New RODI with High TDS....Stumped

[jjstecchino]

Is a cracked membrane an internal issue that cannot be detected from a visual inspection?

May be internal to the membrane. For some reason you have something that is leaching a good amount of unfiltered water into the filtered water compartment of the housing. The few things I can come up with, thinking how the system works is a bad membrane or a bad seal. The functioning of the membrane is quite simple. All it is is a perforated plastic tube wrapped up by a semi permeable membrane. the plastic tube is what goes from the inlet to the waste water of the housing. You apply a flow restrictor to the waste water side and supply water under pressure. Pressure forces pure water through the semi permeable membrane leaving solutes behind. Filtered water fills the space in the housing around the membrane to then exit through the RO outlet. The only places where unfiltered water can mix are at the waste water side of the central pipe. There there are o-rings that should seal as the membrane is seated properly inside the housing. Second place is at the large gasket around the membrane itself that should seal the membrane as it is inserted into the case. This is why you have been already suggested to check the orientation of the membrane. Also make sure the membrane is pushed in completely, otherwise the o-rings at the waste water side may not seal. The 75gpd membrane has a longer exposed center pipe toward the waste water outlet and that could be cracked leaking unfiltered water or the membrane can just be bad allowing more water and solutes out. Having said that a brand new membrane takes a few days to get soaked properly and initial TDS may be high until it start functioning properly and this is normal.
I would check all these points of failure. You have gone through a few membranes already so it is unlikely they are all bad.
I would use the system with just one membrane for the time being. make sure the membrane is fully inserted in the case, check that the ratio of waste vs filtered water is apprrox 3:1. If it is a new membrane let it make water for a day or two until tds go down

 

[AllSignsPointToFish]

May be internal to the membrane. For some reason you have something that is leaching a good amount of unfiltered water into the filtered water compartment of the housing. The few things I can come up with, thinking how the system works is a bad membrane or a bad seal. The functioning of the membrane is quite simple. All it is is a perforated plastic tube wrapped up by a semi permeable membrane. the plastic tube is what goes from the inlet to the waste water of the housing. You apply a flow restrictor to the waste water side and supply water under pressure. Pressure forces pure water through the semi permeable membrane leaving solutes behind. Filtered water fills the space in the housing around the membrane to then exit through the RO outlet. The only places where unfiltered water can mix are at the waste water side of the central pipe. There there are o-rings that should seal as the membrane is seated properly inside the housing. Second place is at the large gasket around the membrane itself that should seal the membrane as it is inserted into the case. This is why you have been already suggested to check the orientation of the membrane. Also make sure the membrane is pushed in completely, otherwise the o-rings at the waste water side may not seal. The 75gpd membrane has a longer exposed center pipe toward the waste water outlet and that could be cracked leaking unfiltered water or the membrane can just be bad allowing more water and solutes out. Having said that a brand new membrane takes a few days to get soaked properly and initial TDS may be high until it start functioning properly and this is normal.
I would check all these points of failure. You have gone through a few membranes already so it is unlikely they are all bad.
I would use the system with just one membrane for the time being. make sure the membrane is fully inserted in the case, check that the ratio of waste vs filtered water is apprrox 3:1. If it is a new membrane let it make water for a day or two until tds go down

This is good general information. His waste to product ratio will be closer to 1.5:1 since he has a 6-Stage BRS system with the water saver option, FWIW.

 

[jjstecchino]

This is good general information. His waste to product ratio will be closer to 1.5:1 since he has a 6-Stage BRS system with the water saver option, FWIW.

Got that, That's why I suggested in one of the posts, he would remove one membrane and check one at a time . This may help him troubleshooting this RO system behaving weirdly..

 

[AllSignsPointToFish]

Got that, That's why I suggested in one of the posts, he would remove one membrane and check one at a time . This may help him troubleshooting this RO system behaving weirdly..

Absolutely agree!

 

[TunnelRatt]

Thanks, great information. Please bear with me, I still need to ask a few questions to make sure I understand correctly.

• What actual part of the membrane (not housing) does pure water exit through? Is it the layered wraps at the bottom near the two o-rings?
• Source water (high TDS tap) should be between the top of the housing and the large seal when I remove the housing cap? If so, does the source water (high TDS tap) also seep through the exposed edges of the wrapped layers that surround the top post of the membrane?

 

[TunnelRatt]

Got that, That's why I suggested in one of the posts, he would remove one membrane and check one at a time . This may help him troubleshooting this RO system behaving weirdly..

