Water reuse system in Xi'an Municipality of China

The distribution of water resources is severely uneven in China; whereas the south is rich in water resources, the North faces problem of water shortage, as Figure 1 shows (Tian 2012). The per capita water resource of the Haihe River, Yellow River, and Huaihe River basins, in the north of China, amounts to less than 500 m³/yr (Wang & Jin 2006). Xi'an, the largest city in the northwest region of China, is located in the Yellow River basin and its per capita water resource totals only 234 m³/yr. With rapid urbanization in China, Xi'an is planning to develop into a Grand International Metropolis, which will have an area of 850 km² and a population of 8,800,000 in the central city in 2020 (Xi'an Government 2009). As shown in Table 1, the total water demand in the year 2010 was 787.7 × 10⁶ m³/yr and is projected to increase to 1,200.9 × 10⁶ and 1,368.0 × 10⁶ m³/yr in 2020 and 2030, respectively. By contrast, the available conventional water resources including surface water and groundwater tend to remain stable over the long term. Nonetheless, the water supply capacity increases slowly mainly because of the planned increasing use of reclaimed water and rainwater. The total capacities of water supply were 766.4 × 10⁶ m³/yr in 2010, and are 825.1 × 10⁶ and 894.3 × 10⁶ m³/yr in 2020 and 2030, respectively (Water Supply Bureau of Xi'an 2010). Table 1 indicates that the present water shortage is 21.3 × 10⁶ m³/yr, which will amount to 375.8 × 10⁶ and 473.7 × 10⁶ m³/yr in the years 2020 and 2030, respectively, accounting for 46 and 53% of total water supply.

On the condition that the available conventional water resource is shrinking, the demand for unconventional water resources including wastewater and rainwater is increasing. Table 1 shows that the planned reclaimed water supply capacity is 215.0 × 10⁶ and 272.0 × 10⁶ m³/yr, which covers 26 and 30% of the total water supply in the years 2020 and 2030. However compared with the urban wastewater production, the rate of wastewater reuse is quite low. The total amount of wastewater can be predicted as 812.0 × 10⁶ and 948.4 × 10⁶ m³/yr in 2020 and 2030, assuming that the water use from domestic, secondary industry, and tertiary industry can be collected, 80% of which is discharged into the sewage system. As a result, the planned rate of wastewater reuse can only reach 26.5 and 28.7% in 2020 and 2030, respectively, which indicates that wastewater reuse can be promoted. Compared with the amount of water shortage of 375.8 × 10⁶ m³/yr in 2020, it tends to be feasible that the water deficiency will be largely alleviated in the event that the wastewater reuse rate reaches 72.8% and the reclaimed water supply amounts to 590.8 × 10⁶ m³/yr. Increasing the rate and the amount of wastewater reuse could not only solve the problem of urban water deficiency, but also reduce the dependence on long distance water transfers, and guarantee the urban water supply.

In this paper, a scheme for the utilization of unconventional water resources is developed, whereby reclaimed wastewater is used to mitigate the water deficit accompanying the development of the Grand Xi'an International Metropolitan Project in year 2020. Wastewater reuse potential is obtained from an analysis of the reuse purposes for the reclaimed water. The supply scheme of the reclaimed water is also formed on the base of distribution of the planned wastewater treatment plants (WWTPs) and their service areas, and the wastewater reclamation capacity.

As mentioned above, sustaining the development of the Grand Xi'an International Metropolis in 2020 requires maximizing the exploitation of all the available and reasonable water sources. The conventional water source is shrinking (Liu & Liu 2002), and rainwater use is constrained by the limited rainfall and seasonal variations in the northwest regions of China (Pei et al. 2010). Wastewater becomes a potentially important water source, which should be emphasized and the use of the reclaimed water must be maximized. Agriculture, industry, municipal, ecological, and indoor uses are the five main reuse purposes of reclaimed water worldwide (Asano et al. 2007), and which can be adopted for the Grand Xi'an International Metropolis in 2020. In this study, we assume that because of the low exposure to human beings the reclaimed water can reasonably supply most of the agriculture, industry, municipal, and ecological water demands. Indoor use of the reclaimed water is limited to toilet flushing, particularly in public buildings. Therefore, analysis of the wastewater reuse potential for every reuse purpose is very important and is implemented in the sections below.

