What is your TDS threshold for changing filters?

Discussion in 'Lighting, Filtration & Other Equipment' started by DAvis, Jul 23, 2011.

  1. DAvis

    DAvis Reefer Madness

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    I have a Pure Flo II 50gpd, and it has been more than adequate. It's an RODI unit I've had almost two years. I've replaced the filters 3x, but have not yet replaced the RO membrane. I do a monthly flush of the system, as recommended. I have a TDS meter reading before and after the RO membrane. My 'before' value is 0 ppm (following the DI resin), my after value is 7 ppm. My membrane is near the recommended change time, but what is your cutoff for TDS values and filter changes. Also, what happens to the RO membrane that causes TDS to rise?
    Thanks!
    David
     
    DAvis, Jul 23, 2011
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  2. DAvis

    little_fish Moderator

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    Basically the membrane gets worn out, and will let other molecules through.

    When you get a reading of 10 or greater on your TDS meter, then its time to change the membrane
     
    little_fish, Jul 24, 2011
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  3. DAvis

    x19 VIP Member

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    I think something is really wrong if the membrane is causing your TDS to rise.. My understanding is that the RO membrane does the majority of the filtering, with the DI cleaning up whatever is left. I replace my filters whenever I get above 5 ppm.
     
    x19, Jul 24, 2011
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  4. DAvis

    bjohanson1234 .........

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    Are you sure that you are reading that right? Usually, the DI resin is after the RO membrane. Atleast it is on my unit.
     
    bjohanson1234, Jul 24, 2011
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  5. DAvis

    DAvis Reefer Madness

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    Thanks all, I may have it backwards in my mind (not the first time!). I'll let you know tomorrow. Have to deal with a headache right now...
     
    DAvis, Jul 24, 2011
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  6. DAvis

    DAvis Reefer Madness

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    No I'm not! I was mistaken in my post, I have the TDS meter following the RO, and one following the DI (like you said should be). SO, my RO membrane is fine, but the filters are needing replacement. I had a bad headache and got confused!

    Will do! Since mine is reading '0' (had the readings mixed up, sheesh) I have another 6 months maybe!

    You are right, I had the numbers wrong 0 after RO, 5 - 7 ppm after the DI


    Thanks all, I will replace all the media cartridges, and wait on the RO. Thanks for your help!
     
    DAvis, Jul 24, 2011
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  7. DAvis

    bjohanson1234 .........

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    Glad we got it figured out!!!!!
     
    bjohanson1234, Jul 24, 2011
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  8. DAvis

    Buckeye Field Supply

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    A good rule of thumb is to replace your sediment filter and carbon block after six months. A more precise way to maximize the usable 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. A good 0.5 micron carbon block for example will remove 99% of chlorine from 20,000 gallons of tap water presented at 1 gpm. Some 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 dependent 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, 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 than 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, Feb 19, 2012
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  9. DAvis

    x19 VIP Member

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    So do you just trowl the web looking for people asking this question? I found this exact same answer by you across multiple sites for the past couple of years... You also always seem to resurrect dead threads.

    It seems like it would be better marketing if you actually built up a relationship with a particular web communities. Instead of essentially wandering around the internet looking for places where you can post your RO/DI answer.
     
    x19, Feb 19, 2012
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  10. DAvis

    DAvis Reefer Madness

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    Actually, I've had (unfortunately) recent resolution to my TDS problems. I have found out by testing my TDS meter along with another, that my meter was so far out of whack that I was adding 100ppm to my tank for months! Apparently my membrane was bad, but I did not detect any problems because the calibration was so far off! THis is why (I assume), I was burning through DI resin in a couple months. I had a bloom of Hair algae that I couldn't explain until now.

    So, lesson learned - periodically calibrate your TDS meter!

    Thanks for all that replied with advice...
     
    DAvis, Feb 19, 2012
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  11. DAvis

    Buckeye Field Supply

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    I appreciate your insight Eric.

    Hopefully the original poster finds the information we provided helpful.

    Its true, for that last 12+ years, in addition to building a growing international water treatment business, we've taken countless evening and weekend hours to give back to folks in this hobby and wherever we find people struggling with water purification problems in many places on the internet. We try to provide thorough, accurate information wherever its needed. Based on the feedback we get, most people find the information we provide helpful.

    We've answered thousands of questions, and the post you're referring to is a reply to our single most frequesntly asked question.

    I'll check with the folks here to see if we have anyone wandering around the internet with one answer in mind just looking for a place to post! ;-)

    Russ
     
    Buckeye Field Supply, Feb 19, 2012
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  12. DAvis

    x19 VIP Member

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    Props for responding. If you'd like to be a part of the living reefs community let me be the first to welcome you. I hope you're around the next time some one has a question.

    DAvis...I haven't checked the calibration for a while. Thanks for the reminder!
     
    x19, Feb 19, 2012
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