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FAQ air diffusers |
air diffuser homepage |
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Resistance of diffuser to fouling, carbonates, fats and bacteria |
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| The air
diffusers are a hybrid between solid stone type diffusers and a flexible
membrane type diffuser. High levels of carbonates and iron present in the
water can foul solid diffusers. If the water is very hard, with an
alkalinity over 400mg/l as calcium carbonate, then the carbonate will tend to come out
of solution and form a precipitate on the diffusers. The carbonate deposit will block solid type diffusers. Flexible membrane diffusers are resistant to
this type of fouling. The Dryden Aqua diffuser is also resistant to this type of fouling because the surface of the diffuser is flexible, and hard
carbonates will tend to crack off the surface. In particularly serious cases of fouling the diffuser are simply lifted out of the water and twisted by hand.
If solid diffusers such as the stone type become fouled by carbonates they can become very difficult to clean, this is not the case with the Dryden Aqua
diffuser. The Dryden Aqua diffusers are often used for the treatment of
water and waste water from sewage works, landfill leachate and industrial
waste. In many cases the waste will have a very high fat content. In new
effluent treatment systems the bacterial cell biomass may not have become
established. In these systems there may be a reaction between the air and
the fats to form hard fat deposits on all surfaces in the aeration tank,
including the air diffusers. The diffusers may need to be removed once every
1 to 3 months and cleaned with a brush or high pressure hose. However after
a period of approximately 4 months, the bacteria in the system should have established, and we find that the bacterial cell biomass in the system actually prevents fouling of the
diffusers by bio-degradable components such as the fats. The growth of bacterial in the system generate acids on the diffuser surface, and these acids also
prevent fouling of the diffuser by carbonates.
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Does the diffuser need to be anchored to the
floor of the aeration tank |
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| The Dryden Aqua diffusers have their own internal ballast, which gives them sufficient weight to stay on the bottom of the aeration tank. Diffusers such
as membrane type need to be anchored on to the base of the tank, and solid stone type diffusers need to be installed on network of pipes. The Dryden Aqua
diffuser is therefore very easy to install. The internal ballast is composed of glass beads. The beads are very smooth and resistant to biofouling. The
beads also move, and this again helps to keep the inside of the diffuser clean.
Our air diffuser is perfect for retrofitting in tanks or for supplemental
aeration of existing tanks. In most cases where our diffusers have
been used to support another aeration system, our clients often remove
the original system and use the Dryden diffusers because they prove to be
more efficient and much easier to maintain |
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Operation of the air diffusers, air flowrate.
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| The Dryden Aqua air diffusers can be made in lengths from 33cm to 500cm. The normal size of diffuser
used for water and wastewater treatment is the 300cm long diffuser. This diffuser has a designed air flowrate of 9 to 12 cubm/hr of air. It is important to
operate the diffuser at the designed flowrate. At a flowrate in excess of 9 cubm/hr the air escapes all around the diffuser, this minimizes the degree of
fouling that the diffuser may experience.
The diffuser can be turned off and left in the wastewater, or it can be operated at the recommended air flowrate, however it should not be operated at a
reduced air flowrate. If the diffuser was operated at less than 9 cubm/hr then the air would not escape all around the diffuser, and it would then be more
prone to fouling by bacteria.
If the diffuser is not operated at the recommended air flowrate, or if it is used in particularly dirty water, then it may be subjected to fouling. In
such cases it may be necessary to remove the diffusers from the water and clean with a high pressure hose. In other cases it may be necessary to soak the
diffuser in a 20% solution of phosphoric acid for a period of 10 minutes once every year. The Dryden aqua air diffuser will not require any additional
cleaning or maintenance.
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Recommendation air pressure. |
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| The Dryden aqua air diffuser has been designed to operate with a high flow of air and low pressure drop. At the
recommended air flowrate the pressure drop through the diffuser will be approximately 2 to 3psi. We normally use the air diffuser in tanks with a water depth of 2 to 5 metres. The hydrostatic pressure head
at 5 metres water depth is 7.5psi. The pressure drop in the diffuser is 3psi, and the pressure drop on the pipework
and hose will be around 3psi. Total pressure drop in the system is usually
around 13psi in tanks with 5m water depth. At air flowrate of 500 cubm/hr to 1000 cubm/hr of air we use 125mm diameter pipework for the air ring main, to minimize pressure drop. You should always
use as large a diameter of pipe as possible in order to minimize pressure drop in the delivery pipe. |
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Size of air bubble |
| The oxygen transfer efficiency from air diffusers is a function of the bubble size from the diffusers. The smaller the bubble the
better the transfer, reducing bubble size from 6mm to 3mm can increase the mixing of the water and oxygen transfer efficient by almost 5 times, bubble size
is therefore very important. The Dryden Aqua air diffuser is one of the most efficient air diffusers available, with a bubble size around 1 to 3mm in most
types of water. However the size of the bubble is a function of the quality and chemistry of the water. In seawater, or water with a high ionic content, the bubble size can be smaller than 1 mm, as the water becomes more fresh, the size of the bubble will
increase to around 3mm in clean freshwater. If the level of fats and lipids is very high, the lipids will tend to reduce water surface tension and increase
the size of the bubble. As a guide in sewage treatment systems, bubble size normally ranges from around 2 to 5mm.
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How much the air can be dissolved. Let us
know typical example |
| The table below provides a guideline for calculating the oxygen transfer of the Dryden Aqua air diffuser in water at different depth against the oxygen
concentration already present in the water. The water type in question equates with sewage effluent to landfill leachate. |
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Oxygen transfer efficiency at different
water depth |
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% oxygen in the water |
1m depth |
2m depth |
3m depth |
4m depth |
6m depth |
30m depth |
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5% |
30% |
35% |
40% |
45% |
50% |
95% |
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10% |
25% |
27% |
30% |
35% |
40% |
90% |
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20% |
20% |
22% |
23% |
25% |
30% |
85% |
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30% |
15% |
17% |
18% |
20% |
22% |
80% |
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40% |
10% |
12% |
14% |
17% |
18% |
75% |
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60% |
6% |
7% |
8% |
12% |
15% |
60% |
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80% |
2% |
3% |
6% |
7% |
9% |
45% |
When we are conducting design calculations, we usually
base the transfer efficiency at 10%. This figure is an under estimate of the
actual transfer efficiency, however it insures that the system will receive
sufficient oxygen, and the tank will be well mixed
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