Apart from wastewater reuse purposes and their reuse potential, the supply scheme for the reclaimed water is another important factor of the water reuse system. This should include the reclaimed water source, water supply service area, and main reuse purposes. In this study, we assume that the reclaimed water can reach the quantity demand for different reuse purposes, and can meet the related water quality standard for reuse requirements. In the supply scheme, the reclaimed water source will be covered from the centralized WWTPs, which are able to implement wastewater reuse in 2020. Water supply service areas will be arranged based on the governmental district divisions and their surrounding WWTPs. The main reuse purposes should be listed according to the function or developmental feature of the service area. Therefore, the supply scheme of the reclaimed water is configured below. An economic analysis will be employed to verify the feasibility and benefits of the newly formed supply scheme.

To analyze the wastewater reuse potential capacity, we collected data from public city plan documents for different reuse purposes. For the agriculture use potential, the irrigation areas of various primary crops were investigated. To determine the industry use potential, data on the water use characteristics of the leading industrial enterprises were collected. For the municipal use, the total gardening and road washing areas were calculated. For the ecological uses, all the main water bodies in the central city were enumerated and the total surface area and volume were determined. Regarding indoor use, we used the per capita toilet flushing water use plus the population to estimate the indoor toilet flushing water consumption.

To configure the reclaimed water supply scheme, we collected data related to WWTPs, pipeline systems of the reclaimed water and districts distribution of the central city. For WWTPs, the capacities of wastewater collection, treatment, and reclamation were obtained. For pipeline systems, the main service areas, delivery capacities, and major users were identified. For distribution by district within the central city, the industry characteristics and developing orientation were investigated, which determined the specific reuse purpose of reclaimed water.

Based on the above-mentioned analyses, costs and benefits were evaluated to verify the feasibility of the planned reuse system from an economic perspective. The costs included the investments in WWTPs construction and operation, and supplying pipelines' construction. For the benefits, on the other hand, we compared the price of the reclaimed water with that of urban water, which was charged differently for agriculture, industry, municipal, ecological, and indoor uses, respectively. Therefore, the benefits were calculated based on the price difference and reclaimed water use quantity.

Wastewater reuse potential capacity (P) was obtained by the summation of the agriculture reuse potential capacity (P₁), industry reuse potential capacity (P₂), municipal reuse potential capacity (P₃), ecological reuse potential capacity (P₄), and indoor reuse potential capacity (P₅).

On the basis of the wastewater reuse potential analysis, we proposed a WWTPs-centered method for the configuration of the reclaimed water supply scheme. According to the location and reclamation capacity of every WWTP as well as the industry characteristics of the different districts, we could arrange the service area of every WWTP and specify the main reuse purposes inside the service area. As a result, a table list for every WWTP and a map for reclaimed water supply in the central city of Xi'an in 2020 can be formed.

Wastewater reuse potential is analyzed on the basis of the calculation that categorizes agriculture use, industry use, municipal use, ecological use, and indoor use as the main reuse purposes of the reclaimed water in Xi'an in 2020. By means of Equations (1)–(5), the demand for reclaimed water for every reuse purpose is calculated.

The total agricultural irrigation area is planned to be 16,151.3 ha, which will be located mainly in the rural-urban fringe zone in 2020. Wheat, vegetables, and fruits are the three main crops (Xi'an Agriculture Bureau 2009). GIQ of the three crops are 1,350, 3,450, and 2,850 m³/yr·ha, respectively (Office of Water Conservation of Shaanxi Province 2011), and the total potential demand for agriculture irrigation is 32.2 × 10⁶ m³/yr. Table 2 shows the overall results based on Equation (1).

