RODI

[Mattiejay6]

So is it true with the 75 and 100gpd RODI units that for every 1 gallon of water you get 4 gallons of waste?

Is there a way to cut that down so there isn't soo much waste?

 

 

And the 150 waste way less?

Edited July 30, 2013 by Mattiejay6

[YHSublime]

Sounds about right. I don't know getting around it, some folks use their waste for laundry, watering plants, etc. just have to plan accordingly to save it instead of drain it.

 

[holedup82]

I have a 75gpd and let the waste go down the drain, if you are able I know a ton of people save it to use for watering the lawn. I tried the whole using it for laundry thing but didn't a really plumb the waste line to the washer. I just put the line in the tub and flipped it on when full. Unfortunately it drained what I filled and re-filled anyways.

[Djplus1]

Get the BRS 150 water saver. It's better than 4 to 1 and makes your water quicker.

[Origami]

There's a dual membrane setup that some people use. Bulk Reef Supply sells a kit and produces a video that shows you how it's "wired." It takes advantage of higher pressure available in some homes and basically piggybacks a second membrane on the waste side of the first membrane.

[holedup82]

I heard 150gpd units give off less waste. I honestly have never measured the waste to pure ratio on my unit.

[Origami]

I heard 150gpd units give off less waste. I honestly have never measured the waste to pure ratio on my unit.

There are two 150 gpd approaches. One uses a single 150 gpd membrane but it's efficiency ratio is about the same, I think. The other way is to use two membranes. Waste is reportedly less in that case but pressure requirements are higher.

[holedup82]

There are two 150 gpd approaches. One uses a single 150 gpd membrane but it's efficiency ratio is about the same, I think. The other way is to use two membranes. Waste is reportedly less in that case but pressure requirements are higher.

My pressure is right at 50 psi as long as the gf isn't taking a shower. I had to build my unit basically from scratch( long story). I'm curious at the pressure loss after each stage now.

[Origami]

I don't have a dual membrane setup, but saw a thread or threads about it a few years back on RC. BRS also produced

showing how to make the conversion using their kit.

 

[holedup82]

The RC threads helped me build my current unit, and I've definitely seen that video on BRS. I'm a huge fan of BRS videos while I'm stuck at my desk instead of out teaching

[Mattiejay6]

Very interesting... I wonder if I can mod that on the AWI typhoon extreme. The waste water just seems very concerning. I feel 4 to 1 is a lot of wasted water and really throws a water bill up there. .

[Buckeye Field Supply]

Let's back up a little.

 

Regardless if we're talking about a 24 gpd, 50, 75, 100, or 150 gpd residential membrane, the manufacturers suggest something near a 4:1 ratio to help assure a reasonable life span on the membrane.

 

That ratio is set by a small $4 piece called a flow restrictor. If you'd like, you can install a different restrictor and get a different ratio. Will take you all of about 2 minutes.

 

We feel it's misleading to tell people they can cut down on waste water by adding a second membrane plumbed in series. Here's why.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, and have something near the factory spec water pressure and temperature, 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 had just 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.  This is 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

[Mattiejay6]

Thanks Russ! That makes perfect sense... I appreciate you taking the time to explain that to me.

 

I guess I need to sit down and think which is cheaper... Buying more membranes or the water bill..

 

I think it might be best to operate the membranes at what's recommended and see if I can do something else with the waste water.

Edited July 30, 2013 by Mattiejay6

[Origami]

Thanks, Russ! So, boiling this down, you're saying that the two membrane setup that we've read about for years is a bit of economic hocus-pocus? That, you either maintain the same 4:1 waste ratio (in which case you may as well go with a single 150 gpd membrane and an appropriate flow restrictor) or faster fouling on the second membrane?

 

Let me ask you this as a hypothetical: Let's assume that we process 100 gallons of 100 ppm water through a single membrane with a 98% rejection ratio and a 4:1 waste:pure output. Through this system, I should get 20 gallons of 2 ppm "pure" water out and 80 gallons of 124.5 ppm "waste" out. Is this correct?

 

I don't know if the way these other systems are configured if the 4:1 ratio is maintained on the second membrane or not - I've never used one but have just beefed up a single membrane to a 150 gpd membrane. However, let's say that the 4:1 ratio is maintained in the second membrane and I now feed the 80 gallons of 124.5 ppm waste water into a membrane similar to the first. Assuming the same efficiencies, it seems that we would get another 16 gallons of 2.5 ppm "pure" out and 64 gallons of 155 ppm waste out. My total output is then 36 gallons of 2.2 ppm "pure" fed to the DI stage and 64 gallons of 155 ppm waste out. I'm just applying a little simple math to the analysis here.

 

In this second example, it seems that the manufacturer recommended 4:1 ratio is maintained and waste water output is reduced at the expense of 1) slightly increased TDS fed to the DI (which exhausts DI resin proportionally quicker), 2) increased TDS being fed into the second membrane, and 3) the added cost of maintaining two membranes (vice 1) with some reduction in waste water costs. Whether there is a cost benefit would remain to be determined.

 

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