High Efficiency RO/DI Systems

[mike31154]

Check your home's plumbing for a Pressure Reducing Valve (PRV). Most homes not on a well, that is on city water, will have one to protect the house plumbing from over pressure. These valves are generally set to keep the pressure in your house from exceeding 60 psi. I have a Watts series 25AUB and those are the specs. It is adjustable however and I've tweaked mine to give me just above 80 psi. I also have two outdoor faucets plumbed in before the PRV so I get full city water pressure for the garden hose. I've measured that recently at about 110 psi. This drops off a bit with a hose running full blast.

In any case, I've found the 80 plus psi works well with my 75 gpd system particularly in the winter months when the input water temperature is down to 8 Celsius. I consider this tweak my 'free' booster pump. It keeps production at a good pace, but I haven't seen much difference in good to waste ratio, although to be honest, I've never taken the time to accurately measure my good to waste ratio, I'm pretty much guessing. I collect all the waste water in a couple of 55 gal plastic drums and use it creatively around the home & garden. I've pretty much given up on any of the magic bullet 1 to 1 solutions and simply don't consider the waste water as such, since I use it for other purposes.

As far as the membrane & wear etc., my system has been running for just over two years on the original membrane. The only thing I've changed so far is pre filters. Just finished a run of RODI water this evening and at the end I recorded 86 psi input @ 8 degrees C. My second pressure guage before the membrane showed 63 psi, so I'm losing 23 psi through the 3 pre filter stages. Probably time to change the 5 micron poly filter. When it's new, the pressure loss is around 20 psi. So it's safe to say that my membrane is working at close to 10 psi above the pressure (50 psi) that most manufacturers use when rating their membranes. Here's a little tidbit of info I saved to a text file on my hard drive, don't recall where I got it, but I'm pretty sure it's good data.

"Membranes are typically rated at 77 degrees and 50 psi.
For every pound of pressure lower than 50(psi) subtract 2% of the output. For every degree(F) below 77, subtract 1.4%.
Example: Let's take 40 psi water at the 77 degree temperature with a 50 GPD membrane.
50 GPD - 20% (40 psi) = 40 GPD.
Now let's take the 50 psi, but reduce the temperature to about 40 degrees:
50 GPD - 51.8% (since it's 37 degrees colder x 1.4) = about 26 GPD"

With the substantially colder water than manufacturer's rating for nominal production, I'm thinking my extra 10 psi helps a little in keeping the flow going somewhat better than if I was using the standard 50 psi house pressure available by default through a PRV. On the positive side, the colder water allows my 75 gpd membrane to get the input TDS of around 210, down to 0 after the membrane, so my DI media is hardly working at all and I haven't touched that either. If I recall correctly, the warmest input water I get is around 15 degrees C and during that time, the best RO reading I get is 1. I used to run a long length of input tubing through a bucket of water with a heater in it to get the temp up, but gave up on that idea some time ago, since I saw no significant benefit in production.

Sorry, long rant, but bottom line is, I choose to save the waste water for other uses rather than pursuing a 1 to 1 ratio using dual membranes, booster pumps etc. The quote you provided from BFS regarding the flow restrictor sounds interesting though, I might give that a go. Change the flow restrictor and maybe try hooking the input directly to the 110 psi city water pressure... Should probably confirm that all the components can handle the extra pressure though, lest I blow the whole system up!