It is used as fertilizer, often not for food consumption. You would be depressed to know how many horrible things are to be found in human excrement, including pharmaceuticals, plastics, fire retardants, etc. To me the greatest use of human waste would be carbon sequestration. Think about it: we have an existing network of pipelines directly from every house to a centralized facility that can collect, dry, and [optionally] store that output in the soil or elsewhere for many years.