Degassing a deep bed filter filter 3

[fordjour]

I am applying permeable filter media ( punched out from reticulated polyurethane) to prefilter a surface water. The filter run time is encouraging , so is the filteration efficiency. I am observing a lot of air bubbles in the column.

One of the causes of the bubbles might be increase in the level of microbial activity.

Will anyone with experience on the use of a permeable filter media offer me guidance on how best to reduce the bubble formation?

 

[BobPE]

fordjour:

Most likely it is being caused by headloss within the filter bed that are reducing pressures to below various gasses vapor pressures. You state that efficiency is high, this also will increase headlosses, it a trade off game with filters.

Guidance, well we can't do thta here in the forums, but you may want to look at pressures within the column ad predetermined locations and compare them with loading rates.

Let us know how you make out....

BobPE

 

[bimr]

It is unlikely that you are getting bubbles from microbial activity. The foam itself is probably impervious (non-nutrient) to microbial organisms and there are not significant amounts of nutrients in clean water. Since you are using a reticulated foam, it seams unlikely that you would be developing any significant pressures as well. Please provide details of your filter as well as what your are trying to accomplish.

 

[fordjour]

Thanks for your response and insights BobPE and bimr. let me provide some further information.

CONCEPT IDEA

I am working on a scheme where only physical means are to be applied to influence particle removal.

Due to setbacks associated with the use of impermeable filter matearial i am applying a synthetic collector punched out from reticulated polyurethane.

This collector will be compressed in a column and run as a deep bed prefiltration unit.

Pathogenic particle presence in the clarified prefiltrate will then be disinfected through physical removal.

SOME OBSERAVTIONS

Filtration efficiency is very high and fairly stable.

The pressure drop or headloss development is very low.

Breakthrough has not set in.

I discontinued the run when the filtration efficiency dropped to 80 % and air bubble size and number increased .


NB. Feed turbidity 30-50 FNU
Collector size 10 *10 mm
Bed density 12 Kg/m³
Bed porosity > 80 %

 

[BobPE]

fordjour:

You say that headloss development is very low, how did you determine this? Are you able to profile the pressure gradient through the column? I really think this will tell you the stroy of your bubbles...

this is interesting, let us know how you progress....

BobPE

 

[fordjour]

by headloss development i mean the preesure diffrential (pd) across the filter section. The increase of this (pd) is small.

Pressure gradient profiling? No i have not .

 

[bimr]

A comment:

The feed turbidity is on the high side for a filter. I assume that is why you are calling it a prefilter.

And more questions:

The problem that I see with your filter is the ability to backwash it. As you are probably aware, when you backwash a convential media filter, it expands the bed and allows the filter bed to be cleaned.

Are you trying to get in-depth filtration? That is why some filters have a less dense anthracite on top of filter sand.

How do you maintain the integrity of your filter? By integrity, I mean the ability to prevent short circuiting through the media and around the sides.

 

[fordjour]

It is a prefilter, like you rightly observed.

Based on the current obseravtions from the work , the next step is to reduce the influent turbidity. What range might be considered appropriate?

Reply (bimr): And more questions???

The loaded column is held in place with a mesh and metal ring.

True, biomass release is aided by a high void fraction. For this work small void fraction and pore size was relied upon in particle capture. Meaning 'normal backwashing' will not work with this process. A different approach for backwashing was tried.

 

[sbush]

Fordjour,

When you mentioned that the size and number of bubbles increased after the filter run was terminated, this is consistent with a phenomenon that occurs when a vacuum is applied during fitration through a media. The vacuum can be developed by a pump on the discharge side of the filter, or by the syphon action that takes place when the filter effluent discharges below grade level. I must qualify that this phenomenon generally applies to filters operating by gravity as oppossed to pressure filters. As soon as the vacuum is releaved and the filter is vented to atmosphere, then off-gassing occurs.

Operating a filter under a slight vacuum actually improves performance. This has to do with van der Waals forces. At one time, Paddock Pool Equipment Company held a patent on vacuum sand filters for large swimming pool applications. They were able to show that vacuum sand filters performed to the same level of filtration as diatomaceous earth without the use of coagulants or filter aids of any kind.

S. Bush

http://www.water-eg.com

 

[sbush]

Further to my post above:

If it turns out that you are in fact using a pressurized filter and not a gravity filter, then the air bubbles may be caused by air entrained in the feedwater. Any air that is present in the feedwater will go into solution when pressurized. As soon as the pressure is relieved, then air bubbles will form and vent to atmosphere. Usually these bubbles start out very fine and then combine to form larger ones. Have you ever drawn a glass of water from a tap and watched as it went from clear to cloudy until all of the fine bubbles slowly dissipated from bottom to top. The feedwater may be naturally high in oxygen or the air might be introduced by a leaky pump seal or some similar form of aspiration.

S. Bush

http://www.water-eg.com