Question on Changing RO Membranes

[Andy V]

If I swap from a 75gpd to a 150gpd RO membrane, the only thing I have to worry about is swapping the flow restrictor, correct? Everything else is the same?

 

[MikeG]

they are all the same, just need to swap the appropriate flow restrictors as you mentioned. Also I think a lot of the higher producing usits do this by using 2 RO membranes, might just want to make sure.

 

[JohnK]

Agreed. Also keep in mind that higher flow may mean a much lower rejection rate wich will cause the unit to blow through DI cartridges much more quickly. (Hence the popularity of the 75gpd membranes with their very high rejection rate) DI can get expensive fast....

 

[Andy V]

What about some of the newer units that claim to have lower rejection rates? Is this because of a better design or just lower rejection that will destroy DI?

 

[JohnK]

Just different membranes as far as I know. The internals of basically all standard RO and RODI filters are interchangable/universal.

A lower rejection rate means less waste water, but more impurities in the waste water. Depends on the applicaion/needs. If you want to make really pure water (like we do) then it's a lot more practical to use a higher rejection rate membrane. If someone were not using DI, and or had very low TDS water in the first place, then a higher rejection rate might be acceptable and would waste less water.

Does this make sense, or am I missing your question?

 

[gweston]

Better membranes have around 98% a rejection rate. Dow Filmtec, which seems very common, runs in that range. There are some other products that run in the 96% range. Unsure if any others are worse than that. Just watch out for what membrane you get when you order. I once ordered a Filmtec and got a knockoff that looks nearly identical to the Filmtec, with a 96% rate. The color and label was essentially the same, but the written name was different. I determined the rejection rate by asking for a product spec sheet from the seller.. the spec sheet was in an ancient microsoft works format. Which I found humorous. It took a few extra minutes to figure out how it could be converted to a usable format.

Maybe the lower the rejection rate, the chance for impurities get to the DI? So maybe DI would last a bit longer with a 98% membrane, than with a 96% membrane? I have -no idea- what that translates to in actual DI life expectancy. Again, that is a guess. No idea if that is correct.

DI can also be wiped out faster if the water quality is sketchy to begin with. My area have very good water, but they do have a turbidity problem. Which I suspect affects life of sediment/carbon filters. The town upgraded their water facility a few years back, so the turbidity issue is FAR less than it was when I first moved to the town <10 years ago. My current filter set is about 1 year old and it seems to be going strong. But my volume is rather low at the moment. Soon it won't be with the tank upgrade.

 

[Andy V]

Ok, so before I was conflating the rejection rate with the amount of waste water. The rejection rate is the amount of material rejected by the RO membrane before it hits the DI resin. So the higher, the better. On the other hand, the lower the amount of waste water, the better.

What I was trying to say is that there are some companies claiming 150gpd with >98% rejection rates, which is what I am ideally looking for. Do I have this all straight now?

 

[JohnK]

Yup, a 98% rejection rate means that 98% of the impurities are filtered out. So if you have 400 PPM tap water and a 98% membrane, you should get 8ppm coming out of the RO with everything working right. With a 96% membrane you would get 16ppm for the DI to deal with, so at 96% you will use up the DI twice as fast.

On brands, (BTW, that's funny yet scary about the look alike ), filmtec makes several membranes. The 75GPD has the 98% rejection rate, but several of their other membranes have lower rejection rates.

I did look at the listings on airwaterice.com and they do show a "new" 150gpd 98% rejection rate membrane, but it does not appear to be made by filmtec. This may or may not be of the same quality, I don't know. IME those guys are very good to deal with and give good advice, I'd suggest calling them and asking if they have the same confidence in the 150gpd membranes.

 

[Cheetos]

If you have a 75 gpd unit you can just piggyback another 75 gpd membrane with flow restrictor and get 150 gpd with a 98% rejection rate. I just bought a second membrane from airwaterice for the reason that I hate waiting for RO water. With the new large tank (125G and 100G sump) I go through a lot of water so this worked out perfectly. You do need at least 65 psi of water pressure to run the two membranes. I had to add a booster pump as I only had 50 psi. I can now make 15 gals of RO in 2 hours with the new setup. 100-110 TDS going in and 1 TDS coming out before the DI.

 

[dcforester1]

If you have a 75 gpd unit you can just piggyback another 75 gpd membrane with flow restrictor and get 150 gpd with a 98% rejection rate. I just bought a second membrane from airwaterice for the reason that I hate waiting for RO water. With the new large tank (125G and 100G sump) I go through a lot of water so this worked out perfectly. You do need at least 65 psi of water pressure to run the two membranes. I had to add a booster pump as I only had 50 psi. I can now make 15 gals of RO in 2 hours with the new setup. 100-110 TDS going in and 1 TDS coming out before the DI.

Thats what I was going to say. I went from a 75, to a 150, and couldnt make water. I finally found out I didnt have enough PSI to run it, so I went back to the 75.