It is not only possible but safer than our current system, google separating toilets and see how this works in the world.
Fossil fueled agriculture is not sustainable even at current population levels besides being incredibly toxic, technologically intensive and inefficient. To comprehend what a population of 15 billion humans require to survive we can look at one person. The average person consumes 1000 pounds of food a year, this varies from country to country but for the purpose of this equation this will work. Now it takes approximately 50 pounds of nitrogen to grow 1000 pounds of food.
Now the average person puts off 49 pounds of sterile, organic nitrogen each year. So what we have here is a Principle Theory of Sustainability, at the individual level. We simply need to recycle our own urine and humanure to create a truly sustainable food supply for an unlimited population. This is ecological engineering to reclaim nutrients and protect the unified water supply which is currently being used as a sewer. read Hard Seed.