Once measures have been taken to reduce water-use as far as possible, there are a number of more major changes that are often advocated to make buildings more water-efficient and/or to reduce pollution.
Reedbeds have earned a reputation as a green icon but are not always appropriate or green. If you have access to mains sewerage then this is the greenest option. If you are away from the mains then the most appropriate solution will depend on many site specific factors. Perhaps surprisingly, a traditional septic tank and leach field is often the greenest option.
The CAT book Choosing Ecological Sewage Treatment gives a good overview of most of the available options: