RO/DI question for the experts

DarG · 04/28/2007, 04:54 PM

Hi all. Finally broke down a got a TDS meter from the Filter Guys.
I have had my RO unit forever and added a DI a little under a year ago. My RO is an old Natures Way unit. Its a 3 stage, 2 prefilters and a RO. Ro is one of the smaller units, 18 gallons per day. Prefilters were just changed first of April, RO membrane is about 9 months old. It's a GE TFC membrane. The DI is also from the Filter Guys, it's a full size 2 stage unit.

The supply water measured at about 150 ppm. After the RO I have 9 ppm and after the DI it's 0. So clearly I'm getting great water after DI but was wondering if my TDS after the RO should be lower. I'm not tearing through DI media or anything like that but I remember reading here about there being a signifigant difference in how quickly DI media is exhausted with relatively small changes in the TDS number after the RO. If 9 ppm after RO is good with 150 PPM as the source water, then great, I'm golden. But if you experts think I should have a lower TDS after RO, I will upgrade my RO system if it will mean a noticeable decrease in DI resin usage. Like I mentioned, the RO system is ancient and if it's not cutting it, I would like to upgrade to a new unit, most likely a 4 stage, to get the TDS lower after RO and save on DI resin.

So does 9 PPM after ro with 150 ppm source water on a 9 or 10 month old RO membrane sound high or does it sound OK?

Thanks

AZDesertRat · 04/28/2007, 05:39 PM

You are fortunate to have a tap water TDS of only 150, when I last made RO/DI water on April 24th mine was 903 ! I am making water right now and it has dropped down to 798 . My RO only TDS averages 6.2 to 6.6 which is much better than average but you should still only be seeing 3 or 4 if your membrane is working efficiently.
To give you an idea of the difference you will experience in the life of your DI cartridges, at a TDS of 9 your first DI cartridge in line will be exhausted at 333 gallons at such time you should rotate the second cartridge into the first position and install a new one in the second canister. If you were seeing a TDS of 3,where I think it should be at 150 incoming, then that same cartridge will last 1000 gallons before exhaustion.
In your case 333 gallons is still a pretty good DI life but 1000 gallons is much better. You are getting 94% efficiency from your RO membrane which is a little low but again with 150 in its not a huge deal unless you are making lots of water. If my RO was only 94% efficient I would be seeing and RO only TDS of 48 to 54 and a normal DI cartridge would only last me 55 to 65 gallons !

DarG · 04/28/2007, 06:27 PM

Well, my first stage DI isnt making it to 333 gallons. I'd say closer to 200-250 gallons. So, I wonder if it's possible that my source TDS is varying. Or, maybe the pressure. When it's good, its good and strong but the water pressure is very low quite often around here and I know that kills efficiency of the RO membrane.

Thanks for the info. I think that I will get a new RO system. It sounds like it will pay for itself after a while in DI resin savings. I'd love to be getting 1000 gallons out of my DI and I really could use more than 18 gallons per day at times. If I am still having some efficiency issues I may look into a booster pump for the system.

fijiblue · 04/29/2007, 07:08 AM

Just remember, the higher the flow per day unit you go, the less rejection rate you get. Up to 75gpd all use the same membrane which is a 98% rejection rate. If you go higher, you get a 92% rejection rate. I hade the same problem with my unit running at 100gpd. I switched out the flow restricter and went to a lower membrane and my DI cartridges last twice as long.

DarG · 04/29/2007, 07:45 AM

Thanks fijiblue. Thats why I was figuring my little 18 gpd unit should be doing a great job. The membrane is a name brand, GE, which I thought was supposed to be a good membrane, and is only about 9 months old, maybe less.

I went with the smallest the Filter Guys had to offer which is a 75 gpd with a filmtec membrane. Its a 4 stage and cost 124 bucks. I got my DI add on from them a while back and they always ship quick and answer all questions. Good guys and good stuff. Bought an additional set of replacement filters plus one extra as I already had a 2 prefilter set and got another bag of DI. I should be all set for the next little while anyway.

Just put a solenoid valve on the unit for automation since it's in the garage and I got tired of climbing over my truck of backing it in and out to turn the supply on and off.

Guess it's RO/DI week. It's always something to spend money on.

Thanks for the input guys, I appreciate it.

BeanAnimal · 04/29/2007, 09:33 AM

The GE membranes are not the same as the DOW Filmtecs. There are a LOT of differences, but in a nutshell, you will get much better rejection out of the DOW.

DarG · 04/29/2007, 10:10 AM

Thanks Bean, good to know. I knew they weren't the same but had heard GE was still a good membrane. I guess not so great.

BeanAnimal · 04/29/2007, 11:47 AM

It is a good membrane, but not so much for our purposes. The combination of average rejection and pressure requirements make the DOW FILMTEC a much better choice.

