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  1. #1

    How to prevent RO/DI TDS Creep and how it will help you save money!!!

    I found this information to be extremely helpful when researching RO/DI units, and understanding how they work. So on that note, let’s begin…

    So what is TDS creep? When water sits in the membrane housing for a duration, TDS rises as it wicks out of the membrane. The initial 60 to 90 seconds when the system turns on will send this burst of TDS into the DI resin, or into the sump if it was plumbed directly (without a DI stage to pass through first). Each time the RO system turns on, this occurs. When one is plumbed directly to the tank, the constant on / off process day-in and day-out reduces the membrane's lifespan and efficiency.

    Does TDS even matter if I'm getting 0 out of the DI stage? My reply would be "How would you like to save some money?" Membranes work best when they run for longer periods of time instead of short bursts. Some LFS (local fish stores) opt to never turn off their water purification systems to maintain the lowest TDS possible from the constancy of water passing through the membrane(s) at a consistent PSI, choosing instead to doing massive water changes in the selling systems to use up any excess water that would otherwise overflow their storage tanks. As hobbyists, we tend to save anywhere we can, including water usage. We care about every drop of water, what filters cost, and how we can avoid being wasteful.

    What's the solution? By installing a tee fitting in the spot between the RO membrane and the DI stage, a short section of tubing with a ball valve is all that is needed to start saving DI resin. When it is time to start making RO/DI water, open up this valve and let it run for 60 seconds or longer to extract the TDS-laden water, then close the valve again. Open the valve after the DI and the water will now be entering the DI stage at the lowest TDS level which avoids wasting the resin. It's that simple.

    Below I have included pictures of how I setup my RO/DI unit. I will try and thoroughly explain how I have my system connected, and the process I use when making water. Hope this helps others save TIME AND MONEY as it has helped me.

    ***PLEASE NOTE –This setup can be used on ANY RO/DI unit***

    Here is a view of the complete system.
    This is the BRS 6 stage - 75GPD. This unit only came with 1 TDS meter, and while ordering this, I also added an additional TDS meter to help me monitor what the input and output of BOTH the RO and the DI is.
    I also have 2 ball valves connected to this system, I will name them Ball Valve 1 (for the RO section) and Ball Valve 2 (for the DI section).

    Setting up the system, lines, connections, etc. -

    Starting from the left side, or RO Section, of the system – The red line is the input line from the home’s water source, the blue line is the output line that feeds into the DI section of the system, and the black line is the “waste” line that goes into the sink drain (I actually have my washing machine right next to the sink, so I use this black line to fill up machine and do laundry (MONEY AND WATER SAVER!!). You will notice I have 2 black waste lines, and I will explain that soon. I connected 1 of the TDS meters to this section, and also added a ball valve. The output (blue) of the RO section runs out of the membrane, then goes through the out-sensor of the TDS meter, and continues to the bottom input of Ball Valve 1. The red input line runs from the water source, and has the in-sensor of the TDS meter connected to it. The black “waste” line goes from the RO section to the sink. The left and right side of the ball valve is the output. I ran another black waste line (from the left side of ball valve) that goes to the sink. Still looking at the ball valve, the blue line on the right side runs to the DI section of the system.

    From the right side, DI Section, of the system – The blue line is the line coming in from the right side of Ball Valve 1, and I put the in sensor of TDS meter 2 inline before the water goes into the DI chambers.

    On the output side of the DI section, I connected the out-sensor of the TDS meter 2, and continued the line up to the input of Ball Valve 2. Remember, the left and right side of the ball valve is the output going to your containers for ATO (auto top off system) or water changes. I have 2 Brute trash cans that I fill, 1 for fresh saltwater, and the other for my ATO (or fresh water).

    I will now go through the steps that I use to make water –

    First thing I do is turn the ball valve 1 so the water coming from the RO section goes to the waste output. This way I make sure the TDS output is as low as possible PRIOR to entering the DI section.
    Next step, turn the water source on, and both black waste line should have water coming out, and not just the 1 line from the membrane (this means the water is going into the DI section early).
    I then wait about 5 minutes (while water is still running out of both black lines), and I turn on TDS Meter 1. I don’t really pay attention to what the input is, but I keep an eye on the output as the TDS is high right at the beginning, and on my system it usually starts in the mid to high 30’s.
    I then turn Ball Valve 2 either left or right, depending on which trash can I need to fill up.
    Once the output on TDS meter of the RO section reads between 3-5, I then turn ball valve 1 to the right to allow water to enter the DI section. Once water begins flowing through the output line, I turn on TDS meter 2 to make sure the output is reading 0.


  2. #2
    Great tip!

    I actually did this as soon as I set my unit up. The only thing is that I didn't do it to save the DI resin, it was so that I can fill water bottles with RO for drinking water.

    Now I'll use it every time to save on my DI resin.

    Thanks for the tip!

  3. #3
    Great info, thanks for elaborating on the details. Most of the online shops sell these as pre-assembled units called "membrane flush kits". Just put in the correct GPD flow limiter and it's ready to go. Here's a link to the one BRS sells:

  4. #4
    Wookie gmguy496's Avatar
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    Quote Originally Posted by Skimmo View Post
    Great info, thanks for elaborating on the details. Most of the online shops sell these as pre-assembled units called "membrane flush kits". Just put in the correct GPD flow limiter and it's ready to go. Here's a link to the one BRS sells:
    I dont think the flush kit will do the same thing as its used to flush the membrane so the membrane isnt rejecting the tds like when in regular use andwill still have high tds coming out but I could be wrong

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