Does flushing membrane also flush TDS creep and carbon dust (RO/DI)

[Opus]

Lol our little tds inline testers are telling a quick head line to the story at best, if your relying on them as your sol indicator your already at a serious disadvantage, I can't believe a 60$ membrane is what puts some people on opposite sides of success. If you hold out for years just think about how long that is for the most important part of your reef tank, they run on water lol it seems important to me tds or not I'm changing and flushing it's super easy to do. It's funny to me the different perspectives of people, I don't think it's about the money, but for 60$ and piece of mind I change my filters and maintain them, just like everything else. Lol it's not a conspiracy!

I've seen arguments over the micron size of the pre-filters, though it has been years since I've seen a good one. Years ago there was a thread that it got so heated one of the commenters was banned.

I'm just going to go with my experiences on this one. I've lived in 2 states and in both places my membranes have lasted for over 5 years before I saw any change in the output tds. Since no one can state how much life you are adding to the membrane if you do flush it, I'm sticking to what I know. If someone showed me it would add a year to the life then I would do it, adding a month or 2 to the life is just not worth it for me.

 

[BeanAnimal]

Since no one can state how much life you are adding to the membrane if you do flush it, I'm sticking to what I know. If someone showed me it would add a year to the life then I would do it, adding a month or 2 to the life is just not worth it for me.

Nobody can tell you because there is no fixed number. The membrane life will depend on (to name a few):
-input water composition
-type and quality of prefilters
-product to waste ratio
-how long it sits between uses
-does it rest under pressure or not
-how much water is processed each use
-total volume processed
-average water temperature
-average operating pressure
-how YOU define a good vs a failed (time to replace) membrane.

Pre-filter type - That too depends on source water. In some places there is very little turbidity and maybe no chloramines. In other places there may be high turbidity with larger particles and yet another with extremely tiny particles and loads of sulfer, etc.

You can manage your RODI system in any fashion you please, but that does not change their fundamental operating properties.

 

[BeanAnimal]

I don’t see any scenario where a clogged membrane lets through anything bad. What it would do is potentially waste water and time by reducing output.

The common failure mode (that results in higher salt migration to output) depends on the type of "clogging" but the most common would be abrasion due to buildup and passage of scale or other hard or crystalline structures. In a similar fashion unwashed or failing pre-filters that pass particles can also contribute to membrane abrasion.

As a membrane "clogs" these abrasion paths become more pronounced. An unrolled membrane will often times show these paths.

Also in contest to the clogging - the remaining area simply gets more wear as well as creates a larger pressure differential between input and output. Some membranes can telescope due to the pressure differential (and housing setup). The telescoping (depending on how much) can allow bypass as well.

Biological fouling (slime gel) is also a culprit in pressure differential damage. (frequent flushing can prevent this too).

There are several other failure modes that may or may not be associated with membrane fouling. GE (or Dow?) has a fairly extensive document explaining failure modes for this type of membrane. It has been years since I looked at it, but the wealth of RO research and development by both GE and DOW is overwhelming.

 

[Aqua Man]

interesting. I do a back flush at the beginning and the end of making RO. I thought that’s what I supposed to do. When I change my carbon blocks, I run the back flush longer. Also thought that’s how it was done to not pollute membrane. My last membrane was 6 years old and creeping up on the TDS. 1-2 TDS after new membrane. I do run it for hrs at a time when I make RO and fill up all my containers. Then it’s sits idle for couple weeks.

 

[BeanAnimal]

It technically isn't a back flush...

Here is a decent drawing

When you "Flush" you are sending a high volume of water BETWEEN the layers and directly to the rejection port (flushing all of the crap that the membrane blocks).

The normal reject water takes a good bit of this away, but flushing (by unblocking the flow restrictor) allows a much much higher flow of "reject" water to quickly and forcibly rinse anything out preferably before it turns to scale or bioslime, etc.

As I mentioned above, the lower your waste to product ratio the more of this "stuff" is left behind and needs to be power flushed. I run 1:1 product to waste, so autoflush very often to perserve the membrane.