TDS Readings Pre-DI/Post-DI - Possible Bad Membrane?

neotactical · Dec 21, 2022

I have a 75GPD BRS 5 stage RO/DI unit. I've upgraded the TDS meter on it and I'm currently running the in prior to going into DI and the out after. The in is reading 14 and out is reading 5.

I realize that this isn't the typical setup for TDS, but I just rebuilt the unit and installed the TDS incorrectly. However - I'm concerned that it's coming in so high after passing through 4 stages.

All cannister cartridges are a month old. The R/O membrane has about 15 months into it. I'm guessing it's the R/O membrane? The D/I resin is also just a month old.

Opus · Dec 21, 2022

Impossible to say since you don't give your tds at the tap. A BRS unit should be giving you about a 98% efficiency but most RO membranes only guarantee 95%. Once you drop below 95% it is best to get a new one. My membrane is about 3 years old and I'm still getting 99% efficiency from it when I checked it a few months back.

As for the DI, with a tds of 14 going into it, you will get about 300 gallons of 0 tds water before it starts going up. If you haven't made 300 gallons then you could also have an issue with the di. If you used bulk di and packed the cartridge yourself, the most common issue is the di media isn't packed tightly enough in the cartridge and it has formed a pathway thru the media.

Randy Holmes-Farley · Dec 21, 2022

Assuming the numbers are accurate, the DI is not doing its job, either because it is partly depleted, or water is channeling through it.

There's no evidence from what you wrote about a membrane problem, but if you measure the tap TDS and the post membrane TDS (14 ppm TDS?) then that will tell.