The path to professional licensure normally begins with students graduating from an engineering degree program accredited by the Engineering Accreditation Commission of ABET, http://www.abet.org. Typically in the final semester of their undergraduate education, students take the 5.5-hour Fundamentals of Engineering (FE) exam. All engineering students take a "General" exam covering fundamental topics such as chemistry, calculus, and thermodynamics during the morning. In the afternoon, there is a subject-specific portion for Environmental Engineering. 
After 4 years of practice under a licensed Professional Engineer (PE) and often further coursework and a Master's degree, engineers take the PE exam. Environmental Engineering is a recognized specialty on professional engineering licensing exams.