Design Check and Resources for RODI/Mixing Station

[StickyThwomp]

Hi All,

I'm in the process of overcomplicating my RODI and Saltwater Mixing Station. For context, the gear and automation is a large part of the appeal of reefing for me, so I'm interested in learning how things work, and creating a system that is convenient, cool, and eventually automated!.

I don't care too much how overkill/unnecessary it may be, since, again, this is my idea of fun. I know its a bit of blasphemy to go this far without actually knowing the TDS of my feed water. I know that there's a chance I'd be perfectly fine with next to nothing for RODI, but minimizing my own work is very much not the point of practically anything I do. Its much more fun to learn and build, even if it's not necessary (or advisable ).

I've come up with the following design, but I wanted to see if any more experienced folks had some thoughts/suggestions on operating/maintaining it efficiently, or could help me with anwers to a few points I'm probably not grasping right. I eventually want to automate the production of RODI water implement auto-flushing and TDS monitoring myself, but for starters I'm thinking manual RODI making is gonna be A-OK.

As to where I am in the design/execution at the moment: I have everything listed below except for the flow restrictor, the DIY ATO, and misc. connectors/valves, and the saltwater mixing pump/heater. The scope of what I'm concerned with at the moment is on the RODI side, not so much the saltwater stuff.

(Woops, I realize I left off the auto-shut-off!)

Questions I have are (See if you can sense the theme):
1. In all the examples I've seen, the flow restrictor on the concentrate lines is a fixed cc/min. Is using a needle/precision valve (like this one) a good/bad idea? Are they reliable or do they wiggle their way out of their settings over time, break easily, etc...? My thought process here is that one piece that I can set to the optimal setting and forget is probably better than buying the wrong one and finding I want a different value, or having to buy a new when I tinker with the system in the future.

2. More about the mechanics... With no flow restrictor in place, is the Feed-to-Concentrate path in an RO membrane more or less unrestricted flow with trivial pressure drop? I have that picture in my head, leading to this "Concentrate staging" approach looking just like a way to scale up the production (and the waste) proportionally, but I'm not sure that's correct (I mean, its sold as "water saving", but what's the math behind that?) What's the meaningful difference between putting a T on the feed and running both RO membranes in "parallel" that way vs feeding the concentrate of one to the feed of the other, would there be a difference in pressure across each membrane?

3. More generally... Seems like there should be a way to say. "Here's my feed pressure, temperature, incoming TDS, and membrane characteristics/configuration. To target X TDS, I'd need Y Flow restriction and have Z waste to product ratio." Where's the magic formula (or the bits and pieces required to get there)?


Thanks for reading! (and thanks in advance for any input you may have!!)

(Edit: Fixed image upload)

 

[theatrus]

1. In all the examples I've seen, the flow restrictor on the concentrate lines is a fixed cc/min. Is using a needle/precision valve (like this one) a good/bad idea? Are they reliable or do they wiggle their way out of their settings over time, break easily, etc...? My thought process here is that one piece that I can set to the optimal setting and forget is probably better than buying the wrong one and finding I want a different value, or having to buy a new when I tinker with the system in the future.

I've been using a fixed restrictor forever. With a relatively stable input pressure, you can use something else. I never bothered to over-optimize this, but there is probably room here.

2. More about the mechanics... With no flow restrictor in place, is the Feed-to-Concentrate path in an RO membrane more or less unrestricted flow with trivial pressure drop? I have that picture in my head, leading to this "Concentrate staging" approach looking just like a way to scale up the production (and the waste) proportionally, but I'm not sure that's correct (I mean, its sold as "water saving", but what's the math behind that?) What's the meaningful difference between putting a T on the feed and running both RO membranes in "parallel" that way vs feeding the concentrate of one to the feed of the other, would there be a difference in pressure across each membrane?

No restrictor in place is flushing through the membrane channels without the pressure to force water through the membrane. The channels add some restriction, but not a ton. These video diagrams may help:



The water saving component is basically doubling the length of membrane that is used so there are more chances for water to cross the membrane before being flushed out as waste. You will get more output generally, though depending on the source water may have higher TDS output. I run a serial membrane as it does cut my water consumption and has no impact to output, but I have low TDS input. Someone on hard well water may have different challenges.

3. More generally... Seems like there should be a way to say. "Here's my feed pressure, temperature, incoming TDS, and membrane characteristics/configuration. To target X TDS, I'd need Y Flow restriction and have Z waste to product ratio." Where's the magic formula (or the bits and pieces required to get there)?

Best advice is to poke around Dow's documentation. Page 98 should help https://www.dupont.com/content/dam/...ents/en/RO-NF-FilmTec-Manual-45-D01504-en.pdf

 

[KStatefan]

Hi All,

I'm in the process of overcomplicating my RODI and Saltwater Mixing Station. For context, the gear and automation is a large part of the appeal of reefing for me, so I'm interested in learning how things work, and creating a system that is convenient, cool, and eventually automated!.

