Everyone is right about the average rejection rates of the membranes (90% for the 100gpd, 98-99% for the 75gpd); but there's more to the story than just TDS. Membranes will basically block anything that's much larger than a water molecule; but anything that's smaller, the same size, or slightly larger than the H2O molecule will easily pass through the membrane. This happens more readily when using the 100gpd unit. TDS is supposed to be a measurement of Total Dissolved Solids, but it's often just a crude way of estimating how pure our water is. A true testing of TDS involves filtering out any suspended matter in the water, then evaporating the water, and what's left in the bottom are the total dissolved solids. Since that isn't exactly easy to test for we rely on another method, which is conductivity. Since suspended particles can't transfer electricity, but dissolved particles can, a simple test of conductivity tells us how many dissolved salts are present in the water. The problem with this test though is that it isn't complete. Not all undesirable compounds will conduct electricity, which means even our RO water that tests zero TDS can still contain compounds that can harm our livestock. RO membranes will "on average" remove the percentage given above. In reality the percentages are different for each compound. For example, copper is almost completely removed; but sodium is only mostly removed, and ammonia isn't removed hardly at all. For these reasons I highly recommend a high-rejection-rate membrane; and I also recommend you use a DI cartridge to mop up whatever gets past the membrane. Don't be fooled by TDS monitors, RO-only water is not completely pure.
Clayton
that price Andy got!! thanks for the help!
