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Thread: Lowering TDS

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    Curious Reefer
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    Lowering TDS

    Hello,

    I have a 240 gallon reef tank that has recently developed a problem with diatoms. I did some research and found that diatoms feed off of silicates, once the silicates are gone the diatoms are gone. So I went out and got a TDS meter and started testing water. My city water is pretty bad having a TDS reading of right around 1000ppm. I then tested my RO/DI water and noticed it was right around 200ppm, which I think is still too high. I recently bought a change of filters for my Kent Marine Hi-S 60gpd and tested that water. The TDS reading did go down but not as low as I expected, it dropped to about 30ppm. I have read that other people change their RO/DI filters when the TDS reading gets above 0ppm. I also checked on the Kent Marine website and found out that my RO/DI unit is optimized for TDS lower than 200ppm. So my question is can I simply put some sort of pre-filter in front of my RO/DI or do I need to get a new RO/DI system? What are my options for fixing this problem? If I need a pre-filter what kind of filter do you recommend and if I need a new RO/DI what brand/model do you recommend?
    Thanks for the help!

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    SPS Reefer / TR Admin lReef lKeeper's Avatar
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    you could add another DI canister just after the 1st one. that should handle the left over TDS that are getting through. it would be a low cost fix to your problem too.
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    Master Reefer poppin_fresh's Avatar
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    how many prefilters are there on your unit? The fact that changing them dropped your tds from 200 to 30 is a good thing. A perfect membrane with 98% rejection (Dow 75gpd) would get you a minimum of 20 TDS before the DI stage. After Di with such a unit should get you near "0" until the resin gets exhausted.

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    Grand Master Reefer Amphibious's Avatar
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    TDS meters need to be calibrated against a known 0 tds sample. It could be off even if it's new.

    Dick
    Amphibious

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    Curious Reefer
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    Thanks for all the comments. I will check the TDS meter, I didn't think of calibrating it since its new. My current RO/DI unit has a total of 4 filters first of which is a 1 micron filter followed by the membrane filter, 0.5 micron carbon filter and a DI resin filter. Do silicates get removed in only the DI filter or are they removed in the prefilters as well?

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    I think that the membrane and the DI remove silicate.
    Carmie


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    Insightful Reefer tdkarl's Avatar
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    what is a ok tds reading after the rodi? At what point should you change your membranes?
    Tony K.
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    Grand Master Reefer CarmieJo's Avatar
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    I'd say <10.
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    Grand Master Reefer PhotoJohn's Avatar
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    zero or there abouts is best

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    0 is the only reading for me
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    Anything other than 0 is when I change DI. I have found the SilicaBuster Di from Spectrapure to be by far the best DI resin for silicates, its designed for just that purpose.
    I am in Phoenix with a tap water TDS in the 800+ range. The only unit I will use is the MaxCap from Spectrapure, its designed for high TDS waters from the ground up. You can use your housings and buy replacement filters easy enough.

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    I guess I've just got to gloat a little here.

    I live in Dunsmuir, Ca. Our water comes from a 10000 year old glacier on Mt. Shasta and is filtered through about 15 miles of volcanic rock. It flows out of a hill about a mile away where it is collected in the city's cistern. I t then flows through the pipes to my house. No filters, no irradiating, no treatments, no chemicals. When it flows into my RO/DI unit the TDS is 35 ppm.
    Here's a shot of Mt. Shasta

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    Insightful Reefer AZDesertRat's Avatar
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    I'd gloat too! The national average TDS is about 250 and rising all the time due to things like groundwater recharge and surface water pollution.

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    Grand Master Reefer CarmieJo's Avatar
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    Beautiful shot!
    Carmie


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    That's a great shot of Mt. Shasta was that taken this year. Man its been a long time since I been there. Thanks for the pic.
    Ray or Raymond
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    Master Reefer Iron E's Avatar
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    Thanks y'all! I feel very fortunate to live here. That shot of My Shasta was taken last new years day. I've lived here for 21 years. I've traveled everywhere in the US and I wouldn't want to live anywhere else.

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    6400 sq miles
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    Last edited by Iron E; 06-16-2009 at 10:01 PM.

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    lol great looking pug hes sure does look cool.

