Upgrading an Existing Sewage Plant

We are working on a major project to improve the Sewage Treatment Plants (STP) in Hyderabad, India.

Most of the STP' s are old and built in the late 80s. I'm looking for ways to upgrade these existing plants to more advanced but affordable technology so that they use less energy and become more efficient, have lower installation and maintenance costs and require less land.

Has anyone worked on upgrading old STPs with advanced technologies? Please share your suggestions and experiences.