In general, the crop area is shrinking, mainly due to the rapid urbanization in most of the cities in China. As well, crop species tend to be simplified and centralized, which leads to the development of an agriculture base characterized by large-scale and centralized crop planting. Like most of the cities in China, Xi'an is planned to develop several modern agricultural zones with central management and intensive farming in its eastern suburb by the year 2020. This will provide favorable conditions for centralized use of reclaimed water from nearby WWTPs through constructing not too long distribution pipelines.

Water is mainly used for production processes, boiler feed and recycled cooling in industry. Of these, water consumption for boiler feed and recycled cooling in many industries including petrochemical, power, steel, and metallurgy industries account for 50–90% of the total consumption, and can be supplied with reclaimed water (Zhang & Su 2007). Different water usage for production processes may vary considerably because of various industries, which have been separated into three categories. First, in some industries such as cement and concrete production, which do not require high-level water quality, water use was encouraged to be supplied by reclaimed water. Second, in some industries such as papermaking and auto manufacture, part of the water use is suitable to be supplied by reclaimed water (Chao et al. 2013). Furthermore, in other industries such as electronic elements manufacture and food production, it is unsuitable to use reclaimed water. Table 3 lists the main industrial products and their water quota (Office of Water Conservation of Shaanxi Province 2011) in Xi'an City as well as their calculated potential demands for reclaimed water based on Equation (2). The total potential demands for industry use of reclaimed water were 89.5 × 10⁶ m³/yr.

In Xi'an, the major industrial enterprises are mainly located in the eastern, western, and northern suburbs. Owing to the local topographic feature of higher altitude in the south than the north, most of the WWTPs are located in the northern, northeastern, and northwestern suburbs, which facilitate the industrial water supply. According to the development plan for Xi'an City, major industrial enterprises are going to be moved to the suburban areas. Consequently, the distance between industrial enterprises and WWTPs will be shortened, which can significantly reduce the costs of reclaimed water delivery.

Municipal water use mainly includes gardening and road washing, which is reasonably covered by reclaimed water. In 2020, the gardening and road washing area is predicted to be 10,243 km² and the reclaimed water consumption will reach 112.2 × 10⁶ m³/yr, assuming unit consumption of 3 L/m²·d (Li et al. 2002) based on Equation (4).

Currently, municipal use has become the major purpose for reclaimed water, surpassing industrial use. This is realized in two ways, one of which is taking reclaimed water from the built-up pipelines for gardening and road washing in the surrounding areas. The other is transporting reclaimed water from WWTPs by watering carts to the city areas for greening and road washing. Owing to continuous construction of reclaimed water pipelines, the coverage area of pipeline networks will be gradually enlarged. Hence, municipal use is believed to be the most important use of reclaimed water under the encouragement of governmental policies for promoting reclaimed water reuse in public facilities.

According to the urban plan, the total volume of water for the central areas in Xi'an City will amount to 73.9 × 10⁶ m³ by the year 2020, and the reclaimed water will be used mainly for urban water replenishment. As mentioned earlier, the water bodies were categorized into two in Equation (4), one of which is the slow-flow or closed water body represented by urban landscape lakes. By calculation, this part of the water volume will reach 5.4 × 10⁶ m³. Owing to the shortage of replenishment by natural water sources, reclaimed water has been applied for lake water replenishment in some areas, and possibly this will become the only way for lake water replenishment in the future. Since this part of the water body is mainly in the central areas of the city landscape, the refresh duration of changing water was chosen as 20 days for the purpose of calculation (Wang et al. 2008). Hence, the potential demand of reclaimed water for landscape amounted to 98.6 × 10⁶ m³/yr.

