The hard part is you cannot ship sewage from cities to farms on the same trucks you used to bring the food in from the farms as sewage contains harmful things you don't want in contact with food.
Situations like this are exactly why trucks carry containers. You don’t want to reuse the containers for those two things but you can absolutely reuse the trucks.
The trucks cost on the order of six figures, the containers an order of magnitude less.
But it can often do so more efficiently than just carrying one container at a time when they're empty - see this company that's solving the "return container" problem by folding up to 4 containers into the space of one for the return trip.
The trucks cost on the order of six figures, the containers an order of magnitude less.