RO/DI Question - RO Membranes in Series

[coralcrazy]

I got the new membrane and am getting 15-19 tds from the ro portion. This is running at 50 psi. Anything else I can do to bring this down?

 

[AZDesertRat]

Run the system for awhile, it should come down further as the membrane becomes "seasoned". Try to make large batches of water when you do run it versus smaller batches where the membrane does not really have a chance to flush itself.
What is your water temperature and your exact measured waste ratio now? Are you using softened water or do you know your calcium carbonate hardness?

 

[coralcrazy]

Run the system for awhile, it should come down further as the membrane becomes "seasoned". Try to make large batches of water when you do run it versus smaller batches where the membrane does not really have a chance to flush itself.
What is your water temperature and your exact measured waste ratio now? Are you using softened water or do you know your calcium carbonate hardness?

Water temperature is approximately 70 - 80 degrees. The water is not softened and I do not know the calcium carbonate hardness. I believe my ratio is 4.33:1, I am going to double check this today.

 

[joekool]

At 50 psi your rejection rate is low you will need at least 600 si to get a 98% rate. Is the pressure reading you are getting before or after membrane. What is it going into the second membrane.

 

[coralcrazy]

At 50 psi your rejection rate is low you will need at least 600 si to get a 98% rate. Is the pressure reading you are getting before or after membrane. What is it going into the second membrane.

Yes this reading is at the input to the RO membrane. Only have one membrane. The spectrapure 90 gpd hand tested.

 

[AZDesertRat]

You don't need 60 psi to reach 98% rejection rate. Why do you keep saying that? The Spectrapure 99% membrane will only drop 1 to 2 tenths of 1 percent between 50 and 60 psi at most. So say from 99% down to 98.8%.

A dry untreated and untested 75 GPD Dow Filmtec should be between 96 and 98% on average and the Spectrapure will be about 1.5-2% higher across the board.

 

[WayneF1]

The dual membrane systems you see all send the waste from the first membrane into the second membrane to increase the GPD, not to save water or further treat the water.
What you describe would require a booster pump since the water exiting the first membrane would be basically at atmospheric pressure and would have to be repressurized to get it through the second.
Do not believe the water saving some vendors claim, dual membranes are not water savers and will fail prematurely if not kept well flushed, this has been demonstarted many times by vendors with much more experience. Note none of the long time vendors, those with say 15+ years in the industry make this claim, they all know better. When you send the concentrated brine from one membrane into a second membrane it is even more important to keep the waste ratio up or both membranes suffer a shortened life driving the cost of ownership up. Dual membranes are to increase production, not to reduce waste.

Membranes are not all the same and some are much better than others. My tap water TDS varies between 550 and 850 and my RO only TDS varies between 2 and 3 always with DI lasting over 1000 gallons per cartridge. You can buy a hand tested 90 GPD RO membrane guaranteed to be 99% rejection rate for only $55 here:
NEW 99 Percent Rejection SpectraSelect Plus? 90-GPD RO Membrane

Here is an article documenting what that 99% means to you:
FAQ p.2

A few questions, what is your tap water TDS? Do you know your water hardness or do you have a water softener? What is your water pressure and water temperature? What is your exact measured waste ratio? Do you have CO2 in your water or if you don't know try using this nomograph to measure it, CO2 eats DI regardless of the T

The dual membrane systems you see all send the waste from the first membrane into the second membrane to increase the GPD, not to save water or further treat the water.
What you describe would require a booster pump since the water exiting the first membrane would be basically at atmospheric pressure and would have to be repressurized to get it through the second.
Do not believe the water saving some vendors claim, dual membranes are not water savers and will fail prematurely if not kept well flushed, this has been demonstarted many times by vendors with much more experience. Note none of the long time vendors, those with say 15+ years in the industry make this claim, they all know better. When you send the concentrated brine from one membrane into a second membrane it is even more important to keep the waste ratio up or both membranes suffer a shortened life driving the cost of ownership up. Dual membranes are to increase production, not to reduce waste.

Membranes are not all the same and some are much better than others. My tap water TDS varies between 550 and 850 and my RO only TDS varies between 2 and 3 always with DI lasting over 1000 gallons per cartridge. You can buy a hand tested 90 GPD RO membrane guaranteed to be 99% rejection rate for only $55 here:
NEW 99 Percent Rejection SpectraSelect Plus? 90-GPD RO Membrane

Here is an article documenting what that 99% means to you:
FAQ p.2

A few questions, what is your tap water TDS? Do you know your water hardness or do you have a water softener? What is your water pressure and water temperature? What is your exact measured waste ratio? Do you have CO2 in your water or if you don't know try using this nomograph to measure it, CO2 eats DI regardless of the TDS.
FAQ

The following link is a test run by Bulk Reef Supply