Ro/DI system

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Aquarium Advice Freak
Mar 5, 2008
Staten Island, NY
How does a RO/DI unit work. I'm not really asking about how it filters through the seperate chambers, but actually how it hooks up and operates. I was looking into buying the absolute best quality/cheap cost balance of a RO/DI system I could, but i have no idea how they work or if it's feasable.
water comes in from the faucet, has 2 exits.. 1 for waste water (~70%) , 1 for pure (20%). Most units are fairly equal, just make sure the GPD fits your needs and that the cartridges are standard (that you can buy them most any place).

They generally have optional hookup attachments so you can hook the up to either a faucet, hose bib, or direct connect to your water line

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You can create a permanent installation by attaching directly to a cold water pipe (the attachment to wrap around and pierce the pipe is generally included). You can also have a temporary installation which is what I do. I remove the kitchen sink faucet aerator and replace it with a garden hose adapter. The RODI unit has a garden hose to 1/4" water line connector that screws in to the garden hose adapter. The 1/4" water line runs from that connector to the intake port on the RODI unit. The waste water is a 1/4" line that runs from the waste output connector to the sink drain. There is another 1/4" water line connector for the RODI output that you run to the container you want to fill. I use a 50' line to fill my 45 gallon top off container and a 25" line to till my pwc container.

Reefkeeper installation guide
I need as little GPD as possible, because i only have a 30G tank and its really only gona be for top off and PWC water. My water needs are minimal, but I am paying 10 bucks for RO water (1G=$1 at LFS) so it will be cost effective for me to get a unit.

Can anyone recommend a RO/DI unit which will suit my needs of very low output.

Also,question... if it is attached to the faucet or whatever, how does it like stop the flow of water or slow it down to what it needs? Will it need like constant attention while it does its thing?
not much attention. get a TDS, or a unit that has one built in. Once you see the TDS counts starting to rise , replace the filters and that's mostly it. If your unit allows you can opt to flush the membrane to get some additional use out of them. Get a 70-100gpd unit, if you have half way decent water your filters will last a long time. I bought one off of ebay for ~$100 shipping included and am very happy. I actually only run one of the 3 DI filters it came with (my water isn't that bad), so I actually have 2 extra DI's stored away as kind of a bonus
I have pretty bad water - (NYC) - so it has unbelievable high phosphates and stuff. Why do I need to get 100 GPD if I only need like 10 gallons every week. Is that correct? I have seen RO/DI units for like 15-20 GPD going for over 150 bucks. How did you get so cheap?
This is where I got my RO/DI unit from. I have had it for over 2 years now and I am very happy with how well it works. I have also purchased replacement filters from this seller.
You pay more for the brand name but the filters are standard and made by the same 3rd party company, so all that "brand name" is getting you is the tubing and the plastic housing. The more GPD the less you'll have to change the filters... The cheaper, though with any you should run it from time to time just to keep everything 'fresh'
You should look for the Dow Filmtec Membrane and a 75GPD unit. The 100gpd does not have as good a rejection rate as the 75 (90% vs. 98%). AirWaterIce (where I got my unit) recommends using a 10 micron sediment filter followed by a 5 micron carbon, followed by a 1 micron carbon. The system is a logical placement of filters. The sediment filters take out all the big stuff and need to be replaced more often than the carbons. The DI unit comes in last, after the RO membrane.

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