Multiple RO membranes

[rjallen]

I would not add a second membrane if my TDS was over 150.



Also hardness of water maters

say you have 200 going in and 300 coming out add another and you now have300 incoming and 400 coming out of that one.

with each additional one you are raising TDS going to the DI, as actual TDS output of clean water is the BEST out of the first membrane.

I have a BRS system with a dual membrane system which provides more product water and less waste water. My TDS incoming city water is about 275 range. I do use 2 sediment filter with progressively smaller micron #2= 0.5 micron. Two carbon filters. My water has cholormines. Post membrane TDS=7 after running for a couple of minutes high on startup is 12-15 TDS.
Two DI canisters. Product water registers zero TDS. Backup TDS meter also shows zero. DI lasts about 9 months. Use color changing DI media. Change the first filter when it registers slightly over 1/2 color change. Rotate second DI canister to the front a place new DI in 2nd canister. I produce a lot of water as the RO side also provides drinking water. Do 15% water change twice a month in my 120 and same for a 29 gallon quarantine tank. Membranes are changed when I see post membrane TDS creeping up. The last 1.5- 2 years.

System is stable and I am getting great service. When I had a single membrane, I provided post membrane waste water around 3-5. So the dual system did increase TDS slightly but I cannot see any effect or higher DI use.

So, at least in my case, I do not think your post is valid. BTW, I live is an area with higher alkalinity [hardness].

RJ

 

[rjallen]

After every membrane the waste water tds will increase. Not the produced ro water.

Yes...+++1

RJ

 

[alexkharden]

Thank you rjallen, that was very helpful info!

 

[outy]

. I don't know what the tds in my house is yet, as I haven't hooked up my system.

That is key right there.

its all a balancing act.


75gpd has the cleanest output over a 100gpd membrane.

So even with decent water a 100gpd membrane will generally use more DI resin


If your over 150-200 consider running parallel instead of series.


I did it to cut down on water usage.

 

[outy]

I have a BRS system with a dual membrane system which provides more product water and less waste water. My TDS incoming city water is about 275 range. I do use 2 sediment filter with progressively smaller micron #2= 0.5 micron. Two carbon filters. My water has cholormines. Post membrane TDS=7 after running for a couple of minutes high on startup is 12-15 TDS.
Two DI canisters. Product water registers zero TDS. Backup TDS meter also shows zero. DI lasts about 9 months. Use color changing DI media. Change the first filter when it registers slightly over 1/2 color change. Rotate second DI canister to the front a place new DI in 2nd canister. I produce a lot of water as the RO side also provides drinking water. Do 15% water change twice a month in my 120 and same for a 29 gallon quarantine tank. Membranes are changed when I see post membrane TDS creeping up. The last 1.5- 2 years.

System is stable and I am getting great service. When I had a single membrane, I provided post membrane waste water around 3-5. So the dual system did increase TDS slightly but I cannot see any effect or higher DI use.

So, at least in my case, I do not think your post is valid. BTW, I live is an area with higher alkalinity [hardness].

RJ

I'm the opposite I have soft water and 17 TDS incoming. I have never had in 25 years, DI resin change color, changed it 3 times. I have 0 TDS out of the membrane before DI even with dual membranes.

But there is a trade off, every water change no matter how old filters and media is, I get GHA every single water change.


Maybe I have cholormines, never checked.

 

[DDon]

After every membrane the waste water tds will increase. Not the produced ro water.

Both waste and product water TDS will increase with each additional RO stage you add.

Let's assume you have 200 TDS on your incoming water, your RO membranes have a 98% rejection rate and your waste to product water ratio is 3:1
Out of the first RO membrane your product water TDS will be 4. You now have 196 parts left in the waste water but in 25% less volume of water. So the waste water which is now feeding your second RO membrane has a TDS of 245 (196 x 1.25). With the same 98% rejection rate your product water from the second stage RO would be 4.9
Hope that makes sense.

