Which RO filter is giving TDS's?

tangtang clown

tangtang clown

New member
I recently purchased a used Oceanus RO/DI system and a DM2 TDS meter. The DI canister is seperate from the RO unit.
I hooked up the meter so it is reading the unit water coming from the RO unit and going out of the DI unit.

The meter was reading 12-16 TDS coming out of the RO and 0 coming out of the DI. I changed the 2 white filters, the 2 that water goes through first, and it dropped the TDS's to 4 and still 0 coming out of the DI side.

I have an extra membrane, should I change it or leave the it like it is?
 
RO filters aren't perfect, seeing a small amount of TDS out of the RO is common. The actual amount depends on the TDS of your supply water and on the waste/product ratio leaving the RO membrane. You should have 4-5 times as much waste water as you do product water with a standard membrane. If your ratio is closer to 2/1 or 1/1 then the result will be more solids being forced through the RO membrane.
 
The membranes have a rejection rating. For example, the dow filmtek 75 GPD unit is rated at, I believe, 96% to 98% rejection. So if the TDS of my water supply is 100, after the membrane the TDS should measure between 2-4 approximately. So having a TDS of 4 after your membrane doesnt tell you much about how well the membrane is working without knowing the TDS of the water supply. If you have a very low TDS, like 50 then 4 isnt all that great with a 98% rejection membrane. If it is 150 then 4 is pretty good.

BTW, I have always gotten 98% atleast with my dow filmtek 75 GPD membranes. My Supply TDS is about 160 and after membrane TDS is 2 or 3. The lower it is post RO the longer your DI resin lasts and even a difference of 1 or 2 ppm TDS makes a difference in resin life.

So anyway, measure the TDS of your supply water and you will then know what the rejection percentage of your membrane is.
 
But the rejection rate won't be right if the waste/product ratio isn't right. My supply TDS is similar yours DarG, if I restrict my waste water line, so the ratio is 1/1 or 2/1 the TDS of my product water jumps from 1 or 2 up to 8 or 10. That's with a 75gpd Dow Filmtek membrane too, so it's rejection rate drops from 99%+ to only 95%. You can force more particles through the membrane than the quoted rejection rate if it isn't set up correctly.
 
<a href=showthread.php?s=&postid=14832293#post14832293 target=_blank>Originally posted</a> by woz9683
But the rejection rate won't be right if the waste/product ratio isn't right. My supply TDS is similar yours DarG, if I restrict my waste water line, so the ratio is 1/1 or 2/1 the TDS of my product water jumps from 1 or 2 up to 8 or 10. That's with a 75gpd Dow Filmtek membrane too, so it's rejection rate drops from 99%+ to only 95%. You can force more particles through the membrane than the quoted rejection rate if it isn't set up correctly.
Click to expand...

No argument. But the fact still remains that you need to know what the TDS in is before you can determine what the rejection rate is. I wasnt contradicting your statement, I just added to it.
 
I have been trying to keep my psi at around 60. The original owner said he ran it at about 55psi. I know my waste is more than my product, but I don't think it is quite 3:1. I think that is my goal for this unit, 3:1.

What PSI are you guys running?
 
Meter was brand new. It said do not touch the sensors with fingers, and that it was factory calibrated. I switched the two sensors and both ways it read 4ppm before DI and 0ppm after DI so I am pretty sure they are right. I am going to hook it up to my feed water and see what % I am getting.
 
You can't calculate you membrane percentage by using the incoming water prior to the prefilters and the good "product" water.

In order to accurately figure out the rejection rate of your membrane you need to know the TDS going in to the membrane and the TDS coming out of the membrane - PERIOD!

All the other prefilters, DI, etc change the numbers and need taken out of the equation.

I also run my unit at 90 PSI
 
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I'm not sure what membrane you're using, but I'm pretty sure the ratio is supposed to be at least 4:1 or 5:1. Many people get away with less than this (i.e. 3:1 or 2:1) but that just means more particles are getting through the membrane and making your DI resin work that much harder. To get a ratio of 4:1 or 5:1 you're probably going to have to put a flow restrictor on the product water line. That being said though, if you're only getting 4ppm out of your RO then that's really pretty good, that doesn't leave much for the DI to have to take out. But, if you want to add a flow restrictor all you need is a little john guest ball valve. Your ratio is already pretty close so you would just have to valve back the product water slightly, and it might drop your TDS down to 1 or 2.

Running the unit at 60psi is probably perfectly fine. Especially if it's a Dow Filmtek membrane in there, they advertise that their membrane's run better at lower line pressure than some of their competitors. Of course they all run fine at higher pressures too. If it's not a Dow Filmtek you might consider replacing your current membrane with a Dow next time you change, but certainly don't rush to do it now or anything. The membrane should run just fine at 60psi, a Dow might be just slightly better for your next choice.

I run my unit at 75psi, and I had to get a booster pump to do that. My line pressure only runs between 35-50psi and is towards the lower end of that range most of the time. It's really quite pitiful :(

Anyway, it sounds like any issues you have are very minor, so good luck! HTH
 
You can just use a capillary tube type flow restrictor on the waste line to fine tune the waste to product water ratio. I havent used one but they are cheap and are meant to be trimmed to fine tune the ratio. Many of the online sites sell them like the filter guys and I believe bulk reef supply may as well. In parts of the country that actually experience different temps throught the seasons, some have a capillary tube type flow restrictor tuned for winter and one for summer ... water temp can affect the membrane performance and adjusting waste water ratio can be used to compensate.

But I agree, with 4 out the RO you are in great shape already. Depending on what your supply TDS is (or what it is after prefilters) that may be as good as it can be expected to get or it may be fine tuneable to slightly better.

I use the standard inline flow restrictor for the 75 GPD membrane and my waste to product water ratio is just about 4:1.
 
I certainly have to run a pressure booster on mine to get it up to 90 PSI. I also dial it in at 4 to 1 waste to product water.

The only other thing I wold suggest doing, especially if you drink the water as well, is to clean out all the housing (not filters or membranes) occassionally with bleach but that's a whole nother story.
 
<a href=showthread.php?s=&postid=14842986#post14842986 target=_blank>Originally posted</a> by BuckeyeFS
Filmtec membranes are spec'ed with a 15% recovery, or about 5:1

Russ
Click to expand...

Thank you.
 
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