Multiple RO membranes

[Syntax1325]

My tap tds is 25. I have two portable tds meters, one is a hannah and one is from Air Water and Ice: Hannah says 25 and Air water and Ice says 23.

 

[CStrickland]

... Has anyone ever used lanthanum cloride i mix water before mixing with salt?

I used tap water for a while, ran phosban as a precaution. IDK if it did much good tho since I only had a API test back then. I think LC would get kind of expensive. I'd be more worried about what else is coming in along with the phos, like you said about ag runoff - could be antibiotics, pesticide, etc.

If chloramine does not produce algae, then I'm stumped.

It doesn't

Ok, so I bought a 5 stage 75gpd RO/DI system, too fast, and then regretted it. So now I'm ordering another DI canister, and a "150gpd RO upgrade kit", which basically runs the RO wastewater directly into another membrane and filters it again.

The question I have is, how many times can this be done? 3 RO membranes? 4?

I think there would be a point of diminishing return, like the others have said.

I'm curious what you regretted about the one you bought? If it's one of those ghetto ebay jobs that you have to buy replacements that are custom-sized, and the membranes don't work well, and the production is too slow, and the build quality is weak etc. It might be wise to cut your losses and buy a decent unit from buckeye, or spectrapure.

Just thinking if you are already going through this trouble and you haven't even made any water, maybe rethink the project.

 

[alexkharden]

No, I bought a pretty nice one from the filter guys. I just decided that I regretted it because street more reading I decided I wanted dual ro and dual di, but the one I ordered was only a five stage. It was a minor regret, no a "holy crap what did I do" regret, lol.

 

[dkeller_nc]

No, I bought a pretty nice one from the filter guys. I just decided that I regretted it because street more reading I decided I wanted dual ro and dual di, but the one I ordered was only a five stage. It was a minor regret, no a "holy crap what did I do" regret, lol.

Then I would say you made little, if any, mistakes. While you can buy customized dual RO membrane, dual DI cartridge systems, the price isn't all that much of a discount over buying the parts and doing it yourself.

With respect to your original question, there are two pieces to the "how many membranes can I run in series" question. The first is pressure; since all hobbyist systems use a flow restrictor on the waste side, then the pressure to your second (or third, fourth, fifth, etc...) will be reduced after each RO membrane. Practically speaking, you can't boost the upstream tap pressure much beyond 130 psig because the fittings and membrane housing of the first stage won't be able to take the pressure and will leak.

The second limitation is the dissolved substances in your water. Certain common dissolved substances, such as divalent cations like calcium, magnesium and manganese, can combine with small amounts of sulfates, phosphates or carbonates in the water to form insoluble precipitates. Because the concentration of these substances is highest at the surface of the RO membrane, that's where they will precipitate and plug the membrane. The 1:4 or 1:3 product to reject ratio that one often sees as the rule of thumb for proper operation is just that - a rule of thumb. If your tap water was free of sulfates, carbonates and divalent cations, then you could run a 9:1 product to waste ratio without issue, giving you a 90% production rate with only a 10% waste. Commercial systems achieve these sorts of ratios with ordinary water by injecting chemicals that prevent the precipitation of calcium & magnesium carbonate and sulfate.

If your incoming water is very low in TDS and also quite low in sulfates and carbonate hardness (municipal water rarely has significant phosphate), you can run your system with a considerably higher product to waste ratio than the common rule of thumb. I'd guess that most in the Eastern US with characteristically low municipal TDS can safely run a 1:2 waste to product ratio - that's how my system runs. In a municipality that gets its primary feed from Colorado river water that's infamously nick-named "liquid rock", a product to waste ratio of 1:6 or 1:8 might be more appropriate.

 

[alexkharden]

Then I would say you made little, if any, mistakes. While you can buy customized dual RO membrane, dual DI cartridge systems, the price isn't all that much of a discount over buying the parts and doing it yourself.

