MECHANICAL AERATORS vs VENTURI PUMP AERATORS 1

[Berko]

Dear All,

Can anyone advise me of the pros & cons of using Mechanical (Surface Mounted) Aerators vs Venturi Aerators (powered by a solids handling pump) for Waste Water applications.

My only (limited) experience of aerators has been with Venturi aerators which I believe are (relatively)efficient devices.

From what info I can find, although they put less Oxygen in per hour than a mechanical aerator, the Oxygen inputted will be more efficiently absorbed (due to longer bubble retension times, as the air travels from the tank bottom to the liquid surface)).

From what I can see Mechanical aerators do not mix the air in as efficiently (as they impart air from the surface, rather than from the tank floor) & I have heard that they can be prone to breakdowns.

If I could throw this open to the forum for opinions/experiences !

Awaiting your comments,

Berko

 

[oxilume]

Dear Berko,

I have written several papers on dissolved oxygen and hold several patents with respect to removing DO from water. Likewise, I have a patent pending utilizing a venturi jet pump for aerating wastewater. In addition, I have developed a new type of jet pump that requires less head pressure to entrain more air and providing a greater lift for dewatering purposes. Finally, I have developed a high speed propellor for use on high speed bass boats and torpedoes. I am a former Naval Officer/Engineer.

Any WWTP superintendent or operator is well aware of the numberous problems and drawbacks associated with propellor type aerators or mechanical type aerators such as diffusers.

The true benefits of a venturi aerator are simply no moving parts and no small holes to plug. Consequently, maintenance is very low. This allows operators to spend more time operating the plant then conducting maintenance and repairs.

The downside of a venturi aerator is that it has a fixed motive (driving fluid) flowrate based upon the nozzle diameter and motive fluid pressure. Thus, the pump providing the motive fluid to the eductor must be sized according to the nozzle size. But please keep this in mind - venturi's use water velocity to produce a vacuum - not simply flowrate (gpm). The goal is to create high velocity thrust - similar to a jet engine (hint: jet pump) to entrain air.

The problem with jet pumps is that an inexperienced operator may use a trash pump to provide head pressure to the jet pump. Many jet pumps require firemain pressure (150 psig) to operate at design capacity. Remember as pressure decreases velocity increases. Thus, a higher pressure motive fluid can have a higher differential pressure when exiting the nozzle. Also, the diffuser should be designed to allow for efficien pressure recovery. The eductor must be capable of pumping the water and air mixture to the bottom of the lagoon, ditch or aeration tank.

Here is an example of the downside of jet pumps and how and why I invented the RamJet venturi pump.

Water Pressure Vacuum
Standard Eductor RamJet
100 psig 11" Hg 27" Hg
50 psig 4" Hg 25" Hg

As you can see by lowering the motive pressure on the standard eductor, the vacuum capabilites were dramatically reduced. On the otherhand, the RamJet still performed well with a low motive fluid pressure. The discovery was in part by accident, but has led to a very efficient and extremely inexpensive eductor.

If you need additional information let me know.

 

[bimr]

In most cases, the mixing power requirements, not the oxygen transfer power requirements are the controlling factor in biological aeration. Therefore, the mechanical aerator is usually the superior device because of the superior mixing.

As you are probably aware, there are countless salemen with countless BS aeration arguements when it comes to the selection of aerators. However, if you conduct a survey of the installations of aeration devices, you will find that most of the larger installations use mechanical aerators. The engineers of record for these installations probably evaluated the options and came to same conclusion, that the mechanical aerator is the superior device.

If you buy quality equipment, you do not have to worry about breakdowns. A quality aerator has the same quality as a quality pump. Fixed aerators are generally considered to be of much higher quality than floating aerators, probably because more expertise goes into them.

The bubble travel time has more to do with the ciculating currents in the tank than with bubble size. And, large bubbles have less transfer surface.

Check Metcalf and Eddy for further information.

