Riddle Me This Q1 - (Reverse) Osmosis Filter

[fluffybeast]

Welcome to Riddle Me This. It is the new plan I hatched. I figured it would be easier to ask people questions than to read the entire internet, but I am cleverly hiding my information seeking and tricking you into answering by couching it in game form. Hurray! Did I mention everyone is doing it? Winner gets a prize of unknown nature provided by an unknown person at some unknown distant time in the future!

Riddle Me This - (Reverse) Osmosis

I've seen passive (regular osmosis) filter bottles for hiking, where you put electrolytes in the clean side, fill the other side up with dirty, and osmotic pressure cleans the water for you (there may be hand pressure versions too). I've read about someone wanting to revolutionize RO capacity and efficiency by putting some salt in the clean side that could easily be precipitated out later - and this was years ago. It seems silly to me that we work so hard to make clean water only to fill it with salt again. Riddle Me This - why don't I see a filter on the market or even hear of a DIY one where the salt mix is on the clean side, and osmotic pressure pulls clean water through to it? Wouldn't this be easier and more passive? Would the membrane last longer and have a higher flow rate? And if so, wouldn't this keep more of the phosphates/etc on the dirty water side and produce cleaner water? Perhaps we could ditch the expensive DI part. Or make cleaner water faster. If the theory is sound, what would be involved in a DIY version?

Let the game commence!

[victoly]

[victoly]

The point of "cleaning" the water, is that you start with completely sterile/pure water that is devoid of any minerals, and that you then put in known quantities of chemicals. Not really sure where you're headed with this.

[wesreyn]

Not a very fun game

[(Bio)³]

I'll stick with my RODI, it takes up little space and they aren't expensive compared to any passive system I have seen.

[bige]

Cleaning water to drink is different then striping the water of everything but h2o. Ro/di water is not drinking water and yes you can make your own but your just assembling pieces. Each "stage" removes a little more. The ro stages are carbon blocks. Then the di resin removes all the rest of the total dissolved solides (Tds). Like said before once the water is sterile, the salt mix makes the water perfect for your reef. Making your experience more enjoyable. You won't stay in this hobby long taking short cuts. But you can buy ro water from local fish stores (lfs) and even premixed saltwater. But it is cheaper and easier in the long run to make it yourself.

[fluffybeast]

Cleaning water to drink is different then striping the water of everything but h2o. Ro/di water is not drinking water and yes you can make your own but your just assembling pieces. Each "stage" removes a little more. The ro stages are carbon blocks. Then the di resin removes all the rest of the total dissolved solides (Tds). Like said before once the water is sterile, the salt mix makes the water perfect for your reef. Making your experience more enjoyable. You won't stay in this hobby long taking short cuts. But you can buy ro water from local fish stores (lfs) and even premixed saltwater. But it is cheaper and easier in the long run to make it yourself.

Thank you for providing an overview and a constructive context. I may quibble with some specifics in a bit.

I'll stick with my RODI, it takes up little space and they aren't expensive compared to any passive system I have seen.

This implies that passive systems exist, which is kind of where I was heading. I've only heard of the camping one on a small scale (and industrial scale ones).

My apologies for not being clear in my post. I will try to be a lot more specific with background and the questions I was looking to answer. I thought it would be a way to stimulate in depth discussions.

The mechanical and chemical filtration we use to clean our water for tanks starts with a water source, for most of us the tap. Most of us are lucky to be on city or well water that is relatively clean. Assuming that for now, we still try to clean the water. Why? There are stuffs in the water we do not want to add to the tank, such as various nutrient salts (phosphates anyone?) and toxic pollutants (lead, synthetic estrogens, pesticides, etc.). We go through a process to clean this water that usually starts with a sediment filter. This is a physical barrier that blocks the larger particles in the water that can clog or damage the stages down the line. Next is a carbon filter, which provides a finer level of mechanical filtration, as well as some chemical filtration to remove smaller physical particles and certain dissolved organics. Next comes the 'reverse osmosis' membrane. In an ideal world, this membrane lets water through but nothing else, giving you pure water. More on this in a bit. Unfortunately, it is not ideal and the membrane lets through unwanted contaminants, and in high enough levels that more discerning consumers, such as reefkeepers, add the extra step of a deionization, which binds some of the remaining contaminants (salts, metals) or exchanges them with less onerous ones to make even more palatable water.

