Dear Sri Sumit Chandla,
Many years ago, there was an article in 'DECCAN HEARLD' daily NEWS papaer. That was very simple - Low cost - space saving - what is more, you can use the excess water for re-charging the ground after utilising the water for your garden. It's basic design is simple.
The method is as follows:
In one lower corner of the site of your home, you construct a TWO walled tank. The inner wall height should be more than the outer tank wall. You discharge all the waste water from your home into the inner tank. The inner tank will have finished bottom. The tank walls are of brick construction and cement finish. You allow some floating water plants in this inner tank and if possible leave a few small size fish in the tank. The over-flow from the inner tank is allowed into the space between the 'Inner Tank wall and Outer Tank wall' which will have NO -repeat- no bottom finish and the bottom is the mother earth. The space between the inner wall and the water wall is filled with 1. Baby Jelly stone layer at the 'TOP', 2. Gravel as second Layer, 3. Coarse Sand as the Third Layer. An out let pipe is inserted into the wall to let the filtered water for your 'Gardening'. The exess water gets into the mother earth for ground charging. The whole expenditure will not run into more than Rs. 5,000/-. If you have more money to spend, then there are other methods- still I call them inexpensive - like using Poly membranes.