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Air scour rate

Air scour rate

Air scour rate

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?

RE: Air scour rate

Yes, it can harm the underdrain by overpressurizing it.  You need to contact the underdrain manufacturer to find their maximum 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

Thank you very much for your help
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

It will make some noise; but, the level will depend somewhat on the pipe size (ie. air velocity exiting it).

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

Don’t think that the change in the air scour volume is going to make any difference.

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

We already built the upflow roughing filter and gravity filter tank. So the backwash area are fixed.

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

I was implying that if you have a separate air scour distribution design, then you probably do not have to change anything with it. Just operate at the higher flow 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

Sending too much air through a header and lateral separate air scour/backwash system could cause problems.  The air should travel through, and be regulated by holes or slots in the nozzle down pipe.  The pipe should extend below the air/water interface.  If you send too much air into the filter, you will lower the air/water interface and the air will travel through the end of the down pipe rather than through the side holes.  This will become a preferential route for the air and you will loses any regulation of the air.  Rather than an even air scour across the bed you will have violent areas of "boiling" water which would disturb you gravel bed.

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

Air scour orifices should be located above the gravel so the gravel will not be disturbed.

Water Quality and Treatment by AWWA is showing air only flow rates of 2-5 scfm/ sf.

RE: Air scour rate

Most of the newer underdrains/nozzles have the air scour orifices built into them and are located under the gravel.  Some nozzless underdrains are now installing caps or screens to eliminate the need for gravel; but, the orifices are built into them none the less.  With appropriately sized gravel and air flow the gravel support layers should not be disturbed.  Providing too much air flow in these drains can interfere with the water flow as rcoop stated and can even rupture the underdrain.  Remember air is a compressible fluid and when compressed can expand violently.

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

I have previously worked for a filter manufacturer in the UK.  We did do custom under drain systems.  The design effort is higher, but the manufacturing effort was not significantly more.  There are pins in the dies to form the air orifices.  Once the size of the orifice is determined the correct pin is fitted to the die and the production run is the same as for a standard size nozzle.  This is cost effective as there will be thousands of nozzles for a single reasonably sized filter block.

RE: Air scour rate

Thanks for the info.  I know some of the filter manufacturers drill the orifices by hand which change out of the bit shouldn't be much either.  I would have thought testing to determine the filter media sizing and equal distribution would have added cost.

RE: Air scour rate

With some minor modifications to the control logic and valve sequencing, you should be able to simply air sour the clarifier at the same time as the filter.  Then shutoff the air flow to the filter and continue air scouring the clarifier at the higher rate.  After air scouring both beds, backwash as usual.  Air scouring the clarifier at half the normal air rate will not harm the internals or media in any way.  You might need to add an air throttling device and air flow meter; but you should already have this equipment regardless of the type of air scour system.

S. Bush

RE: Air scour rate

I wanted to comment on "sending extra air into the filter".  Some automated backwash sequences have only an inch or two of freeboard at the end of the air/water step (the time when air scour is ramping down and water backwash is beginning).  Extra air could send media over the troughs, resulting in media loss.

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