RO and silicates

[oimate84]

I have a reading of my RO/DI water as 1 TDS...But is it possible for silicates to be in the water, or would that easily be picked up by the membrane...Just wanting to know because im have a problem with Dino's, and somesaid it might be from my RO water...thanks

 

[MattM]

In general, RO removes about 60% of silicates. DI is a better method of getting rid of them. That being said,

1. Silicates do normally contribute to the conductivity of water, so they should show on a properly calibrated TDS meter.
2. Diatom algae is normally tied to silicate levels, but a certain amount of diatoms is normal, even in an established tank.
3. Dinoflagelattes can exist for quite a while with virtually no nutrients once they are established in the tank. Best course to combat dinoflagelattes is elevated alkalinity, reduced lighting, and most importantly - patience. Don't go crazy fighting them, it won't get rid of them any faster than doing nothing.

 

[kim]

FWIW, my experience suggests that, while there may not be a problem, you are wise to be suspicious.

I have conductivity which indicates less than 1 ppm TDS but my Salifert test kit shows silicates greater than 2 ppm. Even if I recirculate my water repeatedly thru' my DI it really doesn't get any lower.

I suspect that my probe may not be as precise as I would like at such very low conductivities - it is calibrated to 84 microS/cm, and I am measuring 1 - 2 microS/cm......

Furthermore, the mobility of silicate ion (all wrapped up in water molecules.......) may be low, so perhaps it just doesn't show up so well in a measurement of conductivity. Tied up to this, it's not clear how you convert conductivity to TDS when you don't know what's causing the conductivity !

I need a specialist membrane and a specialist DI resin to correct the situation (both of which I am exploring). My tapwater is off the scale in silicates, I think that because I live in an area with 100 year old pipes it is added to the water supply deliberately.

First thing is to get a silicate test kit, measure your tapwater, and post RO, and post RO/DI water. Then you'll have a better idea.

kim

[Edit...I realise that I've addressed your silicates question only.....I should stress that I have no knowledge of dinoflagate problems or whether in fact silicates could be contributing toward these - hope that's not misunderstood]

 

[taikonaut]

As MattM said, silicate usually is not associate with dinos, but diatoms is.

Anyway, most TDS meter used in the hobby is not very good when it comes to RO/DI water. It is sufficient for RO, BTW. For ultrapure water such as RO/DI, most people familiar with the application usually recommand resistivity meter instead of TDS or conductivity meter. At any rate, I would look into the accuracy of both the Salifert Si test kit and the TDS meter. I am sure you will find that the meter will disappoint you when it comes to RO/DI monitoring, but the Si test kit can also be at fault.

 

[kim]

Just to say, I did check my conductivity meter and Si test kit on pure water from a lab.....both readings were zero, so neither is dramatically at fault. My tapwater is 20 ppm SiO2 (checked with Salifert, confirmed with the water utility).....the readings makes sense.

I agree that it's always best to confirm test results before making any significant response, so thanks for the warning !

kim

 

[oimate84]

thanks for the responses...Ill have to check my silicates out with a test kit...