Need help chosing an RO/DI

[IVPush]

How do I know if I need an RO unit or an RO/DI unit. Also how do you decide to get a 2 stage, 3 stage, 4 stage..and so on. I assume the worse your water is the more filtration you need. I also assume the 5-stager filters out a larger variety of unwanted stuff versus the 2 stage.

So how do I know if I need to filter out just a little or I need to filter out a lot?

My tap water is low in phosphates, Ph is about 8.5, no lead, and no copper. Thats about all Ive tested it for.

[Flame*Angel]

This may not be very helpful but I know how you feel. In my area the (one good) LFS and the only other guy I know with a reef tank tell me that our tapwater is fine and nothing is needed. Considering the money I'm investing in the tank I just couldn't let this go, even just seasonal changes can cause algae blooms. The water you test now might have different results than after a spring thaw.

I found a great deal on a 4 stage RO/DI at http://www.aquariumwaterfilters.com (barracuda model) so I ordered one. Small price to pay for peace of mind.

[IVPush]

Thanks FlameAngle, but lem'me ask you this. Why did you get the 4 stage and not the 3 stage? and what is a TDS?

[Snailman]

We have funkey well water and we use a Kent Maxima Hi-S 60GPD RO/DI. The first stage is a fiber filter to get the heavy junk out of the water to save the other sections. The next stage is the carbon block which then goes to the RO membrane. Out of the RO membrane I have a tee. One side of the tee goes to our RO drinking water tank and the other side goes to the DI section and then on to our RO/DI tank. There should be a check valve on the line to each tank so water can't backup between the tanks. There is a carbon polishing filter on the output of the RO drinking water tank. This feeds our ice maker and an air gap faucet on the sink. I had to put a booster pump on the source water because our well pump does not have enough pressure for the RO membrane. Add in a backflush kit and a few gauges and you have your very own personal water treatment plant.

[Frisco]

It might be good to generalize and say that the different filters pull out different things, and if you truly have very clean water, you won't be losing anything by getting something that's seemingly oversized. If nothing else, it'll give you a little bit of security on the day that something nasty flows through the pipe.

In a four stage, the mechanical prefilter usually pulls out sediment, the carbon pulls out bacteria and very reactive chemicals, the membrane provides a physical (mechanical) barrier to remove less reactive molecules, and the DI resin pulls out fairly unreactive molecules that are small enough to pass through the membrane pores. Generally, the smaller the pore size (in microns) in a sediment and carbon filter is better because there's a higher likelyhood of catching the stuff in the water; larger ones will allow more channeling of the water so that the nasty crud can have a higher chance of just passing through. A RO membrane is similar, a higher rejection rate just means that the pores are smaller and consequently only relatively smaller molecules can pass through. And finally, there are a whole lot of available DI resins in the world depending on what needs to be removed; a mixed bed will remove molecules with both a + (cation like Li+) and - (anion like I-) charge, while a single type of resin will only pull out one or the other. To complicate things even more, DI resins can be tailored to selectively pull out very specific atoms, and the filter that Snailman uses is an example of this - the DI resin is (somewhat) selective about pulling out Si.

Overall, more is better IMO! Water is critical!

[Flame*Angel]

IVPush - I went with the 4 stage because the 3 stage doesn't include the DI, it's just RO. To be honest I don't know what the TDS monitor is, I didn't order one. You should email the company from their site, they're great at getting back to you and will likely call you and answer any questions you have about their products.

[Frisco]

The TDS meter is essentially a salinity (conductivity) meter; it's looks similar to a pH probe... It measures the conductivity of the water, and relates it to the amount of salt (based on NaCl) that would be in the water to give the equivalent conductivity. It's works on the principal that pure water is fairly resistive (doesn't conduct electricity) and when dissolved salts (with their + and - ions) are around, they allow the water to conduct electricity... Depending on the type of probe, some people use them to measure salinity of saltwater (high concentration of salt) and others measure the TDS (usually a low concentration) to understand water purity. HTH

[RobertK]

My understanding is that RO units don't do a very good job of removing silicates, which can be problematic in marine aquaria, thus the added benefit of RO/DI. Therefore I think that an RO/DI unit would be preferable to just an RO unit for most marine aquarists, unless you are sure that silicates are not and will not be present in your source water. The choices do not end there though, you still have different types of DI cartridges (regular, color-change, high silicate removal) and RO membranes (different gpd rates).

The Spectrapure system I bought actually has 7 stages - a 3 stage 35 gpd RO unit plus a separate 4 stage "ultimate DI" unit. The DI has two cartridges each of color-change and high silicate removing media and a built-in TDS meter. This may be overkill but I only wanted to do this once and wanted to know I was getting the purest water possible. Your best bet would be to get an RO/DI unit of 35 gpd or less that fits your budget.

[TnReefer]

RobertK,
I've just read a review about the Spectrapure system that you have I was thinking about getting one sometime in the near future. Where did you get yours and if you don't mind telling how much was it.
TnReefer

[RobertK]

TnReefer,
I bought it directly from Spectrapure, their prices are listed on their website. You might be able to get it a little cheaper from some of the online retailers, but I got some great assistance from Spectrapure's tech support staff and wanted to buy direct from them.

[IVPush]

You all have helped to clear up the functionality of these systems, thank you.

Is there a test kit or a way to find out just how much silicates are in my tap water? Ive tested it for phosphates are there are almost none. But its the silicates im more worried about.

Right now I am looking at either the
Kent Maraine CTA / DI or an
AquaFX 4 Stage RO/ DI Barracuda

any suggestions or comments?

[Ironreef]

It all depends on your water. I use ro and it works for me. Some guy who use to post here only uses tap water ..Ron Shimek. The occassions I used tap in emergancy when i didn't have makeup water some of my corals closed up for a few days and before the ro alga use to grow.So I no the benifits of ro water. I belive ro di would be good but if $$ is tight you can always use ro and add di later.