Comparing OHR and porous air diffusers
The reason why OHR AERATOR never clogs / Porous air diffusers always clog
Rubber porous air diffuser manufacturers claim that “clogging never occurs because rubber works as a check valve to prevent
counterflow of activated sludge” . Then, why continuous periodic washing and replacement is necessary in wastewater
facilities all over the world?
First reason why conventional air diffusers clog.
Temperature of blown air to air diffuser is over 80°C in summer.
Air blower sucks in air and compresses it and discharges it out.
Therefore, the compression heat is added to the suction air temperature, and discharged
air becomes hot. Approximate temperature of the discharged air can be derived by the
following calculation formula.
When the discharged air pressure is 55kPa* and the suction air temperature of the air
blower is 30 °C, 55 + 30 = discharged air temperature is 85 °C hot air.
(*55kPa = water depth 5.0m + rubber porous air diffuser’ s air discharge resistance 50cm)
If the air diffuser has activated sludge inside, it is obvious that this hot air will dry up and
accumulate inside the air diffuser.
Second reason why conventional air diffusers clog.
Activated sludge counterflows into air diffusers during air discharging.
When air is discharged in water, air is intermittently discharged because of airflow
resistance, i.e. water pressure. That’ s why “spherical’’ air bubbles are produced.
(see the right photo)
Activated sludge counterflows into air diffuser due to water pressure during the
intervals of intermittent air discharge.
The claim that "Air is discharged from air diffuser continuously, so activated sludge
does not counterflow into air diffusers" is contrary to the law of nature.
The same applies to the OHR AERATOR. It has only one large air discharge opening.
Air discharging from air discharge opening and activated sludge simultaneously
conterflow into the air discharge opening.
Third reason why conventional air diffusers clog.
When installing air diffuser with a diameter of 30cm at the tank bottom
and the water depth is 5.0m, the water weight is 350kg.
When disc type porous air diffuser with a diameter of 30cm is installed to aeration tank bottom
(water depth: 5.0m), approx 350kg water pressure is applied to the air diffuser in the vertical
direction. This means very strong force is trying always to counterflow the activated sludge into
the air diffuser. There is no way we can stop it by rubber contraction.
There is no rubber that shrinks enough to stop instantaneous counterflow occurring
in each of the many fine pores. In addition, rubber is gradually solidified, and air
discharge openings cannot be closed. Then, activated sludge further counterflows into
rubber porous air diffusers.
This even applies to silicon rubber porous air diffusers which are said to have excellent
durability as shown in the picture below.
Is there space for the activated sludge to dry and accumulate inside the air diffuser?
By observing air diffusers from this viewpoint, you can clearly judge whether it will clog or not.
Rubber porous air diffusers have many fine air discharge openings. So counterflowed activated sludge dries up into the air diffuser by air
blower’ s hot air, and it never goes out because it is blocked by the fine openings. Therefore, porous air diffusers inevitably clog and must be
cleaned and replaced in a few months or years.
On the other hand, the OHR AERATOR has only one large air discharge opening. So counterflowed activated sludge goes out smoothly.
<This means one air discharge opening completely prevents clogging. The air diffuser that has more than one air discharge opening
In addition, inside of the OHR AERATOR, there is no space for the sludge to dry and accumulate anywhere.
So, OHR AERATOR never clogs.
Comparing OHR and porous air diffusers
|Conventional air diffusers
(Porous air diffuser, etc.)
|Air and water are thoroughly mixed by
hydrodynamic process to forcibly
oxygenate the activated sludge.
|In order to oxygenate activated sludge,
air is discharged from many fine pores
to increase the contact area.
After air is blown from a large diameter hole of 30mm,
air and activated sludge are mixed powerfully by
crossing plates and mushroom-shaped separators.
Air is blown from many fine pores that are a few hundred microns
in size. No function for mixing air and wastewater provided, and
whether or not oxygen is dissolved into wastewater is left to nature.
|Many 100μm-sized fine bubbles are generated.||Less than 1/3 amount of 100μm-sized bubbles
generated compared to OHR.
|OHR AERATOR generates over 3.3 times more 100μm-sized
fine bubbles than rubber porous air diffusers.
This is because both air and water are broken down into
microparticles by the two unique structures.
So Alpha value of OHR AERATOR is 1.0.
▶What is the Alpha value ▶Comparison of generated fine bubbles
|Conventional air diffusers have many air discharge openings that
are a few hundred microns in size. But discharged air bubbles
become much larger in size than the air discharge openings
because of water pressure. So porous air diffusers can’t generate
many 100μm-sized fine bubbles because of it’ s structure.
▶Comparison of generated fine bubbles
|Approx 1.0.||Approx 0.2~0.7.|
|The OHR AERATOR performs same OTR in clean water and
OTR of other air diffusers is drastically decreased in actual
wastewater, even if they advertise high OTR which is
taken from clean water.
▶What is the Alpha Value
|Research institutions around the world report that
OTR of conventional porous air diffusers is reduced drastically
by up to 1/5 lower even in low load sewerage wastewater.
In heavily loaded industrial wastewater, the OTR drastically
decreases and the energy loss becomes very large.
|Never clogs.||Always clogs.
(So the OTR will inevitably decrease in time.)
|The reason why the OHR AERATOR never clogs is quite
simple and clear. This is because there is only one large
air discharge opening and activated sludge will not
dry/accumulate anywhere inside the OHR AERATOR.
▶Reasons of never clogs
|"Check valve effect" claimed by rubber porous air diffusers
manufacturers is contrary to the law of nature. When discharging
air in the wastewater, activated sludge inevitably counterflows into
the air diffusers. Counterflowed activated sludge will dry by air
blower’s hot air and accumulate inside of the air diffusers.
▶Reasons of inevitably clogs
|The required number is small for a large coverage area.
(Approx. one-tenth of the number of porous air diffusers.)
|The required number is large
for a small coverage area.
|The standard air blowing capacity of OHR AERATOR AE-130N
model is 1.0 m3/min. This is equivalent to 10 conventional porous
So the number of installation units is reduced, installation costs
are significantly lower. In addition, very strong circulating flows
are generated throughout whole tank. Activated sludge doesn’t
accumulate on the tank bottom.
▶See the specifications
|The air blowing capacity is small so many installation units are
required. And discharged bubbles rise too slowly (about 30cm/sec)
to churn up whole tank, so activated sludge must accumulate.
Then, anaerobic zone (oxygen consumption zone) develops.
|No clogging is recorded even when intermittent
running is performed at MLSS: 50,000 to 60,000mg/L.
|Clogging progresses and the alpha value remarkably
decreases under highly-concentrated MLSS.
|The OHR AERATOR high performance is unmatched in
harsh conditions because it has unique mechanism and structure.
▶See the real case examples
|There is no function to mix water and air. Only discharging
millimeter-sized bubbles from fine pores. Therefore, OTR
decreases drastically and it clogs rapidly in harsh conditions.
|No draining or anchoring
to the tank bottom required.
|Draining and anchoring to
the tank bottom are required.
|Because the main body presses on the tank bottom
by powerful reaction of powerful upward air flow.
OHR AERATORs are installed without draining the tank
in most cases.
|Porous air diffusers hold air inside the air diffuser before air
is discharged. So anchoring is necessary on the tank bottom
to avoid floating. So draining the tank and toxic gas /
accumulated sludge removal are needed when installing.
It is expensive and dangerous work.