I can't speak for your specific area and it's normally an engineering function to determine if an area is "classified", but almost all, if not all, of the sewage treatment facilities of the size that you mention (based on HP) that I have seen, have been classified areas and the Code requires sealoffs when going from a classified area. Cords could very well be legal and can be routed thru seal-offs. You might want to read 501.15(D)(2) for particulars about exposing the conductors on cords in seal-offs.
If the area is not classified, you might still want to consider some sealant (not explosion proof) to prevent harmful vapors entering your enclosures.