Help with RODI configuration

estew09 · Nov 16, 2025

Current water conditions
Water source is City with at TDS of 380-400, I run a water softener and a whole home carbon filter, pentek ep-20 is the cartridge. I currently run several freshwater tanks and I am currently collecting everything to start my IM Lagoon 25... my first saltwater tank ever.
I purchased a 5 stage 75 GPD BRS RODI system and I have 62 psi at the system pressure gauge. I shortly after bought an extra DI canister and a system saver membrane upgrade. I know with high TDS the 2 membrane system is not ideal because of my high TDS so I may remove the second membrane. I am burning through DI like crazy so I am thinking of making it the 3 stage pro DI. My question is, because I have the whole home carbon block filter could I remove one of the RODI systems carbon block filters and use that as one of my DI stages so I don't have to purchase and install another canister? I always test my tap water for chlorine and the whole home carbon filter has always removed any chlorine and I have always tested 0 chlorine. I have attached a picture of my current system. Thanks in advance for any input.

Shirak · Nov 16, 2025

What is the TDS of your RO water before going into the DI? I would start there and make sure you are getting good quality RO water even with the high TDS on your city water. With a high rejection membrane I would be looking for around 10ppm TDS on your RO water after the first membrane.

Next question and possibly why you are burning through DI resin. Are you familiar with TDS creep? If not, it's when the TDS slowly go through the membrane after it's shut off and sitting for a while so the first pint or two of of RO water is high TDS when the system is first turned on again. If you are letting this water go through the DI it will get used really really fast. Solution is to put a bypass valve after the RO line and before the DI cartridge so you can flush the high TDS RO water out for a minute whenever you start making water and then close it so it goes through the final DI stages.

estew09 · Nov 16, 2025

Shirak said:
What is the TDS of your RO water before going into the DI? I would start there and make sure you are getting good quality RO water even with the high TDS on your city water. With a high rejection membrane I would be looking for around 10ppm TDS on your RO water after the first membrane.

Next question and possibly why you are burning through DI resin. Are you familiar with TDS creep? If not, it's when the TDS slowly go through the membrane after it's shut off and sitting for a while so the first pint or two of of RO water is high TDS when the system is first turned on again. If you are letting this water go through the DI it will get used really really fast. Solution is to put a bypass valve after the RO line and before the DI cartridge so you can flush the high TDS RO water out for a minute whenever you start making water and then close it so it goes through the final DI stages.

Shirak,
I am not sure of the TDS out of the 1st membrane but the TDS out of the second is 613 (waste) and the TDS going into the 1st DI is 8 and it gets me to 0 out of the DI. I do have the flush valve (the one that came with the unit) and I run it for a few minutes before and after use is that what you meant or do you mean installing another valve to divert the water completely before the DI?

Shirak · Nov 17, 2025

estew09 said:
Shirak,
I am not sure of the TDS out of the 1st membrane but the TDS out of the second is 613 (waste) and the TDS going into the 1st DI is 8 and it gets me to 0 out of the DI. I do have the flush valve (the one that came with the unit) and I run it for a few minutes before and after use is that what you meant or do you mean installing another valve to divert the water completely before the DI?

The flush valve bypasses the restrictor, allowing for fast flow across the surface of the source water side of the membrane to flush contaminants off. It does not remove the RO water that has been sitting in the membrane housing while the system is shut down. TDS creep happens when the water is not flowing through the filter and high ppm water on the waste water side of the membrane equalizes with the low ppm water on the RO side. For example my well water is around 280 ppm. The RO water first produced when turning the system back on is around 140-150ppm. After 30 seconds or so it’s back around 5ppm where it should be.

Yes install a T and valve to completely divert the RO water before the DI. It’s good for making coffee and what not and removing it will save you a lot in used up DI media. Your RO water should be ready to send back into the DI after about 30 seconds or so.

As far as your other questions. It’s probably fine to remove the carbon as long as your whole house carbon is working. Keep in mind though the carbon block in your RO unit is also a particular filter somewhere around one micron so double check that you are filtering particles down sufficiently with other prefilters if you decide to remove the carbon block.

