TDS/RODI question

speedsport · 03-14-2007, 11:56 PM

Ok I think this has been gone over before. So please forgive me if it has. When you use a holding container (garbage can, rubbermaid etc) and a mechanical float. How do you prevent from using so much DI resin? I know my TDS is not that high. Every time I get water and the float drops so the RO unit starts you can watch the resin change color. It is kinda fast. I was wondering if I took a couple of shut off valves. Put one after the RO membrane with a T befor it then one down the line from the T. Then run that down the drain, letting the first gallon or two go dow the drain. Will that make it so I dont use so much DI resin?

Thanks

smeese · 03-15-2007, 12:23 AM

I have a TDS meter if you want to test your tap water. Mine is 200+ in mankato dialing in at zero with my dual DI resin. I would just buy more resin personally, or continue to test TDS until the last possible moment to stretch life out of it.

David Grigor · 03-15-2007, 12:04 PM

It's all about the membrane. Do you happen to know what brand and type of membrane you have ? Do you use a flush kit on your membrane ?

The overall quality of your membrane is the most critical piece as to how much DI you will use. The cheaper/lesser quality membrane can double or triple your DI usage.

You need to measure the TDS of your water after the carbon cartridge and before the membrane. Then take a measurement after the membrane. See what the rejection % is. Perhaps your membrane is not working well. You want 97-98%.

Even just a few percent less will significantly increase your DI usage.......

speedsport · 03-15-2007, 12:41 PM

Ok. Thanks for the info David. The membrane is from the filter guys so is the DI resin. Also when I get water the first thing I do is turn the flush valve and flush the membrane for a min or two.

smeese I think I would like to try yours.

Thanks

Sea monkey · 03-15-2007, 01:03 PM

maybe ya need to pack the DI tighter ?

David Grigor · 03-15-2007, 01:08 PM

Okay lets go at it from a different angle then. To make sure you don't have an unrealistic expectaction of how long the DI should last.

There is a direct relationship between TDS of water going into the DI and how long it will last.

If you using a standard 10" DI housing/cartidge ( which holds 24oz of resing ) using 18 megaOhms grade your DI should filter a total of 3000ppm.

So if say the TDS out of your membrane is 10 then a DI cartridge should last about 300gallons of filtered water. If output of your membrane is 20ppm then it will be exhausted twice as fast at 150g.

So first thing to do is verify your membrane is working proberly.

Once you have verified your membrane is working to it full potential ( 97%-98% rejection ). Then check the TDS out of the membrane and run through the above calcuation to set your expectation of how long your resins should last.

If you not using a inline TDS meter but a handheld it is also important to remember that the container itself can skew your test results. So be sure to use a clean glass that has been rinsed several times in RO/DI water before collecting the water for the test.

Irenicus · 03-15-2007, 01:17 PM

David, how do you test your membrane to ensure the it is working within the 97%-98% rejection range?

David Grigor · 03-15-2007, 02:29 PM

Quote:

Originally Posted by Irenicus

David, how do you test your membrane to ensure the it is working within the 97%-98% rejection range?

Quote:

Originally Posted by David Grigor

You need to measure the TDS of your water after the carbon cartridge and before the membrane. Then take a measurement after the membrane. See what the rejection % is.

Simply just dividing the TDS out reading by the TDS in reading from the above measurements.

REEFSTOCK · 03-15-2007, 03:59 PM

Doh, that makes sense! Its great to have access to so many people to do the thinking for me!

speedsport · 03-15-2007, 04:20 PM

You know I've all ways said this was a great place.

David Grigor · 03-15-2007, 04:38 PM

Well anytime you have RO/DI questions, AZdesertrat on RC is the water treatment guru. So if you do a search on RC for all post by him releated to RO/DI you will likely get all the info you need on just about every aspect of RO/DI. He has been in the water treatment business ( large scale ) for over 20years.

Much like I trust just about anything from RHF on reef chemistry, I trust everything from AZdesertrat on RO/DI. Of course, Jim ( Fahz ) himself provide great customer service as well and can probably answer just about anything as well.....

Fahz · 03-15-2007, 08:35 PM

Quote:

Originally Posted by speedsport

Ok I think this has been gone over before. So please forgive me if it has. When you use a holding container (garbage can, rubbermaid etc) and a mechanical float. How do you prevent from using so much DI resin? I know my TDS is not that high. Every time I get water and the float drops so the RO unit starts you can watch the resin change color. It is kinda fast. I was wondering if I took a couple of shut off valves. Put one after the RO membrane with a T befor it then one down the line from the T. Then run that down the drain, letting the first gallon or two go dow the drain. Will that make it so I dont use so much DI resin?

Thanks

If you are making small amounts of water at a time TDS creep may be part of the problem. Everytime any membrane starts up it will produce a very high TDS reading in the first minute then fall to normal. You are far better off making 3 or 4 gallons at one time or more rather then a glass. To do that you need a high /low float switch like Reef Fanatic offers. Also if you have low pressure under 50 psi your membrane will perform poorly.

David was a little short on what a full sized cartridge should absorb 6800 ppm. There also is a water chemistry factor I suspect some of the Twin Cities municipal systems have and that is CO2. CO2 has no TDS reading but is a bigtime exhauster of resin. It can cut production down to 60-100 gallons per cartridge.

Jim

jeff262 · 04-08-2007, 10:52 AM

here are a few facts when i called the dow chem. rep about my hight tds with my membrane. ANY back pressure on the membrane will cause high tds. so two things you need to watch, get rid of the resivior tank since it takes pressure to fill and having the RO unit in the basment is bad since it has to push the water up which makes back pressure, so in a perfect world you should have the unit above the tank your trying to fill

russel · 04-16-2007, 03:39 PM

just a neat tip on the TDS meter thing.... I picked a nice one up from ecowater in my town which you very well may be able to do too. I paid 15 vs the 30 online, and you don't have to pay for shipping....

Fish'InMN · 04-16-2007, 10:59 PM

Jim, a question for you:

I've read about the high initial TDS from start-up but haven't seen too many folks truly worried about it. Would it be worth it to, either manually or via solenoids, send the first gallon or so down the drain? Assuming that you continue to make more water after start-up (rather than a quick on-off cycle), is it not worth the effort to drain that first gallon due to the decreasing concentration of the dissolved solids with each additional drop/cup/gallon/etc?

I'm coming up on a prefilter & DI change soon, so you'll probably get a few more questions from me either way!

Regards,
Marty