Air scour rate
Air scour rate
(OP)
We are designing a WTP including gravity filter and upflow roughing filter. Both of them require the air acour wash we only want to use one air blower to supply the air flow.
As the area of the upflow roughing filter is around twice bigger than that of the gravity filter. If we only use one air flow rate for all filters, the air scour rate for gravity fitler will around 6 SCFM/ft2 (normally it is only 3 scfm/ft2).
Will it cause any problem during backwash (we use sperated air-water wash)? or any damage to the underdrain system?
As the area of the upflow roughing filter is around twice bigger than that of the gravity filter. If we only use one air flow rate for all filters, the air scour rate for gravity fitler will around 6 SCFM/ft2 (normally it is only 3 scfm/ft2).
Will it cause any problem during backwash (we use sperated air-water wash)? or any damage to the underdrain system?





RE: Air scour rate
Why not install a flow meter and an actuated blow off valve in the air line to waste the excess air flow to atmosphere. It won't add that much to the cost of the project.
Another thought, I've not seen done; but, is possible. Install a 2 speed or VFD motor to drive the appropriate air flow through the headers for the appropriate filter.
RE: Air scour rate
When the air go through the blow off valve, is it very noisy?
I can't find any two speed blower can supply 1700CFM and 900 CFM.
and The cost of VFD control unit for 125 HP motor is almost as same as two sperate blower.
RE: Air scour rate
Most air scour systems are designed with a blowoff valve that is open on startup and shutdown. This starts and stops the blower against no head and slowly introduces the air into the underdrain driving any water out of the piping. This can be altered to include the flowmeter and a fully actuated (not just open/close) valve. Your filter control panel can be designed to check the flow meter against an air flow setpoint.
My idea on a 2 speed drive is this: If it is a pd blower (roots, kaiser, etc.) the volume of air produced is proportional to the rotational speed of the blower. You can tune the output with various sheave sizes. Having a 2 speed "motor" would change the rotational speed (ie air output) of the blower. Belt slippage should be taken into account as well. You might talk to a blower rep about this idea.
RE: Air scour rate
How the air scour is accomplished depends on your design, which is not done.
If you specify a header lateral air scour distribution with orifices on 6” centers structurally supported from the walls, the volume (or pressure) should not make much difference.
RE: Air scour rate
Bimr,
Do you mean that we shall ask the manufacturer to change (enlarge) the size of orifices on the air lateral and the openings on the underdrain system?
RE: Air scour rate
You have not really posted any details of your design. There are many different designs of air scour systems, some of them put the air through the common underdrain.
It is a good idea to install a simple mechanical type of pressure regulator on the influent line to the filter. Then you can use the pressure regulator to reduce the flow rate of air. You only need 5 psi air pressure for air scour. Air scour is also only used for 1-2 minutes.
RE: Air scour rate
This can be easily overcome by increasing the size of the holes in the nozzles, but you will need somebody who knows what they are doing to size them (I have had nozzle manufacturers get these wrong in the past and had to re-drill them).
It might also be worth while adding an additional layer of larger gravel next to the nozzles to help reduce the risk of the gravel moving.
I don't know if there are any process problems with sending excess air into a filter.
You should bring the air onto the filters at about half normal rate and then increase the air flow rate up to the full rate to ensure the air pattern is set up correctly. You also need to dump air at the end of the air scour before the back wash so you should have an air dump system fitted with an actuated valve, orifice plate and silencer to achieve this.
RE: Air scour rate
Water Quality and Treatment by AWWA is showing air only flow rates of 2-5 scfm/ sf.
RE: Air scour rate
An air grid can be installed above the underdrain and gravel as described by Bimr; but, these are typically retrofits for existing filters.
Unless it is a custom filter underdrain, I think you will have a hard time getting the orifice sizes changed or a manufacture to warranty one you changed. Most underdrain manufacturers have sized their orifices and done extensive testing to insure they are trouble free. To change that would be expensive.
RE: Air scour rate
RE: Air scour rate
RE: Air scour rate
S. Bush
www.water-eg.com
RE: Air scour rate