Are we changing our RO cartridges too soon?

[Ocean Lotion]

So my question is at what tds should we be changing out out ro/di cartridges? At what value of tds is the water detrimental to our tank? 1, 5, 10 ppm? Really is 10 ppm a real suplement to feeding algae? Anyone have any scientific knowledge you could lay on us? I know many change theirs when they get 1 tds out of their rodi system. At what reading do we really need to replace cartridges?
Thanks

 

[nautical_nathaniel]

So my question is at what tds should we be changing out out ro/di cartridges? At what value of tds is the water detrimental to our tank? 1, 5, 10 ppm? Really is 10 ppm a real suplement to feeding algae? Anyone have any scientific knowledge you could lay on us? I know many change theirs when they get 1 tds out of their rodi system. At what reading do we really need to replace cartridges?
Thanks

My LFS changes their's at 5 ppm, I'm pretty sure it doesn't make all that much of a difference, but it still helps to understand what elements and what concentrations are naturally occurring in your water system. If stuff like lead, radon, copper, and other heavy metals come up at or near the max allowable levels from the EPA in your potable water reports, I wouldn't let it go above 5 ppm.

 

[Randy Holmes-Farley Verified Community Expert]

So my question is at what tds should we be changing out out ro/di cartridges? At what value of tds is the water detrimental to our tank? 1, 5, 10 ppm? Really is 10 ppm a real suplement to feeding algae? Anyone have any scientific knowledge you could lay on us? I know many change theirs when they get 1 tds out of their rodi system. At what reading do we really need to replace cartridges?
Thanks

IMO, as soon as it rises detectably. Algae is also not the only concern.

Perhaps surprisingly, when the TDS out of a DI cartridge starts to rise, the concentration of certain elements in the effluent can be HIGHER than the incoming RO water. That is because at that point, ions will be swapping on and off the DI resin, and the most weakly bound will be the ones that mostly get popped off.

Unfortunately, some of the weaker bound ions are also ones that can be an issue, such as ammonia, silicate, and phosphate. Additionally, if the balance is not perfect between the ability to bind positive charges and negative charges, the pH of the final DI effluent can swing wildly up or down as the TDS rises, with most of the positive charges being H+ or most of the negative charges being OH-.

That all said, you can get more capacity out of a DI cartridge if you use two in series, and run the first one (closest to the RO) until the TDS out of it has risen to about half of the incoming TDS (that is, half of the TDS out of the RO). At that point, swap the second one into the first position and put a new one in the second position.

This way the TDS never rises above 0-1 ppm TDS, but the cartridges are used to the point where they are highly depleted.

 

[nautical_nathaniel]

IMO, as soon as it rises detectably. Algae is also not the only concern.

Perhaps surprisingly, when the TDS out of a DI cartridge starts to rise, the concentration of certain elements in the effluent can be HIGHER than the incoming RO water. That is because at that point, ions will be swapping on and off the DI resin, and the most weakly bound will be the ones that mostly get popped off.

Unfortunately, some of the weaker bound ions are also ones that can be an issue, such as ammonia, silicate, and phosphate. Additionally, if the balance is not perfect between the ability to bind positive charges and negative charges, the pH of the final DI effluent can swing wildly up or down as the TDS rises, with most of the positive charges being H+ or most of the negative charges being OH-.

That all said, you can get more capacity out of a DI cartridge if you use two in series, and run the first one (closest to the RO) until the TDS out of it has risen to about half of the incoming TDS (that is, half of the TDS out of the RO). At that point, swap the second one into the first position and put a new one in the second position.

This way the TDS never rises above 0-1 ppm TDS, but the cartridges are used to the point where they are highly depleted.

Nevermind what I said, he's the expert!

 

[chipmunkofdoom2]

TDS of your product water can let you know when it's time to change your DI resin, but it's a relatively poor indicator of when your RO membrane or prefilters need to be changed.

For the most part, our RO/DI systems have three steps. The first is the prefilters, which usually include sediment filters and carbon blocks. These remove large particles and chlorine/chloramine from the water. This is important because the RO membrane, which does 95% of the work of the RO/DI system, is very sensitive to large particles and chlorine. The second part of the system is the RO membrane. Most RO membranes have rejection rates between 95% and 99%. This means that between 95% and 99% of the TDS in your water gets removed by your RO membrane. The final step is the DI stage. The DI stage captures charged molecules that pass through it. Since your RO membrane removes 98% of the junk from the water, the DI resin "polishes off" that last 2%.

You change the first part of your system, the prefilters, every 6 months or so, regardless of your final TDS after DI resins. You can be more intelligent about it and only change the prefilters when the pressure drops or when you can detect chlorine in your RO waste water, but this requires a lot more discipline and testing. If you're not able or willing to do that, a 6 month replacement schedule is likely "good enough" for most people.

You change the second part of your system, the RO membrane, very infrequently. Properly protected by changing prefilters on time, the RO membrane will last many years. To measure your membrane's performance, test the TDS of your tap water, then test the TDS of your RO water AFTER the membrane, but BEFORE the DI resin. Divide the RO water reading by the tap water reading. The result should be less than 0.06 (6%). If the result is greater than 5% - 10%, your membrane may need to be replaced.

You change the third part of the system, the DI resin, as soon as the TDS reads 1 ppm or greater. As Randy mentioned, the trouble with DI resin is it releases weakly-charged molecules as it exhausts, which means the product water could have relatively high levels of ammonia, silicates and phosphates.

 

[Ocean Lotion]

Wow. I feel so much smarter now simply by being a product of the enviroment you guys just created. Thank you. Just ordered new blocks and resin for my rodi.

 

[Grey Guy]

If I let my DI resin go too long, it makes the RO/DI water smell like dead fish. Eeeeeew!

 

[Randy Holmes-Farley Verified Community Expert]

If I let my DI resin go too long, it makes the RO/DI water smell like dead fish. Eeeeeew!

A couple of times I made my limewater/kalkwasser without seeing my DI was depleted, and it stank of ammonia.

 

[saltyhog]

Great post Chimpmonkofdoo!

I'm pretty lucky as my municipal water source TDS runs from 37-42 ppm. My RO membrane is 5 years old and the effluent from it is 0-1 reliably. DI resin lasts a long, long time as well.

 

[Grey Guy]

I thought the original question was, when to change the RO cartridge, a.k.a. RO membrane. I read TDS coming out of the RO unit before it enters the next stage. That’s how I know if the membrane is not working efficiently. A new membrane reads a little above 10 TDS on my system. One of the signs that the membrane needs to be changed is that the DI resin will start getting used up much faster than normal. I have not replaced the membrane since I started monitoring it, but I will keep an eye on the final TDS and the use of DI resin to determine at what TDS I should change the membrane. I expect that when it fails, it fails fairly fast.