Silicates in RODI

[Lavey29]

I seem to have never ending diatoms. Not bad but still an issue. My ICP test showed silicates in my RODI. I use a 4 stage BRS system that reads 0 TDS currently when I make water. Is there something I can do to my filtration to remove the silicates from my RODI water?

 

[DE FISH]

Place an internal filter in your rodi water storage container full of gfo to remove the silicate

 

[Lavey29]

Place an internal filter in your rodi water storage container full of gfo to remove the silicate

I don't really have that. I just fill up 3 of the 5 gallon plastic jugs every week or so. No long term large storage.

Do they make something small for those 5 gallong plastic containers?

 

[ectoaesthetics]

From what I can gather Reef Moonshers method on their website sells a resin that removes silicates. I'm at least 85% confident that that is the case. They sell it because so many people still have issues with their silicates -which can be a big deal... Ran across your thread as I am looking at buying a 6 or 7 stage from BRS and was looking to see if people were still complaining about silicates LOL.

 

[DaJMasta]

If 0 TDS is accurate, your silicate readings are <500ppb, which really isn't a lot. What number does the ICP give you?

There will always be some amount of material that gets by an RODI, so it could be that all of your readings are correct, but the actual amount of silicates in the water is not problematically high.

 

[Lavey29]

I know silicates are getting into my tank via RODI water changes. I get some mild diatoms on the sandbed here and there but after reading some of Randy's posts about the need for silicate in our reef tanks I guess I'm not going to worry about it plus it acts as a barrier against other algae like dinos and cyano in my opinion.

 

[DaJMasta]

But what's the actual number? 50ppb is going to be so minutely small that it probably can't encourage diatom growth in a noticeable way, whereas 500+ppb both could be a problem and would indicate maybe a faulty tds meter reading. We're not talking about a lot of material in the water in either case, but the magnitude of the reading can give you a lot of insight into what the best course of action would be.

 

[Randy Holmes-Farley Verified Community Expert]

Add a second DI in series to the existing one. Any normal DI will work for that, but marketers also promote special silicate binding ones (not sure what that can even mean). Silicate is among the harder ions to remove in an RO/DI and silicate is sometimes added to tap water (and can be naturally high as well), so it can be hard to remove.

While some silicate is good ( I recommend dosing it), too much will allow diatom growth.

Reverse Osmosis/Deionization Systems to Purify Tap Water for Reef Aquaria by Randy Holmes-Farley - Reefkeeping.com

In the DI descriptions above, I did not address the fact that some ions will show a greater preference for attachment to the resin than will others. When the resins are not depleted, it does not matter what the ions’ affinity is, as all are bound. But in a depleted scenario, when there are more ions than ion binding sites, those with a higher affinity for the resin will be retained, and those with a lower affinity will be released. It turns out that silicate is found at the lower end of affinity for anion resins. Consequently, if the DI resin has been collecting silicate for a long period and is then depleted, a large burst of silicate may be released.


and

In the case of silicic acid, some types of RO membranes can be better than others at excluding it, even before it gets to the DI resins. For example, a thin-film polyamide membrane might let only 0.3% of the silicic acid pass, while a similar cellulose acetate membrane might let 12.7% of it pass.