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rosli
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Thickness of Activated Carbon Layers, Fine/Coarse Sand

07/18/2011 4:55 AM

Hi all,

What factor determines the thickness of each layer of activated carbon, fine and coarse sand in a multimedia filter? Any specific formula? I plan to replace all material inside multimedia filter for better function of multimedia filter after premature failure of thin film RO membrane whereby organic foulants passed through the multimedia filters and get stuck inside the membrane unit thus affecting the flow rate.

Thanks
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PWSlack
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#1

Re: Mechanical Engineering

07/18/2011 5:49 AM

In a sand filter, the critical parameter is forward pressure drop, as this determines the backwash point. The area of the filter determines the downflow speed. The backwash frequency is then dependent on the dirt loading, and is a function of allowable pressure drop. Particle size is not as important as bed depth, as the filtration mechanism is that of a labyrinth where the dirt particle sticks to the sand particle, reducing the effective pore size and providing more places at which a similarly-sized dirt particle may impinge.

Carbon filters don't need so much attention as the upstream sand filter, as the activity within this vessel is catalytic and in the aqueous phase. Occasionally, carbon filters will get dirty, so it is more a case of backwashing on time rather than on forward pressure drop.

It would be unusual to combine these two media in a single vessel, as they do different jobs and have different backwash frequency requirements.

Organic foulants on a downstream RO membrane suggests four things:

  • Review the operation of the carbon filter for performance against original design.
  • Review the chemical dosing regime to ensure the correct agents and dose rates, as these are intended to carry organic foulants out of the system in the reject stream.
  • Review the recovery percentage within the RO membrane. If the membrane is being driven to recover in excess of design parameters, then trouble with foulants and membrane life may be expected.
  • Review the chemical cleaning regime for the RO membrane and assess whether this is being carried out at the design frequency or otherwise, for future reference.
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RVZ717
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#2
In reply to #1

Re: Mechanical Engineering

07/18/2011 7:26 PM

Dang,

If I didn't know any better I'd say you really know what your talking about .

Good answer!

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krishnan.ng
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#3
In reply to #1

Re: Mechanical Engineering

07/19/2011 1:09 AM

Your answer is very comprehensive and excellent. You may probably tell us the issues involved in using molded activated carbon filter in stead of sand & carbon powder layers.
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PWSlack
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#4
In reply to #3

Re: Mechanical Engineering

07/19/2011 5:40 AM
  1. They have a different buoyancy, which will screw-up any attempt to backwash them should they be together in the same vessel.
  2. Dirty carbon doesn't work.
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krishnan.ng
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#5
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Re: Mechanical Engineering

07/19/2011 6:24 AM

Thanks!
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CaptMoosie
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#6

Re: Thickness of Activated Carbon Layers, Fine/Coarse Sand

07/19/2011 11:53 AM

PW, excellent post! You get a GA from me.

I just want to add one thing to help out the OP:

I'm assuming that you're employing the filter to remove unwanted constituents from water. Usually, to determine optimum raw water feed rate a well as minimally accepted filter area and media depth one must conduct "bench" trials and review the the test results to find the most economical and effective removals.

I for one would not utilize sand and AC together in the same filter vessel as it would tend to stir-up and mix together the two media types, thereby negating any backwash operations where you will find that you end up with even dirtier filter and filtrate.

In my experience, the only acceptable use of AC and sand in the same filter vessel would be in the slow sand water filtration process, where you do not employ backwashing to clean the media......the AC layer would be buried several inches down below the uppermost sand layer and utilized to remove or reduce the amount of THM precursors (namely suspended and soluble organics) from the raw water prior to final chlorination. Cleaning of the filter is typically done by scrapping the upper limits of the top layer of sand, which is in essence a biologically active zone called Schmutzdecke.

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