Municipal sewage treatment facilities have important implications for cities' sustainable development and water environment protection. This study's aim is to optimize the sewage treatment facilities by evaluating pollution effects and governance demands for Huai'an City in Jiangsu province, China. The township is chosen as the evaluation unit and the spatial pattern, change trend, and environmental impact of pollution sources are analyzed to examine the demands for sewage treatment facilities in the future. By employing the ArcGIS spatial analysis tools, the spatial matching patterns between municipal treatment capacity and sewage discharge are studied. Considering the characteristics of wastewater discharge and the actual ability and designed capacity of the wastewater treatment facilities, the study area is divided into three types, including sewage treatment potential released type, sewage treatment enhanced type, and status quo maintained type. Comparing the quantity of wastewater discharged with the treatment potential capacity, direction for the construction and operation of sewage treatment facilities is proposed. Results of this study provide a scientific basis for site selection and layout optimization of municipal sewage treatment facilities.

INTRODUCTION

Water quality improvement and pollution incident risk reduction have become urgent priorities for water resources management (Duan et al. 2013). As the main sources of water pollution in many countries, point-source pollution is primarily associated with human excreta and industrial water use through wastewater drainage (He et al. 2009a, 2011). In some developed countries, stringent discharge regulations, development of sewerage systems and other relevant efforts in watershed management have led to significant improvements in river water quality in recent years (He et al. 2009b; Takahasi 2009; Luo et al. 2011; Duan et al. 2013). Therefore, the construction of sewage treatment plant as an important infrastructure in cities and towns has become one of the critical measures for preventing and controlling pollution as well as for protecting the current urban water environment at home and abroad. It plays a positive role in guaranteeing the treatment of domestic sewage and partial industrial wastewater in cities and towns, improving the water quality of rivers and lakes, and controlling pollution of the water environment in cities and towns (Shi et al. 2005; Zhou et al. 2007). However, with the growth of city construction, the development of industrial zones, and the acceleration of residence construction in new areas, the aggregate discharge of wastewater and sewage in cities and towns continues to increase, while the actual treatment rate and level of wastewater and sewage are still at a relatively low level. This issue forms one of the main reasons for the severe pollution of rivers, lakes, and reservoirs in the basins at present (Zhang & Qian 1998). Therefore, the planning and construction of urban sewage treatment plants have become a key problem (Xue & Zhang 2008). In terms of strengthening urban sewage treatment capacity, however, the common way is to enlarge and accelerate the construction of urban sewage treatment plants and supporting pipeline networks to strengthen the pollution interception and control effect.

Domestic and overseas research mostly focuses on sewage treatment technology and process (Moosvi & Madamwar 2007; Xie et al. 2007; Zeybek et al. 2007; Colmenarejo et al. 2012; Huang et al. 2012a, 2012b; Wang et al. 2012; Yang et al. 2012) and environmental impact (Ke & Shi 2000; Falletti & Conte 2007; Sadeghpoor et al. 2009; Ba & Wang 2012), while research on the spatial matching of sewage treatment plants and regional pollution control demand is still lacking (Gemitzi et al. 2007; Zhao et al. 2009). In urban planning practices, the factors considered in site selection of sewage treatment plants mainly include wind direction, topography, geological conditions for engineering, potential for expansion related to sewage treatment process, transportation cost, as well as less relocation and occupation of farmlands (Long et al. 1998; Zhang & Jin 2001; Huang et al. 2012a, 2012b). These factors are mainly concerned with individual sewage treatment plants, failing to consider the spatial relationships between sewage treatment plants and between sewage treatment plant and sewage discharge demand. As a result, the present spatial arrangement of sewage treatment plants is disordered and aimless (Ke & Shi 2000), and causes the wastage of plentiful urban infrastructure construction resources (Alexandra et al. 2007; Zhao et al. 2009). Therefore, in order to improve the directivity and orderliness of urban sewage treatment facilities in terms of quantity, scale, construction time sequence, and spatial pattern, the spatial pattern of sewage control demand shall be considered as a factor for the time–space arrangement of urban sewage treatment facilities.

