Dual 150GPD RO Membrane Setup

[Landsailor]

Hi,

This hasn't gotten any responses in the equipment forum, so perhaps the Advanced folks have some insight.

I read about this on some supplier's website, but can't find it now.

What do I need to do to get two 150gpd membranes up and running in a dual membrane system? It's plumbed with the housings in series ala Bulk Reef Supply's Water Saver upgrade (Membrane 1's waste water feeds Membrane 2). We installed them last night, then ran them individually to remove the preservative.

Currently, there is an AquaTec 8800 booster pump that brings the system pressure up to 65psi max. On the old 75gpd membranes that were in there, it could peg the pressure at 120psi (We killed power and backed off the bypass valve dramatically when that happened.) I suspect that there is massive pressure drop between the membranes and that's causing the poor output when they're run in series.

I could put another booster pump between the membranes. I know the 8800 is only rated for up to about 100gpd, but it was working fine on my single membrane system and providing the required flow.

I suspect the 150gpd flow restrictor or the combination of the two high output (relatively speaking) membranes is giving too much flow for the 8800 to boost the pressure enough to help. The output when they're both inline is just crappy, both in volume and TDS.

Any help is appreciated!

 

[Buckeye Hydro]

Landsailor - be happy to help you but the answers to your questions would be better in a conversation rather than on the back and forth of posts.

Feel free to give us a call after 2 PM Eastern time today.

Russ
513-312-2343

 

[Landsailor]

Thank you so much, Russ! I really appreciate the help!

 

[Landsailor]

Wow. As a follow up, I want to add that Russ is a fantastic guy. He spent way too much time helping me with a problem that wasn't his responsibility.

It is very much appreciated and you've definitely earned my business!

 

[Buckeye Hydro]

Thanks for the feedback! Check your email inbox re the specs on the pump we discussed.

 

[dahenley]

can you explain or tell us what you did?
that way we can learn from it.

i too just put a 2nd membrane on my RO/DI unit.
and i currently run a premate pump, but was thinking of doing something different if need be.

thanks

 

[Buckeye Hydro]

Here's some background info you may find useful if you're thinking abut addind a second membrane to "cut down on waste water."

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

 

[dahenley]

Ok, just 1 more question.
if we already have the 2nd membrane, should we change the configuration from Series to Parallel, is that plausible, or will it restrict the pressure thus nulling any good?

thanks.
that does help and explain a bunch!

 

[Buckeye Hydro]

All depends upon why you added a second membrane. If you plumbed them in series and you did it to save water... well, I'd recommend you use a flow restrictor that yields something close to a 4 to 1.

We'd need more info on your situation to give you more specific advise. Hard to do here in posts. Feel free to give us a call and we can talk through various configurations.

Russ