Tds

[shaggy14]

Just got my RODI and wanted to check and see what an acceptable TDS reading is before topping off my tank. Reading before RODI is 610ppm!!!! Reading afterwards is 8ppm. I know everyone tries to have as little TDS as possible, but is 8 ok?

[danferd2002]

It should be 0. For a new ro/di you should toss out the first 10 gallons or so of water, then your tds should start to drop.

[shaggy14]

well i bought it used, it had around 300gallons ran through it. it is a 75gph unit, typhoon. if i fill up a cup and test it, tds reads 6-8. if i run it over the tds meter it reads 2. a gallon of distilled water reads 1

[AZDesertRat]

Always use a squeaky clean clear glass drinking water glass with absolutely no water spots or soap residue, its best to wash it by hand not in a dishwasher and in very hot water to ensure cleanliness.
First test your tap water TDS. Triple rinse the meter and glass in DI or distilled water. (I would use distilled at this point until you determine if the DI is good)
Next test the RO only TDS and again triple rinse the meter and glass with DI or distilled water.
Finally test the final RO/DI TDS and do a final triple rinse in DI or distilled. Put the meter away capped and turn the glass upside down to keep dust out.

It sounds like your DI is shot for sure. You want to replace DI resin at the very first signs of anything other than 0 TDS.
Your membrane should be 96-98% rejection. You can calculate this with the first two TDS readings you get from above.
Take your tap water TDS (610?) and subtract the RO only TDS (lets call it 20 just for the heck of it for now). So 610-20= 590. Now divide that number by the originbal 610 and you get .967. Now multiply that number by 100 for the removal or rejection efficiency. You see at 20 the RO rejection would be 96.7%.

If the membrane is not somewhere in that 96-98% range you will go through DI resin like crazy so it is important it is working well.

[shaggy14]

ok, ill test the ro only

[shaggy14]

ro only was 62, which means im getting an 89.8% rejection. guess ill order some new cartridges. which ones do I need to replace? just the RO membrane, or just all five chambers?

[ALH]

I'm a water chemistry idiot so please feel free to ignore this post. I also have TDS of around 600 from local water supply. I installed a brand new 5 stage RODI unit and after dicarding the initial water output the best TDS I got was 8. I just decided to be happy with that. I am going to have to spend more money replacing filters and resin to keep it there, but that's what I get for living in a town with crappy water. My last FOWLR that I had ten years ago was a tap water tank (I was even more ignorant then) so I figure I'm doin a helluva lot better than that so it's all good.

[AZDesertRat]

If you have the 75 GPD membrane then you are not doing too good at 89%, it should be 96-98%. The difference is really significant when you consider fro every 2% you increase the RO efficiency you double the life of your DI resin. It does not take long for the cost of DI replacements to exceed the cost of a new membrane at 89%.
If the filters are anywhere near 6 months old I would replace everything. I can't imagine the membrane being only 89% if its not that old, they should last 3 years or more if you keep up with the replacements.