I don't care too much how overkill/unnecessary it may be, since, again, this is my idea of fun. I know its a bit of blasphemy to go this far without actually knowing the TDS of my feed water. I know that there's a chance I'd be perfectly fine with next to nothing for RODI, but minimizing my own work is very much not the point of practically anything I do. Its much more fun to learn and build, even if it's not necessary (or advisable ).

I've come up with the following design, but I wanted to see if any more experienced folks had some thoughts/suggestions on operating/maintaining it efficiently, or could help me with anwers to a few points I'm probably not grasping right. I eventually want to automate the production of RODI water implement auto-flushing and TDS monitoring myself, but for starters I'm thinking manual RODI making is gonna be A-OK.

As to where I am in the design/execution at the moment: I have everything listed below except for the flow restrictor, the DIY ATO, and misc. connectors/valves, and the saltwater mixing pump/heater. The scope of what I'm concerned with at the moment is on the RODI side, not so much the saltwater stuff.

(Woops, I realize I left off the auto-shut-off!)

Questions I have are (See if you can sense the theme):
1. In all the examples I've seen, the flow restrictor on the concentrate lines is a fixed cc/min. Is using a needle/precision valve (like this one) a good/bad idea? Are they reliable or do they wiggle their way out of their settings over time, break easily, etc...? My thought process here is that one piece that I can set to the optimal setting and forget is probably better than buying the wrong one and finding I want a different value, or having to buy a new when I tinker with the system in the future.

2. More about the mechanics... With no flow restrictor in place, is the Feed-to-Concentrate path in an RO membrane more or less unrestricted flow with trivial pressure drop? I have that picture in my head, leading to this "Concentrate staging" approach looking just like a way to scale up the production (and the waste) proportionally, but I'm not sure that's correct (I mean, its sold as "water saving", but what's the math behind that?) What's the meaningful difference between putting a T on the feed and running both RO membranes in "parallel" that way vs feeding the concentrate of one to the feed of the other, would there be a difference in pressure across each membrane?

3. More generally... Seems like there should be a way to say. "Here's my feed pressure, temperature, incoming TDS, and membrane characteristics/configuration. To target X TDS, I'd need Y Flow restriction and have Z waste to product ratio." Where's the magic formula (or the bits and pieces required to get there)?


Thanks for reading! (and thanks in advance for any input you may have!!)

(Edit: Fixed image upload)

I use a fixed capillary tube. My pressure remains stale so I set it up for 3:1 in the summer then it moves to 4:1 or so when the winter gets here and the water temp drops.

 

[StickyThwomp]

I use a fixed capillary tube. My pressure remains stale so I set it up for 3:1 in the summer then it moves to 4:1 or so when the winter gets here and the water temp drops.

From a quick google on what the heck a capillary tube is... That looks like a reasonable way to trim down to the behavior you want, and probably much more steady than an adjustable valve. Thanks, I'll keep that in mind!

 

[StickyThwomp]

No restrictor in place is flushing through the membrane channels without the pressure to force water through the membrane. The channels add some restriction, but not a ton. These video diagrams may help:

Thanks! For some reason I was viewing this as a purely Flow from "outside to inside", through all layers of the membrane material, when in fact, it looks like it the water only crosses one layer and then "rides the spiral" all the way to the center. Cool! Not at all what I had pictured.
Running the water perpendicularly through those tightly wound layers, instead of sloshing around in the empty space between RO and housing makes way more sense how there's be a difference in behavior in branching the feed to two filters vs "elongating" the filter as is done in the concentrate staging approach.

Best advice is to poke around Dow's documentation. Page 98 should help https://www.dupont.com/content/dam/...ents/en/RO-NF-FilmTec-Manual-45-D01504-en.pdf

I only saw the several page datasheet, I didn't realize they published an entire manual on these things! Very nice I'll go digging!

Thank you so much for the detailed response!

 

[KStatefan]

Lots of info in that manual. DuPont has been doing a lot of updates to their membranes in the last few years. I assumed they gained some different technology in the Dow-DuPont merger then dis merger. When you are looking at spec sheets make sure you are getting them from DuPont as a number of vendors still have old ones posted

When I first purchased a membrane from SpectraPure their capillary was cut to length to get the correct ratio for your source water

 

[theatrus]

Thanks! For some reason I was viewing this as a purely Flow from "outside to inside", through all layers of the membrane material, when in fact, it looks like it the water only crosses one layer and then "rides the spiral" all the way to the center. Cool! Not at all what I had pictured.

Initially I had the same confusion, as it is how the rest of the canister filters work (carbon, sediment).

 

[StickyThwomp]

Figured I'd test my feed tds, but having no luck. Probes are aligned properly, fully inserted, but the initial reading faded away to zero. I removed the probe from the housing and dipped it in distilled for a while, then back in tap water and it yields around 190ppm before fading back to zero over the course of 30 sec or so. Bounces immediately back to zero when dipped into distilled, but can't get back to a high reading without "charging" in distilled water before switching to the actual water I want to measure. Short soak only raises tap reading moderately, long soak gets me to that peak around 190. Weird... I'm gonna assume that's the approximate tds until I can get a steady measurement. :-( Just mentioning the behavior in case that sounds familiar to anyone. I think this might be a faulty unit, or at least in need of a battery replacement.