RO/DI TDS readings

Phil D. · Mar 23, 2020

So I recently changed my filters, RO membrane and DI cartridges and checked the TDS of all the various stages. Out of the faucet =375, out of the RO membranes, (I have the water saver kit with 2 membranes), 6 and finally out of the dual DI cartridges = 0!! So, I guestimate I will be able to make a lot of water. What do you guys think?

Ron Reefman · Mar 23, 2020

Sounds fairly good to me. The RO reading seems a tiny bit high at 6. Is your dual membrane inline or parallel? That is, does all the water pass through one membrane and then the second, or is the input split in two and water only passes through one membrane?

My system starts with similar TDS and reads 2 or 3 out of the RO. But I have 2 BIG RO membranes. The stainless steel tubes are the RO membranes.

Phil D. · Mar 24, 2020

The dual RO's are set up where the waste water goes from the first one into the second. Then the product water is a "T'd" together output into the DI's

Ron Reefman · Mar 24, 2020

Then I'd say your TDS of 6 isn't bad... not great, but not bad. I'd live with it.

Phil D. · Mar 27, 2020

Ron Reefman said:
Then I'd say your TDS of 6 isn't bad... not great, but not bad. I'd live with it.

Final out of the DI is 0

CJBuckeyes · Mar 27, 2020

I have a dual membrane setup as well, and get a reading of 4 before it goes to DI. My source water is around 600 tds.

I think your reading of 6 is fine. The tds meters likely aren’t that accurate.

Water Dog · Mar 27, 2020

CJBuckeyes said:
I have a dual membrane setup as well, and get a reading of 4 before it goes to DI. My source water is around 600 tds.

I think your reading of 6 is fine. The tds meters likely aren’t that accurate.

Wow! For 600 TDS source water, insanely efficient! What brand RO membranes are you using?

Ewok1 · Mar 27, 2020

Phil D. said:
So I recently changed my filters, RO membrane and DI cartridges and checked the TDS of all the various stages. Out of the faucet =375, out of the RO membranes, (I have the water saver kit with 2 membranes), 6 and finally out of the dual DI cartridges = 0!! So, I guestimate I will be able to make a lot of water. What do you guys think?

0 is always good. Anything above=bad and you’ll get algae and dead corals overtime.

Ron Reefman · Mar 27, 2020

Ewok1 said:
0 is always good. Anything above=bad and you’ll get algae and dead corals overtime.

Whoa there @Ewok1! IMHO that statement is WAY, way out of line.

TDS readings above zero are not a problem unless the cause is something like a heavy metal or some other toxin. Both of which are quite rare, especially from city water sources.

Lots of reefers use water with TDS readings from above zero to as high as 10 or 20 and have no issues.

The most common cause for TDS readings above zero are harmless. A friend of mine has a 200g tank with lots of fish and corals. He uses well water that he does NOT run through an RO/DI system. The TDS starts at very high numbers that change a lot over time, but are alway in the 800 to 1200 range. He then stores it in large open tanks with lots of surface open to the air. He treats the water with chlorine and circulates it for several days and then lets it sit fallow for a day. He then runs the top 98% of the water through a simple filter and ends up with water having TDS readings in the 100 to 200 range. If it's higher he filters it again. He's not killing fish, has great looking corals and he has very little issue with algae.

Miller535 · Mar 27, 2020

Ron Reefman said:
Whoa there @Ewok1! IMHO that statement is WAY, way out of line.

TDS readings above zero are not a problem unless the cause is something like a heavy metal or some other toxin. Both of which are quite rare, especially from city water sources.

Lots of reefers use water with TDS readings from above zero to as high as 10 or 20 and have no issues.

The most common cause for TDS readings above zero are harmless. A friend of mine has a 200g tank with lots of fish and corals. He uses well water that he does NOT run through an RO/DI system. The TDS starts at very high numbers that change a lot over time, but are alway in the 800 to 1200 range. He then stores it in large open tanks with lots of surface open to the air. He treats the water with chlorine and circulates it for several days and then lets it sit fallow for a day. He then runs the top 98% of the water through a simple filter and ends up with water having TDS readings in the 100 to 200 range. If it's higher he filters it again. He's not killing fish, has great looking corals and he has very little issue with algae.

While I do not agree with his statement that it WILL result in algae and dead corals. It can happen. Some places have really clean water, some do not. A tds of say 10 could be harmless, but it may not. And for the average reefer, and all the more water the average reefer uses, changing the filters a little earlier isn't going to make more then a few bucks difference in a year. So why even take the chance?

K7BMG · Mar 27, 2020

Ron Reefman said:
Whoa there @Ewok1! IMHO that statement is WAY, way out of line.

TDS readings above zero are not a problem unless the cause is something like a heavy metal or some other toxin. Both of which are quite rare, especially from city water sources.

Lots of reefers use water with TDS readings from above zero to as high as 10 or 20 and have no issues.

The most common cause for TDS readings above zero are harmless. A friend of mine has a 200g tank with lots of fish and corals. He uses well water that he does NOT run through an RO/DI system. The TDS starts at very high numbers that change a lot over time, but are alway in the 800 to 1200 range. He then stores it in large open tanks with lots of surface open to the air. He treats the water with chlorine and circulates it for several days and then lets it sit fallow for a day. He then runs the top 98% of the water through a simple filter and ends up with water having TDS readings in the 100 to 200 range. If it's higher he filters it again. He's not killing fish, has great looking corals and he has very little issue with algae.

Respectfully. Yes and thats fine FOR HIM.
But that does not mean that will work for anyone else.

If your TDS meter reads 0 on the output theres no doubt you will be ok.
It does not matter what it reade on the source but its nice to know.
It does not matter what the reading is in between your RO and DI canisters. But agan its nice to know.

The nice to knows will help determine when things need replaced.
As long as you have 0 on the output you know your golden.

Miller535 · Mar 27, 2020

CJBuckeyes said:
I have a dual membrane setup as well, and get a reading of 4 before it goes to DI. My source water is around 600 tds.

I think your reading of 6 is fine. The tds meters likely aren’t that accurate.

Most of them can be calibrated. BRS even sells calibration solution for them. The HM that's on mine I BELIEVE says to calibrate it every couple of years. But I admittedly have not.