Urban rivers represented the other category of water body. These kinds of water bodies are mainly located in the surrounding areas of the city, some of which are near the centralized WWTPs. Others are the receiving water bodies of the WWTPs. These have been the main reclaimed water users and the total scale of this kind of water body amounted to 23.6 km². Furthermore, the reclaimed water was mainly used for river base flow maintenance along with natural replenishment. By calculation, the potential demand of reclaimed water could amount to 47.2 × 10⁶ m³/yr, based on the calculation of water loss as 2 m/m² per year, and considering the water loss of evaporation and leakage (Wang et al. 2011). Hence, the potential capacity of reclaimed water for urban water replenishment could reach 145.8 × 10⁶ m³/yr.

The potential of reclaimed water for ecological use was high. Nonetheless, compared with the other reuse purposes, it is difficult to define the specific ecological users. Similar to the closed water body for landscaping, the administrative units of the parks may pay the water fee. However, for the other water bodies such as rivers for urban landscaping, it is possible that nobody will be willing to pay the fee of water replenishment. With the expectation for a rise in the public perception of urban water environment quality, the government should make efforts to promote the utilization of reclaimed water for water replenishment.

Toilet flushing is the key component of the indoor use of reclaimed water. Research has shown that the amount of toilet flushing accounted for about 30% of the total water consumption per capita (Matulova et al. 2010). The utilization of reclaimed water for indoor toilet flushing depends on the construction of a dual water supply system. The larger the coverage area of the dual water supply system is, the higher the usage rate of reclaimed water for toilet flushing. By calculation of the indoor reclaimed water reuse potential based on Equation (5), the total population size was 8.8 × 10⁶ and the unit water use for toilet flushing amounted to 40 L/d per person (Chu et al. 2007). Similar to the utilization ratio of reclaimed water for toilet flushing, since the popularity degree of reclaimed water was much higher in public buildings than residential buildings, the ratio of public building areas including commercial buildings, cultural and recreational buildings, sports buildings, medical buildings, and education and research buildings to the total urban land area was assumed as the utilization ratio of reclaimed water for toilet flushing, which was 37% in this study. By calculation, the potential demand of reclaimed water for indoor reuse was 47.5 × 10⁶ m³/yr.

The popularity of reclaimed water for toilet flushing was not only related to the construction of a dual water supply system inside the building, but also related to the public acceptability of the reclaimed water for toilet flushing. Hence, in order to promote the indoor utilization of reclaimed water, it is important to reinforce the construction of a reclaimed water delivery system and a dual water supply system. It is also important to encourage the public to use the reclaimed water in place of tap water for toilet flushing. Certainly, other factors including the rise of water quality standards, the reduction of usage risks, and price increases for tap water will also promote the popularity of reclaimed water use for toilet flushing.

Based on the urban planning of Xi'an City, the 15 centralized WWTPs including WWTP1–7, WWTP9–11, WWTP13–16, and WWTP-Chanba being put into operation before 2020 realizes 100% of municipal water collection. All the 15 WWTPs are located in the fringe fields of the urban area surrounding the city. We propose a WWTPs-centered reclaimed water supply scheme, which synthesizes WWTPs locations, district divisions and main reuse purposes.

As is shown in Table 4 and Figure 2, the main reuse purposes and service areas of every WWTP vary. Since the northern and western suburbs are featured as the combination of old and new industry bases, the WWTPs located in the western, northern, northwest, and southwest suburbs are mainly for the supply of industry use. On the other hand, the eastern and southern suburbs are featured as newly developed ecological and tourist areas, so that the plants in these regions are mainly for the supply of municipal use including gardening and road washing. In addition, the reclaimed water is also one important source for river replenishment in Xi'an City, including the Zaohe River, Weihe River, Fenghe River, Juehe River, Chanhe River, and Bahe River surrounding the central city; Xingfu Trench, Caoyunming Trench, and Taiping River inside the city; the lakes of Weiyang, Hancheng, Guangyun, Xingqing, Kunming, Tang Paradise, South Lake, and Taiye. Moreover, the WWTPs located in the Baqiao District, Weiyang District, and Yanta District could also serve for the supply of crop irrigation. Indoor use of reclaimed water for toilet flushing depends on the buildings’ distribution and indoor pipeline construction, which is not specified in Table 4 and Figure 2.

