RO/DI troubles

rachelcb80 · #1 07/07/2007, 08:27 AM

I bought my RO/DI unit last October. I changed out the filters at the end of May (a little over, I know but the resin didn't start changing color until then and I still had a 0 reading). My resin has already half changed color and I'm getting a 1 TDS reading after just a month with these filters. I can't afford to start buying filters and resin every month. Why would they get used up so quickly this time around?

REV · #2 07/07/2007, 09:26 AM

I can't say for sure, but you may want to check your RO membranes rejection rate = check tap TDS and after RO, before DI. From what I have seen most RO membranes recommend a specific amount of water pressure (water temp. can also affect performance) My pressure is a little low. I would be getting a pressure booster pump if I wasn't selling everything. :-( Anyway, good luck!

BuckeyeFS · #3 07/08/2007, 07:17 AM

Quote:

Originally posted by rachelcb80
I bought my RO/DI unit last October. I changed out the filters at the end of May (a little over, I know but the resin didn't start changing color until then and I still had a 0 reading). My resin has already half changed color and I'm getting a 1 TDS reading after just a month with these filters. I can't afford to start buying filters and resin every month. Why would they get used up so quickly this time around?

Need a little more info to troubleshoot your system:
tap water tds?
ro water tds?
di water tds?

How much pressure do you have?

What is your waste water to purified water ratio?

Russ

rachelcb80 · #4 07/08/2007, 08:44 AM

Russ,

My tap water tds is 306. I'm not sure how to test just my RO or DI water (do I have to pull some tubes out or swap some around?), I just know I get a tds reading of 1 with the two combined. My pressure runs between 60-70 psi. I have a 1.5:1 waster water to purified water ratio.

Thanks!

vessxpress1 · #5 07/08/2007, 08:51 AM

Quote:

Originally posted by rachelcb80
Russ,

My tap water tds is 306. I'm not sure how to test just my RO or DI water (do I have to pull some tubes out or swap some around?), I just know I get a tds reading of 1 with the two combined. My pressure runs between 60-70 psi. I have a 1.5:1 waster water to purified water ratio.

Thanks!

1.5:1 is really low. Usually it takes more like a 3:1 or 4:1 ratio to produce RO/DI.

My unit has ball valves to switch between regular RO or let it go on through the DI resin. Mine's usually running at 60-70 psi also.

There's two different types of RO membranes. One can be used for chlorinated water and the other type cannot and should only be used for well water. Are you sure you have the right membrane? That's the only thing I can think of right now.

jdieck · #6 07/08/2007, 09:00 AM

The usual waste to product water is 4:1 so you may be restricting the waste too much which increases the TDS out of the membrane and increase the consumption of the DI resin.

To measure the RO TDS disconnect the line between the membrane and the DI filter and run the system collecting the sample of water comming directly from the RO membrane without the DI.
The % rejection ratio is calculated the following way:

% rejection = (TDStap - TDSro) x 1000 / TDStap

Where:
TDStap is the TDS of your inlet water (Tap or your 306 ppm)
TDSro is the TDS at the outlet of the membrane

If your membrane is a 75 gpd or less the rejection rate should be 90 to 93%. If lower adjust your rejection ratio and test again, if still lower then check that you do not have an internal leak (bypassing the membrane), If the membrane seals inside the housing seem OK and the membrane seats in the housing properly then you may need to replace the membrane.
Membranes get damagd by chlorine so if you waited too long to replace the carbon filter that could accelerate the membrane deterioration.

rachelcb80 · #7 07/08/2007, 09:29 AM

I measured the ratio as I was typing my response. I had one quart of purified water and 1 1/2 quarts of waste water. That's a 1.5:1 ratio, isn't it? Is there something I can do to the unit to change the waste water restriction?

I'll go and see about getting the RO tds reading.

When I bought my unit it didn't say anything about being for well water only so I'm assuming it is the correct membrane.

rachelcb80 · #8 07/08/2007, 09:50 AM

Okay, my RO tds reading was 13. So if I'm doing that formula correctly, my rejection rate is 95.7%. (I have a 75 gpd unit)

vessxpress1 · #9 07/08/2007, 10:14 AM

Ok, I gave you the wrong description. Here's from a FAQ page on RC:

"What are CTA, TFC, and PVC?

There are three types of RO membranes on the market: Cellulose Triacetate (CTA), Thin Film Composite (TFC), and Poly-Vinyl Chloride (PVC). Almost all of the membranes sold for aquarium use in the US are TFC. PVC membranes are currently available only outside the US. The notable difference between these types is how they are affected by chlorine: CTA membranes require chlorine in the water to prevent them from rotting. TFC membranes are damaged by chlorine and must be protected from it. PVC membranes are impervious to both chlorine and bacteria. This FAQ assumes you're buying a TFC membrane."

So if you have a TFC membrane, it is possible it's been damaged by chlorine and is letting TDS pass which is exhausting your DI resin. There's only so many explanations there can be for what's happening with it. Good luck.

jdieck · #10 07/08/2007, 10:27 AM

Quote:

Originally posted by rachelcb80
Okay, my RO tds reading was 13. So if I'm doing that formula correctly, my rejection rate is 95.7%. (I have a 75 gpd unit)

That rejection is not bad but with a TFC 75 gpd membrane the specification is at 98%+
As a comparison, your DI resin will last 5 times more if you get a rejection of 98% rather than 95.75%
To change the waste to product ratio you will need to adjust or change the flow restrictor installed at the waste outlet of the membrane housing.
Different units have different types of restrictors, some have adjustable restrictors (valve type) some have fixed restrictors and in this case will have to be chnaged. Some others have a restrictor in the form of a small diameter tubbing inserted inside the 1/4" tubing, by using a longer or shorter tubing the amount of restriction can be changed, if you have one of these (Tpical on Spectrapure units) you can just pull the tubing out and cut small portions at a time to reduce the restriction until you gat approx 4:1 ratio.
Check with the unit manufacturer for your specific instructions.