Environmental engineering is the branch of engineering which deals with developing solutions for environmental problems. Careers in environmental engineering is one of the fastest growing trends and with growing environmental concerns ,the demand is expected to grow. The best way to get a post graduation degree in Environmental Engineering would be through a part time course in the degree as many colleges provide such courses specifically for working professionals. There is also distance education available but such courses are not considered legal as engineering courses done through distance are not considered as legal . Therefore the best way to pursue the course is through part time courses .
Some colleges that provide part time in environmental engineering are :
1. Jamia Millia Islamia University also offers an M.Tech degree in environmental science and engineering for part time. The course is for a duration of three years with six semesters in total. The course is specifically designed for working professionals . The fee structure is aroind ₹13000 for a year. You must have a minimum percentage of 60% in your B.E / B.tech degree and  appear entrance test as well as interview. According to the department at the University,the ultimate goal of the degree is to provide advanced learning with enhanced analytical ability  to solve problems that are interdisciplinary in nature and help in protecting the environment. The institute also provides technical advices on live engineering problems  to government and private sectors . As such ,these projects are ultimately used as training projects for the students at the postgraduate level. It also has collaboration with several universities abroad.

2. Government college of Engineering,Amravati also offers part time M.Tech degree in Environmental engineering. The total fee is  2 lakhs and  the duration is for three years.

3. Malviya Institute of Technology has a part time post graduate degree in Environmental Science engineering. It has 6 semesters which is offered by the department of civil engineering. The fee for the course is ₹8500. Also one must have an undergraduate degree in B.E/B.tech to be eligible for the course.

4. Bengal Engineering and Science University also offers a master of engineering degree in Environmental Engineering for part-time . One must be a B.E or B.Tech graduate with atleast two years of work experience. The minimum percentage is 55% to  be eligible for the course. The fee for the course is ₹15000.

5. Anna University Chennai provides a part time post graduate degree(Ma s ter of engineering) in environmental engineering. One must be a B.E/B.Tech graduate with atleast three years of work experience. Admissions are based on Tamil Nadu Common Entrance Test conducted every year.

6. Vellore Institute of Technology also offers a part time master degree in environmental engineering. One must have be an B.E/B.Tech in engineering or applied sciences . Admissions are done on entrance tests or through GATE score.
I hope these details are helpful to you .