How many RO membranes?

[DanielJay]

New to saltwater and have a question about RODI filters. I currently have the BRS unit with the 150GPD upgrade and a booster pump as I am on well water. I have almost burned through my DI stage (color is changing fast) with only making about 100g (DI still has about 1/3 left). My dirty water line into the RODI unit is showing about 370 for TDS and then by the time it leaves the RO it is down to about 10-20. By the time it leaves the DI it is at 0 TDS. Would putting a 3rd RO membrane on the system be beneficial to get TDS down farther before it gets to the DI stage?

Just trying to find ways to not burn through filters so fast.

[BeanAnimal]

370 input at 98% rejection would be about 7.5 tds in the RO output. You have "10-20" indicating that your rejection is somewhere in the 95% range. Not so bad, but not so great.

Keep an eye on the TDS at the DI output, as the color change in the resin is not always a good indicator of the life or usage. You may find that you get more than you expect from the resin, regarldess of the "color".

Have you checked the product to waste ratio to see where it is at? I am not sure (to lazy to look) what the 150 upgrade at brs is. I assume a second membrane? Great, but was the flow restrictor changed? You need to be at around 4:1 waste to product ratio.

Adding another membrane will reduce waste a bit, but can (will) increase the aggregate TDS sent to the DI as the membranes rejection ratios are fixed and you are in essence feeding more concentrated water to each membrane in series.

Adding more membranes

[reefergeorge]

You are running ro membranes in series? I don't think that works, but for a definite answer post in the filter guys, or bulk reef section. They are always happy to help.

[BuckeyeFS]

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 as Bean mentioned above.

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

[TheFishTeen]

Ok, so with that said adding a second membrane is NOT an effective solution to increasing the efficiency of the RO system?

That puts BRS in a questionable situation.

http://www.bulkreefsupply.com/store/...grade-kit.html

[breon]

Second membrane is a no no in my opinion

[patbob]

.. somehow ended up posting to a thread I wasn't reading..?

[Ripperj]

I believe he is saying that if you add the second membrane and do not change the restricter, that it would have been cheaper to keep a single membrane and reduce the waste ratio- the end result of both of these options is reduced membrane life due to excess fouling.
The second membrane with the proper flow restrictor is an option from most manufacturers

[DanielJay]

Ok so I have finally been able to test my output of clean product water out of the RO vs flush water. Looks like I am at a 1:1 ratio instead of a 1:4 product/flush water.

I took 2 buckets with QT marks on it and they both measure the same. I can say that my RO unit is mounted on a wall away from my water source and drain. I have an adapter for the water line connected to the blue pex line to the utility sink and then the 1/4" line runs up to the ceiling and then over the door and back down. The flush water line follows the same path except it just dangles in the sink for the moment. Will the head pressure from having to push the flush water up cause problems?

Below is a photo of the room before I put the RO unit in. It is just out of the picture on the right past that door.

[Floyd R Turbo]

Well I don't know what to tell you but my input in my 150 BRS membrane is >500 TDS and the output (running @ 55psi so it's slow) gets down around 10, and my DI cansiter (BRS also) has been running unchanged for at least 500 gallons, still 0 TDS and it's only maybe 2/3 used up (color changing nuclear resin). Something else is up IMO.

[ehaleyjr18]

DanielJay, My son and myself are new to this as well, but we installed a 50 GPD 7-stage system with a buster pump and a permate pump. We get a 3:1 ratio which I thought was due to the permate pump releiving the pressure on the back of the membrane. Now after seeing BeenAnimal and BuckeyeFS's post I might not be getting the desired 0 TDS. But I can say is that something is definately wrong with your filtration system if you are seeing such filter degredation after so little RO water delivered. Can the MFG. help you with this?

[ehaleyjr18]

Booster pump.

[ehaleyjr18]

I'll have to look at my instructions when I'm at home but I believe the booster pump has to be mounted within several feet from the outlet.That might have something to do with the 1:1 ratio.

[BuckeyeFS]

Originally Posted by Ripperj

I believe he is saying that if you add the second membrane and do not change the restricter, that it would have been cheaper to keep a single membrane and reduce the waste ratio- the end result of both of these options is reduced membrane life due to excess fouling.
The second membrane with the proper flow restrictor is an option from most manufacturers

The second membrane with the IMPROPER flow restrictor is an option from many vendors.

[BuckeyeFS]

At a 1:1 ratio expect a short, and perhaps very short, life span on your membranes.

One issue you have is relatively high feedwater TDS: 370 ppm.

You're getting a reasonable rejection from your membranes (~95%), but you should get a more precise measurement than "10 to 20" to see how the membranes are working.

If we assume an average RO water TDS of 15 ppm, then you could reasonably expect to get about 350 gallons from a single 10" x 2.5" DI cartridge. If you burned through a DI cart in 100 gallons, then something else is going on.

You mentioned you are on well water. Have you checked for CO2? Does your water have a "rotten egg" odor?