Urban centralized sewage treatment facilities in Huai'an City of Jiangsu Province have mostly realized full coverage in urban areas of county level and above. However, on the one hand, current construction of sewage treatment plants is separated from pollution control demand; consequently, pollution discharge is by far larger than the treatment capacity, and regional pollution control is insufficient. On the other hand, insufficient supporting pipeline networks have resulted in the low operating efficiency of sewage treatment plants. Therefore, with Huai'an City as an example, based on analyzing the impact of pollution source pattern and the demand on sewage treatment facilities, and according to the spatial matching pattern of sewage discharge and governance capacity, this paper discusses the guide for spatial optimization of sewage treatment facilities, and provides a theoretical and methodological basis for the site selection and spatial layout optimization of economical and high-efficiency urban sewage treatment facilities.

SITE DESCRIPTION

Huai'an is located on the east of Jianghuai Plain, the north-central part of Jiangsu Province, and in the Yangtze River Delta region represented by Shanghai, with a northern latitude 32°43′00′–34°06′00′ and an eastern longitude 118°12′00′–119°36′30′. It is connected with Lianyungang in the north, adjacent to Yancheng in the east, linked with Yangzhou and Anhui's Chuzhou in the south, and close to Suqian in the west. Consisting of four districts (Qinghe, Qingpu, Chuzhou and Huaiyin) and four counties (Jinhu, Xuyi, Lianshui, and Hongze), Huai'an City covers an area of 10,072 km2, accounting for 9.82% of the total area of Jiangsu Province. In the city, there are densely distributed water networks, including two water systems (Huaihe River and YiShuSi River), Hongze Lake, and more than ten inflow and outflow rivers of Taihu Lake. In recent years, this region has enjoyed fast economic development, and the regional total output value has increased by four times from 29.1 billion Yuan in 2000 to 169.0 billion Yuan in 2011. In 2011, the total population here reached 5,432,400 and the population density was 539 people/km2, more than four times higher than the national average population density of 139 people/km2. By the end of 2011, the whole city had a total sewage discharge of 265,656,700 t and completed the construction of 13 urban centralized sewage treatment plants, which basically realized the full coverage of the county district, but only had an actual sewage treatment capacity of 122,202,000 t. Huai'an is stepping into the development stage of fast industrialization and new urbanization, which will surely bring a great increase in the discharge of wastewater and sewage. Therefore, there is great significance for protection of the water environment of urban rivers to reasonably plan and construct sewage treatment facilities and raise sewage treatment rate.

METHODOLOGY AND DATABASE

The methodology including prediction of discharge of wastewater and sewage, determination of sewage treatment capacity, and matching pattern of sewage treatment facilities is presented; the evaluation unit, the data sources and processing are introduced.

Wastewater and sewage discharge and treatment capacity

Prediction of discharge of wastewater and sewage

Method I: Per capita water consumption and the production and discharge coefficient of wastewater and sewage, with formula as shown below: 
formula
1
where a1 and a2 indicate the sewage collection rate of central urban areas and small towns; X1 and X2 indicate the per capita water consumption of central urban areas and small towns; R1 and R2 indicate the population of central urban areas and small towns; b indicates the loss rate of water supply; and c indicates the sewage discharge coefficient. In accordance with the Code for Urban Water Supply Engineering Planning (GB50282-98) and the Overall Planning of Huai'an City 2009–2030, 60% and 50% are taken as a1 and a2, 800 L per capita and 400 L per capita every day as X1 and X2, 10% as b, and 0.8 as c. Among the parameters, R1 and R2 are variables and the most influential factors to the discharge of wastewater and sewage.
Method II: Water consumption of unit construction land and production-discharge coefficient of wastewater, with formula as shown below: 
formula
2
where a indicates the area of construction land, b indicates the sewage collection rate, X indicates the comprehensive water consumption of unit construction land, c indicates the loss rate of water supply, d indicates the repeated utilization rate, and e indicates the sewage discharge coefficient. In accordance with the ‘Code for Urban Water Supply Engineering Planning (GB50282-98)’ and the ‘Overall Planning of Huai'an City 2009–2030’, 10,000 m3 per km2 in a day is taken as X, 60% as b, 60% as d, 10% as c, and 0.8 as e. Among the parameters, a is the only variable and the most influential factor to the discharge of wastewater and sewage.

According to prediction, by 2020, the increment of wastewater in the whole city will be 464,300 t/d, wherein that of urban areas will be 290,100 t every day, accounting for 62.48%, and that of four counties will be 174,300 t/d, accounting for 37.52%.