So let's back up to how the membrane works. With osmosis, water moves from areas of low salt concentrations to areas of high concentration. For our purposes, this is the wrong direction, we want the water to move from our higher concentration input water to the lower concentration output water side. So, we add physical pressure, which when stronger than the osmotic pressure, pushes the water through the membrane and leaves the contaminating salts and other molecules on the dirty water side. We continually flush this dirty water so that the salt concentration does not get too high (increases pressure needed, reduces flow rate), and so that other physical damage doesn't result.

So now we are getting into the meat of things. Essentially, we take mildly salty water, we try to get all of the salt and other stuff out, and then we add lots more salt. I point this out only because I find it poetically amusing, but I understand it is necessary as we want a very specific salt mixture at the end and do not want what we start out with. What I question is why do we fight against osmotic pressure, when in the end we are wanting to go from a low concentration to a high concentration? From a macro level, we are fighting that pressure with energy inputs (think of desalinization, why isn't it more widespread? too much energy). On a more personable level, while we all know water and energy are free and in limitless supply ad infinitum, what if it weren't? Thus part of the discussion.

I also want to look at the membrane itself. If it is only supposed to let water through, how do other things get through? Well, 1) imperfections in the membrane, 2) gaps in the seals between the membrane and housing, and 3) 'other'...membranes degrade over time (why?). So, I *suspect* that the high pressure needed to overcome the osmotic pressure leads to the accelerated failure of the gaps and degradation of the membrane itself. I also suspect that the pressure causes more contaminants to get through the membrane. Overcoming the pressure also reduces the flow rate, similar to the head on a pump (gravity ~ osmotic pressure).

So, since we want a salt mix on the other end anyway, why don't we put that on the output side of the membrane so that osmotic pressure is working FOR us instead of AGAINST us. That should give us a better flow rate at least. I suspect that it would keep the contaminants from coming through, or at least significantly reduce the amount that breach the membrane. Few enough contaminants, and perhaps the DI stage is not needed, which I understand is even freer than the osmotic membrane and energy and water (not that we'd use the DI membrane with the salt on the output side..but just saying it might be rendered unnecessary for quality purposes). We could reduce the pressure (ie energy) required, or even eliminate it.

So, Q: why isn't there a cheaper filter out harnessing osmotic pressure instead of fighting it? Would it not work faster, last longer, and make better quality water?

The tiny quibbles: the water is not completely sterile, imperfect as each stage is, some bacteria and viruses breach them including the membrane, nor would it matter that is not the goal for reefkeeping; the goal is more chemical purity. The resin does not remove all TDS; it has affinities for certain salts and metals but lets most nonpolar organic molecules that made it that far through (in theory, they would have been adsorbed by the carbon or rejected by the membrane).

So, there we go. I don't know the specifics of how the membrane fails, and whether harnessing rather than fighting osmotic pressure would improve quality or membrane life. So, school my newbie brain as to what I am missing here. I assume I am missing something because I haven't seen or read about something like this yet. Your move.

Not a very fun game

Indeed, most games not played are not. I hope it is slightly more interesting now. Jump in!

[(Bio)³]

This implies that passive systems exist<I didn't imply anything you are the one who said they existed and I based it off of what I've seen "passive" wise (think brita filter in fridge) they require double the space (side for dirt and side for clean), which is kind of where I was heading. I've only heard of the camping one on a small scale (and industrial scale ones) <The camping ones exist because they are portable....for camping.... not gonna plug my pressure pump into a tree and have it send river water into my RODI, also the camping ones I have seen are either hand driven or have a pump/filter and aren't passive.>

My apologies for not being clear in my post. I will try to be a lot more specific with background and the questions I was looking to answer. I thought it would be a way to stimulate in depth discussions.

The mechanical and chemical filtration we use to clean our water for tanks starts with a water source, for most of us the tap. Most of us are lucky to be on city or well water that is relatively clean. Assuming that for now, we still try to clean the water. Why? There are stuffs in the water we do not want to add to the tank, such as various nutrient salts (phosphates anyone?) and toxic pollutants (lead, synthetic estrogens, pesticides, etc.). We go through a process to clean this water that usually starts with a sediment filter. This is a physical barrier that blocks the larger particles in the water that can clog or damage the stages down the line. Next is a carbon filter, which provides a finer level of mechanical filtration, as well as some chemical filtration to remove smaller physical particles and certain dissolved organics. Next comes the 'reverse osmosis' membrane. In an ideal world, this membrane lets water through but nothing else, giving you pure water. More on this in a bit. Unfortunately, it is not ideal and the membrane lets through unwanted contaminants, and in high enough levels that more discerning consumers, such as reefkeepers, add the extra step of a deionization, which binds some of the remaining contaminants (salts, metals) or exchanges them with less onerous ones to make even more palatable water.

