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• Dear Vatsal,
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•    You can definitely do M. Tech in Environmental Engineering after completing B.Tech in civil engineering.
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• One of the eligibility criteria for seeking an admission in M.Tech Environmental Engineering is pocessing a Bachelors degree in any of the engineering streams like Civil engineering, Chemical Engineering, Ceramic Engineering. Technology, Mechanical, Metallurgy, Mining, Environmental, Bio-Chemical Engineering/Bio-Technology, agricultural Engineering etc. Apart from this you have to appear in the entrance exam help by the respective colleges/universities. Yiu can also appear in GATE exam to carry on with your M.Tech in the IITs. 
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• Wishing you all the best !

Yes, definitely we can first complete our graduation with B.tech in Civil engineering and then go for masters in environmental engineering.

For this, you can give GATE exam in the civil engineering subject, get a good score in GATE exam and then apply in different NITs and IITs for doing M.Tech in environmental engineering. There is a provision of changing the streams and you can easily do it.

Feel free to ask if you have any more queries. All the best!!