 

[SASC]

Berko & Bimr,

Have you looked at multi-stage centrifugal blowers? If not, why not? They seem to hold court in the major aeration/diffuser set-up in the industry today.

Regards,
SASC

 

[bungayman]

For a wastewater the main differences that may effect your choice are:

1) Water Depth
2) Noise
3) Aerosol Production
4) Maintenance
5) Solids content
6) Tank Size
7) Tank Geometry

There will always be an appropriate time to use different styles of aeration, no single method is always the best depending of the application. In one application jet or venturi aeration may be better, in another surface aeration may be better, fine bubble diffused air vs coarse bubble diffused air etc.

1) Water Depth

Generally the oxygen transfer efficiencies will be similar. The deeper you go jet aeration will become favoured, as the surface mixer will simply not be able to mix the water sufficiently. My own choice would be 3-4m deep use surface aeration, 4m + use jet aeration.

2) Noise

The surface aerator produces a whipping/mixing noise that you cannot easliy control or deal with, other than by fitting cowels or covering the process tank. If noise is an issue jet aeration may be favoured.

3) Aerosol Production

If aerosol production is an issue do not use surface aeration.

4) Maintenance

Depending on the size of the tank different issues will be presented regarding how the equipment will be accessed and isolated for routine maintenance, or for mechnical breakdowns.

5) Solids content

If there are high solids in the waste is there a risk of blockages? Are there fibres in the waste that may build up over time? Can anything, such as calcium be precipitated out of the water?

6) Tank Size

As has been said previously is the tank is large, surface aeration would be preferential. In smaller tanks both may be acceptable. However, there are some quite sophisticated surface aerators now that can be used for dual mixing and aeration, or just mixing.

7) Tank Geometry

If the tank was an oxidation ditch I would use horizontal surface aerators such as mammoth rotors.
If the tank was rectangular I would use either multiple vertical surface aerators, or jet aeration.
If the tank was circular or square I would use vertical surface aerators.
If the tank was concentric around say a final tank I would use horizontal surface aerators if the annulus was wide enough, or venturi aerators.

SUMMARY

There has been a general trend towards fine bubble diffused aeration due to improved oxygen transfer efficiencies, and subsequent power reductions. However, these present potentially high maintence problems, and short operational life spans compared with other aeration devices. As the solids content goes up, fine bubble should be replaced by coarse bubble aeration. Again there are coarse bubble aerators such as a Helixor that operates as both a coarse bubble aerator and a mixer. The advantage of CBDA is that any maintenance should be outside the tank at the blowers.

The down side of both surface aeration and venturi aerators is that you tend to be at the mercy of the supplier to what the oxygen transfer and mixing capabilities are, which makes checking your designs calculations a bit more tricky.

If I had to use surface aeration or jet/venturi aeration I would consider all the above factors before making a process choice, which doesn't help you that much. Venturi aerators are a good method of supplementing an existing aeration system. For example BOC use venturi aeration in their pure oxygen systems. In this situation the comments above regarding tank geometry are less relevant.

 

[HudCameron]

Not all mechanical surface aerators are depth limited. Aeration Industries claim their new "Triton" Blower assisted aspirators/mixers have fine bubble plumes to 28ft depth.

We have supplied their various units with good results here in NZ downunder.

You can contact them from

http://www.aireo2.com

 

[mmo]

Hi this is a mesage for oxilume, would you explain a bit more what is the main difference between your RAMJET and a standard ventury.
I am quite interested in new aeration devices

Thanks
mmo

 

[oxilume]

Dear MMO,

I can send you a powerpoint presentation that will take you through the hydrodynamics of how and why the RamJet is an efficient aerator, pump, vacuum system, chemical mixer or venturi scrubber.

It can also be used as a mini-dredge for dredging lagoons or to supplement a lift station's capacity.

Please send me an email so I can forward the powerpoint presentation to you.

Sincerely,
Todd Foret
toddforet@usa.net