 

[rjallen]

I'm the opposite I have soft water and 17 TDS incoming. I have never had in 25 years, DI resin change color, changed it 3 times. I have 0 TDS out of the membrane before DI even with dual membranes.

But there is a trade off, every water change no matter how old filters and media is, I get GHA every single water change.


Maybe I have cholormines, never checked.

You can get a cheap test kit from BRS and probably other online dealers. Or, your LFS may be able to test your water for Cholormine. I doubt that would affect algae before causing other problems.

You might have a source of phosphate in your water supply. That is common in rural communities where agricultural practices can pollute the ground water and streams because of run-off. Our water has that problem. A RODI system will lower this but some can still get through. Your live rock can give off phosphate as well as an old sand bed.

Has anyone ever used lanthanum cloride i mix water before mixing with salt?

RJ

 

[outy]

Both waste and product water TDS will increase with each additional RO stage you add.

Let's assume you have 200 TDS on your incoming water, your RO membranes have a 98% rejection rate and your waste to product water ratio is 3:1
Out of the first RO membrane your product water TDS will be 4. You now have 196 parts left in the waste water but in 25% less volume of water. So the waste water which is now feeding your second RO membrane has a TDS of 245 (196 x 1.25). With the same 98% rejection rate your product water from the second stage RO would be 4.9
Hope that makes sense.

Thank you.

That is how I should have explained it

 

[rjallen]

Both waste and product water TDS will increase with each additional RO stage you add.

Let's assume you have 200 TDS on your incoming water, your RO membranes have a 98% rejection rate and your waste to product water ratio is 3:1
Out of the first RO membrane your product water TDS will be 4. You now have 196 parts left in the waste water but in 25% less volume of water. So the waste water which is now feeding your second RO membrane has a TDS of 245 (196 x 1.25). With the same 98% rejection rate your product water from the second stage RO would be 4.9
Hope that makes sense.

Yes but most of the increase will be on the waste side of second membrane. Actual increase on product side should not be enough to worry about under average water supply conditions.

Another thing.... I have seen the term parallel and series used as to how the membranes are plumbed. At on my system it is neither. It is a combination. Water enters the 1st membrane. It waste water goes to the 2nd. Clean product water goes directly to DI. The second membrane re-processes the waste water from the first. Waste water with more concentrated TDS exits system as waste. Clean water from 2nd membrane [now with higher TDS] exits and joins the clean water from the first membrane. [combination of the two flows gives the water a TDS higher than product water from membrane #1 and lower than product water of #2 membrane.] Product water from two 75 gpd membranes= equal 150 gpd. Because you "recycle" the waste water from the 1st membrane you use far less total to produce 150 gallons of water.
I know most of you are familiar but some of us may be a bit confused.

RJ

 

[outy]

You might have a source of phosphate in your water supply.
RJ

Ive tested it bud and it showed zero. Maybe I need better then salifert

Even suspected my Rubbermaid sump as leeching, and tested that and my Rubbermaid green garbage cans. Both had water in it for a week.

Came out zero

If chloramine does not produce algae, then I'm stumped.


This has been a battle for 25 years. I did water changes once a year because of this. Works out OK but made SPS long term a bad idea. 3 years is all I can do with SPS. LPS thrives and I sell it I grow so much

 

[outy]

. on my system it is neither. It is a combination.

RJ

That is generally what we call series as the waste goes to the next membrane.

Parallel would be clean water going to each membrane.

 

[rjallen]

Ive tested it bud and it showed zero. Maybe I need better then salifert

Even suspected my Rubbermaid sump as leeching, and tested that and my Rubbermaid green garbage cans. Both had water in it for a week.

Came out zero

If chloramine does not produce algae, then I'm stumped.


This has been a battle for 25 years. I did water changes once a year because of this. Works out OK but made SPS long term a bad idea. 3 years is all I can do with SPS. LPS thrives and I sell it I grow so much

Wow, cholomines just kill stuff because of the free amonia. It could act as a fertilizer but things should be very sick cause of the new cycle. If your rocks are 25 years old, I assume they have finished leaching anything a long time ago. What does you phosphate run...what test kit do you use?

