Improving RODI filter water conversion

Discussion in 'DIY' started by zeroinverse, Jul 16, 2013.

  1. zeroinverse

    zeroinverse Guest

    For those curious, I posted this on ReefCentral a while back in the Wine Country Reefers forum.

    In the cold winter months RODI water filters do not work that well due to the cold water being denser and significantly lowering your filtering ability and increasing time to filter water.

    Normally, you may get a 1:4 ratio of filtered water vs waste water. With the denser cold water, your ratio may go to 1:6 to 1:8 or more. That means (a) you are wasting more water down the drain. For a 20 gal water change that means maybe 120-160 gal of waste water instead of 80 gal.

    Also, it takes longer time to make the same water. A 75 GPD (filtered water device may decrease to a 40GPD filter, etc.

    The culprit is water temperature/density.
    RODI filters are meant to be run optimally at 70-80 deg F. Cold water, especially during winter is like 45-50 deg. That's 20-30 deg difference.

    My solution, use an anti-sweat valve.

    Anti-sweat valves are used in areas MUCH colder than California where very cold water enters warm houses (in particular toilets). This causes the cold water to cool the toilet reservoir so much that water condenses on outside of the toilet reservoir and causes a lot of "sweating" onto your bathroom floor.

    Anyways, anti-sweat valves work by combining cold water with an adjustable hot water. You could do this with a T-fitting, but you would probably end up with higher cold water back pressure pushing cold water into your hot water pipes. Not a good idea if your RODI will be running for 4-10 hours to make water and you try to take a shower. You'll get luke-warm water.

    Anti-sweat valves have one-way valves built into each inlet. This means no cold water can enter hot water inlet line and no hot can enter the cold inlet line.

    Anti-sweat valves allow you to adjust HOW much hot water you want to mix/combine. So you basically adjust it to around 70-80 deg F outlet temperature.

    NOTE: remember, when it is summer, your cold water line will get warmer, so either you need to adjust it again as weather warms up, or you need to lower the mixed outlet water temperature now a few degrees (during winter) so water can rise a bit and not exceed 90-100 deg F.

    Too hot water temperature (above 100-120 deg F) will ruin your RO membrane.


    So how do you make it and how much does it cost...

    Anti-sweat valve ($15 on Amazon)
    I bought a used one on Amazon - $8 (free shipping)
    This uses 1/2 FIP for inlet & outlets.

    Dual outlet shutoff valve ($18 each, need two, one for hot, one for cold water)
    (I used dual outlet with teo knobs so I can independently shutoff the water lines to RODI).
    My house copper pipes are 5/8, so I got a 5/8" compression inlet with two 3/8" compression outlets.

    3/8" compression to 1/2" FIP braided faucet connector (12" long in
    my case). ($6 each, need two, one of hot, one for cold).

    1/2" FIP to 1/2" FIP braided pipe ($8, new one)
    Connects outlet of anti-sweat valve to my RODI filter (in this case, it goes to a ball valve & then RODI Johnny Quest 1/4 plastic piping.


    Total cost:
    $71 total cost.

    Technically, you should already have some of the parts already if you hooked up your cold water faucet line to RODI.

    For me, it cost me about $50 for the modification.

    And no I can make water faster (in case I forget to make it ahead of time). And less water goes down the drain (volumetrically).

    Yes, some people will argue I am dumping lukewarm water down the drain and therefore also wasting natural gas that is used to heat my water heater. But for me, I just want to cut my RODI water production time in half (during winter months).
    neuro likes this.
  2. sfsuphysics

    sfsuphysics Guest

    So the hot water line warms up the water, but where does that hot water actually go? The picture shows the hot and cold from the wall go into the anti-sweat valve, and the output of that goes through a hole in the wall to the RO unit(?) (other 2 holes in the wall miscalculations on where to drill?? :) ). Does it mix hot and cold together in the output?

