Needs of water There are various needs for water re-use. For example in the newly formulated"Master Plan of Beijing Municipality (2004-2020)"(Beijing Municipality, 2004), treated domestic wastewater is set as an important water resource in Beijing, mainly for urban gardening, sprinkling, non-drinkable domestic use(car washing, toilet flushing etc. )and industrial cooling water supply. As mentioned in the former sections, low annual rainfall nd high potential evaporation is the characteristic of cities in north China. For these reasons, a large amount of water is used in the urban environment. Table 3 shows the current conditions and future predictions of urban environmental water use in the central urban area of Xi'an city The current daily water supply in Xi'an city is approximately 1.2 million m"/day From Table 3, it is understood that nearly 100,000 m /day or 8.4% of the supplied water is used for gardening and sprinkling. This is equivalent to the domestic water consump- tion of more than 400,000 people at the present level. Supplying water of drinkable qual ity for gardening and sprinkling not only wastes high quality water resources, but also wastes money for the unnecessary treatment of this amount of water. Treated wastewater can be used as a substitutive water source for this purpose Strategies of wastewater re-use For the above-mentioned reasons, nowadays many cities in north China are practicing or planning urban wastewater projects. In Xi an city, pipelines have already been provided for using treated domestic wastewater for gardening along the newly rehabilitated large streets in the city area though the centralised re-use water supply system is still under planning. The cost for such a large scale centralised wastewater treatment and re-use sys- tem is always questioned because it often requires long distance transfer of collected wastewater from the city area to the sewage plants, which are usually located in the out- skirt of the city, and the treated wastewater back to the city area. This is the main diffi culty faced by these cities and becomes an economic restriction on decision making On the other hand, the coverage of wastewater collection and treatment systems is still ery low in north China. In many newly urbanised towns and villages, there are almost no such systems provided. The government persuades each newly constructed sewage plant to introduce measures for treated wastewater re-use. This, in fact, increases the financial burden for these towns to improve their sanitation conditions. In addition to cen- tralised systems, decentralised systems should be put into consideration in formulating strategies of wastewater treatment and re-use. At least three advantages can eseen with the decentralised systems: (1)small service area and therefore small-scale system; (2)onsite treatment and onsite re-use, and (3)no long distance transfer of wastewater and treated effluent Table 3 Water demand for urban environment in Xi'an city(present conditions and predictions of 2020) 2020 Central (km2) Population(million) 2.578 Gardening area(km2? 3882 58.23 Road and 13.61 29.13 later for gardening(m"/da 58230 Water for sprinkling(m"/day) 87,390 otal water demand(m" /day) 99,060 174.735 Water consumption for gardening at 1.5 Um / day or 1, 500 m /km?/day ater consumption for sprinkling at 3o U/m/day or 3, 000 m/km dayNeeds of water re-use for urban environment There are various needs for water re-use. For example in the newly formulated “Master Plan of Beijing Municipality (2004–2020)” (Beijing Municipality, 2004), treated domestic wastewater is set as an important water resource in Beijing, mainly for urban gardening, sprinkling, non-drinkable domestic use (car washing, toilet flushing etc.) and industrial cooling water supply. As mentioned in the former sections, low annual rainfall and high potential evaporation is the characteristic of cities in north China. For these reasons, a large amount of water is used in the urban environment. Table 3 shows the current conditions and future predictions of urban environmental water use in the central urban area of Xi’an city. The current daily water supply in Xi’an city is approximately 1.2 million m3 /day. From Table 3, it is understood that nearly 100,000 m3 /day or 8.4% of the supplied water is used for gardening and sprinkling. This is equivalent to the domestic water consump￾tion of more than 400,000 people at the present level. Supplying water of drinkable qual￾ity for gardening and sprinkling not only wastes high quality water resources, but also wastes money for the unnecessary treatment of this amount of water. Treated wastewater can be used as a substitutive water source for this purpose. Strategies of wastewater re-use For the above-mentioned reasons, nowadays many cities in north China are practicing or planning urban wastewater projects. In Xi’an city, pipelines have already been provided for using treated domestic wastewater for gardening along the newly rehabilitated large streets in the city area though the centralised re-use water supply system is still under planning. The cost for such a large scale centralised wastewater treatment and re-use sys￾tem is always questioned because it often requires long distance transfer of collected wastewater from the city area to the sewage plants, which are usually located in the out￾skirt of the city, and the treated wastewater back to the city area. This is the main diffi- culty faced by these cities and becomes an economic restriction on decision making. On the other hand, the coverage of wastewater collection and treatment systems is still very low in north China. In many newly urbanised towns and villages, there are almost no such systems provided. The government persuades each newly constructed sewage plant to introduce measures for treated wastewater re-use. This, in fact, increases the financial burden for these towns to improve their sanitation conditions. In addition to cen￾tralised systems, decentralised systems should be put into consideration in formulating strategies of wastewater treatment and re-use. At least three advantages can be foreseen with the decentralised systems: (1) small service area and therefore small-scale system; (2) onsite treatment and onsite re-use, and (3) no long distance transfer of wastewater and treated effluent. Table 3 Water demand for urban environment in Xi’an city (present conditions and predictions of 2020) Item Present 2020 Central urban area (km2 ) 186.97 275 Population (million) 2.578 3.5 Gardening area (km2 ) 38.82 58.23 Road and square area (km2 ) 13.61 29.13 Water for gardening (m3 /day)a 58,230 87,345 Water for sprinkling (m3 /day)b 40,830 87,390 Total water demand (m3 /day) 99,060 174,735 a Water consumption for gardening at 1.5 L/m2 /day or 1,500 m3 /km2 /day b Water consumption for sprinkling at 3.0 L/m2 /day or 3,000 m3 /km2 /day X.C. Wang and P.K. Jin 42