My RODI starts off with high TDS, then gradually reaches 0

Seeko

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I have the BRS 4 Stage Value 200GPD Water Saver RO/DI System and when I first got it, my water always read 0 TDS, from start to finished whenever I used it, but since the first time I changed the filtration media, it starts off really high, like around 10TDS, and then creeps down to 0.

What happened here? How do I fix this? I have really bad cyanobacteria growth in my tank, and I believe this is why. I've done a Google search, but haven't really come up with anything definitive.

Thanks
 

Halal Hotdog

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It isn’t unusual for the initial few gallons to be higher TDS as you are flushing the membranes out. At this stage you do NOT want your DI connected as it will be depleted prematurely.
 

SPR1968

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I think it’s probably as contaminates clear out, mine does it when it’s been turned off for a few days.
 

Water Dog

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What you’re seeing is called TDS creep. Do you have an inline TDS meter sensor between the RO membrane and the DI stage? A TDS reading between these stages of your RODI unit are essential for keeping tabs on TDS creep and preserving your DI resin life. If you don’t have an in-line TDS meter, I’d suggest getting a triple inline TDS meter and place the first sensor between the carbon block and the RO membrane, the second sensor between the RO membrane and the DI stage and third sensor after the DI stage.


TDS creep occurs because when a system is shut down, you have dirty, high TDS water on the waste water side of the membrane and very low TDS water on the permeate (product water) side of the membrane. As a unit sits idle, the low TDS product water on the permeate side of the membrane will aggressively seek to reach equilibrium with the dirty waste water side of the membrane. So as it sits, TDS is drawn from the dirty side of the membrane, through osmosis, until it reaches equilibrium with the clean permeate side of the membrane.

Install a 3 way valve, otherwise known as a DI bypass valve between the RO membrane and the DI stage. Upon startup of a system, you use the DI bypass valve to flush out the TDS creep water that has accumulated on the permeate side of the RO membrane. This drastically improves DI resin life by not allowing this high TDS creep water to go through your DI resin. Upon startup, I usually let the bypass run for a minute or two. I make sure to position my TDS sensor between the membrane and the bypass valve so I can see the permeate side go from a high TDS creep number to it normal RO product water number (in my case, it’s 1). After all the TDS creep water has been diverted out of the system, only then will I allow the low TDS RO water go through my DI resin.


Remeber, the DI bypass valve should not be confused with the membrane flush valve that is on the flow restrictor. The membrane flush valve is different from a DI bypass valve and accomplishes a totally different task.
 
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najer

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It isn’t unusual for the initial few gallons to be higher TDS as you are flushing the membranes out. At this stage you do NOT want your DI connected as it will be depleted prematurely.

This except when I flush the new filters I leave the old resin in place for an hour and then change the resin.
 

redfishbluefish

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Excellent write-up @Water Dog .....TDS Creep or TDS Breakthrough...happens every time you turn on your unit. One of the reasons I disagree with "On Demand" water systems....will eat your DI in no time.

I do exactly what @Water Dog recommends...on start up I turn the valve to waste for seven minutes.

Three way Valve ROBI.JPG


I've found that seven minutes drops my TDS from around 50 or so to 2. Flip the valve and make good water. I also make a day of making water. I get a year plus out of my DI resin.
 
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Seeko

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Thank you very much. I'll give this a shot and let you know how it goes!
 

highpoint

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What you’re seeing is called TDS creep. Do you have an inline TDS meter sensor between the RO membrane and the DI stage? A TDS reading between these stages of your RODI unit are essential for keeping tabs on TDS creep and preserving your DI resin life. If you don’t have an in-line TDS meter, I’d suggest getting a triple inline TDS meter and place the first sensor between the carbon block and the RO membrane, the second sensor between the RO membrane and the DI stage and third sensor after the DI stage.


TDS creep occurs because when a system is shut down, you have dirty, high TDS water on the waste water side of the membrane and very low TDS water on the permeate (product water) side of the membrane. As a unit sits idle, the low TDS product water on the permeate side of the membrane will aggressively seek to reach equilibrium with the dirty waste water side of the membrane. So as it sits, TDS is drawn from the dirty side of the membrane, through osmosis, until it reaches equilibrium with the clean permeate side of the membrane.

Install a 3 way valve, otherwise known as a DI bypass valve between the RO membrane and the DI stage. Upon startup of a system, you use the DI bypass valve to flush out the TDS creep water that has accumulated on the permeate side of the RO membrane. This drastically improves DI resin life by not allowing this high TDS creep water to go through your DI resin. Upon startup, I usually let the bypass run for a minute or two. I make sure to position my TDS sensor between the membrane and the bypass valve so I can see the permeate side go from a high TDS creep number to it normal RO product water number (in my case, it’s 1). After all the TDS creep water has been diverted out of the system, only then will I allow the low TDS RO water go through my DI resin.


Remeber, the DI bypass valve should not be confused with the membrane flush valve that is on the flow restrictor. The membrane flush valve is different from a DI bypass valve and accomplishes a totally different task.

Bringing this thread back to life.

I have the 6 stage system with the RO drinking water 3 gallon tank. When I turn my unit on my RO quickly spikes close to 100 and back down to normal (below 10) in maybe less than a minute.

Where should I place the valve so the waste isn’t going into my RO drinking water tank?
 

KStatefan

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Bringing this thread back to life.

I have the 6 stage system with the RO drinking water 3 gallon tank. When I turn my unit on my RO quickly spikes close to 100 and back down to normal (below 10) in maybe less than a minute.

Where should I place the valve so the waste isn’t going into my RO drinking water tank?

After the membrane.
 

Buckeye Hydro

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KStatefan

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Here's some data from tests we ran with the most common membrane used in this hobby: the 75 gpd Filmtec.
TDS Creep.PNG

You'll note that the permeate TDS was down to where you'd want it in 60 seconds.

A TDS Creep bypass Kit is a good choice if you're physically going to be standing next to the unit when you kick it on. https://www.buckeyehydro.com/tds-creep-bypass/

This discussion also highlights the need for a THREE probe TDS meter - the TRM1 https://www.buckeyehydro.com/trm1-tri-inline-tds-meter/
That is why I removed my drinking water system from my RODI unless you are standing the to bypass each time is seems like all you are sending to the bladder tank is TDS creep water.
 

Buckeye Hydro

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Yep - you do not want to send RO water from a bladder tank to the DI.
For one, a substantial percentage of the water in that tank is TDS creep water
Also remember the concept of Net Driving Pressure. When a system operating at for example 60 psi, with a standard ASOV, is filling a bladder tank, when the tank is nearly full it is exerting 40 psi of back pressure.
60 psi feed water pressure
-40 psi back pressure
=20 psi net driving pressure

You can imagine how poorly the membrane will perform @ 20 psi.

This is all a non-issue for straight drinking water applications... but for ultrapure RODI systems it's a huge deal.
 
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