RO Membranes Filters & TDS Meters

FirstContact · Mar 27, 2018

Hi Everyone,

I'm running a double RO membrane on my unit and have about 60 psi. I typically use a 1.0 micron sediment and 0.5 micron carbon filter. Should I be using 0.5 sediment and 0.5 carbon? I typically would use 2 75 gpd membranes, but right now my first is a 100gpd Puratek and the second is 75gpd DOW. My combined TDS membrane output is about 15, so I think one of the membranes is blown. My state water service does not use chloramines, so I am confused why one of the membranes would have blown so prematurely. I just set up new filters and added a new membrane and switched to double membranes last August or so. I change pre-filters every six months and my system is 220g plus 70g in 2 QT tanks.

Also, I need a new in line TDS meter. I was thinking about using a triple and putting one after each membrane output and one after my first DI stage.

Appreciate any advice or suggestions.
Thoughts? Advice?

DesertReefT4r · Mar 28, 2018

Your membranes need to be the same gpd rating. Contact BRS or Spectrapure for help on anything RO related.

JoshH · Mar 28, 2018

What kind of TDS do you have entering from the street? I wouldn't be concerned about running two different types of membranes, I do with no issue.

KJAG · Mar 28, 2018

Sounds like you’re relying on readings from an inline TDS meter, which are usually inaccurate. I wouldn’t add more, I would stop relying on any of them. I literally cut mine off from my Spectrapure Maxcap 180 and bought a handheld HM Digital meter. Also what is your rejection rate. Chloramines aside, what is your incoming TDS? What sediment and carbon filters are you using? I would also look into your local jurisdiction water source, as in my State(CA) it varies from city to city. You’re saying your TDS statewide is the same? I doubt it. 15 TDS RO isn’t great but isn’t entirely unacceptable, and anyone telling you that their RO values are zero are simply wrong. Your DI should be 0 however. I’ve had excellent luck with the Spectrapure string woven sediment filters and the Enduro/ Mega Max DI cartridges.

FirstContact · Mar 28, 2018

I have a handheld TDS meter but haven’t had a chance to decouple the lines an measure the membrane outputs independently. I can measure the tap water and individual membrane outputs tonight. Can’t remember the value last time I measured; it was pretty good, but things change. Sorry, my municipality/water provider does not use chloramines. I look up their reports online and have called them to confirm. I feel something is wrong/different because when I use a single membrane and have for a decade in my current location, the output from a 75g membrane is normally 2 TDS. Either a membrane is blown, or it’s the 100g membrane not operating as efficiently, or it’s 2 membranes operating together with the waste line going into the second, or the municipality water provider has changed something. The filters I use are described in my OP. Guess I’ll test tonight with the handheld.

chipmunkofdoom2 · Mar 28, 2018

FirstContact said:
My state water service does not use chloramines, so I am confused why one of the membranes would have blown so prematurely.

Chlorine is actually much more harmful to Dow TFC membranes than chloramines. From Dow's literature:

Eventual degradation may occur after approximately 200-1,000 hours of exposure to one mg/l of free chlorine (200-1,000 ppm-h tolerance).

However, regarding chloramines:

For chloramine the tolerance of the FT30 membrane is 300,000 ppm-h, which implies that dechlorination is not required. However, since chloramines are formed by adding ammonia to chlorine, it is possible that free chlorine will be present. Since this free chlorine can be damaging to the membrane, dechlorination should still be considered.

So, per Dow's own literature, 1ppm of chlorine going into your membrane can ruin it in as little as 200 hours, while dechlorination is technically not required for chloramines. Obviously, chloramines are problematic in our application, so we would want to dechlorinate anyway. But still, the point is that chlorine is much more harmful to TFC membranes than chloramines. For this and other reasons, unless you feel like doing regular and thorough testing, your prefilters should be replaced every 6 months on a schedule. Prefilters are mainly to protect your RO membrane. You'll never tell that your prefilters are shot by looking at your final TDS. You'll only be able to tell after you start eating DI resin because you ruined your membrane with chlorine.

As to whether your membranes are functioning properly, you can't tell this by the post-membrane TDS alone. You need to know your tap water TDS and compare the two. Take a measure of the TDS of your tap water. Then, divide your post-RO membrane TDS by your tap water TDS. This will give you the amount of solids making it through the membrane. Let's say your tap TDS is 400ppm. 15 / 400 ppm = 0.0375. Per Dow's manual, your membranes should be rejecting at least 96% of solids, meaning 4% can make it through. If your tap water is 400 ppm, only 3.75% of solids are making it through, so you're good. If instead your tap water TDS is 200, that's 0.075, or 7.5% of solids making it through. This is too high, and one or both of the membranes may need to be replaced.

Keep in mind that dual membrane setups are incredibly hard on the second membrane. Additionally, there's no reliable way to ensure that the membranes are getting the waste water they need to stay clean. Expect a shortened service life from the second membrane at the very least in a dual RO membrane setup.

FirstContact · Mar 28, 2018

Okay! Did some testing. Tap water is 175 ppm. First membrane a 100gpd Puratek is producing 2 ppm and the second membrane a 75gpd dow is producing 35 ppm. So, I'm getting a better than 98% rejection rate on the first membrane and a n 80% rejection rate on the second membrane. The second membrane is definitely blown. Not sure if it's because of the membrane age, or if it was in my single set up and I ran the pre-filters too long, or if it's just the tandem set up that wore it out. Have to go back and read my logs.

JoshH · Mar 28, 2018

35??? Yeah definitely gone lol how old is the 75GPD membrane??

FirstContact · Mar 29, 2018

Yup! I probably stored it too long in the basement before using it when it should have been in the fridge with some water in a zip lock bag.

Buckeye Hydro · Apr 8, 2018

175 is your feedwater to the first membrane... but the feedwater to the second membrane is going to be much higher than that.

FirstContact · Apr 8, 2018

Thanks Russ. I should get around to measuring that too. Hopefully, this new membrane lasts a lot longer.

croix67 · Jun 20, 2018

can any add some diagrams on how to add a second membrane want to use two 50 gallons

Buckeye Hydro · Jun 20, 2018

Would you tell me what your goal is in adding a second membrane?

Russ

croix67 · Jun 21, 2018

you cut down on waist water

JoshH · Jun 21, 2018

BRS did a video on it let me have a look, what kind of membrane do you have now??

Buckeye Hydro · Jun 21, 2018

In most situations. it is not a good idea on these low flow, low pressure systems to plumb the membranes in series without changing the flow restrictor... regardless of what you may have heard/seen. There are exceptions.