RO/DI advice from AZDesertRat

[cd77]

AZDesertRat (or WaterKeeper),

You helped me out some time ago in another thread, and with your advice I purchased a nice RO/DI system through The Filter Guys.

As I mentioned in the previous thread, my current fixture (3 stage RO, with an additional single stage DI) has a membrane that is now a couple years old and has not been replaced. Everything else (cartridges, DI) is relatively new. You had expressed some concern regarding the age of the RO membrane.

I'm still getting a TDS of 0 (my COM-100 unit won't come in until tomorrow) but the cheapy eBay one I bought is still showing zero. Obviosly I'll have a more accurate reading tomorrow after the better TDS meter comes in.

So here is the question -- should I continue running the current RO/DI unit (with the old membrane) until I start seeing a TDS>0 before switching to the new unit, or is there a good reason to throw out the current unit and start using the new one immediately?

[AZDesertRat]

Check your RO only TDS before the DI cartridge. If its still low enough so DI cartridges both last and can get the effluent to 0 or close to it then you are still good. Calculate your RO rejection rate ie. the percentage removal between the tap water and the RO membrane, if its in the 90+% range with 95+% being better then its still performing acceptable. A good rule of thumb is for every 2% you increase the RO membrane efficiency you double the life of your DI resin. If the old membrane is down in the 90% range then DI will not last nearly as long and the replacement costs start to hit the old wallet. If its performing better then it does not cost nearly as much to operate. It becomes a trade off at some point between the cost of a new membrane/system or the cost of replacement DI resin/cartridges.

[cd77]

Thank you, sir! :)

I'll check again when the better meter comes in, but last time I checked, RO output was around 29 I think (before I even added the DI), so I was at ~94% on the RO -- I'm actually shocked it has lasted this long considering how high my tap TDS is! The DI seems to be exhausting fast (if the color is the indicator) in which case I imagine it won't be long until the new unit goes into operation.

Again, thanks! :) Much appreciated!

[AZDesertRat]

At 29 DI won't last long especially if you have any CO2 in your water like we do in the Phoenix area. I was only getting 150 gallons per 20 oz refill with a TDS of 13-15 going into it if that gives you any idea.

[cd77]

Understood -- And it puts things into perspective a little more for me as well. I only pull out the RO/DI unit for water changes, to rinse food, fill up topoff reservoir and for 5G/wk water changes -- so I've still got some time before I even get to 75 gallons, but sounds like it won't be too long. :)

[Duff Man]

How do you know if you have CO2 in your water? Is that something that can be tested for or is it something on the water quality report?

[AZDesertRat]

Look in the Chemistry Forum, there is a way to calculate CO2 in a recent thread.

[cd77]

<a href=showthread.php?s=&postid=11098676#post11098676 target=_blank>Originally posted</a> by Duff Man
How do you know if you have CO2 in your water? Is that something that can be tested for or is it something on the water quality report?

The formula I've plugged into my spreadsheet and have used for my plant tanks is:

3 x KH x 10^(7-pH)

A quick google search gave me a link with a table: http://www.csd.net/~cgadd/aqua/art_plant_co2chart.htm

IIRC, this formula does not work when certain buffers (phosphates for instance are commonly used to keep pH low in preference of carbonate in unplanted discus tanks) are present.

Either way, follow AZ's advice and search the chem forums :)

Using that formula, I'm fortunate to have a fairly low CO2 level in my tap -- which also makes maintaining my discus tank more difficult than the marine tank.

[Duff Man]

Thanks guys. I have seen that CO2 affects DI, but couldn't find info on the water report.

[cd77]

Ok, well I finally got around to testing my RO performance after re-filling my top-off reservoir. To my surprise (mind you, this membrane is ~2 years old -- the DI unit being new, and fairly new carbon pre-RO cartridges) these are my results:

Tap:
-- HM Meter: 473ppm
-- COM Meter: 525ppm

RO:
-- HM Meter: 11ppm
-- COM Meter: 12ppm

DI:
-- HM Meter: 0ppm
-- COM Meter: 0.8ppm

If my calculations are correct, I'm getting ~97.7% rejection rate from my RO unit. I was expecting to see something closer to 85-90%, so this blew me away.

I just purchased the recommended 382ppm calibration fluid for my new COM-100 from The Filter Guys to (hopefully) get a slightly more accurate reading on the RO and DI water.

Either way, with numbers like this, I think I may opt to go through another cheapy DI attachment before ditching the RO membrane -- and before I put the new Filter Guys super-dee-duper RO/DI system in use.

I figure I might as well run the current system in to the ground before switching to the nicer unit -- I also figure that it is possible that the new pre-filters are causing me to get far better results and I'll likely burn through them faster because of the ~500ppm source water.

Is my logic flawed here? Any suggestions?