Is the OTR of porous air diffusers really high?
Porous air diffusers vs. OHR AERATOR.
Porous air diffusers are commonly used for wastewater treatment around the world especially rubber made ones.
Why do so many people use porous air diffusers even though they 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 a lot of fine
bubbles are generated. But is this expectation realistic? Is the OTR really high?
Verification 1: Result of Porous vs. OHR fine bubble generation test.
Verification 2: Real case examples of comparing Porous vs. OHR at two wastewater treatment plants. / What is the Alpha Value.
|1. Which air diffusers generates the largest number of micron-size fine bubbles?|
|Comparative test using two transparent acrylic tanks.|
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 tanks at the
|1||Immediately after starting aeration.|
|2||How the two air diffusers work.|
|3||The condition of the two tanks after 60 secs of aeration.
Pictures taken immediately after switching off the air blowers.
|4||Why the OHR AERATOR generates over 3.3 times more micron-size fine
bubbles than porous air diffusers.
The OHR AERATOR is the world’s only air diffuser having a powerful mixing function. Therefore, a lot of micron-size fine
bubbles can be generated.
|2. Real case comparisons of Porous vs. OHR at two wastewater treatment plants.|
|In actual wastewater treatment plants, OHR AERATOR is much more efficient.|
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 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) Real case in a paper mill.
OHR AERATOR used 30% less air than a porous air diffuser which should have twice the OTR as OHR.
*Wastewater amount : 12,000m3/day *BOD value : IN 400mg/L OUT 10mg/L *BOD load : 4,680kg-BOD/day
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 as the OHR AERATOR, the required air
amount should be less than 1/2. However, actually, 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 actually calculates with half the OTR.
B) Real case in a bean sprout manufacturing factory.
Due to low efficiency of a rubber porous air diffuser, the DO didn’t rise sufficiently and the wastewater treatment conditions became bad.
*BOD value : IN 3,000mg/L *MLSS : 12,000~13,000mg/L *Water depth of aeration tank : 4.0m
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 ones 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 ones 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.
When using a porous air diffuser in wastewater,
the very high OTR taken from clean water is dramatically reduced.
The properties of wastewater differ for each site (factories, houses, etc) so uniform performance data of air diffusers can not
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.
To get the "actual OTR in wastewater", multiply the "Alpha value" by the "OTR in clean water".
Air diffuser manufacturers advertise high OTR knowing in fact their OTR is low in wastewater.
“Alpha value” is “How much the OTR in clean water decreases in actual wastewater” and isn’t publicized by air diffuser
manufacturers. This value is an important indicator to demonstrate the true performance of air diffusers because even if 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 that the OTR drops to 1/4.
In the above two cases, how much is the "Alpha value"?
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 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 drop down 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.)
OTR of the porous air diffusers dramatically drops in actual wastewater.
This is a worldwide assessment.
(1) Singapore’s National Water Agency views : 0.23 ～ 0.68 (for Singapore sewage water )
(2) An example in the report of the *EPA (*United States Environmental Protection Agency) : Average 0.4
*Alpha value of the porous air diffuser at the sewage water treatment plant in New Jersey state was avg. 0.4
on weekdays high load time.
(3) OHR’s views : Approx 0.2 ～ 0.5 (=OTR decreases to about 20 ～ 50% in industrial wastewater not sewage.)
*Industrial wastewater has a high BOD value of several hundreds to several tens of thousands of mg/L, so the Alpha value
will be considerably lower at 0.2 to 0.25. However, the BOD value of sewage water is very low at about 200 to 350mg/L,
so the decrease in Alpha value is relatively small compared to industrial wastewater. But it is gradually becoming evident
worldwide that OTR dramatically decreases even in sewage water.
(OHR AERATOR is used only for industrial wastewater not sewage water in Japan, so the "Alpha value of 0.2 to 0.5" is only
for industrial wastewater.)
Alpha value of OHR AERATOR is approx 1.0.
Efficiency of OHR AERATOR does not decrease even in actual wastewater.
The Alpha value of the OHR AERATOR is approx 1.0 this means the OTR doesn’t change between clean water and
actual wastewater. The proof is as follows.
- Evaluation report from an engineering company.
● From the 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 decreasing in OTR".
● In the test report, it is described as follows.
"In order to evaluate OTR in actual wastewater, firstly 5.0ppm of surfactants 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 about 30% compared with in 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 using 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.
Why the OHR AERATOR’s OTR doesn’t change between clean water and wastewater.
When the OHR AERATOR and rubber porous air diffuser are operated under the same conditions, OHR AERATOR generates over
3.3 times the amount of fine bubbles (approx 100μm size) than rubber porous air diffusers.
OHR AERATOR’s original strong air-wastewater mixing function and mechanisms probably contribute to the high Alpha value
performance of 1.0.