It's particularly insane to flush our waste into our limited clean water supply. Given high population density, do you really want to drink the water from your municipality? How bizarre is that concept to begin with?

If you want to look at real "doom" do any rational perusal of scientific study of the growing clean water shortage. That's far more realistic. We may be fighting over oil now, but in the future, it likely will be over water rights.

It's true, we can't keep mining rock phosphate in sufficient amounts to provide enough food for the world. We particularly can't produce enough grains if we end up creating ethanol for fuel.

The real issue is probably to alter our agricultural practices and most like (horrors) give up meat consumption. I'm a true carnivore, but I anticipate for economic reasons that if the world population keep increasing, especially in density, that fewer and fewer people will feed grain to livestock to produce meat. It's dumb.

Of course we probably will simply change our diets and farming methods, and the need for chemical fertilizers will go by the wayside since we'll run out. It's that or discover new sources.