Determination of sewage treatment capacity

Based on the designed capacity of existing sewage treatment plants, the factors determining the actual sewage treatment capacity mostly include the operating load factor and centralized sewage treatment rate of urban sewage treatment plants, and their parameters are mainly determined according to the following.

Determination of a treatment plant's operating load factor

According to the Opinions of the Ministry of Construction on Strengthening the Operation Supervision of Urban Sewage Treatment Plants, the actual treatment load of urban sewage treatment plants after being put into operation shall be no lower than 60, 75, and 85% of the designed capacity in one year, three years, and more than three years, respectively. By the end of 2010, the average operating load factor of urban sewage treatment plants nationwide was up to 78.95%. In 2011, the average operating load factor of urban sewage treatment plants was up to 75%. According to the ‘12th Five-Year Special Planning for Sewage Treatment and Recycling in Urban Areas of Wuxi’, the total operating load factor of sewage treatment plants in Wuxi was up to 75.65% at the end of 2010. Therefore, according to the requirements of the Ministry of Construction and the actual situations of Jiangsu Province, the average operating load factor of urban sewage treatment plants in Huai'an City in 2020 is finally determined to be 85%.

Determination of centralized sewage treatment rate

According to the requirements of the ‘12th Five-Year Planning for the Construction of Nationwide Urban Sewage Treatment and Recycling Facilities’, the sewage treatment rate of cities all over the country shall be up to 85% by 2015. Moreover, in the ‘Planning for Urban Sewage Treatment and Rainwater Drainage of Huai'an City’, it is proposed that the centralized sewage treatment rate of Huai'an City will reach 90% in 2020. Comprehensively considering nationwide and Huai'an-specific planning requirements, the centralized sewage treatment rate of Huai'an City in 2020 is finally determined to be 90%.

Research on the matching pattern of sewage treatment facilities

Based on the discharge of wastewater and sewage as well as the comparison of actual treatment capacity and designed treatment capacity of sewage treatment plants, the spatial analysis module of ArcGIS9.3 is adopted to treat related data of sewage treatment plants. Spatial analysis commands like Zonal Statistics are used to segment information on the discharge of wastewater, the designed treatment capacity, and the actual treatment capacity into small town evaluation units, which will be divided into sewage governance potential releasing area, sewage governance strengthening area, and current state maintaining area (as shown in Table 1).

Table 1

Types of space matching between sewage discharge and treatment capacity

Type Dividing basis 
Potential releasing area Discharge of wastewater and sewage is greater than water environment capacity, the actual treatment capacity is smaller than the actual discharge, but the designed treatment capacity is greater than the actual discharge 
Strengthening area Discharge of wastewater and sewage is greater than water environment capacity, the designed treatment capacity is smaller than the actual discharge, but the actual treatment capacity is smaller than the actual discharge 
Current state maintaining area Discharge of wastewater and sewage is smaller or greater than water environment capacity, and the actual treatment capacity is greater than the actual discharge 
Type Dividing basis 
Potential releasing area Discharge of wastewater and sewage is greater than water environment capacity, the actual treatment capacity is smaller than the actual discharge, but the designed treatment capacity is greater than the actual discharge 
Strengthening area Discharge of wastewater and sewage is greater than water environment capacity, the designed treatment capacity is smaller than the actual discharge, but the actual treatment capacity is smaller than the actual discharge 
Current state maintaining area Discharge of wastewater and sewage is smaller or greater than water environment capacity, and the actual treatment capacity is greater than the actual discharge 

Evaluation unit, data source and treatment

With small towns as evaluation units, we have collected the administrative maps of the researched area, including 116 small town (street) units in 2011, related attribute data of sewage treatment plants in the area, and sewage discharge data. Wherein the data of sewage treatment plants mainly include the coordinates of latitude and longitude, construction time, designed treatment capacity, actual treatment capacity, and sewage discharge; the data about the current water quality of rivers are excerpts from the ‘Environmental Quality Report of Huai'an City in 2010’, and the data about target water quality are obtained from the ‘Functional Zoning of Surface Water Environment of Jiangsu Province’; the data about sewage discharge originate from organizing and analyzing the environmental statistical data of 2011, and mainly include the industrial wastewater discharge, domestic sewage discharge, and gross sewage discharge of each small town (street) unit. Other social and economic data are mainly from the ‘Statistical Yearbook of Huai'an City 2012’.

