Have 2 RO/DI units, can I run Waste water from one to the other?

[raynist]

I have 2 ro/di units and was wondering if I can take the waste water from one and run it to the other unit to make more water with less waste?

The first unit is a 4 stage unit with a prefilter/carbon block/RO/DI. The membrane is a 150gpd filmtec.

The second unit is a 5 stage with a 75gpd filmtec membrane. If this could be done, I would think I would not need the prefilter or the carbon block since this is already done on the first unit.

My water pressure is sky high if that matters at all.

Let me know your thoughts.

Thanks
Ray

[bnmir]

It seems to bme that the waste from unit 1 would have considerably higher tds than your raw input water, so the reject rate from unit 2 is likely to be extremely high.
The added backpressure on the output of unit 1 (caused by unit 2) will have a detrimental effect on the output of the 1st unit.
You might be able to overcome this with a permeate pump, though.

[a2fire2i]

Do you know what your starting TDS is?

[raynist]

I will have to check but I think my incoming TDS is around 170 and the outgoing (waste) is around 220 or so.

Having 2 different membranes may be a hurdle.

[frank2926]

You could feed both membranes with the water after sediment and carbon, if your pressure is good enough. Tee off the incoming to membrane one and feed the second membrane then tee them back together before the flow restrictor. You may need a different size flow restrictor or an adjustable one at least.

[Roland Jacques]

Yes, you can. you will end up with less waste water .

Ive been doing it over a year with good results. whats your starting TDS from your tap?

[AZDesertRat]

The answer is yes you can, its done all the time. But, you will not end up with less waste and get the same life out of the RO membranes. This has been tried over and over by the large manufacturers and better vendors and it only works for a time before the membranes fail due to insufficient flushing action. You must waste about 4:1 or they will fail sooner than later guaranteed.
My system uses a 150 GPD RO membrane and a 90 GPD RO membrane for a total of 240 GPD when water temp and pressure are right. In the summer months I can actually push more like 270 GPD out of it at 99.23% rejection rate. I kep the waste ratio at the recommended 4:1 and expect to get 5 to 7 years or more out of the membranes. If I reduced the waste ratio it has been proven in my area of the country I would get less than 18 months out of the membranes.

The only long term proven system that has sucessfully reduced the waste to 1:1 or even a little less than that and has probably in excess of 2000 hours on the membranes is a Spectrapure Ultra High Efficiency unit. It is a very unique system and you pay for the research that has gone into it but it really works.

Rolands situation is very unique in that he has almost pristine water to begin with, and I HATE him for that . In most places it would never work for very long.

[rpeeples]

Originally posted by Roland Jacques
Yes, you can. you will end up with less waste water .

Ive been doing it over a year with good results. whats your starting TDS from your tap?

Impressive setup......well done.

[Icefire]

AZDesert, you have sky high TDS so it doesn't count.

You also waste way less even by keeping the 4:1 ratio as if you wire the waste from the 150 to the input of the 75, you will have 4:1 waste from each (with proper restrictor) but you will have a final waste ratio of ~2:1(54 good, 96 waste for 150G entered in the 1st membrane)

[Roland Jacques]

In the South GA, AL,.. we have a pretty bad drought. I believe big water makers should conceder this multi series RO water making option. For me it has paid for itself in water savings cost the first year. I use a lot of water. If I had to replace all my membranes it would have taken longer to recoup. (for now my membranes are still the original and are working good)

PROS
1. 80% Less water use

2. less prefilter changes = 80% less cost on pre filters and 80% less time changing cartage

3. less load on my septic system (not an issue for most but big issue for me)

CONS
1. You'll have to change membranes more often
(this is where me and D Rat disagree, I believe you'll loose about 30% of membrane life. D rat indicates (above) about 400% shorter life)

2. Higher up front cost
(and the Each membrane should have it's own bypass valve, for maintenance flushing, periodic fast flush of membranes)





Right now my RO has made over 18,000 gallons of water. ( for some/most that may equal 10-15 years of water for thier reef.

I'm pretty sure each one of my membrane is in fact getting a 4:1 flushing. I believe I've confirmed this in a couple of ways.

I believe my first membrane will have the same life as it would if it where the only membrane. The second " should have 20% less life than the first. And the third 20% less than the second.

the first last 5 years
the second should last 4 years
the third should last 3.2 years

My Tap TDS is in fact VERY LOW. About 50 TDS. D Rats Tap TDS is about 800-900 TDS. They say you can stucco a house with just a garden hose and Phoenix water.

[Icefire]

The thing about 20% less life is not real, you would need to have a membrane that would have a life of XXX Gallons @ XXX ppm.

Membrane don't work like that, it depend on way too much factor.

Using your logic that would say a guy with TDS of 50 would get 5 years while someone with 200 would get 1.25 years? that's just stupid.

Most membrane will last over 5-7 years if your TDS are below 500 if I remember correctly. If you have a higher TDS, like AZDesertRat, I would use a 50% rejection membrane to reject most of the salt and have better life from the 99% membrane. I think they sell low ratio membrane for that.

