Honestly I would suggest calling your local Building/Engineering/Env. Health department and requesting that info, whoever regulates that for you there. Its all public information and should guide your decision.
Unfortunately, there is no national standard for these types of systems (To my knowledge), so I cant really be more helpful. When I worked in Env. Health in IL, septic and city sewer diagrams I reviewed varied in depth, material, size, etc. and it all depends on site condition factors such as soil permeability, soil composition, distance to groundwater, slope, flood hazard, etc. Of course, budget plays a role in that as well.
Papaya would probably be a decent shallow rooted option, or its relatives. But yeah regarding citrus, I think it just depends whats in the ground and how deep it is. I want to assume youre fine for a city line, but if problems come in the future dont blame me! lol.