RESULTS AND ANALYSIS

From the two levels of time and space, this paper comprehensively analyzes the impact of pollution sources, the demand on sewage treatment facilities, and the spatial matching pattern of sewage treatment facilities, proposing optimization suggestions.

Current quality of the regional water environment and distribution of pollution sources

The water environment in Huai'an City is of good quality in general. Among the 38 surface water monitoring sections of the whole city in 2010, 33 sections were proved to have good-quality water; and the sections with relatively poor water quality and severe pollution were mainly distributed along Qing'an River, Xun River, and seaward waterways (Figure 1). Among more than 10 pollution-taking rivers in the whole city, Qing'an River suffers from the severest pollution, and is determined to be a Class-V river. In 2010, Qing'an River received up to 19,458,000 t wastewater and sewage. Its pollutants mainly comprised domestic sewage from central urban areas and development zones and industrial wastewater from 109 key polluting enterprises, of which Dongli Dyeing (5,987,000 t) and Longxing Printing and Dyeing (3,000,000 t) discharged relatively large quantities of wastewater. Following Qing'an River is Xun River, which is a Class-IV river with light pollution receiving 18,281,000 t wastewater and sewage. Its pollutants are domestic water from Hongze County and Saline and Alkali Industry Gathering Park, as well as industrial wastewater from 27 key polluting enterprises such as Daimengte Chemical and Hongda Industry and Trade. Yan River, Huaihe-Changjiang Waterway, Hongze Lake and some others generally meet the standard of surface water function areas in terms of water quality at present, but they receive a large amount of pollutants due to the impact of domestic sewage discharged from surrounding cities and towns and wastewater discharged by key polluting enterprises (Figure 2). In particular, Huaihe Chemical and Hongze Organic Chemical have a relatively great impact on the quality of water in the east of Hongze Lake, and industrial wastewater discharged by Jinlian Paper Industry and partial domestic water from Jinhu County have a relatively great impact on the water quality of Pianhong in the west of Huaihe-Changjiang Waterway (Figure 3, Table 2).

Table 2

Quality of main pollutant-receiving water environment and quantity of received pollutants

Water receiving pollutants Whether water quality meets the standard Quantity of waste (000 t) Main polluting enterprises and quantity of their pollution (000 t) 
Qing'an River No 1,945.8 Dongli Dyeing (598.7), Longxing Printing and Dyeing (300.0) 
Xun River No 1,828.1 Daimengte Chemical (700.0), Hongzeyinzhu Chemical, Foster Chemical 
Hongze River Yes 935.7 Huai rever Chemical (890.0) 
Weiqiao River Yes 777.7 Nuopu Paper Industry, Fanli Pipe 
Yan River Yes 712.7 Baimei Suger Industry (224.5) 
Huaihe-Changjiang Waterway Yes 669.1 Jinlian Paper Industry (625.0) 
Zhangfu River Yes 423.9 Huai'anyuan Powder, Pacific Ocean Chemical 
Nanliutang River Yes 280 Jinshiyuan Wines, Jinyue Paper Industry 
Huai River Yes 98.4 An'ge Medicine Industry, Hongguang Chemical 
Water receiving pollutants Whether water quality meets the standard Quantity of waste (000 t) Main polluting enterprises and quantity of their pollution (000 t) 
Qing'an River No 1,945.8 Dongli Dyeing (598.7), Longxing Printing and Dyeing (300.0) 
Xun River No 1,828.1 Daimengte Chemical (700.0), Hongzeyinzhu Chemical, Foster Chemical 
Hongze River Yes 935.7 Huai rever Chemical (890.0) 
Weiqiao River Yes 777.7 Nuopu Paper Industry, Fanli Pipe 
Yan River Yes 712.7 Baimei Suger Industry (224.5) 
Huaihe-Changjiang Waterway Yes 669.1 Jinlian Paper Industry (625.0) 
Zhangfu River Yes 423.9 Huai'anyuan Powder, Pacific Ocean Chemical 
Nanliutang River Yes 280 Jinshiyuan Wines, Jinyue Paper Industry 
Huai River Yes 98.4 An'ge Medicine Industry, Hongguang Chemical 
Figure 1

Distribution of water quality.