[Roland Jacques]

Originally posted by Icefire
The thing about 20% less life is not real, you would need to have a membrane that would have a life of XXX Gallons @ XXX ppm.

Membrane don't work like that, it depend on way too much factor.

Using your logic that would say a guy with TDS of 50 would get 5 years while someone with 200 would get 1.25 years? that's just stupid.

.

I agree with you on this part. I was trying to give a worse case scenario, and keep it simple. I believe it would be much less than 20% also.

How would you estimate the life of the membranes running in series? 1St, 2nd, 3rd,

I do no one guy who has been set up like this 2 membranes for 3-4 years with the same membrane last I checked.

[Roland Jacques]

Originally posted by Icefire


Most membrane will last over 5-7 years if your TDS are below 500 if I remember correctly. If you have a higher TDS, like AZDesertRat, I would use a 50% rejection membrane to reject most of the salt and have better life from the 99% membrane. I think they sell low ratio membrane for that.

This i do not agree with. this would not work in series.

I assume you are suggesting taking the "good water" from the first 50% membrane and run it as the tap/source water to the 99% membrane. I don't think this would work. besides having higher waste water than a straight 4:1. i dont think you would have the pressure needed to run the second membrane.

I think this is a different subject and It is just mudding the water.

[Icefire]

If you use 2 membrane with the correct 4:1 ratio you should get the same life as using a single membrane if you are still in spec with the manufacturer. Localy I have 125ppm TDS, IF I would use 2 membrane, I would have 150ppm at the second, not very high still and I doutb it would change something.

[jdieck]

Originally posted by raynist
I have 2 ro/di units and was wondering if I can take the waste water from one and run it to the other unit to make more water with less waste?

The first unit is a 4 stage unit with a prefilter/carbon block/RO/DI. The membrane is a 150gpd filmtec.

The second unit is a 5 stage with a 75gpd filmtec membrane. If this could be done, I would think I would not need the prefilter or the carbon block since this is already done on the first unit.

My water pressure is sky high if that matters at all.

Let me know your thoughts.

Thanks
Ray

Yes and no

To get any advantage you need to use two membranes of the same capacity.
If you use diferent capacity, then to be able to keep the membranes operating within their specification 4:1 to 5:1 you will need to use one restrictor for each membrane sized for their individual capacity, this means the waste of the first membrane will be at low pressure (after being restricted) Thus you would need to boost pump it higher to opearate as an input to the second membrane.
At the end you will get the same volume and waste as if you operate both systems independently but at the same time.

On the other hand if the membranes are of the same capacity and specifications (both filmtec, both same production rating, both same rejection) then they can share a single restrictor (sized for a single membrane) and the first membrane will not require one (See diagram below), In other words two 75 gpd membranes can share a single restrictor sized for a membrane of 75 gpd.

The advantage of this operation is that:
The life of the membranes is not afected, as a matter of fact the life of the first membrane is improved as it will operate at or close to a 5:1 ratio while the second will operate at the 4:1 ratio.
In this configuration the waste of the first membrane has almost the same input pressure (is only 2 to 4 psi lower than the rated 70 psi inlet)

The diagram below shows aproximate volume and purity balance, in summary for every gallon produced you will waste about half than before (2 gallons instead of four), you will be using 60% of the sediment and carbon filters that you used before because you need to filter less waste but you will be consuming 10% more DI resin per gallon than before as the delivery purity from the combined membranes will degrade by about 10% (In the example below purity goes from 6 ppm to 6.6 ppm)

Note that the cost savings in metered consumption lowering the wasted volume will not pay back for the cost of a second membrane but the reduction in the consumption of sediment and carbon filters will make it a wash over the life of the membrane.

I have been using the shown configuration with 75 gpd filmtec high rejection membranes for 4 years with tap water that fluctuates within 120 to 140 ppm at a pressure of 70 psi at the membrane inlet and I still get more than 99% rejection from both membranes combined and get depending on the temperature between 140 and 180 gpd.

[Roland Jacques]

Originally posted by Roland Jacques

PROS
1. 80% Less water use

2. less prefilter changes = 80% less cost on pre filters and 80% less time changing cartage

Bad math it should be
60% less water
60% less prefilters

jdieck, i'm glad to here you are using the same membrane a was not positive you were

[AZDesertRat]

The waste is still 4:1 at the final point of waste. I make 240 GPD, I waste 240x4 or 960 gallons, its not less. its still 4:1 of the total production. Membranes will not last as long without doing , Dow and Spectrapure don't stay in business by not bench testing things like this for years and years. Short term it will work but eventually it will catch up to you guaranteed.

[jdieck]

Originally posted by AZDesertRat
The waste is still 4:1 at the final point of waste. I make 240 GPD, I waste 240x4 or 960 gallons, its not less. its still 4:1 of the total production. Membranes will not last as long without doing , Dow and Spectrapure don't stay in business by not bench testing things like this for years and years. Short term it will work but eventually it will catch up to you guaranteed.