Equations (5) and (6) processed the economic analysis of the reclaimed water reuse system. For the cost calculation, empirical data of existing WWTPs in Xi'an City were chosen as the construction unit price and operation price per m³. The potential amount of reclaimed water for reuse was assumed as the scale of wastewater reuse, which was 427.2 × 10⁶ m³/yr or 1.2 × 10⁶ m³/d. As well, the construction cost per km of water supply pipelines employed the empirical data of tap water construction. To present the effect of maximum wastewater reclamation, the length of the tap water supply pipeline was assumed as the ideal goal of reclaimed water pipeline construction. The total costs are shown in Table 5.

The results of the benefits are shown in Table 6. For the calculation of benefits (Equation (6)), the reclaimed water price was assumed at the current price of 1.7 RMB/m³. The price difference between the water for various reuse purposes and the reclaimed water could be influenced by several factors. First, for agriculture use, the agricultural areas are mainly located in the eastern suburbs and its development was mostly restricted by water deficiency, as mentioned before. Exploiting the groundwater was the main way to supplement urban water in those areas. Currently, the average cost of groundwater exploitation is 5 RMB/m³, which gives a price difference with reclaimed water as 3.3 RMB/m³. Second, for industrial use, the industrial water price is 5.8 RMB/m³, which gives a price difference with reclaimed water of 4.1 RMB/m³. For municipal use, the water price of greening and road washing was assumed as the residents’ water price, 2.9 RMB/m³, which gives a price difference with reclaimed water of 1.2 RMB/m³. For ecological use, the economic benefits were mainly from the slow-flow water body replenished by reclaimed water. Since the economic benefit from urban rivers' replenishment was difficult to calculate, its benefit was neglected in this calculation. The slow-flow water body was mostly replenished by extra water from water plants, and its price was assumed as the residents' water price, 2.9 RMB/m³. The price difference with the reclaimed water was 1.2 RMB/m³. Lastly, for indoor use, the price difference between tap water for toilet flushing and reclaimed water was 1.2 RMB/m³.

Through the comparison of Tables 5 and 6, it is demonstrated that if the reclaimed water use was maximized, the construction and operation costs of the production facilities of reclaimed water were stable. However, the coverage area of the reclaimed water network had a great impact on the balance of costs and benefits. When the construction length of reclaimed water pipelines reached 50–80% of the total length of tap water supply pipeline, the costs and benefits of the constructed wastewater reuse system could achieve a good balance.

From the analysis of the potential demands for reclaimed water we can find that agriculture use makes only a small contribution to the total reclaimed water consumption, which is 32.2 × 10⁶ m³/yr and accounts for 7.5%. This is because only a small agricultural area in the central city is irrigated. Industry use is an increasing demand for reclaimed water along with the water price rising and the reclaimed water quality improving. The demand in 2020 is predicted to be 89.5 × 10⁶ m³/yr and account for 21% of the total consumption. Municipal use forms an important reuse purpose for the reclaimed water, which is 112.2 × 10⁶ m³/yr and accounts for 26.3% of the total consumption. The high municipal use is a result of the construction of large green belts and squares in the city. Ecological use should be another important reuse purpose for the reclaimed water because the demand for water bodies' replenishment will increase rapidly with the expansion in construction of the livable city area in Xi'an in the near future. The reclaimed water consumption in 2020 is 145.8 × 10⁶ m³/yr and accounts for 34.1% of total consumption. Indoor use is the most flexible reuse purpose for reclaimed water because it depends on investment in indoor dual pipe systems. It can make a more notable contribution if more attention is paid to it.

Based on the analyses above, the potential demands for reclaimed water were predicted to be 427.2 × 10⁶ m³/yr in the central city of Xi'an in 2020, and the corresponding wastewater reuse rate is 52.6%. By fully exercising the potential for reclaimed water reuse, the whole configuration of urban water supply will change and form a new balance, as shown in Figure 3.