Figure 1

Distribution of water quality.

Figure 2

Grading of major water pollutants.

Figure 2

Grading of major water pollutants.

Figure 3

Distribution of key polluting enterprises.

Figure 3

Distribution of key polluting enterprises.

Current state and future trend of pollution discharge

Change in aggregate

The aggregate discharge of wastewater and sewage in the whole city continues to increase (Figure 4), and the annual average growth rate was 8.6% in 2000–2010. The aggregate in 2010 was 265,657,000 t, wherein the annual average growth rate of industrial wastewater and domestic sewage discharge was 5.1% and 11.8%, respectively, accounting for 39.5 and 60.5% of the total discharge of wastewater and sewage at present. Along with the constant acceleration of industrialization and urbanization, the discharge of industrial wastewater and domestic sewage will continue to increase.

Figure 4

Trends of wastewater discharge and treatment.

Figure 4

Trends of wastewater discharge and treatment.

Spatial change

The main industrial pollution sources in the whole city are distributed in a relatively centralized way. Accounting for 73.99% of the whole city's total discharge, the 310 polluting enterprises cluster in Huai'an Economic and Technical Development Zone, Huai'an Economic Development Zone, Huaiyin Economic Development Zone, Huai'an Industry Park, Qingpu Industry Park, and provincial economic development zones in Lianshui, Hongze, Jinhu and Xuyu counties. Along with the reintegration of industrial function blocks, the Saline and Alkali Industry Gathering Park in east expansion area and south optimization area, and Lianshui Heavy Chemical Industry Park in north industry and urban agglomeration area will be the areas seeing a great increase of industrial wastewater in future. The discharge of wastewater and sewage in the city is mainly concentrated in the central urban area and the urban areas of four cities (Figure 5). Wherein, the discharge in the central urban area is around 300 million tonnes, accounting for 75%, and that in the urban areas of four counties is around 100 million tonnes, accounting for 25%. According to the allocation of urban population, central urban areas, especially new urban areas, will experience rapid population growth in the future and the discharge of urban domestic sewage will greatly increase. The spatial change of the aggregate discharge of wastewater and sewage in the whole city will present a more centralized trend (Figure 6).

Figure 5

Distribution of wastewater discharges.

Figure 5

Distribution of wastewater discharges.

Figure 6

Distribution of wastewater discharge increment.

Figure 6

Distribution of wastewater discharge increment.

Analysis of aggregate and spatial demand of sewage treatment facilities

Aggregate analysis

In recent years, the construction of centralized sewage treatment facilities has been accelerated and the sewage treatment rate of the whole city has been continuously and significantly increasing, from 4.1% in 2001 to 56.7% in 2010. In 2010, there were still 120 million tonnes of wastewater and sewage not subject to centralized treatment. In 2011, the total designed treatment capacity of 13 centralized sewage treatment plants (more than 20,000 t/d) was 170 million t/year, and the operating load factor was around 70%. According to prediction, by 2020, the aggregate discharge of urban domestic sewage and industrial wastewater in the entire city will be around 400 million tonnes; it follows that, according to the objectives of a centralized sewage treatment rate of 90% and operating load factor of 85% of sewage treatment facilities, the gap in the treatment ability of urban wastewater and sewage in the whole city will be around 200 million tonnes. The rural domestic sewage discharge of surrounding small towns will be around 150 million tonnes, while the present sewage treatment ability is only 50 million tonnes, leading to a gap of nearly 100 million tonnes (Figure 7).

Figure 7

Distribution of sewage plant.

Figure 7

Distribution of sewage plant.

Spatial demand analysis

According to the predicted discharge of urban wastewater and sewage and the comparative analysis of the actual treatment capacity and designed treatment capacity of sewage treatment plants, regions where the discharge of wastewater and sewage is higher than the designed sewage treatment capacity mainly include old urban areas, new urban areas, urban areas of four counties, and Lianshui Heavy Chemical Industry Park. The discharge of wastewater and sewage is lower than the designed sewage treatment capacity but higher than the designed treatment capacity mainly in the regions including the Saline and Alkali Industry Gathering Park, and New Materials and Equipment Manufacturing Park in the south. In addition, in key towns such as Maba, Gaogou, and Dailou, the discharge of domestic sewage and industrial wastewater is higher than the designed sewage treatment capacity, and the gap is around 100 million tonnes; while in other small towns, the discharge of wastewater and sewage does not increase greatly and the sewage treatment capacity generally meets the wastewater governance requirements.

