There are two possible reasons why the TDS may be higher now:
1. The municipal water supply might have changed, and it’s higher in TDS content now.
2. If the leftover water from the bottom tank hasn’t been emptied after each filtration cycle but topped off on the previously filtered water. Then the concentration of impurities will go up in the bottom tank resulting in higher filtered water TDS.
Because the alkaline post filter returns the minerals into the purified water, getting the actual membrane rejection rate directly from the top tank is tricky.
One of our customers wrote a detailed review on how he did it:
This filter DOES reduce TDS properly; you just have to test the water correctly (updated review).
I tested the water myself using a TDS meter because I saw some other users in reviews talk about it failing to reduce TDS to the proper levels for an RO filter. I thought I had confirmed that the RO filter didn't reduce TDS as expected and was about to return the device, thinking they were falsely advertising the performance of the RO system. Still, thankfully I had a back-and-forth correspondence with the company owner dissecting my testing method and found that my testing methodology was incorrect for this system. Let me explain here for any users wanting to verify for themselves the filter's water quality.
First, for verifying the reduction of TDS, you cannot test the water output from the top tank because it passes through a post-filter for remineralization (which increases the TDS at the end). You must open the back of the machine and use the tube that exits the RO filter and enters the post-filter by disconnecting it from the post-filter entry point (see instructions for how to change filters). You must also let the machine run a couple of complete cycles and throw away the water if these are new filters (per the instructions).
At this point, if you turn on the cycle and collect a cup of water from the tube and test it, you may think this is the correct TDS reading, but that also would be wrong. When you begin the cycle, you must collect some initial water from the tube and set it aside because the initial spurt of water will have higher TDS (due to TDS creep). This small amount of higher initial TDS would be dispersed entirely if you were instead collecting from an entire reservoir of filtered water. If you collect about a cup of water from the tube and set it aside, then collect and test a second cup, you will find the proper TDS level for an RO filter (under 50ppm).
In other filters without a mineral post filter, you would test the filtered water from the reservoir and find the correct TDS right away. However, the post-filter (which is a huge advantage of this system due to the replenishing it does of critical minerals), makes it more difficult for those that like to verify the RO water quality. I hope this review helps give others the confidence I now have that this filter does indeed produce excellent quality reverse osmosis water! See for yourself using the above method!