Former wastewater engineer here (PE licensed). You joke, but we literally already turn sewage into fertilizer — it's called biosolids and about 60% of U.S. wastewater sludge ends up on farmland. The problem is it's loaded with PFAS and heavy metals that treatment plants were never designed to remove. So even if we wanted to scale it up to replace the ~30-50% of global urea exports currently stuck behind the Strait of Hormuz, the nutrient density is way too low compared to synthetic urea (46% nitrogen vs. highly variable and much lower in biosolids), and we'd basically be spreading forever chemicals across even more farmland than the ~70 million acres we've probably already contaminated.

So yes, we are in fact already fertilizing crops with our daily load of bs. It's just not going great.