TDS is typically parts per million. If you have a TDS of 100 you have 100 parts of dissolved solids for every 1 million parts of water. If you have a gallon of water with 100 TDS you can dump half of it out and the rest will still be 100 TDS.
If the filter actually clears the water to 0 TDS mixing 1/2 filtered and 1/2 tap with the tap being 192 TDS the mixture will be 96 TDS. The KH and GH will be reduced if the filter removes them. I think you better read back through the description of the faucet filter. I couldn't find where they actually stated what it removes. I think it's different than the other zero water products. They say in one place that it is not designed to remove 98.6% of TDS. Further down the page it says, " While this product can remove various contaminants the filter technology was not made to remove 100% of the total dissolved solids from your water". If you look at some of the 1 star reviews they refer people complaining back to the page with these statements and tell them they are trying to remove stuff that this is not designed for. I think you better check this product out a little better. It doesn't look like it removes much. If you could get a faucet filter for $50 that did the same thing as an RO unit for 400 gallons of water, why would anyone ever buy a small RO unit that takes up a lot more space, costs considerably more and not produce the tremendous amount of waste water that most $RO's produce. One other thing, when you add back GH and KH your increasing your TDS.
If the filter actually clears the water to 0 TDS mixing 1/2 filtered and 1/2 tap with the tap being 192 TDS the mixture will be 96 TDS. The KH and GH will be reduced if the filter removes them. I think you better read back through the description of the faucet filter. I couldn't find where they actually stated what it removes. I think it's different than the other zero water products. They say in one place that it is not designed to remove 98.6% of TDS. Further down the page it says, " While this product can remove various contaminants the filter technology was not made to remove 100% of the total dissolved solids from your water". If you look at some of the 1 star reviews they refer people complaining back to the page with these statements and tell them they are trying to remove stuff that this is not designed for. I think you better check this product out a little better. It doesn't look like it removes much. If you could get a faucet filter for $50 that did the same thing as an RO unit for 400 gallons of water, why would anyone ever buy a small RO unit that takes up a lot more space, costs considerably more and not produce the tremendous amount of waste water that most $RO's produce. One other thing, when you add back GH and KH your increasing your TDS.
