RO Question

[mikev63]

I just got a new membrane for my RO system and ran the system for over 1 hour and I am reading 8 TDS on the meter? Should I be getting 0 since this is a new membrane?

The membrane I have is a TFM-75.

Has anyone expierenced this ? Is this normal?

Mike

 

[poppin_fresh]

It depends on the incoming water TDS. If your 75gpd membrane is rated for 98% rejection and incoming water is 100, outgoing will be no better than 2. This number goes up as TDS of incoming water goes up. You will not get 0 from RO only, you will need to add a DI unit to get 0.

 

[mikev63]

The incoming water is 295. I do have a DI unit also. Thats is why I was thinking I should get a 0 reading.

 

[poppin_fresh]

Your DI resin might be tired. How old is it?

 

[purelyh2o]

MikeV

MikeV

A membrane will not give you 100% rejection. 8 is a good number. you are using a 75 gpd Dow Filmtek membrane which is the best you can buy for rejection. Are you testing this water after the DI or before? If you are reading 8 after the DI you do need a new DI cart

Bryan Crenshaw

 

[purelyh2o]

Also,

Also,

Try this,
Take a calculator and take your incoming TDS which is 295 and minus 98%

295- 98% = 5.9 Not quite 98%

295- 97% = 8.85 better than 97%

So your membrane is rejecting better than 97% of the TDS from the water which is definitely acceptable.

If you want to acieve 0 TDS you will need to get an add on DI kit and add that to the line after the RO. The good news is that with a TDS level of 8 going into the DI it will last a long time


Bryan Crenshaw

 

[dchao]

Also, it's winter now, the tap water are a lot colder, membrane performance is reduced when the temp of the source water is colder (The 98% rejection rate is tested at 77Ã"šÃ‚°F).

 

[AZDesertRat]

You don't say if the 8 TDS is after the membrane or after the membrane plus the DI. You need to test after the membrane only to tell if it is working correctly.
Test your raw water TDS, then your RO only TDS and finally the RO/DI TDS. You should see a 98% reduction between the raw and RO and RO/DI should be 0 or no more than 3. I suspect your DI resin is shot and in need of replacement.

 

[ahill3780]

295?! Wow, is this common? My TDS comming in from my garden hose is 31 and out from DI is 0. I hadn't thought Atlanta's water would be so 'clean'!

 

[AZDesertRat]

Nationwide 150 to 250 is pretty common. Here in Phoenix and much of the Southwest in varies from 500 to over 1000 depending on the source and blend of the waters. Last week my raw water was 623, my RO only output was 8 and RO/DI was 0.

 

[dchao]

San Fran is 33ppm, not too bad either, I love soft water. I hardly get anything out of RO output