Your septic systems needs to be professionally designed, but you can hire that separate from the install.
In fact I would advise you to hire the design first separately and only after you have a design get bids on the install. My grandpa got bids from several design/installers, and only after the contract was signed did he discover that all the design/install bids in the county were for a more expensive mound system. My uncle next door learned and hired the design first and then tracked down the one installed in the county that would do the cheaper system that was designed.
That is what I am trying to do. The designer doesn't seem to be helping much with getting the price down though. He is basically just following the state requirements exactly and not putting any thought into any exemptions that might lower the price.
In fact I would advise you to hire the design first separately and only after you have a design get bids on the install. My grandpa got bids from several design/installers, and only after the contract was signed did he discover that all the design/install bids in the county were for a more expensive mound system. My uncle next door learned and hired the design first and then tracked down the one installed in the county that would do the cheaper system that was designed.