We've made many posts on this issue of plumbing membranes in series to "save water."
Say you have hard water and use a 75 gpd membrane with a 75 gpd restrictor and your system runs @ factory spec pressure and temperature. All makes sense. You’d have something around a 4:1 ratio of concentrate to permeate and get a reasonable lifespan from the membrane.
Now say a vendor sold you a kit that has you plumb a second 75 gpd membrane in series (so now, functionally, you have a single long 150 gpd worth of membrane), and they have you continue to use the 75 gpd flow restrictor you’ve always used. Hmmm. A 150 gpd membrane with a 75 gpd restrictor… will you have a lower concentrate to permeate ratio? Sure. Will you get a reasonable life span from the membranes? Nope.
The concept of running membranes in series is applied commonly/routinely in commercial RO systems. How do they do it and still get a good membrane lifespan? If you read the specifications regarding required feedwater quality, you’ll see this:
Maximum Feedwater Hardness: 0 grains per gallon.
This is why commercial RO systems
have a softener before them.
If you have a functioning softener that is set up for 0 gpg hardness (no bleed through), or if you have naturally soft water, then YES you can get away with a 1:1 or 2:1 ratio of concentrate to permeate.
You don't need to plumb in a second membrane if all you're looking to do is reduce concentrate flow by 50%. Simply buy a tighter flow restrictor for $4. The end result will be the same in terms of membrane life, but you'll have some extra $ in your pocket.
So… what’s a guy with hard water (most of us) to do if he wants:
-Reasonable membrane life
-Less concentrate (less overall water use)
-Faster production
but he doesn’t want to spend the money to install a water softener to treat all the water coming into his house?
Yes- there are solutions. Give us a call if you're in the
market.
Russ