Spatial demand matching pattern and optimization of sewage treatment facilities

Based on the predicted discharge of wastewater and sewage as well as the relationship between the actual treatment capacity and designed treatment capacity of sewage treatment plants, the whole city is divided into three types of areas, namely current state maintaining type, potential releasing type, and sewage governance strengthening type (Figure 8).

Figure 8

Types of space matching.

Figure 8

Types of space matching.

The current state maintaining area includes 46 small towns such as Liujun Town, Nanzha Town, and Fuxing Town of Huai'an District, Shihu Town and Tangji Town of Lianshui County, and Guiwu Town and Heqiao Town of Xuyi County, covering a total area of 3,874.92 km2 and accounting for 38.64% of the city's total area. Areas of this type are mainly distributed at the edge of the city and have a relatively small discharge of wastewater. By 2020, the increment of wastewater is predicted to be 6,900 t/d, accounting for only 1.49% of the whole city's increment. The discharge of wastewater in 2020 will be smaller than the actual sewage treatment capacity; therefore, under the precondition of guaranteeing the normal operation of the existing town (street) sewage treatment plants, it will be able to meet the wastewater discharge demand in the near future without building, reconstructing or expanding sewage treatment plants.

The potential releasing area includes 10 small towns (streets) such as the urban area of Qingpu District, Nanmachang Town, Xuyang Town, Huangji Town, Heping Town, Fanji Town, and Donghu Street, covering a total area of 384.06 km2 and accounting for 3.83% of the city's total area. By 2020, the increment of wastewater is predicted to be 87,700 t/d, accounting for 18.90% of the city's increment. Most areas of this type are clustered with industrial enterprises and have a high demand on the discharge of industrial wastewater. As the enterprises in economic development zones and salt chemical industrial zone are put into operation successively, the existing sewage treatment facilities cannot meet the demand of wastewater discharge any longer. Therefore, we should accelerate the construction of sewage pipeline networks and related supporting infrastructure of Huai'an Economic and Technical Development Zone Sewage Treatment Plant, New Salt Chemical Industry Zone Sewage Treatment Plant, Hongze County Qingjian Sewage Treatment Plant, as well as raise the operating efficiency of sewage treatment facilities. We may also properly construct artificial wetland treatment systems or adopt micro-exposure oxidation ditch and biological aerated filter technology for further ecological purification of the tail water from the sewage treatment facilities of fine chemical enterprises, strictly observing the up-to-standard discharge criteria and protecting surrounding rivers so that they suffer from minimal pollution.

The sewage governance strengthening area mainly includes 66 town and street units such as Wangying Town and Xindu Town of Huaiyin District, Xincheng of Qinghe District, Huaicheng Town and Chengdong Town of Chuzhou District, Liancheng Town, Zhuma Town and Chenshi Town of Lianshui County, Gaoliangjian Town of Hongze Town, Yicheng Town and Maba Town of Xuyi, covering a total area of 4,729.73 km2 and accounting for 47.16% of the city's total area. By 2020, the increment of wastewater is predicted to be 369,600 t/d, accounting for 79.62% of the whole city's wastewater increment. According to the predicted increment of wastewater in 2020, areas of this type are characterized by a wastewater discharge that is higher than the designed sewage treatment capacity, mostly distributed in central urban areas and future urban expansion areas with only a small portion dispersed in surrounding undeveloped towns. As a result of the low urbanization level, the supporting municipal infrastructure is lagging behind. The existing sewage treatment facilities cannot meet the demand of wastewater, especially domestic sewage, treatment in future. Towns with a large sewage discharge may build, reconstruct or expand sewage treatment facilities according to their own conditions, while neighboring towns without large sewage discharges that are upstream and downstream along the same river may cooperatively or jointly construct a sewage treatment facility. In addition, towns with a centralized rural population, high degree of agricultural industrialization, and rivers, lakes and wetlands may greatly enhance the regional sewage treatment rate by means of constructing oxidation ponds.

