After completion of a higher degree like MSc, ME/MTech, and Ph.D. in Environmental science, you can work as independent environmental consultants or work for any consultancy. For example, ecological consultancies like RAMKY, ENZEN, etc.
Almost all the university and Govt research organization has an Environmental Department. You can start working as a JRF or Project Assistants for various projects.
You can also join government PSU like NTPC, IOCL, CIL, etc. all have environment departments. They prefer MSc and M.Tech in Environmental Engineering.
You can also start your career in the private sector. Many Industries like Reliance, Adani Port. WIPRO etc. all have environmental departments, and they prefer environmental students for multiple positions.