Don’t like GAC used as sediment filter

Discussion in 'Water Softener Forum, problems, installation and r' started by shopco, Apr 24, 2014.

  1. shopco

    shopco New Member

    Messages:
    45
    Location:
    Riverside, CA
    I know from previous experience that a sediment filter at my house point of entry will collect a fair amount of rust and silt. I now want to install a backwashing GAC filter. It seems to me that rust and silt are heavier than the 20x50 mesh GAC so how could they be backwashed out without also washing out the GAC?
  2. Reach4

    Reach4 Active Member

    Messages:
    1,897
    Location:
    IL
    For the silt, it would be a matter of particle size. I hope your rust particles are small too.
  3. LLigetfa

    LLigetfa DIYer, not in the trades

    Messages:
    3,828
    Location:
    NW Ontario, Canada
    Is there another reason to install a GAC filter? Install an iron filter to remove rust and sediment.
  4. shopco

    shopco New Member

    Messages:
    45
    Location:
    Riverside, CA
    I am installing the GAC filter to remove chlorine and thihalomethanes. I do not have iron in the water, just rust particles. I am thinking about putting a Big Blue 5 micron sediment upstream but I know a backwashing GAC filter is sometimes used as a sediment filter as well. i don't really understand how that works because GAC is light weight.
  5. Reach4

    Reach4 Active Member

    Messages:
    1,897
    Location:
    IL
    If that BB housing precedes your activated charcoal filter, I would go with a 100 or at least 50 micron filter in the BB housing, or use http://www.twistiiclean.com instead of the BB. If you put a BB in place and you don't actually need that filtering, you can leave the housing empty. Bigger stuff will settle into the empty housing. I put my 20x4.5 BB filters after my backwashing filter. There was not much for the filters to catch. I went with 3 housings (overkill): empty, 50..5 micron, and finally 1 micron. After 13 months, the 50..5 micron looked almost new, and the 1 micron looked totally new. Silt is smaller than 63 microns. http://en.wikipedia.org/wiki/Silt I don't know how large your rust particles are. But if they are large, a pre-filter would take that out.

    50 mesh is about 297 microns and 20 mesh is about 835 microns. http://en.wikipedia.org/wiki/Mesh_(scale) Note that specific gravity plays a part in how backflow will lift a particle, but so does size. The smaller particles will be caught up in a flow more than larger particles.


    Also, while plumbing, consider a pressure gauge or two to help gauge filter condition. A "boiler valve" can serve to hold a pressure gauge and it could serve as a handy faucet potentially.

    When ordering a BB, get plumbing silicone grease for the O-rings. I use nitrile gloves when handling that grease. Also get an extra O-ring in case you need it when you open the filter and close it back up later. I did not need my spare O-rings when I changed my BB filters, but it is good to have them.

    If you want to compare specific gravity effects vs size effects, play with a Stokes Law calculator. I expect the higher the terminal velocity from the calculation (because it is the falling velocity), the less likely it is for a particle to wash out. It is not going to be an exact calculation, but it could be useful in relating the importance of SG vs particle size for settling vs washing away. I is not trivial rounding up the values for the calculation.
    Last edited: Apr 24, 2014
  6. ditttohead

    ditttohead Water systems designer, R&D

    Messages:
    1,789
    Location:
    Ontario California
    Once 12x40 mesh GAC is wetted, it is not that light. We have some complex equations for determining the problem you are discussing, surface area, Specific gravity, expansion rates, etc, but in general, iron that has been oxidized is fairly easy to backwash out of GAC, Birm, and other medias that are considered light weight. It has a high surface area, allowing it to be easily backwashed from GAC.

    Hope this helps.
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