Why do you need to hire a Professional Civil Engineer to design a Septic System?

Because they know how shit works

Because you don’t want to have a crappy system.

And actually, septic systems are rather complex arrangements that are affected by stuff like what type of soil you have where the drain field needs to go, the slope of the land, what specific sort of system you are installing (gravity, pressure, alternative), how much sewage is expected to go through the system, the various regulations that control what you can and cannot do when installing a septic system, and so on.

Installing the wrong system for your location can be an expensive and unhealthy error.

For some background information on generally what is involved in designing a septic system, see http://www.eco-nomic.com/indexsdd.htm .