Shirak · Nov 17, 2025

Here is a shot of my setup with the DI bypass T and Valve. I circled the bypass T/Valve in red. Added a shutoff after that to stop flow to the DI circled in green. That way I can get just RO out the bypass and when ready switch the two valves to flow through the DI. FWIW I have two cation DI, off the left (not pictured) are two sets of two anion bigger canisters and then a final small mixed bed (the small one left of the two small cation).

estew09 · Nov 17, 2025

Shirak,
Thanks for the picture and the help. I will be doing this tomorrow evening.

estew09 · Nov 18, 2025

Shirak said:
Here is a shot of my setup with the DI bypass T and Valve. I circled the bypass T/Valve in red. Added a shutoff after that to stop flow to the DI circled in green. That way I can get just RO out the bypass and when ready switch the two valves to flow through the DI. FWIW I have two cation DI, off the left (not pictured) are two sets of two anion bigger canisters and then a final small mixed bed (the small one left of the two small cation).

So I got my valve today, installed it and did a test run... I had heard of TDS creep bud didn't really understand the severity of it until tonight. If you zoom in on my picture you can see in my picture the TDS is at 467 and that was after 30 seconds or so. It took 3-4 minutes before it fell to around 7-8. So now I understand, pushing several hundred TDS through the DI that is designed to convert sub 10 TDS will definitely blow through the DI Resin. I will be replacing this one this weekend and will see how much longer it lasts. THANKS!!

Buckeye Hydro · Jan 2, 2026

Shirak said:
Here is a shot of my setup with the DI bypass T and Valve. I circled the bypass T/Valve in red. Added a shutoff after that to stop flow to the DI circled in green. That way I can get just RO out the bypass and when ready switch the two valves to flow through the DI. FWIW I have two cation DI, off the left (not pictured) are two sets of two anion bigger canisters and then a final small mixed bed (the small one left of the two small cation).

Hmmmmm.

Is that a 4" x 21" membrane? If so, you should be sending a minimum of 3 gpm to drain. No way can you feed that membrane well with 1/4" tubing. Depending upon which membrane you're using, what is your spec feedwater pressure? Will be at least 80 psi... and that is the very max you'd want to push through that 10" clear carbon block housing.

Give us a call if you want to noodle through what you've put together there.

Russ

Your feedwater flow dramatically exceeds the recommended max flow for any 10" x 2.5" carbon block.

Shirak · Jan 2, 2026

Hi Russ,

Feed on the left side 1/2" from the well line which is 1" then 3/4" into the main house line. I have the main line pressurized to around 65psi max. The well water goes through a series of particle filters down to 1 micron. The carbon block is post RO to eliminate any funky chemicals the RO may have missed. I am running around 4 minutes for 1gal of RO. My well water is cold... 55 degrees and TDS is around 280ppm Hard! Lots of calcium magnesium etc in the limestone rock here. It's a AMI extra low energy 1000gpd membrane. 4" x 21" https://www.appliedmembranes.com/pages/tap-water-extra-low-energy It works really well even with my cold water and lower than ideal pressure and the fairly high waste/RO ratio which helps a ton in keeping it from fouling up because my sediment filters don't catch everything.

Buckeye Hydro · Jan 2, 2026

Shirak said:
Hi Russ,

Feed on the left side 1/2" from the well line which is 1" then 3/4" into the main house line. I have the main line pressurized to around 65psi max. The well water goes through a series of particle filters down to 1 micron. The carbon block is post RO to eliminate any funky chemicals the RO may have missed. I am running around 4 minutes for 1gal of RO. My well water is cold... 55 degrees and TDS is around 280ppm Hard! Lots of calcium magnesium etc in the limestone rock here. It's a AMI extra low energy 1000gpd membrane. 4" x 21" https://www.appliedmembranes.com/pages/tap-water-extra-low-energy It works really well even with my cold water and lower than ideal pressure and the fairly high waste/RO ratio which helps a ton in keeping it from fouling up because my sediment filters don't catch everything.

I see. You biggest problem will be with what is dissolved in that water - especially the calcium and magnesium (= hardness). This is what will scale the membrane. Looks like they have spec'ed that membrane at 100 psi at 8% recovery, meaning about an 11:1 ratio of concentrate to permeate. Ugh.

We stock membranes spec'ed at 80 psi . I'd rather see you running a 2.5" membrane rather than a 4" membrane. They come in 14", 21", and 40" in length. Min concentrate flow on a 2.5" is 1 gpm, as opposed to 3 gpm for the 4"

You'd also benefit from a pump prior to the membrane.