Question regarding ro/di unit???

[dipan]

There are other things you can do to minimize waste, if anyone is interested. My brine : permeate ratio is 2:1. Using a booster pump to ensure 80-90 PSI at the membrane (after the prefilter stages) and using an auto flush valve on the waste line with a tighter restrictor (built into the auto flush valve) are two things that you can do to increase efficiency.

[BuckeyeHydro]

In my opinion dual 75 is the way to go! Fast, efficient and much less waste water. Love it

First - remember that what folks call "waste water" really would be better thought of as "flush water" in that this water serves the important purpose of internally flushing the surface of the semipermeable membrane to keep the membrane from fouling/scaling.

When you configure a system with two membranes in series (the waste from the first membrane going to the "in" port on the second membrane), for this discussion let's say it's two 75 gpd membranes, the system behaves like you have a single long (75 gpd x 2) 150 gpd membrane.

Now - if you use a proper flow restrictor, that is, one for a 150 gpd membrane, you'll have about a 4:1 waste to product ratio. Sounds familiar, right?

If however you don't change the flow restrictor - meaning you keep using the same restrictor you were using when you just had one 75 gpd membrane, then you'll see a waste to product ratio much lower than 4:1. But remember that the recommendation for a ~4:1 ratio comes from the membrane manufacturer. They are telling you that you need about a 4:1 ratio to keep the membrane flushed and keep the membrane from fouling or building up scale. Run the system with a lower ratio and you will foul/scale the membrane(s) quicker than would have otherwise been the case.

Instead of adding a second membrane to lower that ratio, you could have just changed out your flow restrictor ($4) instead. A much less expensive approach to get you to the same endpoint in terms of saving on waste water.

Now, to confuse things just a bit. Filmtec specs call for the 4 to 1 ratio on the basis of assumptions about the water that will be supplied to the membrane. If you have very soft water you MAY be able to get a decent service life from the membrane running at a ratio lower than 4 to 1 (e.g., 3 to 1). Remember that the waste water from the first membrane is about 25% harder than your tap water.

Bottom line: If what you are after is reduced waste water, experiment with a different flow restrictor for $4 instead of messing around with a second membrane plumbed in series.

As a side note, you can also lower the ratio by increasing the pressure delivered to the membrane (with a booster pump), because flow restrictors are sized assuming you are providing factory spec conditions (50 psi and 77 degrees for Filmtec membranes). Increase the pressure and you'll drive more water through the membrane and viola - less waste water. But as I mentioned above, if you do this (just like over-restricting a membrane) - the lower the waste to product ratio, the shorter the lifespan on the membrane.

Makes sense?

Russ

[Big_Pun]

thanks Russ very good points

[justahobby]

thanks Russ very good points

X2, thanks for taking the time to explain it so thoroughly

[Europhyllia]

Bottom line: If what you are after is reduced waste water, experiment with a different flow restrictor for $4 instead of messing around with a second membrane plumbed in series.

Maybe I am missing something (wouldn't be the first time lol).
The way I read this is: on the cheap you can reduce waste water by using a different flow restrictor but you run the chance of ruining your membrane faster (if I get this correct.The savings upfront by just using a different flow restrictor could be negated later on when you need a new membrane sooner) Am I misunderstanding this?

Plus it still takes forever to make water (I have a 215 and a 60 gallon to make water for so I use a lot and don't want to wait days to get it)

Here's where I am coming from:
I want a lot of water and I want it fast. I don't want to replace DI all the time. I want to reduce waste water. I don't want to have to replace the membranes prematurely.
In my case I found the dual-75 to perfectly fit my needs
(By the way I am also using the things dipan described -booster pump, flush valve, etc.)

That being said I agree that it's nice there are different approaches for different budgets and requirements and something else may fit somebody else's needs better

[BuckeyeHydro]

bottom line - any time you over restrict a membrane, you risk shortening its life span.

Plumbing two membranes in series, and over restricting them gives you the same situation.

If you want water faster, we typically prefer to plumb the membranes in parallel, with a flow restrictor on each. This way we can adjust/control the ratio as needed on each membrane. If we plumb membranes in series we are careful to install a flow restrictor that yields the correct waste to purified water ratio.

Russ

[dipan]

Karin, are you using one of these auto flush valves, or just a regular manual flush valve? Is your booster on when you flush the membrane? My understanding is that the auto flush valve when used with a booster pump and tighter restrictor can help to mitigate membrane fouling while increasing efficiency by "power flushing" the membrane automatically at full boosted water pressure both on startup and every hour of operation (flushing is for 15 seconds).

http://aquatec.com/auto-flow-restricters.html

[350gt]

I HAVE THE 75 ALSO AND MAKE 5 GALLONS IN ABOUT 45MIN

just thinking how bad that water is in that neighborhood.....