You need to ensure no noticeable amounts of pathogens remain else they’ll be present in sme of the crops which then sicken people who eat them.

Apparently reusing waste water for agriculture already a thing, but there are standards about the average concentration of coliform bacteria per unit volume for different classes of reused water, and for multiple classes the standard matches one used for drinking water (2.2 MPN/100mL).

https://www.epa.gov/waterreuse/california-treated-municipal-...