What is the best & economical way of municipal waste water treatment?

Dear all,

As we know ​that this is ​the era of ​smart city with ​smart solutions ​& the Indian ​government ​trying to ​developing ​Indian cities ​in smarter way. ​So, i am trying to ​search, "what ​is the best & ​economical way ​of municipal ​waste water ​treatment?" And which Indian city municipal water tretamnet is most advance in term of technology? Any one knows?