RO/DI Question - RO Membranes in Series

[AZDesertRat]

A 35-40 psi raise in pressure results in about a 1% rise in rejection rate, your numbers are flawed. The Dow Filmtec 75 GPD
membrane is rated at 96% sustained rejection rate at 50 psi. At 60 psi this rises to about 96.1%, at 70 psi you might see an additional 0.2%, nothing like your numbers.

Here is a study Spectraure did under controlled lab conditions. Note the increase in pressure from 35 psi up to 85 psi, a 50 psi increase, only resulted in a 1.2% increase in rejection rate:
Another perspective on Booster Pumps... - Reef Central Online Community

A 1.2% increase in rejection rate is significant though since every 2% increase results in a DOUBLING of your DI life In addition to that an increase of 50 psi from 35 to 85 on a 75 GPD RO membrane results in a 87 GPD increase at 77 degrees F water temperature:
Calculator

I run my RO booster at 95-100 psi and get around 150 GPD out of a 90 GPD Spectraure RO membrane but the rejection rate only rises about 1% over what it is unboosted at 68 psi. It is not as significant as you claim unfortunately.

 

[joekool]

So your saying that I'm not getting a rejection rate of 98.6 on my Dow filmtec 75gpd membrane. I believe your numbers are flawed the rejection rate increases the higher the psi. Maybe your membrane only increases that much. My post membrane tds is only 1.4% of the total tds in.

Also a increase of only 2% depending on your number could be a good amount less tds output
Obviously you know everything about this and I can't do math. I now what my system does and he has the same setup you don't.

A booster pump is what he needs.

It's not even worth my time to go back and forth with you. Bye

 

[AZDesertRat]

Some membranes are better than 98% out of the box. This is why vendors like Spectrapure and Buckeye Hydro test membranes and offer the higher rejection rate membranes at a slightly higher price to cover the cost of the service. You obviously got a good one. Call and talk to both, most membranes do not fit in this category so are sold for things like hydroponics or drinking water where TDS is not so critical. The average Dow 75 tests out at around 96 to 96.5% consistently but again there are good ones so the are hand picked for reef use by those who test them.

2% is very significant, it will double the life of your DI resin. But, you will nt get a 2% gain by boosting pressure 20 or 30 psi. You will increase the GPD sgnificantly but not the rejection rate so much. A booster pump is a good thing but not a miracle worker.

 

[coralcrazy]

Just tested my ratio's. For every 1/2 cup of RO I produce 1 1/4 cup of waste water. This is at 56 psi.

 

[AZDesertRat]

That is less than 2.5:1 waste ratio. With your high TDS and non softened water you need to be at 4:1 or even slightly higher. A capillary tube flow restrictor is $5-$6 at places like Spectrapure and Buckeye Hydro. You would remove your fixed restrictor then install the capillary tube and measure your waste and permeate then trim the length of the restrictor with a new razor blade according to the included instructins to get it to 4:1. Your membrane is not being properly flushed and with high tap TDS will fail quickly. Properly flushed membranes on softened water can easily last 7-10 years, if not taken care of it is 18-24 months as I discovered with a previous name brand popular RO/DI system, twice in fact.

 

[coralcrazy]

That is less than 2.5:1 waste ratio. With your high TDS and non softened water you need to be at 4:1 or even slightly higher. A capillary tube flow restrictor is $5-$6 at places like Spectrapure and Buckeye Hydro. You would remove your fixed restrictor then install the capillary tube and measure your waste and permeate then trim the length of the restrictor with a new razor blade according to the included instructins to get it to 4:1. Your membrane is not being properly flushed and with high tap TDS will fail quickly. Properly flushed membranes on softened water can easily last 7-10 years, if not taken care of it is 18-24 months as I discovered with a previous name brand popular RO/DI system, twice in fact.

Spectrapure doesn't make one for 75 GPD.

Is this what I new.

http://www.buckeyehydro.com/capillary-flow-restrictors/

 

[AZDesertRat]

The Spectraure 90 GPD is a 75 GPD Dow Filmtec RO membrane that has been specially treated using a proprietary process to increase both its rejection rate and its GPD then either batch tested for their normal membranes or individually hand tested and guarateed in writing t be 99% rejection rate or better for their Select series membranes.
The 90 restrictor is the same as a 75 restrictor. Make sure to get an untrimmed one with either vendor as you want to fine tune it yourself.

 

[coralcrazy]

The Spectraure 90 GPD is a 75 GPD Dow Filmtec RO membrane that has been specially treated using a proprietary process to increase both its rejection rate and its GPD then either batch tested for their normal membranes or individually hand tested and guarateed in writing t be 99% rejection rate or better for their Select series membranes.
The 90 restrictor is the same as a 75 restrictor. Make sure to get an untrimmed one with either vendor as you want to fine tune it yourself.

The 90 gpd regular membranes are on sale. Should I just upgrade my membrane with the restrictor?

