Hii,i am here to help you according to your query.Well, i can say there are many private companies you can join

-Many PSUs need environmental enginners

-You can start your own consultancy firm

-You can be into research and get into institutes like NEERI and work as a scientist

-You can be an engineer in any PCB(Pollution Control Board)

-Pursue PhD and become a professor.Almost every project requires an environmental clearance and all industries require environmental engineers, So there is a lot to after pursuing M.tech in environmental enginnering.

Hope,this information will helpful for you.Goodluck.