| In the arid and water deficient urban area, such as Xi'an in the northwest region of China, gardening and forestation often use large amount of tap water. Therefore, there is a need for treated wastewater reuse for such purpose to mitigate urban water shortage, especially in the newly developed housing area, where a high green coverage is often required for both commercial value and living condition improvement. Supply of the treated wastewater through a centralized system which has been planned and partially constructed is one measure to meet such need, but it may require an extension of the distribution system for a full coverage of the whole city area. A supplementary measure is to construct decentralized wastewater treatment and reuse (DESAR) systems in areas that are distant from the planned centralized system. In order to optimize the plan of wastewater reuse for housing development in the urban area, the authors analyzed the existing plan of centralized wastewater reuse and the envisaged plan of housing development in Xi'an urban area A mathematic model was proposed for evaluation of the costs and benefits of wastewater reuse for a housing project through the centralized and decentralized ways, taking into account the project location, project scale, option of reuse, and environmental factors. As a result, a critical distance was obtained and a distribution map was drawn to show the boundary between the zone suitable for housing development projects using the treated wastewater supplied by the centralized system and the zone suitable for housing development projects with implementation of DESAR systems. An example was also given to show the advantage of a DESAR system for a housing project in Xi'an urban area Some policy suggestions, concerning municipal plan, technology Integration, system selection, institutional construction, water price reform and public orientation, were proposaled finally.
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