I do not know how you have your system connected or if you are using individual restrictors for each membrane or if you adjusted a single overall restrictor to give you 4:1 overall but in my system that is not the case.
The overall total waste is not 4:1 it is 4:2 (Look at the mass balance in the diagram above) which is equivalent to 2:1 (Two gallons of waste for each gallon of product for the combined system)
Despite this lower overall ratio the flow used to flush membrane 1 is 5 times its product which is better than what the specification of 4:1 calls for. The flow used to flush membrane 2 is 4 times it's product volume which for membrane two will make it 4:1.

By the way I do not claim to have designed this myself, it is a design I got from Scott's (from Spectra) published diagrams.

I remember you saying back then when I first presented this about two years ago about eventually catching up "guaranteed" and that no one has tested it long term.
Well here we are again but it has not caught up yet after 5 years for membrane one and 4 or so years for membrane two (which I installed after I had the system in operation for about a year).

The thing I really do not understand is were do you get that the flushing ratio of each membrane has been reduced when in reality has been increased for membrane 1. The only difference is that membrane two is getting water with 20% higher ppm but still well within the membrane's specifications specially if the TDS of the tap water is relatively low.

[trmiv]

I've been seriously considering adding a second membrane to my RO/DI unit because of the nasty drought we are having here in NC. Lots of good info in this thread, thanks! The filter guys have a dual membrane conversion kit I've been considering for $77. I already have a booster pump, so it looks like I just need that kit and I'm in business.

[Fishbulb2]

jdieck,
Is it possible to combine the system you use with a permeate pump to further reduce waste water when filling a pressurized storage tank? Or would the dual membranes each require their own pump ect?

[jdieck]

Originally posted by Fishbulb2
jdieck,
Is it possible to combine the system you use with a permeate pump to further reduce waste water when filling a pressurized storage tank? Or would the dual membranes each require their own pump ect?

I do not see why not just note that in the above configuration the pressure out of the waste on the second membrane will be about 10% lower than using a single membrane. There is another possibility that may not save waste or filters but can use a lower rejection membrane for drinking and a higher rejection for and DI for the reef.
Here is such a diagram. Many of the reservoirs and drinking lines will be optional of course but it will give you the idea.
M1 could be a nano filter 100 gpd or a 150 gpd HR membrane and M2 could be a 75 HR membrane.
The main advantage of this system is not filter or waste savings but you get a very fast recovery on the pressurized tanks even when you are producing water for your reef which becomes a real nuisance not to have drinking water when there is production for the reef.
It will also be recommended to use a booster pump if your income water is below 70 psi which most of us suffer from.

[AZDesertRat]

Think about it guys. if it were that easy Spectrapure would be selling tons of dual membrane 1:1 or 2:1 systems at $200 a pop and so would everyone else. Those vendors and manufacturers who have been in business for 25 years like them and others have been down this road and every other one you can dream up. Its not that simple. Thats the reason the Spectrapure UHE is $800 retail. I am sure they would like to cut the price to be more competitive but the engineering and components that go into building the system cost big bucks. Take a close look at all it takes to make it work, its very complex and the only proven system around because of that.

[jdieck]

Originally posted by AZDesertRat
Think about it guys. if it were that easy Spectrapure would be selling tons of dual membrane 1:1 or 2:1 systems at $200 a pop and so would everyone else. Those vendors and manufacturers who have been in business for 25 years like them and others have been down this road and every other one you can dream up. Its not that simple. Thats the reason the Spectrapure UHE is $800 retail. I am sure they would like to cut the price to be more competitive but the engineering and components that go into building the system cost big bucks. Take a close look at all it takes to make it work, its very complex and the only proven system around because of that.

Multiple membrane combinations have been used in industrial systems for years, even spectrapure sells the piggy back system to install the second membrane and filterguys have their supersaver.
The fact that they do not promote it as much does not necessarily means it does not work, it may just mean that they will be making less money

[AZDesertRat]

Sure, they all sell them, but not as a way to save water. They are sold as a way to increase production and thats it. I am working on two large 7.5 and 6 million gallon per day multiple membrane systems as we speak bu they run 15% recovery just like every manufacturer recommemds. Thats more like 6:1 waste and its actually what Dow, GE and Applied recommend.

[Fishbulb2]

Originally posted by jdieck
I do not see why not just note that in the above configuration the pressure out of the waste on the second membrane will be about 10% lower than using a single membrane. There is another possibility that may not save waste or filters but can use a lower rejection membrane for drinking and a higher rejection for and DI for the reef.
Here is such a diagram. Many of the reservoirs and drinking lines will be optional of course but it will give you the idea.
M1 could be a nano filter 100 gpd or a 150 gpd HR membrane and M2 could be a 75 HR membrane.
The main advantage of this system is not filter or waste savings but you get a very fast recovery on the pressurized tanks even when you are producing water for your reef which becomes a real nuisance not to have drinking water when there is production for the reef.
It will also be recommended to use a booster pump if your income water is below 70 psi which most of us suffer from.

Wow, that's an intense diagram. It took me a while to follow the lines and really see what the logic was but now I get it. They are just using two membranes, one specifically to store the pressurized water with the permeate pump, and the other just for the reef with no back pressure. Cool design.