grumpus, It depends on the Nation, State, County, City, where you want to work and the requirements set-up by the various agencies involved. Here in Illinois all engineers/operators, be they POTW or Industrial Pretreatment, must receive state approved academic training, serve a period of apprenticeship, then take the State liscensing exam. There are various classes of liscense's granted by the State that reflect the type of Treatment Plant involved and type of process streams involved.
As for Engineering credentials, it 's like any State controlled engineering discipline, academic credentials/experience, passing the State P.E. exam,
and passing area of expertise exam. At which time the State will grant an Official State P.E. Liscense. Here in the Chicago area (this area has got the largest POTW and Industrial Pretreament operations on the face of the planet)all types of engineering disciplines are involved, from maintenance personnel, draftsman , designers, EIT's and P.E. In all disciplines, Mech. Civil, Elec., Chem., Arch., Comp., etc., and the various R/D centers at the U of I, Northwestern, U of C. and the likes of Argonne and Fermi Nat'l. Labs.
Check with the governmental agencies in your area, they should be able to tell you what is involved.
Goodluck!
saxon