04/28/2007, 04:54 PM	#1
DarG Registered Member Join Date: Dec 2006 Posts: 5,515	RO/DI question for the experts Hi all. Finally broke down a got a TDS meter from the Filter Guys. I have had my RO unit forever and added a DI a little under a year ago. My RO is an old Natures Way unit. Its a 3 stage, 2 prefilters and a RO. Ro is one of the smaller units, 18 gallons per day. Prefilters were just changed first of April, RO membrane is about 9 months old. It's a GE TFC membrane. The DI is also from the Filter Guys, it's a full size 2 stage unit. The supply water measured at about 150 ppm. After the RO I have 9 ppm and after the DI it's 0. So clearly I'm getting great water after DI but was wondering if my TDS after the RO should be lower. I'm not tearing through DI media or anything like that but I remember reading here about there being a signifigant difference in how quickly DI media is exhausted with relatively small changes in the TDS number after the RO. If 9 ppm after RO is good with 150 PPM as the source water, then great, I'm golden. But if you experts think I should have a lower TDS after RO, I will upgrade my RO system if it will mean a noticeable decrease in DI resin usage. Like I mentioned, the RO system is ancient and if it's not cutting it, I would like to upgrade to a new unit, most likely a 4 stage, to get the TDS lower after RO and save on DI resin. So does 9 PPM after ro with 150 ppm source water on a 9 or 10 month old RO membrane sound high or does it sound OK? Thanks

04/28/2007, 05:39 PM	#2
AZDesertRat Moved On Join Date: Jan 2002 Location: NW Phoenix Posts: 16,621	You are fortunate to have a tap water TDS of only 150, when I last made RO/DI water on April 24th mine was 903 ! I am making water right now and it has dropped down to 798 . My RO only TDS averages 6.2 to 6.6 which is much better than average but you should still only be seeing 3 or 4 if your membrane is working efficiently. To give you an idea of the difference you will experience in the life of your DI cartridges, at a TDS of 9 your first DI cartridge in line will be exhausted at 333 gallons at such time you should rotate the second cartridge into the first position and install a new one in the second canister. If you were seeing a TDS of 3,where I think it should be at 150 incoming, then that same cartridge will last 1000 gallons before exhaustion. In your case 333 gallons is still a pretty good DI life but 1000 gallons is much better. You are getting 94% efficiency from your RO membrane which is a little low but again with 150 in its not a huge deal unless you are making lots of water. If my RO was only 94% efficient I would be seeing and RO only TDS of 48 to 54 and a normal DI cartridge would only last me 55 to 65 gallons !

04/28/2007, 06:27 PM	#3
DarG Registered Member Join Date: Dec 2006 Posts: 5,515	Well, my first stage DI isnt making it to 333 gallons. I'd say closer to 200-250 gallons. So, I wonder if it's possible that my source TDS is varying. Or, maybe the pressure. When it's good, its good and strong but the water pressure is very low quite often around here and I know that kills efficiency of the RO membrane. Thanks for the info. I think that I will get a new RO system. It sounds like it will pay for itself after a while in DI resin savings. I'd love to be getting 1000 gallons out of my DI and I really could use more than 18 gallons per day at times. If I am still having some efficiency issues I may look into a booster pump for the system.

04/29/2007, 07:08 AM	#4
fijiblue Moved On Join Date: Mar 2007 Location: unfortunately here Posts: 1,895	Just remember, the higher the flow per day unit you go, the less rejection rate you get. Up to 75gpd all use the same membrane which is a 98% rejection rate. If you go higher, you get a 92% rejection rate. I hade the same problem with my unit running at 100gpd. I switched out the flow restricter and went to a lower membrane and my DI cartridges last twice as long.

04/29/2007, 07:45 AM	#5
DarG Registered Member Join Date: Dec 2006 Posts: 5,515	Thanks fijiblue. Thats why I was figuring my little 18 gpd unit should be doing a great job. The membrane is a name brand, GE, which I thought was supposed to be a good membrane, and is only about 9 months old, maybe less. I went with the smallest the Filter Guys had to offer which is a 75 gpd with a filmtec membrane. Its a 4 stage and cost 124 bucks. I got my DI add on from them a while back and they always ship quick and answer all questions. Good guys and good stuff. Bought an additional set of replacement filters plus one extra as I already had a 2 prefilter set and got another bag of DI. I should be all set for the next little while anyway. Just put a solenoid valve on the unit for automation since it's in the garage and I got tired of climbing over my truck of backing it in and out to turn the supply on and off. Guess it's RO/DI week. It's always something to spend money on. Thanks for the input guys, I appreciate it.

04/29/2007, 09:33 AM	#6
BeanAnimal Registered Member Join Date: Feb 2003 Location: Pittsburgh Posts: 20,772	The GE membranes are not the same as the DOW Filmtecs. There are a LOT of differences, but in a nutshell, you will get much better rejection out of the DOW.

04/29/2007, 10:10 AM	#7
DarG Registered Member Join Date: Dec 2006 Posts: 5,515	Thanks Bean, good to know. I knew they weren't the same but had heard GE was still a good membrane. I guess not so great.

04/29/2007, 11:47 AM	#8
BeanAnimal Registered Member Join Date: Feb 2003 Location: Pittsburgh Posts: 20,772	It is a good membrane, but not so much for our purposes. The combination of average rejection and pressure requirements make the DOW FILMTEC a much better choice.