With respect to your original question, there are two pieces to the "how many membranes can I run in series" question. The first is pressure; since all hobbyist systems use a flow restrictor on the waste side, then the pressure to your second (or third, fourth, fifth, etc...) will be reduced after each RO membrane. Practically speaking, you can't boost the upstream tap pressure much beyond 130 psig because the fittings and membrane housing of the first stage won't be able to take the pressure and will leak.

The second limitation is the dissolved substances in your water. Certain common dissolved substances, such as divalent cations like calcium, magnesium and manganese, can combine with small amounts of sulfates, phosphates or carbonates in the water to form insoluble precipitates. Because the concentration of these substances is highest at the surface of the RO membrane, that's where they will precipitate and plug the membrane. The 1:4 or 1:3 product to reject ratio that one often sees as the rule of thumb for proper operation is just that - a rule of thumb. If your tap water was free of sulfates, carbonates and divalent cations, then you could run a 9:1 product to waste ratio without issue, giving you a 90% production rate with only a 10% waste. Commercial systems achieve these sorts of ratios with ordinary water by injecting chemicals that prevent the precipitation of calcium & magnesium carbonate and sulfate.

If your incoming water is very low in TDS and also quite low in sulfates and carbonate hardness (municipal water rarely has significant phosphate), you can run your system with a considerably higher product to waste ratio than the common rule of thumb. I'd guess that most in the Eastern US with characteristically low municipal TDS can safely run a 1:2 waste to product ratio - that's how my system runs. In a municipality that gets its primary feed from Colorado river water that's infamously nick-named "liquid rock", a product to waste ratio of 1:6 or 1:8 might be more appropriate.

Well, I think my tds is pretty low here, <70, if I got an accurate reading. Will my RO membrane automatically produce at whatever rate it's capable of, given my water? Or do I need to adjust the pressure or the membrane or something to get a higher product/waste ratio?

 

[shifty51008]

Temp and water pressure will determine how much your membrane can produce. You can make a 75god membrane produce 90gpd with good water pressure and temp. But you will always want to have a 4:1 waste to good, or you can kill the membrane prematurly as it isnt flushing properly. While you can do a 1:1 a membrane that may last 4-5 years with proper 4:1 a 1:1 membrane will maybe last a year or less.

 

[dkeller_nc]

Well, I think my tds is pretty low here, <70, if I got an accurate reading. Will my RO membrane automatically produce at whatever rate it's capable of, given my water? Or do I need to adjust the pressure or the membrane or something to get a higher product/waste ratio?

Your waste to product ratio is fixed - it's a result of the restrictor insert that's in the waste side of the line. And as shifty notes, the production rate of the membrane is a function of the temperature of the incoming water (higher = faster production rate) and pressure. Generally, the production rate of an RO membrane goes up with increasing inlet pressure up to a point - you generally don't want to run the differential pressure across the membrane any higher than 80 psig. For a single-stage RO set-up, that differential pressure is the incoming tap water pressure minus the outlet pressure, which is generally zero. For a two membrane set-up, it's a bit more complicated; the differential pressure is the incoming tap water pressure minus the pressure upstream of the 2nd membrane, which will be considerably above zero. You'd need to install a pressure gauge on the 2nd membrane housing to calculate the differential pressure across the first membrane.

 

[alexkharden]

Wait, I thought I read somewhere that higher temperatures would destroy the membrane, but higher temperatures will cause it to produce more? I assume only up to a point, then the destruction happens.

 

[shifty51008]

Correct, to hot of water can kill a membrane so trying to get the temp of incomming water is hard to do and why most people dont even worry about it and just run the cold water only. I have seen people run a long tube from the tap to the unit and have the excess tube go into a bucket of water that is temp controlled, while it may help i dont think it is worth doing, i dont think the ammount of extra water produced will make up for the extra electricity to keep the water at temp.

 

[alexkharden]

Gotcha, thanks!

 

[Buckeye Hydro]

Yeow there is a lot of "kinda-right-but-not-quite" info in this thread...

Consult a trusted vendor before you buy!

Russ