air fans for backwash 1

[Karbone]

Guys,

In my limited experience in water treatment, I am seeing for the first time air fans used to assist in the backwash of filtering and softening equipement. How important is the pressurized air in fluidizing the beds? and is the backwash water pressure not enough to loosen the bed?

I am new to eng-tips and I am marvelled and humbled by the level of expertise on this forum.

Best regards,

Karbone

 

[quark]

Heard it for the first time. I can't think of any advantage, however.

 

[kchayfie]

Are you talking about air scour blowers? If so, we always use them. I understand its because they are more effective at fluidising the bed and loosening the solids prior to the water wash. I guess just straight water wash would do the same thing but probably requires more water and possibly bigger pumps.

 

[bimr]

Air scour-assisted backwash came into common usage 30-40 years ago to improve the effectiveness of the backwashing operation of filters. That equates to a shorter backwash time and a cleaner bed; saving water and time.

Water softeners do not use air scour because the ion exchange media has a lower specific gravity and the ion exchange media will float out with the backwash water. However, other ion exchange processes do utilize air for mixing the ion exchange bed.

 

[SteveWag]

I am and have used a combined air/water backwash for conventional rapid sand and softening filters. You must be very careful, as to much air/water will lift the bed and carry the media over the trough walls. I constructed two trailer mounted units with three transparent rapid sand filters and the effects of the introduction of air was dramatic. The B/W becomes very violent and the backwashed filter exhibited lower initial headloss.

Steve Wagner

 

[Catcher66]

To alleviate the media loss concern, the following backwash sequence may be used:

-Close filter influent valve
-Allow filter to drain to below the washwater trough
-Close filter effluent valve
-Run blowers to agitate media without backwashing filter
-Shut off blowers as filter effluent valve is opened
-Continue with backwash without air scour

 

[rhkarim]

how can i calculate the amount of air which needed for backwashing? and how much pressure is needed?

 

[semo]

Air scour is typically performed at a rate of 2 scfm/sf of bed area. Your head pressure depends upon the piping, water level, and blower location.

You must have an appropriate underdrain for air scour or you must install a grid type of system strictly for the air scour that lies on top of the underdrain or gravel support media.

DO NOT TRY TO FORCE AIR INTO AN EXISTING FILTER UNDERDRAIN THAT ISN'T DESIGNED FOR AIR. Not unless you want to blow it out of the filter basin.

As Bimr and SteveWagstated earlier, the air scour is very agressive and will improve the cleaning by giving the media more agitation to break up mudballs, etc. It also does it in a shorter time frame than water only and saves time and backwash water.

Media guards can also be incorporated into the trough design to help retain media at higher air/water wash rates.

Another hint is to bring the air header above the filter water level to prevent a leaking air valve allowing water into the blower.