does 2 membranes actually save water?

johnfallon135 · 01/20/2016, 03:27 PM

i can easily see why it would as it's using the rejected waste water for the next membrane however i was told by my lfs that this is actually a false myth can anyone confirm this?

vhuang168 · 01/20/2016, 03:30 PM

Did they tell you why?

Mrramsey · 01/20/2016, 03:39 PM

Without getting too technical with it it is true and false. I have one but the trade off as I understand it essentially you get higher TDS out of the membrane into the DI. I am actually converting my BRS 5 Stage "Water Saver" back to a single membrane from Spectrapure. To get my TDS lower out of the membranes currently I had to get it set back to a 4:1 ratio anyway. It was nice for the initial fill but I have no need anymore for a 150GPD capacity. Just my $0.02.

dartier · 01/20/2016, 05:37 PM

Like most things in life, I think it depends. From what I remember when Iooked into doing this, the trade off was that you needed a lot more pressure to make it work effectively, which usually requires the addition of a boost pump.

The main advantage of these arrangements is that you are using water that has already been through your pre-filter and carbon stages.

In the end I decided the added complexity was not worth it and did not add the second membrane.

Dennis

outy · 01/20/2016, 05:42 PM

Quote:

Originally Posted by johnfallon135

i can easily see why it would as it's using the rejected waste water for the next membrane however i was told by my lfs that this is actually a false myth can anyone confirm this?

Depends on your incoming TDS

I would not run two if I was over 200 TDS

I have 32 TDS so I run two and could probably do 3, I have 0 TDS before DI.

I also use a 50gpd restrictor instead of the 75-100 I originally was using.

So mine is an example that saves water.

Mrramsey · 01/20/2016, 05:52 PM

Quote:

Originally Posted by outy

Depends on your incoming TDS

I would not run two if I was over 200 TDS

I have 32 TDS so I run two and could probably do 3, I have 0 TDS before DI.

I also use a 50gpd restrictor instead of the 75-100 I originally was using.

So mine is an example that saves water.

By using a 50gpd restrictor you are increasing the waste water ratio therefore you have a higher wastewater to production water value correct.? You may be generating less good water but I would think that you have a higher waste ratio...

DDon · 01/20/2016, 11:55 PM

Assuming the membranes are connected in series, you will use less supply water to make a given volume of ro with 2 membranes than with 1. Though as has been mentioned, you will have a higher total tds and will use more di resin. If your supply water tds is reasonable the net increase should be relatively minor and cost may be offset by the increased life of the sediment and carbon filters (on a per unit of product water basis).
Now if the membranes are connected in parallel then the lfs is correct and there is no savings.

Here is an example:
Assume your system has a 3:1 waste to product water ratio. If you put 100 gallons of supply water through your single membrane, you get 25 gallons of product and 75 gallons of waste. With a second membrane in series, that 75 gallons of waste will give you another 18.75 gallons of product for a total of 43.75 gallons from 100 gallons of supply. With a single membrane you would have the original 25 gallons.

01/20/2016, 03:27 PM	#1
johnfallon135 Registered Member Join Date: Oct 2015 Posts: 442	does 2 membranes actually save water? i can easily see why it would as it's using the rejected waste water for the next membrane however i was told by my lfs that this is actually a false myth can anyone confirm this?

01/20/2016, 03:30 PM	#2
vhuang168 Registered Member Join Date: Feb 2015 Location: San Jose, CA Posts: 2,136	Did they tell you why? __________________ Vincent 40g custom AIO --> DSA190Pro

01/20/2016, 03:39 PM	#3
Mrramsey NEO Reefer Join Date: Jan 2014 Location: Hudson Ohio Posts: 1,801	Without getting too technical with it it is true and false. I have one but the trade off as I understand it essentially you get higher TDS out of the membrane into the DI. I am actually converting my BRS 5 Stage "Water Saver" back to a single membrane from Spectrapure. To get my TDS lower out of the membranes currently I had to get it set back to a 4:1 ratio anyway. It was nice for the initial fill but I have no need anymore for a 150GPD capacity. Just my $0.02. __________________ Mike My 120 Build: http://www.reefcentral.com/forums/showthread.php?t=2389417 Current Tank Info: 120g In-Wall \| BA Overflow \| 55g Sump \| SWC Extreme 150 Skimmer \| DIY ATO \| 2 Jebao RW-8\| Fluval SP6 \| Photon 48v2 LED \| GFO and Carbon

01/20/2016, 05:37 PM	#4
dartier Registered Member Join Date: Aug 2012 Location: Brampton, ON, Canada Posts: 958	Like most things in life, I think it depends. From what I remember when Iooked into doing this, the trade off was that you needed a lot more pressure to make it work effectively, which usually requires the addition of a boost pump. The main advantage of these arrangements is that you are using water that has already been through your pre-filter and carbon stages. In the end I decided the added complexity was not worth it and did not add the second membrane. Dennis __________________ 560G Miracles tank in process making a DIY DyMiCo style filter (for 560G) Current Tank Info: 560G Miracles tank in progress, 80Frag Temporary

01/20/2016, 11:55 PM	#7
DDon Registered Member Join Date: Feb 2014 Location: Brentwood,CA Posts: 776	Assuming the membranes are connected in series, you will use less supply water to make a given volume of ro with 2 membranes than with 1. Though as has been mentioned, you will have a higher total tds and will use more di resin. If your supply water tds is reasonable the net increase should be relatively minor and cost may be offset by the increased life of the sediment and carbon filters (on a per unit of product water basis). Now if the membranes are connected in parallel then the lfs is correct and there is no savings. Here is an example: Assume your system has a 3:1 waste to product water ratio. If you put 100 gallons of supply water through your single membrane, you get 25 gallons of product and 75 gallons of waste. With a second membrane in series, that 75 gallons of waste will give you another 18.75 gallons of product for a total of 43.75 gallons from 100 gallons of supply. With a single membrane you would have the original 25 gallons.