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How do RO membranes work.

Discussion in 'Scientific Section' started by seangee, Nov 11, 2019.

  1. seangee

    seangee Member

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    No I don't mean the filtration aspect. I have a 50GPD membrane and my weekly water change is around 200 litres. 6 months on and the performance is still great in terms of water quality - but the output (and recovery percentage) is frustratingly low, and even more so now that the water going in is at winter temps.

    Since the filters are all the same size is it really as simple as putting in a higher capacity membrane? My system is electrically pumped so I don't think input pressure will be an issue.
     
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    Back in the fold Fish Herder

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    Not real sure but I think the way they work is that the membrane pores are microscopically small. The water molecules are able to pass through these tiny pores but minerals, being larger, are not able to. Over time the outside of the membrane gets crusted over with what you are filtering out. There is a thing you can get that reverses the water flow and flushes this crust off and down the wastewater drain. Yours may already have this. It is a standard feature on deluxe models. Also, the pre-filters have to be changed out on a regular schedule. I'm going to buy an R/O unit this month and have been reading up on them. Another thing that is handy to have is an electronic TDS meter. These are inexpensive and will tell you when your filters need to be changed. A dirty clogged filter makes your membrane work harder or just plain be inefficient.
     
  3. seangee

    seangee Member

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    Yeah I have an inline input/output TDS meter. Its still coming out at around 9ppm from an input of >300. My under sink tank is supposedly 14 litres so when I start filtering my first 10 litre jerrycan fills quite quickly. But thereafter it takes about an hour for 10 litres.

    I am pretty sure everything is working as it should - I just want the subsequent cans to get done faster :), and hopefully waste less water.
     
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    Back in the fold Fish Herder

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    Coming down to 9 PPM from 300 is good. You got no complaints at all. So it's a little slow. So what ?
     
  5. seangee

    seangee Member

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    So can anyone answer the original question?
    Last weekend I changed all 3 tanks on Sunday evening. I went to the office every day until Thursday and finished refilling my vats on Thursday before leaving home.
    I worked at home on Friday so did the 2 small tanks on Friday. And managed to replace what I used by mid morning Saturday at which time I changed the big tank. Right now I'm sitting in the departure lounge at Heathrow and the last thing I did before leaving home was turn off the tap as it just finished in time.

    I know I could change 50% instead of 75 and maybe even do alternate weekends. But I don't want to. If a 100 gpd membrane would get the water through twice as fast that would solve my problems. If the water quality is a bit lower I can live with that, if its too low I'll just add a di filter at the end.

    Is this worth doing?
     
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    Back in the fold Fish Herder

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    It sounds like you need more water. Am I right ? So you have two ways to go. Either get a bigger storage container and let your current unit run longer or upgrade to a 100 gallon per day unit. The water quality is the same for either, one is just bigger. I would stay with your current unit and get a 50 gallon or larger storage tank and equip it with a float valve for automatic shut off. You would always have 50 gallons because the R/O unit would kick on automatically when the level went down. I've been researching R/O units because I am going to get one and something I've found out is that a deionizer cartridge is nice but they don't last long and they are very expensive to run. The disposable D/I cartridges last for 15 to 25 gallons. You can get a refillable D/I cartridge but the D/I resin is very expensive too. Plain old R/O water is more than adequate for guys like us. I am told that a good R/O membrane should last 2 years and that a USA or UK manufactured membrane is superior to Chinese ones. As far as the waste water goes, it's just something you have to live with. Nothing can reduce it as far as I know.
     
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    Back in the fold Fish Herder

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    Space is a problem for me so I am going to have a 30 gallon storage tank, fill it overnight, do my water changes and fill a couple 5 gallon jugs for whatever else then I can put the 30 gallon thing away for a couple days until I have to do it again. The things we do for our fish. If the roles were reversed they would just eat us.
     
  8. seangee

    seangee Member

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    Hmmm - are you making your own tank or buying one? Anything over 4G (which I have) seems eye wateringly expensive. For comparison I checked on a couple of US sites and they are more than double the price here. Space is also an issue. The 4G fits nicely under the sink in the utilility room. Only complaint is from SWMBO about the cupboard she has lost :rolleyes:. Would need quite a lot of work to relocate the storage tank.

    Conversely a 100GPD membrane costs less than another 4G tank, so unless someone can point me at cheap tanks that's the way I'm leaning :dunno:

    When I say here I actually mean there (UK). I am still in Belgium, and they aren't cheaper here either.
     
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    Back in the fold Fish Herder

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    I'm going to use a plain old Rubbermaid plastic garbage can. $20 bucks at the hardware store. It can sit out overnight, I use it the next day and then it goes in the spare room closet out of the way until the next time I need it. She Who Must Be Obeyed has even given this the seal of wifely approval.
     

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