In the new balance, the available water resource will increase substantially owing to the maximized use of reclaimed water. The water supply scheme will be altered as follows: (1) the surface water, groundwater, and inter-basin water are mainly supplied for domestic use as well as part of the secondary and tertiary industry which have higher requirements for water quality; (2) the rainwater is mainly used for ecological water replenishment; and (3) the reclaimed water could not only be supplied for the primary industry dominated by agriculture and ecological replenishment, the required quality of which is not high, but also could be supplied for part of the secondary and tertiary industry.

As a result, the total water supply capacity can reach 1,037.3 × 10⁶ m³/yr including surface water, groundwater, inter-basin water, reclaimed water, and rainwater, which could reduce the water shortage from 31.3 to 13.6% of the total demands.

Although the Xi'an Urban Planning (2009–2020) stated that reclaimed water could replenish urban water and introduced the total amount of utilization, the main approach and total demand for wastewater reuse have not been confirmed. This study analyzed the main approach of urban wastewater reuse and its potential capacity. It also confirmed the main reuse purposes for the reclaimed water and its optimal distribution for various purposes. Furthermore, the goals for the construction of new WWTPs have been mentioned in the plan, but the reuse scheme of the reclaimed water has not been confirmed. This study presented a WWTP-centered reuse scheme for reclaimed water based on the layout of administrative districts and industrial structure. It also confirmed the service areas of different WWTPs and the main reuse purposes for reclaimed water. Therefore, this study effectively supplemented the Xi'an Urban Planning and enhanced its operability. It also accomplished the specific goals of the plan, while providing guidance for policy-making and operation of the plan.

The operation of a wastewater reuse system in this study will involve a number of urban administrative departments. As to water supply, the Water Supply Bureau is in charge of the production of reclaimed water, and the Construction Bureau the reclaimed water pipeline, respectively. As to water demand, the agriculture use, industrial use, municipal use, and ecological use falls under the Agriculture Bureau, the Industry and Information Bureau, the Municipal Public Utilities Association, and the Bureau of Park and Woods as well as the Water Ssupply Bureau, respectively. Although these departments will all participate in the promotion of urban wastewater reuse under the guidance of Xi'an Urban Planning, they still lack systemic operation and effective communication. This study will help to establish close relationships among different departments, and build a rounded system incorporating production, distribution, and utilization, thus forming an ideal working mechanism characterized by multi-disciplinary cooperation to promote the reclaimed water reuse.

With the prospect of severe water shortage in 2020, maximized wastewater reuse should be implemented to mitigate water stress. A water reuse system formulated for the Xi'an municipality in this study included an evaluation of reclaimed water reuse purposes, reuse potential demands, and a supply scheme for the reclaimed water. Agriculture use, industry use, municipal use, ecological use, and indoor use should be the five main reuse purposes of reclaimed water. The reuse potential capacity was deduced to be 427.2 × 10⁶ m³/yr, which makes a significant contribution to increasing the total urban water supply capacity and mitigating water shortages. A supply scheme for the reclaimed water composed of reclaimed water sources, water supply service areas, and the main reuse purposes was configured. A WWTPs-centered reclaimed water supply system was proposed, which defined the main reuse purposes of the 15 WWTPs to be put into use in 2020 and their service districts. Based on the results of an economic analysis, the costs of the water reuse system could be covered by the benefits when the total length of reclaimed water supply pipelines accounts for 50–80% of that of the tap water pipelines, a perfect coverage condition of a distribution network for reclaimed water. This study provided theoretical basis and implementation strategies for a system configuration of water reuse in Xi'an City and also contributed to mitigating the water-deficiency problems related to rapid development in China.

This study was sponsored by the National Natural Science Foundation of China (No. 51308439), Shaanxi Provincial Program for Science and Technology Development (No. 2013KJXX-55), and Program for Innovative Research Team (No. 2013KCT-13).