There are standard design equations for sewage treatment.
You need this information:
-Strength of waste to be treated (BOD,TSS,TKN,pH, etc.)
-Effluent quality you need to achieve (BOD,TSS,NH3,NO3,P)
Then the simplest thing to do is contact a firm that sells aeration equipment and biological processes and they will be able to help you size your necessary tanks and equipment. There are a lot of designs to choose from depending on your plant effluent requirements, capital, space, etc.
This will give you a good idea of the size of plant and equipment costs very quickly. You can then go out and hire a consulting firm to prepare all the construction documents and drawings using equipment selected.
Check out
to see process equipment used in sewage treatment.
Good Luck