Jump to content

Reverse Osmosis Membranes - Theoretical question


Recommended Posts

My understanding of how RO's work is limited but I had an idea last night and wondered if anyone knows the likelihood of it working.

(It is completely theoretical, ignore the obvious things like "why bother? Use resin", "would be a faff", "would take longer")

If my RO reduced tap water from 350+ ppm down to around 10ppm or less. Could that product water be stored in IBC, then pumped back through RO (with enough pressure) bringing it down to 1 / 2 ppm?. 

As the initial TDS of the product water is only 10ppm, could the RO be run with a much lower waste ratio, say 10/90 or 20/80? 

Or would the 10ppm contain only dissolved solids that are too fine for the membrane to catch? 

 

Just wondering what options I'd have if a resin shortage comes and I get stuck without any. (I know apparently 10ppm could be used, but closer to 0 ppm less worries)

Link to post
Posted (edited)
7 hours ago, con95a said:

My understanding of how RO's work is limited but I had an idea last night and wondered if anyone knows the likelihood of it working.

(It is completely theoretical, ignore the obvious things like "why bother? Use resin", "would be a faff", "would take longer")

If my RO reduced tap water from 350+ ppm down to around 10ppm or less. Could that product water be stored in IBC, then pumped back through RO (with enough pressure) bringing it down to 1 / 2 ppm?. 

As the initial TDS of the product water is only 10ppm, could the RO be run with a much lower waste ratio, say 10/90 or 20/80? 

Or would the 10ppm contain only dissolved solids that are too fine for the membrane to catch? 

I would say agree that this. You might reduce the tds by a point or two depending where the membrane catches the water the second time around.

Just wondering what options I'd have if a resin shortage comes and I get stuck without any. (I know apparently 10ppm could be used, but closer to 0 ppm less worries)

This was a question asked many years ago on another forum.

A membrane consists of tiny microscopic pores that allow water to migrate through under pressure but stop particles bigger than the pores. The water diverted to waste flushes those particles away so they don't block up these tiny pores. As a membrane gets older it 'wears'. These tiny pores become bigger. This translates into a less efficient membrane as the tds of the product water rises.

But there are some dissolved particles smaller than the pores in the membrane and these pass through the membrane along with the product water. It was likened to a tea strainer. Particles of tea leaves get caught by the mesh the strainer is made from. But there could be very fine particles of tea leaves - a 'dust' will pass through the strainer along with the tea. (It was such a good illustration that I sometimes use it myself.)

The product water from the membrane contains these fine 'dust' particles that we polish off with resin. If we passed that water through the membrane again it would just allow those microscopic particles to pass through again. So the second pass wouldn't do much to improve the product's water quality.

You have mentioned the other aspect of the second filtration. You still need to have flush water which would be wasted as it wouldn't achieve much. Could you reduce the flush ratio? Probably, but I don't know the answer for sure. It doesn't really matter anyway because the second pass is a waste of time and resources.

There was another facet of this that you haven't mentioned. What happens if you try to filter the waste water you are putting down the drain? Some suppliers use that process with small Ro-Man type r/os. They feed the waste water from the first membrane into the second and then feed that waste into the third. They will tell you that its a water saving method but its at the cost of membranes no2 and no3.

I'm sorry for you guys whose water is hard like yours. Our tap water tds is 122 atm. I have just replaced the resin from a 25 liter bag I purchased 4 years ago. The last resin change was done in March 2019. I have a 6 liter di vessel.

I didn't have enough resin to fill the di vessel, but remembered I still had a small bucket of resin I purchased over 8 years ago from Ro-Man. I used that to fill up the remaining space in the di vessel. It will be interesting to see if I still get another year out of this resin mix.

 

 

 

Edited by spruce
Link to post
Posted (edited)

Spruce. Great explanation. Always adding value for others.

Just reading this I like how you mention about the wearing of the hole size. Perhaps a worn RO could be used as ro1 and with a severely adjusted waste after it passes through ro2. I use an adjustable service valve so can set the waste  which helps.

 

edit. It would perhaps be best to test the output pressure of ro1 pure first though. Not sure if it would drop off a little due to the waste ratio. 

Edited by Omega
Link to post
Posted (edited)
42 minutes ago, Omega said:

Spruce. Great explanation. Always adding value for others.

Just reading this I like how you mention about the wearing of the hole size. Perhaps a worn RO could be used as ro1 and with a severely adjusted waste after it passes through ro2. I use an adjustable service valve so can set the waste  which helps.

 

edit. It would perhaps be best to test the output pressure of ro1 pure first though. Not sure if it would drop off a little due to the waste ratio. 

The credit for this explanation must go to someone else whose name has long since been swallowed up in the sink hole of my brain. The tea strainer has to be one of the best symbolism I've heard to explain the workings of a membrane.

Chlorine in the water also eats away at the membrane and increases the size of the pores.

As the waste valve is placed after the last membrane (no3) pressure in all the membranes should balance out.

 

Edited by spruce
Link to post
6 hours ago, spruce said:

This was a question asked many years ago on another forum.

A membrane consists of tiny microscopic pores that allow water to migrate through under pressure but stop particles bigger than the pores. The water diverted to waste flushes those particles away so they don't block up these tiny pores. As a membrane gets older it 'wears'. These tiny pores become bigger. This translates into a less efficient membrane as the tds of the product water rises.

But there are some dissolved particles smaller than the pores in the membrane and these pass through the membrane along with the product water. It was likened to a tea strainer. Particles of tea leaves get caught by the mesh the strainer is made from. But there could be very fine particles of tea leaves - a 'dust' will pass through the strainer along with the tea. (It was such a good illustration that I sometimes use it myself.)

The product water from the membrane contains these fine 'dust' particles that we polish off with resin. If we passed that water through the membrane again it would just allow those microscopic particles to pass through again. So the second pass wouldn't do much to improve the product's water quality.

You have mentioned the other aspect of the second filtration. You still need to have flush water which would be wasted as it wouldn't achieve much. Could you reduce the flush ratio? Probably, but I don't know the answer for sure. It doesn't really matter anyway because the second pass is a waste of time and resources.

There was another facet of this that you haven't mentioned. What happens if you try to filter the waste water you are putting down the drain? Some suppliers use that process with small Ro-Man type r/os. They feed the waste water from the first membrane into the second and then feed that waste into the third. They will tell you that its a water saving method but its at the cost of membranes no2 and no3.

I'm sorry for you guys whose water is hard like yours. Our tap water tds is 122 atm. I have just replaced the resin from a 25 liter bag I purchased 4 years ago. The last resin change was done in March 2019. I have a 6 liter di vessel.

I didn't have enough resin to fill the di vessel, but remembered I still had a small bucket of resin I purchased over 8 years ago from Ro-Man. I used that to fill up the remaining space in the di vessel. It will be interesting to see if I still get another year out of this resin mix.

 

 

 

Thanks for the thorough answer. I was half expecting it to be the case. The tea strainer is a good illustration 👌🏼 oh to have softer water... 

Link to post

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Restore formatting

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.