    Quote Originally Posted by Iron E
    7 stoplights
    lol get this where my cabin is up in Plumas county I think its pretty close to you has one stop light and just added another one last year man its great to say the towns are that small don't you agree I love it.
    Ray or Raymond
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    Master Reefer Iron E's Avatar
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    Quote Originally Posted by rayme07 View Post
    lol great looking pug hes sure does look cool.



    lol get this where my cabin is up in Plumas county I think its pretty close to you has one stop light and just added another one last year man its great to say the towns are that small don't you agree I love it.
    Absolutely! The town I live in is Dunsmuir. Population 1800, no stoplights! The biggest political issue right now is wether or not to force the owner of the California Theater, (now deep into an advanced state of decrepitude) to repair the marquee since it was damaged by a produce truck.

    Tom

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    Assistant Moderator rayme07's Avatar
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    I know Northern California is great. You are so lucky you live there.
    Ray or Raymond
    There is no elevator to success in marine tanks. You have to take the stairs.


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  20. #20
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    Quote Originally Posted by NDReefer View Post
    Hello,

    I have a 240 gallon reef tank that has recently developed a problem with diatoms. I did some research and found that diatoms feed off of silicates, once the silicates are gone the diatoms are gone. So I went out and got a TDS meter and started testing water. My city water is pretty bad having a TDS reading of right around 1000ppm. I then tested my RO/DI water and noticed it was right around 200ppm, which I think is still too high. I recently bought a change of filters for my Kent Marine Hi-S 60gpd and tested that water. The TDS reading did go down but not as low as I expected, it dropped to about 30ppm. I have read that other people change their RO/DI filters when the TDS reading gets above 0ppm. I also checked on the Kent Marine website and found out that my RO/DI unit is optimized for TDS lower than 200ppm. So my question is can I simply put some sort of pre-filter in front of my RO/DI or do I need to get a new RO/DI system? What are my options for fixing this problem? If I need a pre-filter what kind of filter do you recommend and if I need a new RO/DI what brand/model do you recommend?
    Thanks for the help!
    This may help (from the FAQ's on our web page):

    A good rule of thumb is to replace your sediment filter and carbon block after six months. A more precise way to maximize the useable life of these two filters is to use a pressure gauge to identify when pressure reaching the membrane starts to decline. This is your indication one or both of the filters is beginning to clog.

    Also be cognizant of the chlorine capacity of the carbon block. The Matrikx+1 (“Chlorine Guzzler”) for example will remove 99% of chlorine from 20,000 gallons of tap water presented at 1 gpm. Original equipment suppliers commonly provide carbon cartridges rated at 2,000 to 6,000 gallons.

    Regarding your RO membrane and DI resin, use your TDS meter to measure, record, and track the TDS (expressed in parts per million) in three places:
    1. Tap water
    2. After the RO but before the DI
    3. After the DI.

    The TDS in your tap water will likely range from about 50 ppm to upwards of 1000 parts per million (ppm). Common readings are 100 to 400 ppm. So for sake of discussion, let's say your tap water reads 400 ppm. That means that for every million parts of water, you have 400 parts of dissolved solids. How do we go about getting that TDS reading down to somewhere near zero?

    If you do some experimenting with your TDS meter, you'll note that your sediment filter and carbon block filter (collectively called “prefilters”) do very little to remove dissolved solids. So with your tap water at 400 ppm, you can measure the water at the “in” port on your RO housing and you'll see it is still approximately 400 ppm.

    The RO membrane is really the workhorse of the system. It removes most of the TDS, some membranes to a greater extent than others. For instance, 100 gpd Filmtec membranes have a rejection rate of 90% (i.e., they reject 90% of the dissolved solids in feed water). So the purified water coming from your 100 gpd membrane would be about 40 ppm (a 90% reduction). Filmtec 75 gpd (and below) membranes produce less purified water (aka “permeate”), but have a higher rejection rate (96 to 98%). The life span of a RO membrane is dependant upon how much water you run through it, and how dirty the water is. Membranes can function well for a year, two years, or more. To test the membrane, measure the total dissolved solids (TDS) in the water coming in to the membrane, and in the purified water (permeate) produced by the membrane. Compare that to the membrane’s advertised rejection rate, and to the same reading you recorded when the membrane was new. Membranes also commonly produce less water as their function declines.

    After the RO membrane, water will flow to your DI housing. DI resin in good condition will reduce the 40 ppm water down to 0 or 1 ppm. When the DI output starts creeping up from 0 or 1 ppm to 3 ppm, 5 ppm, and higher, you know that your resin needs to be replaced. Sometimes people complain that their DI resin didn't last very long. Often the culprit is a malfunctioning RO membrane sending the DI resin “dirty” water. This will exhaust the resin quicker then would otherwise have been the case. Sometimes the problem is poor quality resin – remember that all resins are not created equal!

    Russ @
    Buckeye Field Supply

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