RO/DI effeciency

[Tbonekilla]

I have a 6 stage BRS RO/DI with the drinking water tank between the membrane and DI. My tap water is TDS is 126 and my RO TDS is 2. DI is TDS 0. My fridge water dispenser, ice maker, and sink drinking water faucet are all off the RO. We drink a fair amount of water and use the ice a lot. All the fish tanks in the house will be about 400 gallons total. I'm currently paying almost $5 per 1000 gallons on the water bill. Water pressure is at 60 PSI.

I'm looking to decrease my waste water output. After reading about sediment filter combinations and getting a booster pump I'll be changing my filter configuration to:

5 micron sediment > booster > .5 micron sediment > .5 micron carbon

My question is what would be the best membrane configuration? Two 75 GPD 98% rejection membranes or one 150 GPD 98% rejection membrane. Both would cost about the same to upgrade to.

Also, permeate pumps, anybody use them? How much do they help?

My priority will be bang for my buck first with the goal of having everything in the end run as far as efficiency is concerned.

Thanks.

 

[Two beer cans]

I run 2 x 75gpd membranes I run the waste water from the first into the second membrane this has halved my waste and doubled my ro output . My pressure is 105 psi my tap (faucet)

 

[bdr]

maybe a different rodi unit if that one isnt efficient? I am new to saltwater but i bought a vertex 100gpd unit and good to waste is 1:1 can you get better?

 

[Tbonekilla]

I wish I had 105 PSI tap. I would save me $138 lol.

bdr, 100 GPD at 1:1? What's your water pressure at? I'm curious the Filmtec 100 GPD membranes run at 90% rejection rate. Which would burn though DI resin because of a higher TDS out of the membrane.

Just for reference my Filmtec 75 GPD 98% rejection membrane @ 60 PSI gives me a 1:4 ratio as intended.

 

[BeanAnimal]

I run 2 x 75gpd membranes I run the waste water from the first into the second membrane this has halved my waste and doubled my ro output . My pressure is 105 psi my tap (faucet)

Lets process 100 gallons of water in a perfect world where each membrane is set to 4:1 waste to product.

Single membrane:
100 Gallons in turns into
80 gallons of waste
20 gallons of product
4:1 waste to product

Second membrane:
75 gallons in turns into
60 gallons of waste
15 gallons of product

Total
100 Gallons consumed:
65 gallons of waste
35 gallons of product
1.8:1 waste to product


The problem is the pressure drop over the first membrane and the reduced output of the second membrane. If the input TDS is low, then the membrane life (of both membranes) will be decent. However, at higher TDS input, the second membrane is going to take a beating and in many cases the consumable cost will be more than the wastewater cost of a single membrane setup. In the same fashion, reducing the waste to product ratio can save water but will eat up DI resin and likely cost more in consumables than the wasted water at 4:1

Just food for throught.

 

[thor109]

I contacted watts premier and was told that the different membranes have different efficiency. They said the 75 and 100 are best.
No sure if running 2 in series would be cheaper when you water supply is only 126 tds. Mine is 480, it might save me money. I currently run a single 100 gpd.

 

[bdr]

im on a well so my pressure goes between 45-65 psi but the vertex rodi comes with a premounted booster pump

 

[Tbonekilla]

Ok I did some measuring after reading Bean's post on waste ratios and second membrane life.

Right now I have 56 PSI, 126ppm tap, 4ppm RO, and 169ppm waste.

So at 98% rejection with 126ppm tap I should have 2.52ppm RO. Mine is within margin of error of the meter. If I were to send the 169ppm waste to a second 98% rejection membrane it should have a 3.38ppm RO. Which would dilute together with the other membrane's output to an overall RO of 2.95ppm.

I don't think I'm going to run into premature second membrane problems or burning through DI. Most of our RO goes towards drinking anyways and doesn't hit the DI.

Could a permeate pump get that 1.8:1 down even further? Was reading that the permeate pump keeps the pressure differential high in the membrane to increase efficiency and reduce TDS creep when the tank fills up.

 

[Buckeye Hydro]

I have a 6 stage BRS RO/DI with the drinking water tank between the membrane and DI. My tap water is TDS is 126 and my RO TDS is 2. DI is TDS 0. My fridge water dispenser, ice maker, and sink drinking water faucet are all off the RO. We drink a fair amount of water and use the ice a lot. All the fish tanks in the house will be about 400 gallons total. I'm currently paying almost $5 per 1000 gallons on the water bill. Water pressure is at 60 PSI.

I'm looking to decrease my waste water output. After reading about sediment filter combinations and getting a booster pump I'll be changing my filter configuration to:

5 micron sediment > booster > .5 micron sediment > .5 micron carbon

My question is what would be the best membrane configuration? Two 75 GPD 98% rejection membranes or one 150 GPD 98% rejection membrane. Both would cost about the same to upgrade to.

Also, permeate pumps, anybody use them? How much do they help?

My priority will be bang for my buck first with the goal of having everything in the end run as far as efficiency is concerned.

Thanks.

Do you have your system configured such that water from the pressure tank never makes it to the DI?

