A recent paper published in the AWWA Journal (Olmos & Lodge, 105:2,February 2013, pp 41-42) discusses achieving zero water use and reasonable payback periods for a housing development in Davis California. Net zero water use as defined here is that annula water use equal annual rain fall. This would make the development sustainable from teh stand point of water.
The study concluded that such an objective can be obtained by increasing teh efficiency of indoor water use and landscape water conservation. principle savings came from reducing the need to heat water for indoor use. Simple residential greywater systems for landscape irrigation showed relatively short payback periods, about 1-3 years. Improving the efficient use of potable water for landscape irrigation has alonger payback period.
This February issue of the journal also discusses other water conservation and resue applications in San Francisco. The city published a guide to designing greywater water treatment systems. For a one or two unit residence directly using washing machine effluent, no permit is required for installing a grey water system if the water comes directly from the washing machine and does not require a change in the buildings plumbing.
Treatment of residential grey water, at least from the washing machine, seems to be a low cost and easy way to make an immediate impact on the sustainability of water resoruces
In larger greywater projects, for example large buildings, more extensive treatment may be required, especially if the water is going to be stored for some period of time since this could lead to biological growth. Ozone has been found to be effective in these types of water reuse situations.