Hey folks! Chad mentioned this thread to me so I'd figure I'd hop in.
When it comes to unusually high TDS like in your situation it can be tricky. The 100GPD membranes aren't the most efficient in the terms of rejection ratio. So for example with a GE Osmonics 100 GPD 96% rejection rate membrane filtering 1000 TDS water, you'd end up with 40 TDS making it's way through the membrane at
perfect conditions. In this case the 60 TDS you are seeing would be normal.
Now, when it comes to running dual membranes, we don't recommend doing so on source water that has a TDS higher than 300. The problem is that the second membrane in series is fed with the waste water from the first which has a higher concentration of TDS.
This video here explains a bit more, and starting at around the 55 second mark gives a good example of what I'm talking about
:
https://youtu.be/GE0Dhp5DZv8
And this video here just gives some general tips on overall DI conservation:
https://youtu.be/x6f07WzT2Ng
Now, what would I do in this situation? I would probably swap out your membrane and matching flow restrictor with something more efficient. Our 75GPD membrane, albeit 25 GPD slower, has a 99% rejection rate. This means you could possibly expect TDS as low as 10 under perfect conditions. You'll need to purchase a 75 GPD flow restrictor as well to have that membrane perform properly.
http://www.bulkreefsupply.com/75-gpd...w-filmtec.html
http://www.bulkreefsupply.com/membrane-flush-valve.html