In an arid and water deficient urban area, such as Xi'an in the northwest region of China, gardening and forestation often use large amounts 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 method was proposed for selection of a feasible way of reclaimed water reuse from two options, namely centralized and decentralized ones, by introducing a critical distance L0 which depends on the relationship between the cost for DESAR system installation and that for water delivery pipeline construction. If the distance from the project site to the nearest access point of the centralized system L is shorter than L0 then using reclaim water from the centralized system becomes more feasible, and otherwise DESAR system installation becomes more feasible. A distribution map was thus obtained to show an optimized plan of centralized and decentralized wastewater reuse systems for housing development in Xi'an city. An example was also given to show the advantage of a DESAR system installed.

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