[bluGill]

True, but that doesn't get back the farms in the same quantity that nutrients left. some farms near the city get more than they need while others get very little. Most fruit in the stores (US at least) is grown in Chili - they have a great climate for this (there is lots of reason to debate this that would take books to get into, so for now lets accept this simplistic statement), but how much sewage from the US goes back to that area? Cattle produce a lot of manure, some farms sell that to local farms and it goes back to the soil (dairy farms typically do this - often they own the fields the cow feed comes from), but other cattle end up on giant feedlots where there are not enough farms around to take the generated manure and so the local fields end up over fertilized while more distance farms don't.

Note that this waste is a bio hazard. So the obvious, just send the waste back to the fields in the empty truck cannot work. Once a tank has been used for waste you cannot use it for food again. Thus you end up with twice the trucks gong back and forth if you try this - and this in turn presents more CO2.

[nuc1e0n]

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.

[bluGill]

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

[nuc1e0n]

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.