There and back would be 4 times the distance through more rock and deeper depth, plus a desalination plant and steam driven pumps along with massive lift stations for any deliveries along the way, and it would not be much more than a simple gravity drain?
I’ve worked projects that had similar thoughts for cost estimates.
They didn’t go so well.
Just thinking out load here (sort of...)
The tunneling would be the major part of the project.
A reflective mirror boiler should make team so reverse osmosis would not be required.
150# or 300# steam should supply most of the power needed.
Effluent steam could be re-superheated and piped to altitude, then condensed for the down hill trip to users.
I still think it is doable and much more realistic and needed than a Gov. Moonbeam train to no where... The 64 billion for the train would be a good start on building it.