What is the best low cost solution for sewage treatment plant for low cost affordable housing

We are developing mass scale low cost housing at Ahmedabad, Gujarat, India that requires treatment of 1.8 MLD sewage water daily. The treated water will be reused for flushing toilets and gardening through porous pipe. What is the best low cost and low maintenance solution? Should we go for anaerobic digestion and gasification or another surface bed treatment system. Initial cost and running maintenance should be low and affordable.  Ar Jignesh Goyani jgoyani@yahoo.com +919898246050