of 20% to 30% at 5.0m water depth. The OTR of OHR AERATOR is 10% at 5.0m water depth. So when compared only by advertised OTR, the OHR AERATOR would appear to be a very inefficient air diffuser.
The advertised OTR of the OHR AERATOR in clean water is surely lower than porous air diffusers, but this is meaningless in wastewater. The OHR AERATOR has a much higher OTR than the porous air diffusers at the actual site where there is wastewater, not clean water.
See the following two real case examples:

A paper manufacturer compared the OHR AERATOR and E company’s rubber porous air diffuser. The advertised OTR of both air diffuser companies at 5.5m water depth is as follows:

If the E company’s rubber porous air diffuser is really more than twice the OTR of the OHR AERATOR, the required air amount should be less than half. However, in reality, the OHR AERATOR requires 30% less air than the porous air diffuser when comparing the ‘required air volume’ submitted to customers by each company.

Why the big contradiction?
It is clear when you look at the calculation formula submitted by E company.

While E company advertises a high OTR for the public, it is multiplied by 1/2 in calculating the required air volume.
In other words, it is actually calculated with half the OTR.

A bean sprout manufacturing factory installed M company’s rubber porous air diffusers in their aeration tanks.
However, the wastewater treatment conditions became worse due to DO (Dissolved Oxygen) in the aeration tanks not rising sufficiently. The customer hastily replaced the porous air diffusers with OHR AERATORs.
The advertised OTR of each air diffuser at 4.0m water depth is as follows:

The rubber porous air diffuser’s OTR should be more than twice the OHR AERATOR, so when replacing the porous air diffusers with OHR AERATORs the DO should not increase.
However, in fact, the DO value more than doubled after replacing the porous air diffusers with OHR AERATORs.

The properties of wastewater differ for each site (factories, houses, etc) so uniform performance data of air diffusers cannot be collected from wastewater. When the intended use of an air diffuser is for wastewater treatment, the OTR taken from clean water is meaningless because actual wastewater contains many impurities, making it more difficult to dissolve oxygen than in clean water. This means OTR dramatically decreases when using porous air diffusers.

Alpha value is “How much the OTR in clean water decreases in actual wastewater” and is not publicized by air diffuser manufacturers. This value is an important indicator to demonstrate the true performance of air diffusers, because even when high OTR is claimed in a catalog, if the Alpha value is low, it is actually a low-efficiency air diffuser. However, neither air diffuser manufacturer nor wastewater treatment company releases the Alpha value of their air diffusers. If the Alpha value is 0.5, it means that the OTR decreases to 1/2, and if the Alpha value is 0.25, it means the OTR drops to 1/4.

In case A, Company E multiplied their OTR by 1/2. This means the Alpha value is 0.5.
In case B, the Alpha value of M company’s rubber porous air diffuser is less than 0.25.
(M company’s air diffuser should have a higher OTR, more than twice that of the OHR AERATOR, but in fact their OTR is less than 1/2 the OHR in wastewater. This means the advertised OTR of M company’s air diffuser dropped to 1/4 in wastewater, and assuming that the Alpha value of the OHR AERATOR is 1.0, the Alpha value of M company’s is less than 0.25.)

The Alpha value of the OHR AERATOR is approx 1.0 this means the OTR does not change between clean water and actual wastewater. The proof is as follows:

Evaluation report from an engineering company

From water treatment engineering company S, we received a test report saying that "even when the OHR AERATOR is used in actual wastewater, there will be almost no decrease in OTR".

In the test report, it is described as follows.
"In order to evaluate OTR in actual wastewater, firstly 5.0ppm of surfactant was added to clear water to make simulated wastewater, and the rate of decrease in OTR in the simulated wastewater was measured. The OTR of a conventional air diffuser was reduced by approximately 30% compared with clean water. However the OTR of the OHR AERATOR showed almost the same value as clean water. From this result, it is assumed that the OTR of the OHR AERATOR will be nearly unchanged when used in actual wastewater."

Design calculations have been used for more than 30 years with an Alpha value of 1.0

OHR has used design calculations with an Alpha value of 1.0 for more than 30 years and we have never had a case that resulted in inferior treatment due to insufficient air blowing amount. In addition, we clearly show customers the design calculation formula for the required air blowing amount based on the Alpha value of 1.0.

When the OHR AERATOR and rubber porous air diffuser are operated under the same conditions, the OHR AERATOR generates over 3.3 times the amount of fine bubbles (approx 100μm size) as rubber porous air diffusers.
The OHR AERATOR’s original strong air-wastewater mixing function and mechanisms almost certainly contribute to the high Alpha value performance of 1.0.

Verification 1: Result of Porous vs. OHR fine bubble generation test   Mechanism