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Thread: RO membrane, how often to change?

  1. #1
    DIYer, not in the trades LLigetfa's Avatar
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    Feb 2011
    NW Ontario, Canada

    Default RO membrane, how often to change?

    What happens to a membrane over time? Does it clog and get tighter, letting less through or does it wear and get looser, letting more stuff through?

    Mine starts with a 5 micron followed by a 1 micron. The 5 micron gets the dirtiest and the 1 micron gets a small bit of iron staining. The 3rd stage is a carbon filter and it looks like carbon leaks through to the RO membrane as the RO chamber had black staining after the carbon element. After the RO, there is one last carbon element.

    In the past, I would change the lowest numbered elements more often than the higher numbered ones, so the 1st stage 5 micron got changed the most often, the 2nd stage 1 micron less often and the 3rd stage carbon element even less often. The 4th and 5th were never changed before now. Anyway, the wife complained about the taste so I did a complete cleaning and replaced all 5 stages including the RO membrane.

    Is there value in the 3rd stage carbon element? Does it improve taste or eventually add taste? I am on well water so there is no chlorine to remove. Would it warrant more frequent replacement or should I replace the 5th stage carbon more often?

    My water has lots of iron and manganese so it smells and tastes horrible. After aeration, the iron filter, and the softener, it tastes/smells better but still not what I consider good. When I was using potassium it had an undesirable taste and smell. When I switched to MortonŽ System SaverŽ II salt, it got better but still had a slight odour in the shower. Now that I switched to MortonŽ Rust Remover Pellets, I don't notice the smell any longer.

    I've made other changes as well. I reduced the house pressure so that the micronizer aerates through most of the pressure range. I increased the iron filter backwash from twice a week to three times. I also increased the softener regen from twice to three times but also reduced the amount of salt per regen.

  2. #2
    Water systems designer, R&D ditttohead's Avatar
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    Jan 2012
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    Wow, detailed question. I will answer it in a simple manner.

    Membrane, it will foul over time thus reducing its flow rate, but possibly increasing its rejection of dissolved solids. Over time, it is not unusual to see a membrane produce less, but higher quality water.

    It can also do the opposite. Over time, it can get microtears, or oxidation damage that slowly degrades the filtration surface that will allow more production, and lower quality water.

    All that being said, many years ago a residential membrane cost 5-10x as much as they do now. Considering the low cost, it is now considered a consumable filter, not some fragile, high priced filter that we should spend too much time worrying about.

    it was common practice to soften water ahead of small commecial membranes. now many companies simply consider the cost of salt compared to the cost of simply replacing the membrane more often, the salt is usually nuch more expensive than the membrane.

    It is much more complex than this, it would take dozens of pages, but since the cost of the membranes is so low and theior durability is so much higher than it has been in the past, the need for training people how to protect their residential or light commercial membranes has been virtually eliminated. Simply replace the membrane when the performance drops out of specification.

    Change all of the filters annually. For a well, a 3 or 4 stage RO should be fine. A simple 5 micron sediment filter, membrane, and a post carbon should suffice.

    What is your water like coming out of your water treatment system? Do you have a water analysis you can post?

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