How to tell if u need to change your RO membrane?

Discussion in 'Filters, Pumps, etc..' started by Magnus, Jun 13, 2011.

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  1. Magnus

    Magnus Sharknado

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    So, I've come to realize that my TDS is at 5, which is quite high. I'm not sure how long is the membrane supposed to last and I can't give out an exact (or close enough guess) on the amount of water I had made with it. I haven't changed my pre filter or carbon in about 7 months, but getting ready to install the new ones. I also bought a membrane, but I'm not sure whether I "need" to change it or if it still has some life left.

    What is your experience and opinion on good visual sings that are a dead give away when its time to change it? I'm guessing my actual TDS of 5 is one of them, but it could also be the pre filter or carbon block, right?

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  3. johnc101

    johnc101 Feather Duster

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    I ususally change out the pre-filter and carbon block first to see if that works and gets me back to 0 tds before replacing my maxcap di and silica buster, if that doesnt work I then change out the maxcap di and silica buster and if that doesnt work I usually then change out my membrane. I only had to do this once in over a year while changing out the pre filter and carbon block a few times............ hope this helps........
     
  4. Magnus

    Magnus Sharknado

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    I see what you're saying. So its a change and see what happens kind of deal?

    I went ahead and changed all 3 cartriges, including the ro membrane. I will be more conscious about my pre filters and will buy some extra today since they're the cheaper ingredient in the dish. I also installed a dual TDS meter with the IN probe after the filters and the out after the membrane. Instruction said to install the IN on the tap water hose, but I figured I already know what that is with a hand held tester. I believe this way it will give me more of a warning system when my pre filter isn't performing as well.

    Please, do keep the suggestions coming about this, so I can know better when to change them. Something I was absolutely disgussed by was the color of the old pre filter compared to the color of the new one. It's only an RO unit and I drink this water as well, so I'm glad I didn't drink all that nasty crap the filler caught.

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  5. evolved

    evolved Wrasse Freak

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    Sounds as though you do not have a DI filter post-membrane? An RO unit will (almost) never give 0 TDS; this is what the DI filter takes care of.

    Membranes are supposed to last a few years, but you can always monitor their performance via the TDS gauge you just installed. Many people will consider them spent once the output goes above 20 TDS or they begin to give a poor rejection rate (says less than 95-96%).
     
  6. Magnus

    Magnus Sharknado

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    That's correct! It's only an RO unit and I had thought about buying the DI kit to add to it, but I need ro figure out where I can fit it under the sink. Not too roomy down there. I saw a 4 stage RO/DI on spectrapure.com for $120 on a special offer, but even though the price is good, I still have the issue with the space I have available :-(

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  7. AZDesertRat

    AZDesertRat Giant Squid

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    Prefilters and carbons have very little to absolutely nothing to do with TDS. They are there to protect the RO membrane from suspended solids, TSS not TDS and chlorine. TDS is dissolved solids, things in the 0.0001 micron range versus suspended solids in the 1.0 micron range.

    To determine your membrane condition you need two TDS readings, tap water TDS and RO only TDS.
    As an example say your tap water TDS is the national average of about 250 ppm and your RO only TDS is 5 as you are seeing. Subtract the RO TDS from the tap then divide it by the tap TDS and multiply by 100 to get the rejection rate or removal efficiency.

    250-5=245, 245/250=.98, .98x100= 98% rejection which is good.

    If your tap TDS was 100 then 5 would not be so good as it would only be 95% rejection which is a little low.

    Other ways to tell if the membrane needs replacing is a slowdown in production due to fouling or plugging or a major increase in production meaning the membrane material has been compromised or burst and is bypassing the membrane, this usually results in higher TDS but if you have a DI filter and do not monitor RO only TDS you may not see it other than having to replace the DI resin more frequently resulting in a higher operating cost.
     
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  9. Magnus

    Magnus Sharknado

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    I'll go and check my rejection rate now. So adding the IN line of the dual TDS meter to the water hose after the pre filter and carbon block won't change a thing on the TDS reading. Is it just as if it was hooked to the tap line?

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    Last edited: Jun 13, 2011
  10. AZDesertRat

    AZDesertRat Giant Squid

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    Correct. It may drop 1 or 2 TDS when it removes the chlorine but chlorine residuals are only 0.5 to 2.0 ppm most places so its minimal. The TDS meter tells you nothing about prefilter and carbon block condition or when to replace them and they have no effect on TDS.
    You replace the prefilter and carbon block every 6 months like clockwork regardless of how much or how little water you have made. The only other ways to know when to change them is by monitoring pressure loss due to plugged filters or with a low range chlorine test kit to monitor chlorine breakthru on the carbon. Its easier to stick with the 6 month replacements and remember to disinfect the system while you have it down for replacements.
     
  11. DarkEarth

    DarkEarth Spanish Shawl Nudibranch

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    If you keep a log for your aquarium. Just note the pressure when you get new filters. So that when the pressure rises to a pre-determined amount. Then you change them.

    If I only knew how many bars its got to go up before recomended replacement? I bet the manufacturer can tell you exactly when.