DISCUSSION

Using ArcGIS spatial analysis tool, starting with the current quality of the regional water environment, the distribution and impact of pollution sources, and demand on sewage treatment facilities, and according to the comparison of actual and designed urban centralized sewage treatment capacity and wastewater and sewage discharge of town units, the paper divides the researched area into current state maintaining area, sewage governance strengthening area, and potential releasing area. According to the gap in demand on sewage governance, this paper proposes a guide for the optimization of sewage treatment facilities' construction and layout. Based on the above research, the paper mainly draws the following conclusions.

The sections with relatively poor water quality and severe pollution in the city are mainly distributed on Qing'an River, Xun River, and seaward water ways, and more than 400 key polluting enterprises are the main pollution sources of the aforementioned rivers. Paper manufacturing, textile, food production, and petrochemical industries are the severest pollution sources. Treated industrial wastewater has a low up-to-standard level, the discharge of pollutants has a relatively great impact on the water quality and environment of rivers, and the layout of some key polluting enterprises also has a certain impact on the safety of important ecological function reserves delimited by the province and the city. Along with the constant acceleration of industrialization and urbanization, the discharge of industrial wastewater and domestic sewage will increase continuously at index rate. Besides industrial structure adjustment and optimization, accelerating the construction of sewage treatment projects is an important means for reducing the total discharge of major pollutants, improving the environment of rivers and lakes, and preventing environmental risks.

The centralized sewage treatment facilities have generally realized full coverage in urban areas of county level and above, but the total designed treatment capacity is by far lower than the total discharge of wastewater and sewage and the centralized treatment capacity is in severe shortage. Park-based sewage treatment plants that give priority to industrial sewage governance have a low load factor. For them, the focus should be on strengthening the pipeline network construction and raising the operating efficiency of industrial park sewage treatment plants. Urban sewage treatment plants that give priority to urban domestic sewage and industrial wastewater governance are mostly in high-load operation. There is an urgent need to expand the sewage treatment plants, giving priority to surrounding waters with the simple function of discharging tail water, and further enhancing the implementation standards for the discharge of pollutants.

In accordance with the spatial matching of the treatment capacity of sewage treatment facilities and the discharge of wastewater and sewage, for towns (streets) in the urban area belonging to the potential releasing area, the existing sewage treatment facilities cannot meet the demand of wastewater discharge any longer; it is necessary to accelerate the construction of sewage pipeline networks and related supporting infrastructure as well as raise the operating efficiency of sewage treatment facilities. For towns (streets) at the edge of the city belonging to the current state maintaining area, there are fairly complete sewage treatment facilities and supporting pipeline networks. It is not necessary, therefore, to build, reconstruct or expand the sewage treatment plants; rather, it suffices to maintain the current state to meet future demands. For towns (streets) with a large urban population and rapid urbanization and industrialization development belonging to the sewage governance strengthening area, it is necessary to expand, re-build, and reconstruct the facilities according to higher standards and to strengthen the regional ability to govern wastewater and sewage. In addition, for towns in rural areas or far away from central areas, the construction of pipeline network facilities lags far behind. It is thus possible to strengthen the sewage governance ability by building new treatment plants, cooperatively constructing sewage treatment plants, or by means of oxidation ponds.

CONCLUSIONS

The research provides a new perspective for discussing scale determination and spatial site selection of urban sewage treatment plants. It may also provide certain theoretical evidence for the spatial optimization and construction of urban municipal infrastructure and the protection of the regional water environment. However, the research needs more depth in the following areas: during the research process, the researched area is taken as an enclosed area, thus the contribution of the sewage treatment plants outside the researched area to the regional internal sewage governance is not considered. This aspect is worthy of further in-depth research. In addition, although the coverage scope of the pipeline networks of sewage treatment plants is sufficiently considered, the point attribute data of sewage treatment plants are adopted for the estimation of sewage governance capacity. Precisely estimating the sewage governance capacity of surrounding areas is impossible and methods need to be further discussed.

ACKNOWLEDGEMENTS

The authors acknowledge support from: Knowledge Innovation Program of the Chinese Academy of Sciences (KZZD-EW-10-04); 135 Strategic Development Planning Project of Nanjing Institute of Geography and Limnology, CAS, No. 2012135006; National Natural Science Foundation of China, No. 41130750; the State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, CAS; Key Laboratory of Watershed Geographic Sciences, Nanjing Institute of Geography and Limnology, CAS.

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