The system has been ran in both dual and single membrane configuration. Unfortunately I am still getting fluctuating tds levels that get higher , lower, and then higher, etc. Also, the waste / product ratio still not right.

 

[AllSignsPointToFish]

Thanks, great information. Please bear with me, I still need to ask a few questions to make sure I understand correctly.

• What actual part of the membrane (not housing) does pure water exit through? Is it the layered wraps at the bottom near the two o-rings?
• Source water (high TDS tap) should be between the top of the housing and the large seal when I remove the housing cap? If so, does the source water (high TDS tap) also seep through the exposed edges of the wrapped layers that surround the top post of the membrane?

Here's a diagram that might help with your first question:

In the next diagram, you can see that high TDS water is intended to enter the membrane from the area you mentioned in the second question:

 

[jjstecchino]

Thanks, great information. Please bear with me, I still need to ask a few questions to make sure I understand correctly.

• What actual part of the membrane (not housing) does pure water exit through? Is it the layered wraps at the bottom near the two o-rings?
• Source water (high TDS tap) should be between the top of the housing and the large seal when I remove the housing cap? If so, does the source water (high TDS tap) also seep through the exposed edges of the wrapped layers that surround the top post of the membrane?

Let me give you some more details. As I read my previous post I realized my explanation was clear as mud. Ill try to do better this time with pictures.

Here is a RO membrane:

The right side is the unfiltered water side. The center pipe is not continuous but plugged close to the unfiltered water side.
Unfiltered water enters this side of the membrane

travels through the membrane layers leaving solutes behind. Purified water collects into the center pipe and waste exits the membrane as in the picture below:

If these O-rings are broken or the membrane is not inserted completely, waste water will mix to RO water.

If you look at the housing:

Waste water comes out from the upper outlet, RO water from the lower one.

Hope this is more clear and helpful.

 

[TunnelRatt]

Boy am I confused. The diagram and what was previously posted doesn't seem to match, unless I misunderstood the post.

• In the second diagram, it appears that feed water (tap) seeps through the exposed wrapped layers on the top of the membrane, but does feed water also get in the center tube once it enters the ro housing cap?
• So feed water travels the length of the membrane and RO water slowly drips into the permeate tube?
• All RO water stays within the permeate tube and NEVER enters the membrane housing thanks to dual o-rings at the bottom of the membrane?

Please correct me if I am wrong. I definitely have a lot to learn.

 

[TunnelRatt]

Sorry jjstecchino, I did not see your post before I posted. My apologies.

Thank you for clearing that up...I was really confused.

 

[jmc1974]

See this video to learn how an RO membrane works

 

[jjstecchino]

Boy am I confused. The diagram and what was previously posted doesn't seem to match, unless I misunderstood the post.

• In the second diagram, it appears that feed water (tap) seeps through the exposed wrapped layers on the top of the membrane, but does feed water also get in the center tube once it enters the ro housing cap?
• So feed water travels the length of the membrane and RO water slowly drips into the permeate tube?
• All RO water stays within the permeate tube and NEVER enters the membrane housing thanks to dual o-rings at the bottom of the membrane?

Please correct me if I am wrong. I definitely have a lot to learn.

I think you got it.

Please disregard my first explanation as it was not very clear. Refer to the one with pictures.

This may clear up things further:

 

[TunnelRatt]

Yes it does. Thank you

 

[TunnelRatt]

Okay, last time I opened the membrane housing, I noticed some air bubbles resting on the outter edge of the large membrane seal where it meets the membrane housing wall. Is this normal?

I also noticed an area on the top of the membrane (exposed wrapped layers) with a difference in thickness and a small gap. Would this cause an issue?

I'll try to post a photo later today.

 

[TunnelRatt]

Air bubbles:

Gap between blue wall and layers:

Difference in thickness and small gap:

What do you guys think?

 

[AllSignsPointToFish]

I don't see any issue with that. There has to be some way a relatively large amount of rejected water is being introduced to the collection tube. Those don't look like unusually large manufacturing variations.

 

[TunnelRatt]

I didn't think it was an issue. I'm guessing it's something internal. Going to give BRS a call tomorrow.

Thank you

 

[AllSignsPointToFish]

Here's a photo of an old membrane of mine. Doesn't look that different from yours. Mine was producing elevated TDS (~10-15 ppm) after years of use.

 

[TunnelRatt]

What is your TDS before the rodi? Also, thank you for the photo.

 

[AllSignsPointToFish]

That's an old membrane. I will have to get a TDS measurement before the membrane when I get home.

In the past, it's run 300-400 ppm.