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Double pass reverse osmosis.

Discussion in 'Saltwater Hardware' started by Attloyden, Aug 12, 2019.

  1. Attloyden

    Attloyden New Member

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    Is it worth it, can it be done?

    I would really like to know if anyone has that type of system, if it can be run on normal 40-80 psi water pressure. I do not like the idea of having to repeatedly buy mixed bed resin filters to get the water quality where I want it when the resin has been used up after 20 gallons of water. Even after reading up on TDS creep and installing the valve to siphon off the high TDS water that first comes out of the RO filter.

    I have hard water 450ppm TDS most of it is calcium but the rest of it is nitrate in excess of 80ppm. The first pass with the membrane alone knocks the TDS down to around 44pmm, most of it is nitrates since the test from API matches in color to about 40ppm. If I pass that through a resin bed then I get like, 5ppm nitrates at the end but like I said, it's too expensive for me to do that way.

    I'm just worried that the pressure from the pure water line isn't going to be high enough to pass through a second membrane. even if i do buy a 150gpd membrane and run the pure water line to a 50gpd membrane.

    If i must i will buy an additional water pump, though I have not decided between electrical or one of those that runs off the waste water, it sounds like the waste water permeate pump will do the trick since the water should not have as hard of a time passing through the second filter in theory.

    I have not found any information online about double pass reverse osmosis other than what it is. As far as I know there is no documentation about someone setting one up, save for commercial applications that use it for desalination of seawater.

    I only have a 10 gallon fresh water tank, heavily planted, Three shrimp, two sparkling gouramis, five neon tetras, three bandit cory catfish and two guppies.

    I'm using the Aquatic Life RO Buddie system.
     
  2. seangee

    seangee Member

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    Is it worth switching to a pumped RO system to try to improve efficiency? I haven't measured my pressure but mine reduces the input water from a TDS of 350 to around 10. In my case the source has 50ppm of nitrates and this is not measurable after filtering. I have traces of Ca left and don't bother with DI.

    Prior to switching to RO I used the Pozzani filter @Colin_T mentioned in your other thread. This lasted me about 800 - 1000 litres using water straight from the tap (sorry I don't understand gallons ;)). SInce you already have an RO unit you could put a DI canister in at the end which should mean it will last longer than mine did. The resin is actually rechargeable.

    Just FWIW guppies are hard water fish and all your other fish are softwater fish, the shrimp will depend on the species. I usually change 75% per week, I recommend 50-75% per week.
     
    #2 seangee, Aug 12, 2019
    Last edited: Aug 12, 2019
  3. Back in the fold

    Back in the fold Fish Crazy

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    Not sure if I understand but I think that an RO unit sometimes needs a pump to boost the water pressure to the membrane. The membrane doesn't work as efficiently under low tap pressure. Check your owners manual.
     
  4. seangee

    seangee Member

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    That's what I meant - you just explained it better :). I ordered the non-pumped version but they were out of stock so I ended up with a pumped one for the same price.
     
  5. Attloyden

    Attloyden New Member

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    Well I measured the water pressure before getting the system to see if it would work, I get between 40-82 PSI depending on if the water pump had just been turned on or not, I actually had to install a pressure regulator to stop it from going above 70PSI. So I do not have an issue filling up a 5 gallon bucket overnight. Go to bed, turn the system on, go to sleep and wake up with a full bucket. I do have the automatic shut off valve installed and only run it when my bucket is empty.

    What I want to do, is replace the Resin filter with another RO membrane instead, so I am doing double pass reverse osmosis. But i have not found anyone that has done this, and would like to find out if it's possible to even accomplish on a small scale before buying the equipment.

    However it's interesting to know that the resin in that system is rechargeable.

    I also just found out that yes, i ruined my first bottle of test solution for nitrates by not shaking the second bottle before adding drops, and because i used so much before i started to shake it, it's now coming back as double the ppm. So my test at home showed 80ppm at the tap, the test at petsmart showed 40ppm.

    Just to be sure though i bought a brand new nitrate test kit from API, will be testing it after I eat but i am pretty sure it's going to be around 40ppm, if not less.
     
  6. rjthevagabond

    rjthevagabond New Member

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    I have a question here. could I use a airstone or bubble bar in a bucket of tap water for 3 days to make a type of ro?
     
  7. essjay

    essjay Moderator
    Staff Member Moderator Global Moderator

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    No. Air bubbling through tap water will not remove anything except chlorine if your water company uses that. But if they use chloramine, it won't do anything.
    The RO process removes just about everything that was dissolved in the original tap water, from hardness minerals, to nitrate, to dissolved organics. Bubbling air through tap water cannot do this.
     
  8. rjthevagabond

    rjthevagabond New Member

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  9. seangee

    seangee Member

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    The frustrating thing about those is the range for when your levels are high. My colours are ...20/40/80... and I struggle to differentiate between 40 and 80. My tap water is 50 and I can't tell that using the test :(
     
    • Agree Agree x 1
  10. Attloyden

    Attloyden New Member

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    Yeah i know right? there is yellow- orange, 5ppm, then orange 10ppm, then another, exactly the same orange color 20ppm, then red 40ppm, then exactly the same red color again, 80ppm, then a dark red for 160ppm.

    See what I do, is when I check the color, if it's orange i just see if it's closer to the color in front or the color behind, so if it's orange but closer yellow-orange then it's 10ppm. if it's orange but looks closer to red then it's 20ppm. Same goes for red.

    Still a pain in the rear.

    Right now my reverse osmosis filter alone is producing water with about 7.5 ppm nitrates, I'm still on the fence about buying a Pozzani nitrate filter for a polish, or trying Double Pass reverse osmosis filtration.

    After all I found out that reverse osmosis filters can be cleaned with chemicals, and if the Pozzani resin can be recharged, it would be a hard choice between the two, save for the fact the second RO membrane will probably need a permeate pump.
     
  11. seangee

    seangee Member

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    At 7.5 ppm I would probably not bother.

    To be clear I never successfully recharged a Pozzani cartridge. These are sealed so the only way you could do this is to pump a large volume of salt water through it. The nitrate specific resin (Purolite A-520) can be recharged using brine. Standard mixed bed resins do require acid and I'm not brave enough to try that.

    Here is a link https://www.devotedly-discus.co.uk/acatalog/A-520-Nitrate-Resin---by-the-litre-7_0010__.html
    Disclaimer: I switched to using RO before I tried this. I know that one of our members @AbbeysDad does recharge his homemade nitrate reducing filter.
     
  12. Attloyden

    Attloyden New Member

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    Looks like that Resin is a cheaper alternative though it would not work in my current in-line DI resin cartridge. It needs to "swirl" and sounds like quite a bit would end up inside my bucket. And Seachem purigen can be recharged with bleach but, I want no part of any bleach coming near my tank so that's out as well.

    So far Nitra-zorb is winning, that's what @AbbeysDad is using. I need to find a way to measure how much my housing will hold and how much Nitra-zorb I need.

    I would prefer to get my nitrates down as far as possible before putting the water in my tank.
     

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