What's an acceptable TDS meter reading?

[nmbeg]

I have had my RODI going for a year now on a 125-gallon system, and never changed the filters.

The TDS reading is at about 3-4 now.

When is it time to change the filters? Which filters do i change?

[Randy Holmes-Farley]

Yes, I'd change it (the DI). I recommend changing when the effluent rises to 1 ppm TDS or so.

I detail why here:

Reverse Osmosis/Deionization Systems to Purify Tap Water for Reef Aquaria
http://reefkeeping.com/issues/2005-05/rhf/index.htm


from it:

If you are evaluating an existing RO membrane and can collect water from the tap and after the RO membrane, the conductivity (in mS/cm or ppm TDS) should drop by a factor of more than 10 across it (to as much as 100), relative to the tap’s water. If the drop is less than a factor of 10, it is not working properly, and may have holes in it.

Monitor the DI resins by measuring the effluent’s conductivity, either with an inline meter (set to its most sensitive level), or by measuring the effluent manually. If you are using a TDS or conductivity meter, then the measured value should drop to near zero, or maybe 0-1 ppm TDS or 0-1 mS/cm. Higher values indicate that something is not functioning properly, or that the DI resin is becoming saturated and needs replacement. That does not necessarily mean, however, that 2 ppm TDS water is not OK to use. But beware that the flow of impurities and the conductivity may begin to rise fairly sharply when the resin becomes saturated. Do not agonize over 1 ppm versus zero ppm. While pure water has a TDS well below 1 ppm, uncertainties from carbon dioxide in the air (which gets into the water and ionizes to provide some conductivity; about 0.7 mS/cm for saturation with normal levels of CO2, possibly higher indoors) and the conductivity/TDS meter itself may yield results of 1 or 2 ppm even from totally pure water by not being exactly zeroed properly. Also note that the first impurities to leave the DI resin as it becomes saturated may be things that you are particularly concerned with (such as ammonia if your water supply uses chloramine or silica if there is a lot in the source water).

[nmbeg]

i read the article, but a few questions remain:

1. how are you sure that it's the DI causing the high reading, not any of the other components? how do i know when the sediment filters need replacing?

2. noob question: which one is the DI? the horizontal one on the top?

[AZDesertRat]

To check the membrane condition, take a tap water TDS reading and an RO only TDS reading before the DI filter. You should see a 96-98% reduction in TDS with a properly functioning RO membrane. If the RO only TDS is in the acceptable range then its the DI only that needs changing. Next compare the final RO/DI TDS reading to your RO only TDS reading. You should see it drop from whatever that is to 0 with good resin in a properly packed cartridge. DI has a relatively short life compared to the other components and needs to be monitored on a regular basis.

The prefilter and carbon have very little to absolutely no effect on final TDS so its best to stick with the 6 month rule. Their purpose is to protect the membrane which is the workhorse of the system. It is possible in some locations with heavy sediment or particulates to have a pressure drop due to a plugged or fouled prefilter or carbon. If your system has an inline pressure gauge at the membrane, compare that pressure reading to your normal house pressure. Normally changing at 6 month intervals catches filter problems before they reach that stage though.

As for which is the DI filter it depends on what system you have. some have smaller horizontal, usually clear but sometimes white filters on top. If its clear you will see the resin which appears like tiny beads inside. A better unit will have a standard 10" vertical canister just like the prefilter and carbon block.

[Randy Holmes-Farley]

how are you sure that it's the DI causing the high reading, not any of the other components?

A properly functioning DI will always make 0 ppm TDS even if all other filters are removed.