So let's back up to how the membrane works. With osmosis, water moves from areas of low salt concentrations to areas of high concentration. For our purposes, this is the wrong direction, we want the water to move from our higher concentration input water to the lower concentration output water side. So, we add physical pressure, which when stronger than the osmotic pressure<osmotic pressure inside RODI isnt that much to overcome actually>, pushes the water through the membrane and leaves the contaminating salts and other molecules on the dirty water side. We continually flush this dirty water so that the salt concentration does not get too high (increases pressure needed, reduces flow rate), and so that other physical damage doesn't result.

So now we are getting into the meat of things. Essentially, we take mildly salty water, we try to get all of the salt and other stuff out, and then we add lots more salt. I point this out only because I find it poetically amusing, but I understand it is necessary as we want a very specific salt mixture at the end and do not want what we start out with. What I question is why do we fight against osmotic pressure, when in the end we are wanting to go from a low concentration to a high concentration? From a macro level, we are fighting that pressure with energy inputs (think of desalinization, why isn't it more widespread? too much energy). On a more personable level, while we all know water and energy are free and in limitless supply ad infinitum, what if it weren't?<Please realize that water is not endless/limitless. energy could be argued but not the place here.> Thus part of the discussion.

I also want to look at the membrane itself. If it is only supposed to let water through, how do other things get through? Well, 1) imperfections in the membrane,<Items smaller than the membrane exist and pass through(think sugar sand in pasta sieve)> 2) gaps in the seals between the membrane and housing,<not sure if your talking RODI or some otehr filter but in RODI the seals are really snug in mine, silicone grease helps to set the seals> and 3) 'other'...membranes degrade over time (why?)<Everything has a shelf life, sediment builds up, pores get clogged and stretched>. So, I *suspect* that the high pressure needed to overcome the osmotic pressure leads to the accelerated failure of the gaps and degradation of the membrane itself.<Most membranes are actually killed from sediment build up from short on off cycles of the unit not allowing time to flush off the sediment.> I also suspect that the pressure causes more contaminants to get through the membrane. Overcoming the pressure also reduces the flow rate, similar to the head on a pump (gravity ~ osmotic pressure).

So, since we want a salt mix on the other end anyway, why don't we put that on the output side of the membrane so that osmotic pressure is working FOR us instead of AGAINST us. That should give us a better flow rate at least. I suspect that it would keep the contaminants from coming through, or at least significantly reduce the amount that breach the membrane. Few enough contaminants, and perhaps the DI stage is not needed, which I understand is even freer than the osmotic membrane and energy and water (not that we'd use the DI membrane with the salt on the output side..but just saying it might be rendered unnecessary for quality purposes). We could reduce the pressure (ie energy) required, or even eliminate it.

So, Q: why isn't there a cheaper filter out harnessing osmotic pressure instead of fighting it?<100 mg/L of TDS is equal to 1 psi (rough rule of thumb> I have about 160 TDS... again not that much pressure to overcome> Would it not work faster, last longer, and make better quality water?<your filter has no waste water, no way to clear off particles and there for clogs faster, promotes breakdown faster by clogging pores and increasing pressure and probably harbors bacteria and algae.>

The tiny quibbles: the water is not completely sterile, imperfect as each stage is, some bacteria and viruses breach them including the membrane, nor would it matter that is not the goal for reefkeeping; the goal is more chemical purity. The resin does not remove all TDS<160 TDS In to my RODI 000 out of the RODI, 005-006 before entering resin. Not sure where you getting at here.>; it has affinities for certain salts and metals but lets most nonpolar organic molecules that made it that far through (in theory, they would have been adsorbed by the carbon or rejected by the membrane).

So, there we go. I don't know the specifics of how the membrane fails, and whether harnessing rather than fighting osmotic pressure would improve quality or membrane life. So, school my newbie brain as to what I am missing here. I assume I am missing something because I haven't seen or read about something like this yet. Your move.

