Not being able to return sewage to where food is grown is a consequence of shipping food long distances. This also causes more CO2 emissions. I think we need to grow food more close to our population centres.
We can't while supporting the current population. Food takes a lot of land and effcient high production needs each region to specialize. Disasters like drought require long distance transport to handle
I think we could with solar panels, LEDs, hydroponics and sewage reclamation. It wouldn't take so much land if all this was in tall buildings or multiple levels underground. Any methane involved could be put into biofuels for vehicles or lighting/temperature control of the hydroponics.