TDS creep post membrane

[t5Nitro]

Had a question on post membrane TDS creep to see if this is normal.

When I start up my RODI system, I have it a bypass that discards pre DI water to the drain until the TDS creep drops, but then the longer the unit runs, the higher the post membrane TDS will be. I’m a bit concerned that it’ll exhaust the expensive resin filters earlier and earlier.

Initial startup of the unit: TDS 200-250
Flush and waste for several minutes: TDS drops to baseline of 4-5. This is post membrane
Post DI TDS 0
The unit runs for a while and the TDS post membrane creeps again. TDS 10-15.
Post DI stays 0. I don’t want to be exhausting DI stages early when the tds should be 5 coming into them. Why does it creep back to the teens?

 

[cooltowncorals]

The incoming water may not be the same level of TDS always, so if it fluctuates 100 ppm TDS you could see a variance on the way out.

Down here in the Phoenix valley I start above 500 all the time and feel pretty good about running through the DI anything below 40.

 

[cooltowncorals]

If you want to maximize your resin make sure your flow through is slow so you exhaust it all and don’t have tunnels and mixed colors when you change it.

 

[t5Nitro]

All good info.

I don’t think the incoming TDS is too highly variable because it occurs in the same sequence every time the unit is run.

Start 240
Turn to the DI at 4-5
Check in a few hours and the pre resin is back to 10-15 (this number never returns to single digits unless the unit is powered down and the I make rodi water on another day. Then it will follow the same sequence. I usually turn it off when I see the pre resin hitting 15 and then I’ll make more another day).
Post 0 with new resin changed each time it starts to show 1

I’ve tried changing pre filters and RO membrane in the past as well, but the same pattern occurs. More reassured to run 15 through the resin if you’re good running anything under 40 through it.

I won’t think too much more about it if 10-15 is fine to run into the DI resin.

 

[cooltowncorals]

As long as it is going through slow enough to get enough contact to get back to 0 TDS your good.

Only concern and a valid one is the higher the TDS going in, the faster you burn through the resin, which is spendy.

 

[KStatefan]

Membrane rejection does not normally change like that while operating with constant parameters.

 

[Buckeye Hydro Verified Sponsor]

Kstatefan is correct.

Does your water pressure (reaching the membrane) decline over time when you run the system?

 

[ROharvester]

When the membranes feed water is cut off reverse osmosis stops and osmosis takes place equalising the concentration on permeate and concentrate side. when it restart to reverse the osmosis the low tds water push out the equalize water in the permeate chamber until its all low tds coming out.