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View Full Version : Addind a second membrane to an existing RO system.


MadBeach79
07/31/2010, 02:04 PM
What are the pros and the cons to adding a second membrane parallel to the existing membrane? How about in series? Waste water from first mebrane feeding second membrane...

Thanks,
Danny

raynist
07/31/2010, 04:14 PM
I don't think parallel will do anything.

Series will cut back on waste water for sure.

Buckeye Hydro
07/31/2010, 05:52 PM
We get this question a lot - I think we need to add this to our FAQ list: http://buckeyefieldsupply.com/FAQ.asp

We feel it is more than a little misleading to tell folks they can cut down on waste water by adding a second membrane. 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, you'll have about a 4:1 waste to product ratio. Sound 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.

Bottomline: 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?

Buckeye Hydro
07/31/2010, 05:53 PM
Adding a second membrane in parallel will produce RO water faster, assuming you have adequate feedwater flow and pressure.

Russ

MadBeach79
07/31/2010, 08:38 PM
Russ, thanks for the in-depth explanation, exactly what I was looking for. So, what I get from it, you don't really recommend the series method. Here is some info into my system as is now:

40 psi Tap Water Feed (Chloramines) > GE Model FXHTC 25 micron granulated activated carbon whole house filter > 5 Micron Sediment Filter > 1 Micron Sediment Filter > Carbon Block Cartridge > Carbon Block Cartridge > Aquatec Booster Pump (boosts pressure to 60 psi) > TDS of 210 ppm > 100gpd TFC RO Membrane > 100gpd TFC RO Membrane > TDS of 9ppm > DI Canister > DI Canister > DI Canister > DI Canister > TDS of 0 PPM

I am trying to produce 200gpd + but I would like a lower tds level before entering my DI media. Any recommendations? I have installed a flush kit and try to flush the membranes about once a week for about 15-20 minutes.

Anything you would do different? My original question was because I was thinking of running the membranes parallel to achieve a higher gpd of product water. It looks like that's the only way to go.


Thanks so much for your time.
Danny

raynist
07/31/2010, 08:52 PM
I guess I am not understanding something. If membrane one is set to make water at a ratio of 4:1 and then the water from here is going to the second membrane making 4:1 isn't the total product output 4:2 or 2:1 which is saving quite a bit of waste water?

Thanks
Ray

Buckeye Hydro
08/01/2010, 04:26 AM
Madbeach - a few comments for you:
1. You have to be careful with a whole-house dechlorination system. It results in a situation where all the water in all the pipes in your house is dechlorinated. Think about that faucet in the faucets that may not get used very often. Or the water sitting in the pipes while you are away for two weeks on vacation. Microbes love dechlorinated water. Your showerheads are likely full of them!

2. Think about switching to higher rejection membranes. Either two 75's or one 150.Your rejection is pretty good now at about 95%, but could be better.

3. Crank up the pressure on your booster pump. You may already have done that.

4. I'd probably drop down to 2 DI stages. Running 4 sure isn't hurting anything, but it is complicating the O&M process.

5. I like that you've preceeded the booster pumps with sediment filters.

6. Especially since you already have a booster pump, I'd probably go with a 150 gpd membrane and an autoflush flow restrictor. The flow restrictor plugs right in to the aquatec wirings harness. This should give you higher rejection, about 200 gpd, and would automate the flushing.

Russ

Buckeye Hydro
08/01/2010, 04:30 AM
I guess I am not understanding something. If membrane one is set to make water at a ratio of 4:1 and then the water from here is going to the second membrane making 4:1 isn't the total product output 4:2 or 2:1 which is saving quite a bit of waste water?

Thanks
Ray

Ray - Adding multiple membranes in series without installing the proper flow restrictor WILL reduce the waste water to product water ratio. But....

Just so I don't repeat myself here, might want to take another read through my long post above.

Russ

MadBeach79
08/02/2010, 12:57 PM
Russ, I appreciate all the advice.

Couple of things:

1. The whole house filter is not actually being used as a whole house filter, I am using it only as a prefilter to the RO/DI system.

2. I will definitely contact you when it comes time to replace my membranes.

3. By cranking the pressure on the booster pump, you are talking about the little screw on the face of the pump?

4. What do you mean by "it is complicating the O&M process"?

5. How much are we talking for the 150gpd membrane and the autoflush flow restrictor?

Thanks again for everything.

Danny


Madbeach - a few comments for you:
1. You have to be careful with a whole-house dechlorination system. It results in a situation where all the water in all the pipes in your house is dechlorinated. Think about that faucet in the faucets that may not get used very often. Or the water sitting in the pipes while you are away for two weeks on vacation. Microbes love dechlorinated water. Your showerheads are likely full of them!

2. Think about switching to higher rejection membranes. Either two 75's or one 150.Your rejection is pretty good now at about 95%, but could be better.

3. Crank up the pressure on your booster pump. You may already have done that.

4. I'd probably drop down to 2 DI stages. Running 4 sure isn't hurting anything, but it is complicating the O&M process.

5. I like that you've preceeded the booster pumps with sediment filters.

6. Especially since you already have a booster pump, I'd probably go with a 150 gpd membrane and an autoflush flow restrictor. The flow restrictor plugs right in to the aquatec wirings harness. This should give you higher rejection, about 200 gpd, and would automate the flushing.

Russ

Buckeye Hydro
08/02/2010, 05:10 PM
Check the head of the Aquatec 8800 boster pump. You'll see a small raised area about the diameter of a dime. Right in the center of that is a small allen screw - by turning the screw you can adjust the output (pressure) of the pump. Don't go any higher than 90 psi.

O&M = Operations and Maintenance. I'm assuming that you are moving each of the DI stages when the first stage is exhausted. More moves/more work/more cartridges/more expence/more to sanitize/more opportunity for leaks with more cartridges. At 200 gpd 3 cartridges is more than sufficient. 4 is probably ovrekill. But heck - if you have the time and money, certainly no harm done with 4.

$150 gpd membrane will run you about $55. Autoflush flow restrictor about $50.

Russ

MadBeach79
08/02/2010, 07:21 PM
Russ, one last question, is the 150gpd membrane the same size as all the regular membranes? and if so, what kind of rejection rate are they rated at? Could you run two of those membranes in series? is there a restrictor made for them if I were to run two of them in series?

Okay, that was more than one question, lol. I really do appreciate you helping me out with all of this.

Buckeye Hydro
08/02/2010, 07:38 PM
Hey - no problem.

Yes - the 150 gpd membranes fit in a "standard" RO membrane housing. They need 65 psi minimum, and the factory spec is 98% rejection - just like the 75's. We don't have a flow restrictor for a (2x150) 300 gpd membrane...