RJ

 

[DDon]

Yes but most of the increase will be on the waste side of second membrane. Actual increase on product side should not be enough to worry about under average water supply conditions.

Another thing.... I have seen the term parallel and series used as to how the membranes are plumbed. At on my system it is neither. It is a combination. Water enters the 1st membrane. It waste water goes to the 2nd. Clean product water goes directly to DI. The second membrane re-processes the waste water from the first. Waste water with more concentrated TDS exits system as waste. Clean water from 2nd membrane [now with higher TDS] exits and joins the clean water from the first membrane. [combination of the two flows gives the water a TDS higher than product water from membrane #1 and lower than product water of #2 membrane.] Product water from two 75 gpd membranes= equal 150 gpd. Because you "recycle" the waste water from the 1st membrane you use far less total to produce 150 gallons of water.
I know most of you are familiar but some of us may be a bit confused.

RJ

I agree that the increase in product water is small and the savings in water outweighs any increase in DI resin use. I was just giving an example that there is an actual increase in product water TDS, albeit small.

Your system would be considered run in series as that is how the RO membranes are connected, regardless of the joining of the product water streams. Parallel would be splitting the incoming feed to both RO membranes.

 

[outy]

. What does you phosphate run...what test kit do you use?

RJ

Salifert always reads 0

But I quit testing regularly as I can generally look at algae/film on glass and any GHA as a guide and make decent guesses.


Running chaeto now so I know it wont show any. But when I do a 100g water change on my 300g system I will get a slight increase in GHA

 

[Syntax1325]

Ok, since this thread has gone in a few directions please excuse this brief interjected question: I have dual membranes and in line tds meters that say that my water coming out of both membranes (have a tds meter on each output of each membrane) is 0 tds. My question is: How long will my DI last filtering 0 tds water? I make approximately 40 gallons every two weeks.

 

[albano]

Ok, since this thread has gone in a few directions please excuse this brief interjected question: I have dual membranes and in line tds meters that say that my water coming out of both membranes (have a tds meter on each output of each membrane) is 0 tds. My question is: How long will my DI last filtering 0 tds water? I make approximately 40 gallons every two weeks.

I doubt that anyone can answer that, especially without knowing TDS of input water...
Are you saying that you have 0 TDS before/without the DI?

 

[Syntax1325]

I doubt that anyone can answer that, especially without knowing TDS of input water...
Are you saying that you have 0 TDS before/without the DI?

Yes, my tds is 0 after the membranes and before the DI resin.

 

[albano]

Yes, my tds is 0 after the membranes and before the DI resin.

Sounds like the DI will last a long, long time! Are you sure that the TDS meters are accurate? Do you measure input water TDS?

 

[Syntax1325]

Sounds like the DI will last a long, long time! Are you sure that the TDS meters are accurate? Do you measure input water TDS?

Nope, not sure; however, is anyone here sure that their TDS's meters are 100% accurate?

I don't have a tds meter on my input, but when I flush my membranes my tds rises to about 20 and declines to 0 over about 2 minutes so I'm fairly confident that my water is 0. I also have a meter after the DI but that is inconsequential to this question.

 

[rjallen]

Yes, my tds is 0 after the membranes and before the DI resin.

Everyone's system is different. I guess you don't know the TDS or your city water. I suggest you get another TDS gauge and check it. Unless you have very, very clean water, something is wrong. A RO membrane will NOT, under normal conditions, reduce your TDS to zero. The standard of a good system is the membrane plus good sediment filters and good carbon cartridges is 90% of the starting TDS. Some do a little better, some systems do a little worse.

My city water runs around 275 TDS. With double sediment filters, double carbon filters and 75 gpd membranes, I get around 7 TDS. According to the standard rule as above it should be around 27-28 TDS. I don't know if all the extra filters really lower it that much more. And yes I have double checked with a portable TDS gauge. So, as the above poster said, no one can tell you what your system should do without knowing far more and then it is somewhat of a guess.

RJ