    Other option if you have the water pressure (or spring for a booster pump) is to piggy back 2 RO membranes, the waste water of the first goes into the input of the second, net result is you cut your waste production by roughly half. But you do need pressure that's in the 70-80psi range IIRC.
  3. Coral reefer

    Coral reefer President

    I do this. It's awesome. Thinking of adding a third membrane. Also makes your prefilter stages last about twice as long!
  4. zeroinverse

    zeroinverse Guest

    My goal was to
    1) increase my rate of water production
    2)reduce "heating costs" of getting temperature up to "usable"

    The double RO membrane helps with water production, but if water temperature is low, like in the winter, your RO membranes will still underperform. Something about the density of water and the RO membrane pores cause your waste water to nearly double.

    That means
    A) more wasted water dumped into your drain
    B) longer time to make RO water
  5. zeroinverse

    zeroinverse Guest

    Yes, they were mis-drills.
    And yes, the anti-sweat valve mixes hot and cold water together.

    The MIXING has a few impacts...
    1) Increase water temperature that helps improve your RO production rate.
    2) Using hot water line does increase the TDS slightly in the input water, but your RODI system should still get your to zero to very low TDS.
  6. denzil

    denzil Webmaster

    I'm wondering where the larger tradeoff is between manual management overhead, heating (from the tap or an actual heater), and wastewater. I'm curious what the optimized solution is. :)
  7. zeroinverse

    zeroinverse Guest

    I did it more for the "production throughput" than the "warm water" element. Last winter it was pretty cold and it took me a whole day 8+ hours to make 35 gallons on a 50~75GPD RODI unit.

    The cold water temperature was at like 50~60s. When RO membranes are optimized around 80~90 degrees.

    And I am too lazy to run a heater. Always worried I will accidentally leave it running dry and burn the heater & the plastic. Instead, I turn on my "saltwater mixing pump" in the container and that injects enough heat to warm it up for my water change purposes.

    I don't try to "calibrate temperature" before putting it into my sump during water changes. I figure my 150 gallons can tolerate the 10 gallon water change.
  8. sfsuphysics

    sfsuphysics Guest

    Could buy a 1000 feet or so of vinyl tubing and simply leave it sitting in the room in a bucket with water to heat it up, or put it in your sump to heat it up (at the cost of electricity since it will cool your tank... maybe too much :D)
  9. Coral reefer

    Coral reefer President

    My point was you could do the temp mod AND the multi membrane trick. also, I thought that 77 was the best temp for the membranes?
  10. zeroinverse

    zeroinverse Guest

    Yes, you could do BOTH. ^_^ Just more expensive to buy more parts.

    Maybe 77 deg F is just the standard at which they "rate" the production output for.

    For those curious about effects of different parameters on RO membranes...
  11. Coral reefer

    Coral reefer President

    Very cheap to add an extra membrane if you have good pressure. About $60 total for a housing and a membrane.
  12. Marc

    Marc Guest

    Just saw this thread and thought it was interesting...

    Could also use one of these electric heaters... only turn on and off when water is flowing

  13. rygh

    rygh Supporting Member

    Very interesting.
    I moved my RODI inside the nice warm garage for totally different reasons, and I remember thinking it seemed to fill
    the barrel a little bit faster, but really thought nothing about it.

    PS: (After looking it up.)
    Not density related. Density of water only changes about 1% over that temp range.
    Diffusivity on the other hand changes a lot.
  14. anathema

    anathema Supporting Member

    Depending on the age of your house, I'd be concerned with adding a lot more gunk from the water heater.

    I know my hot water has black goo in it, I'm constantly scrubbing it out of the shower.
  15. Coral reefer

    Coral reefer President

    Yeah, definitely wouldn't use hot water output.
  16. Marc

    Marc Guest

    Have you tried flushing your water heater out? Probably a lot of build up in there or could also be old pipes.

    Sent from my iPhone using Tapatalk.
  17. anathema

    anathema Supporting Member

    I've had the landlord drain it a few times. It's black decomposed rubber, I think. Or algae? Or aliens. Probably aliens.

    Comes back right away, each time I have it flushed it's gone for a bit but comes back. In any case I'd assume it'd trash any prefilters pretty quickly.
  18. Marc

    Marc Guest

    Yeah that's sucks. Sorry you have to put up with that.

    Sent from my iPhone using Tapatalk.
  19. Coral reefer

    Coral reefer President

    Even with a clean water heater more stuff will be in there due to hot water in the pipes pulling out stuff like little amounts of solder etc... At least that's my understanding

Share This Page