John, how many gallons per day does your unit turn out? I was reading up on RO/DI units, and it seems that the higher the output, the more work for the Deionization resins to do. 2 ppm I believe is acceptable, however, I like mine at zero. Here is some information I had put together...
There are 2 types of reverse osmosis membranes. One is called cellulose triacetate (CTA), and the other is called thin film composite (TFC). CTA membranes don't filter as well as TFC membranes do....CTA rejection rates ~90-95%, while TFC rates at ~95-98%. CTA membranes also break down from bacterial attack, where TFC membranes are impervious to it. One more thing about TFC membranes....they are sensitive to chlorine and it can damage the membrane if there isn't a properly functioning carbon pre-filter. Reverse Osmosis works by the force of water under pressure against the membrane, which will allow the passage of water molecules, but reject most of the larger contaminants. By using reverse osmosis prior to deionization, the resin of DI will last longer then without RO. RO still allows some contaminants through in trace amounts....which the DI will pick up and clean out the rest of the water.
There are 2 types of DI units. One is a mixed bed, which is a single chamber with a mixture of anion and cation resins. The other is a separate bed unit, which contains 2 chambers - one for anion resins and the other for cation resins. DI utilizes a 2-stage process that removes ionic material remaining in water after RO, for example. The resins only last so long before they need to be replaced. What happens is the Cation resin exchanges positively charged ions for H+, and the Anion resin - negatively charged ions for OH-. Cation resins release hydrogen (H+) in exchange for cations, while the Anion resins release hydroxide (OH-) for anions. The hydrogen and hydroxide then combine to form H2O.
- I know, I know....what did she say? lol
Something interesting I came across was that silica seems to be one of the first contaminants to leak past RO/DI. This is because silica is loosely bound to the resins, so when stronger binding materials come in, the silicates will lose their place and start to break free of the exhausting resin.
I found an article about
A Study of Silica and RO. I'll touch on a couple of points. Silica is found in 3 forms: reactive, colloidal & suspended particles. The colloidal silica causes problems when it comes to water treatment because of its stability as an un-ionized compound....the difficulty is in removal by means of ion-exchange processes. Silica breakthrough (as mentioned just before) is usually the first to occur, and the use of reverse osmosis prior to deionization aids the process by reducing the silica load on the resins. As a side note, some think silicone dioxide (SiO2) is a challenge due to a similarity of H2O.
Fundamentals of Osmo Systems There is a chart of typical rejection rates towards the bottom of the page. If you look at the cation and anion charts you will see there are several that pass through. These would then be taken care of through the deionization process.
So, what does all of this mean? Deionization is a way to finalize the process of removing contaminants. Reverse osmosis does a fine job, but for the 1-2% (maybe more in some cases) that get through, it would be worth it to remove the remaining bits. For someone like me who has a TDS reading of 456 (taken just a little bit ago) going into the unit - I'm going to rely heavily on the DI process. Someone with cleaner water would probably make it through the RO process relatively clean, but the deionization would finish off the remaining contaminants.
Hope this didn't contaminate the RO/DI water
? (lol, you know .... I hope this didn't muddy the water? LOL)
