Join Date: Jun 2014
Location: Cape Coral, Florida
First, the comment on TDS of 0-1 is for RODI water, and it sounds like you have RO water.
RO water should be removing 95% or better (usually better) of what's in the water, so if your TDS of incoming water is 300, expect to see around 15 TDS. Note I don't know how that plays with water softeners. But I think 25-40 is way too high.
Now for the "however". This sounds like it is a drinking water unit? Often a drinking water system may actually have a "filter" for adding mineral content back in to make the water taste better. Make sure yours doesn't.
Also, the ones with a pressure tank are not great in terms of keeping TDS low. An RO unit by its nature has relatively high TDS water come out when it first has water flow through it from any stoppage. What happens is the normally reverse osmosis process is reversed when the pressure drops by either turning off the input water, or (in some ways worse) just letting pressure build on the output line. This reversal (i.e. forward) pumps contaminants back into the filtered water. They do this on home water systems so it is always ready, feeding a tank, you take some from the tank, it puts more in, and for a home system who cares if it is 10 or 40 or 80 -- in fact some water companies selling RO water set their target around 50-80. For people that's not only pure enough, but tastes better, and doesn't corrode things as much.
In general for the cleanest water you want to run water through the RO membrane for a while before you start using water from it. Same with RODI -- it's helpful to divert RO water for the first 5-10 minutes before running it through the DI filter.
If you are dealing with a tank, that's harder, but the thing you want to find out is whether the outflow from the tank is FASTER than the RO production. If so that's good -- it means the RO membrane is not starting and stopping. If it's slower, however, that is bad -- you do not want (when making a bunch of water for your tank) to have it stop/start/stop/start/etc. It's not terrible, but the TDS content will be easily 2-5 times as high in those first minutes. But just make a bunch of water all at once and the later hours overshadows the first few minutes.
Latest tank build:
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