I would change your pranned configuration to put the booster AFTER all the prefilters (and after a strainer).

I'd go with 2 75 gpd membranes plumbed in parallel, or in series (as long as you stick to a 4:1 ratio).

Permeate pumps work well. The key thing is to isolate the pressure tank from the DI.

 

[Buckeye Hydro]

I run 2 x 75gpd membranes I run the waste water from the first into the second membrane this has halved my waste and doubled my ro output . My pressure is 105 psi my tap (faucet)

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

 

[Buckeye Hydro]

I contacted watts premier and was told that the different membranes have different efficiency. They said the 75 and 100 are best.
No sure if running 2 in series would be cheaper when you water supply is only 126 tds. Mine is 480, it might save me money. I currently run a single 100 gpd.

Here's some data:

 

[Tbonekilla]

Do you have your system configured such that water from the pressure tank never makes it to the DI?

I would change your pranned configuration to put the booster AFTER all the prefilters (and after a strainer).

I'd go with 2 75 gpd membranes plumbed in parallel, or in series (as long as you stick to a 4:1 ratio).

Permeate pumps work well. The key thing is to isolate the pressure tank from the DI.

Yes, I have the drinking water side isolated with a check valve.

I assume two 75 GPD membranes in series with a 75 GPD flow restrictor would last longer than one 75 GPD with a 36 GPD flow restrictor due to more surface area of two membranes instead of one. Based on your description of two 75's being a long 150 membrane.

 

[Buckeye Hydro]

I assume two 75 GPD membranes in series with a 75 GPD flow restrictor would last longer than one 75 GPD with a 36 GPD flow restrictor due to more surface area of two membranes instead of one. Based on your description of two 75's being a long 150 membrane.

Right. Better yet would be to install a 150 gpd restrictor with your (2x75) 150 gpd membrane.

 

[James77]

What are the signs of membranes fouling because of lack of adequate flushing?

I piggybacked an additional 75 gpd membrane onto another, but kept the same 75 flow restrictor. It obviously doubled my output. Recently I am getting a very foul odor and taste right after the membranes, before the DI.

 

[Buckeye Hydro]

Inadequate waste water->scaling or fouling. Sounds like you have biofouling. Don't drink that water. Time for a new membrane.

When you take the membrane out feel the outside - is it slimy? Smell the ends - do they smell bad?

 

[James77]

Sounds like you have biofouling. Don't drink that water. Time for a new membrane.
When you take the membrane out feel the outside - is it slimy? Smell the ends - do they smell bad?

I just checked one membrane, while it doesn't feel slimy, it definitely stinks, especially in the housing itself. Like rotten onion or something, tastes the same too(had about a sip and a half before I noticed it a couple days ago). Yick.
What causes the biofouling, and the best ay to prevent that?

 

[Buckeye Hydro]

In most instances, just keep that waste to permeate ratio at about 4 to 1. You were about half that, right?

 

[James77]

In most instances, just keep that waste to permeate ratio at about 4 to 1. You were about half that, right?

About half....I think I measured when I first installed it and it was something like 1.8 to 1 waste to permeate. No wonder they fouled....I'd never had any problem like that before in almost 10 years of all sorts of RODI systems on the same city water supply. It was just over a year ago I intalled the "water saver" upgrade.

 

[Buckeye Hydro]

Yep. We don't recommend the "water saving" configuration some vendors support - for that reason.

 

[DamonG]

I think the issue is the restrict that was not adequate. I know my particular vendor that I did my upgrade with (2x100 membranes) came with the membrane,a correct sized restrict (as they explained the exact same from the beginning), tubing, brackets etc. they also mentioned the required psi. At the time,I was at a newer home that had higher psi (larger water lines), but my home now has lower, so I needed a little boost. The efficiency is still good, as I ask well aware of a few things:

-Any tds out of di and you are past (well past) needing to change your sediment and carbon filters.
-The importance of color changing resin.
-Take a look at carbon filters as they do different things if you advance past"just a plain old filter"and as such, have different lifespans.
- In my case my dealer supplies matrikx filters and run two of those after a 1 micron sediment. I run a 10k cr1 & a 20k +1, then the two membranes, and lastly the di.
-The tds of my ro actually comes out at zero for a long time, and the di really lasts a long time.
-I change the sediment regularly (every 3 months) and the others once a year as I do 30 gallon batches maybe every two Weeks.
-also,I flush the membranes frequently. Best to have auto flush and I believe this helps tremendously with the membrane life.

But for running ro units, the best thing I recommend is looking att the aftermarket blocks and learning what the different models do and their strengths and weaknesses over the standard blocks and sediment that come with the unit unless they are purpose designed for your application by verifying the model number with the manufacturers site as a "I need to know what I ask giving my fish and family". If they are just generic who knows what the dealer has (nothing against them at all) in them. Contact the mfg (yes,I am that crazy about the companies I deal with and I wasn't to know or at least hear some technical/working knowledge of their product and what they recommend based on your needs) and see what they know.

Sorry about the long post, but I have learned that ro/di is not add simple as it seems...

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