Another RO Question.....

[ohioreef]

I tested my RO water after it came out of the membrane, but before it went through any of the post filters or DI.

The first water I tested was like 100TDS!!!! I tested some more and it dropped to around 80TDS!!! I tested again and it came out to around 34TDS and stayed there.

Is it normal for the first bit of water to be high in TDS?

[RicksReefs]

if it a brand new unit, then yup, it's normal. you need to toss the first 5-10 gallons to get to the good stuff.

[ohioreef]

Yes, it's a new unit, but I tossed the first about 3 gallons and I've probably used another 5-10 since then.

I thought I read somewhere here on RC that when the RO first starts up the first bit of water has a higher TDS for some reason, but I'm not sure why.

I was wanting to confirm whether that was true or not.

I still get 1ppm after the DI, but I'm afraid that I'm seriously shortening the DI life if it is getting hit with 100ppm water when I first turn it on.

[Ixthys]

I can't remember what happens at a chemical level, but yes, every time you turn on your ro/di unit, the tds will be up for the first 5 or 10 minutes.

[Micki]

So what happens with an auto top off? Does that mean that every time it tops off is is giving high TDS to the tank?

[redpyro]

autotop off should be DI water ...so only 1ppm

[Micki]

So the higher TDS was via the RO only and not DI?

My top off is via the DI as well.

[ohioreef]

Micki,

The high TDS was RO before the DI so the actual TO will be 1ppm, you just exhaust your DI resin a little quicker. I was just wanting to make sure there wasn't an issue with my membrane.

Just cirious, how long did you have your unit before you had to replace your DI resin? What is your tap TDS?

[ohioreef]

Originally posted by Ixthys
I can't remember what happens at a chemical level, but yes, every time you turn on your ro/di unit, the tds will be up for the first 5 or 10 minutes.

Could it be that while the water sits idle, some of the TDS on the "dirty" side leaches over to the clean side trying to equalize the levels on both sides? (Everything in nature likes to be in balance.) Then when you start the unit you get that burst of high TDS until the membrane housing flushes out.

Sounds good anyway, makes sense.

[ohioreef]

Wow, I was right!! I found the following in the RO/DI FAQ here on RC:

"Always let the unit run for a few minutes before measuring TDS on the output. The first half-gallon or so will normally have an elevated reading. That's because impurities will equalize across the RO membrane over time when the unit is idle."

[thrlride]

:thumbsup: good find! I've always heard that, just didn't know why.