 

[AZDesertRat]

Given your poor rejection rate a new membrane would be a cost effctive upgrade. Using your 450-500 tap TDS and 35-40 TDS RO TDS the rejection is 92%. Increase that to 98% or even better with their 99% guaranteed Select membrane a like $50 and your DI should last like 16 times longer.
I owned a Tyhoon III RO/DI that would only give me a RO TDS of 13-15 even after a brand new 75 GPD membrane. My DI lasted 150 gallons before it was at 2-3 TDS and needed changing. I switched to Spectrapure and my first DI cartridge lasted 830 measured gallons!
My present MaxCap UHE gets over 1000 gallons per MaxCap cartridge and 3000 gallons per SilicBuster cartridge and it is almost 7 years old now on the original treated and tested membrane. Thats 99.43% rejection rate still with my softened tap water over 550 TDS. The extra couple of $$ pays off in the long run, usually in less than a year.

 

[coralcrazy]

Given your poor rejection rate a new membrane would be a cost effctive upgrade. Using your 450-500 tap TDS and 35-40 TDS RO TDS the rejection is 92%. Increase that to 98% or even better with their 99% guaranteed Select membrane a like $50 and your DI should last like 16 times longer.
I owned a Tyhoon III RO/DI that would only give me a RO TDS of 13-15 even after a brand new 75 GPD membrane. My DI lasted 150 gallons before it was at 2-3 TDS and needed changing. I switched to Spectrapure and my first DI cartridge lasted 830 measured gallons!
My present MaxCap UHE gets over 1000 gallons per MaxCap cartridge and 3000 gallons per SilicBuster cartridge and it is almost 7 years old now on the original treated and tested membrane. Thats 99.43% rejection rate still with my softened tap water over 550 TDS. The extra couple of $$ pays off in the long run, usually in less than a year.

Are the un tested membranes just as good or is the tested ones the only ones to get?

 

[AZDesertRat]

The treated and batch tested membranes prove to be much better than a dry off the shelf membrane but there are no guarantees.

The handtested 99% membranes come with a written guarantee.
Here is a write up showing the potential cost savings

FAQ p.2

It does not take long to pay for itself.

 

[redfishbluefish]

I'm a little late to the party, but never, ever use oil based grease or vasoline on rubber parts. These oil based lubricants will make the rubber swell and eventually cause it to fail. I would highly recommend plumbers grease or pool lube. I personally use Leslie's Pool Lube....best bang for the buck....a large tube for about seven bucks. The rubber stuff that I've treated with this have never failed.

 

[AZDesertRat]

Homebrewing shops sell Keg Lube, a food grade silicone grease made specifically for O rings and rubber seals and gaskets. Not very expensive and I have even seen it in hardware stores too. The Leslies stuff is pretty good too but not ANSI/NSF food rated.

 

[redfishbluefish]

I have to throw in a caveat here about plumber's grease. It's been a number of years since my plumbing days, when all plumber's grease was silicone based....and used on everything rubber. It would appear, in perusing the HD site, that they now make a petroleum base plumber's grease. This is not good. If you go the plumber's grease route, make sure it is silicone based, and not petroleum. Petroleum and rubber don't get along.


Again, I have been using Leslie's for over 25 years on my fish tanks and have never had an issue.

 

[dereks16]

Joekool. I'm not 100% sure if it's post or pre. Never noticed. But it's just the meter that comes on the plus unit.

 

[coralcrazy]

UPDATE

So I installed a manual flush kit and got a new capillary flow restrictor. With new flow restrictor I am getting 75 ml of ro to 325 ml to waste. Before DI chamber my TDS is still 33 ppm. How can I get this down?

 

[AZDesertRat]

Did you switch the system to softened water and raise the incoming pressure?

 

[coralcrazy]

Did you switch the system to softened water and raise the incoming pressure?

No. I'm still at 50 psi. Think I need a booster pump?

 

[AZDesertRat]

I would spend $50-$60 on a better membrane guaranteed to be 99% rejection rate before I spend $130 or more on a booster pump that may only get you another 1% to 1.5% rejection rate at 90-100 psi.

Personally I think you wasted money on the flush kit since they have no proven documented value but it wasn't very expensive. You would be better off with a DI bypass valve to flush the TDS creep out.

50 psi is very common and your membrane should function well at that, 40 psi is about the minimum and 60-65 psi is very good but 50 will work just fine.

 

[coralcrazy]

I would spend $50-$60 on a better membrane guaranteed to be 99% rejection rate before I spend $130 or more on a booster pump that may only get you another 1% to 1.5% rejection rate at 90-100 psi.

Personally I think you wasted money on the flush kit since they have no proven documented value but it wasn't very expensive. You would be better off with a DI bypass valve to flush the TDS creep out.

50 psi is very common and your membrane should function well at that, 40 psi is about the minimum and 60-65 psi is very good but 50 will work just fine.

Ok I'll try the new membrane.

What does the DI bypass look like?