Your missing www.google.com

Not a very fun game

I agree, I'm done playing.

[victoly]

I guess i could further this a bit. If your point is, how can we increase efficiency of the RODI process, what is to be gained from the efficiency You can already add another RO membrane and increase output while decreasing waste water, it's just that you can't really payback the RO membrane cost because water is cheap. Generally, the only people who run the 150gpd dual membrane units have very large tanks, so it's a question of total gallons produced per day.

[fluffybeast]

I guess i could further this a bit. If your point is, how can we increase efficiency of the RODI process, what is to be gained from the efficiency You can already add another RO membrane and increase output while decreasing waste water, it's just that you can't really payback the RO membrane cost because water is cheap. Generally, the only people who run the 150gpd dual membrane units have very large tanks, so it's a question of total gallons produced per day.

It is a start down the rabbit hole. Well, here is what might be gained. Exact same RO membrane, now lasts 20k gallons instead of 2k gallons. DI resin gone. Your 75pd flow, through the same membrane, is now 230gpd. Everything else is the same, you still drain to waste, except now your waste:output ratio has gone from 4:1 -> .5:1

[Robb in Austin]

I can't answer your question but I will echo some of the others.

Your post was very vague and had a hint of being argumentative. If you want to ask a question; ask it. Interesting topics/questions will beget constructive dialog without being labeled a 'game''.

Clearly you're an intelligent person, but as you know, tone is lost in the written form. Eschew obfuscation, , and all will be well.

[victoly]

I guess i could further this a bit. If your point is, how can we increase efficiency of the RODI process, what is to be gained from the efficiency You can already add another RO membrane and increase output while decreasing waste water, it's just that you can't really payback the RO membrane cost because water is cheap. Generally, the only people who run the 150gpd dual membrane units have very large tanks, so it's a question of total gallons produced per day.

It is a start down the rabbit hole. Well, here is what might be gained. Exact same RO membrane, now lasts 20k gallons instead of 2k gallons. DI resin gone. Your 75pd flow, through the same membrane, is now 230gpd. Everything else is the same, you still drain to waste, except now your waste:output ratio has gone from 4:1 -> .5:1

Seems to me that if this was a possibility, an RODI unit manufacturer would have jumped on it and stolen the market. If it's feasible, and hasn't been done, jump on and make that money.

[DerrickH]

Whos on first.....

[fluffybeast]

I can't answer your question but I will echo some of the others.

Your post was very vague and had a hint of being argumentative. If you want to ask a question; ask it. Interesting topics/questions will beget constructive dialog without being labeled a 'game''.

Clearly you're an intelligent person, but as you know, tone is lost in the written form. Eschew obfuscation, , and all will be well.

Indeed! I was originally aiming at joking and fun instead of being argumentative, but yeah it didn't work. I love your 'eschew obfuscation' Wasn't intending to obfuscate though, just jumped into the issue without the context.

[Timfish]

I'm going to throw my two cents in and try to comment on the original post as if I haven't read all the others (although there are some good points and funny gifs). As I understand the original post the if the filtrate side is bathed in a salt solutoin it will increase the efficiency of the RO membrain elliminate the need to have pressure on the membrain and concurrently increase it's life expectancy. With the RO and RO/DI units currently used the effeciency of the membrain is increased with pressure on the feed side. It seems to me that unless new technology is used a much larger RO membrain will be needed to get the quantities required. Some kind of storage container will be needed on the filtrate side to hold the salt solution if line pressure is used on the feed/waste side otherwise two storage containers will be needed. If line pressure is not used on the input/waste side two pumps and two storage containers will be needed, this is assuming the membrain is housed in side some kind of container like they are with our RO units now. Alternatvely a special cotainer can be design with the membrain unrolled and seperating the salt solution and the feed water. I don't see how the membrain can be kept clean in either of these scenarios. Membrains are kept realtively clean with the compination of mechanical prefilters and pressure flushing drebris and bacteria off it but they still get clogged. It seems to me running them in a scenario where there is no pressure on it some mechanism must be in place to keep it from getting clogged up. It's an intriguing idea but I keep seeing this getting more and more cumbersome, at this point I would want to see some specifics on the research showing a salt solution on the fitrate side actually does improve effeciency of the RO membrains we use.