3.1. Spatial Pattern of Provincial Urban Wastewater Discharge
The spatial variation of nonylphenol, nitrogen, and dichlorodiphenyltrichloroethane (DDT) discharges were studied on a basin scale [32
]. Water quality variation through space was also investigated [35
]. However, water quality might not give insight into the spatial variation of the wastewater discharge because different water bodies have different dilution capacities. Recently, the spatial characteristics of total wastewater [3
] and industrial wastewater discharge [8
] were assessed. However, studies on urban wastewater discharge are limited.
To fill this research gap, we used the natural breakpoint tool in ArcGIS 10.6 (Esri, Redlands, CA, USA), and we divided the urban wastewater discharge of the 31 provinces for the period 2011–2015 into five levels. Among these levels, the low discharge ranged from 0.04 to 0.5 billion tons, the low and middle discharge ranged from 0.5 to 1 billion tons, the middle urban wastewater discharge ranged from 1 to 2 billion tons, the middle and high from 2 to 3.5 billion tons, and the high discharge ranged from 3.5 to 7.5 billion tons. We chose 2011, 2013, and 2015 as the first, middle, and last year of our study to analyze the spatial variation of the urban wastewater discharge (Figure 1
From Figure 1
and Figure 2
, it can be seen that the amount of urban wastewater discharge within each province increased from 2011 to 2015. The number of high wastewater discharge provinces also increased. Guangdong was a high-level discharge province throughout the study period, while the urban wastewater discharge from Jiangsu and Shandong provinces increased significantly, making them high discharge provinces gradually. The urban wastewater discharge in Henan, Hebei, Hubei, Hunan, and Anhui also increased considerably.
Relatively high discharge provinces were concentrated in the central and eastern parts of the country. It shows a tendency to spread from the eastern coast to the central and western parts of the country.
The eastern part of China includes most of the economically developed provinces [38
]. These provinces’ wastewater discharge was relatively high (Figure 2
). Hence, provinces in the eastern part of the country should be pioneers for introducing advanced urban wastewater treatment and management methods, and also should be the leaders in innovating new and improving existing urban WWTP technologies. Policymakers in the provinces located in the central and western parts of China should determine the appropriate rate of urbanization to avoid explosive urban population growth and disorderly urban expansion, which may cause a rapid increase in urban wastewater discharge. In addition, strict environmental standards need to be put in place in these areas [39
The Moran’s I index results (shown in Table 2
) describe the spatial relationship among provinces. From Table 2
, all the P(I) (Moran’s I index) values passed the 5% significance test. For the period from 2011 to 2015, the value of Moran’s I index remained positive, within the range 0.188–0.176. These values were relatively stable or did not fluctuate that much, indicating that the spatial agglomeration of provincial urban wastewater discharge was relatively stable during the study period.
We used GeoDa software to analyze further the urban wastewater discharge’s spatial agglomeration and identify the provinces that need to implement strict water pollution control because of their intensive urban wastewater discharge. By comparing the Moran scatter diagram for the period from 2011 to 2015, we found that the number of provinces that belong to HH, HL, LH, and LL categories did not change. The results can be seen in Figure 2
As shown in Figure 3
, the HH category included Jiangsu, Shanghai, Zhejiang, and Fujian provinces, which are gathered in the eastern coastal part of the country, and Anhui, Jiangxi, Hubei, Hunan, and Sichuan provinces, which are located in the central parts of China. The LL category included provinces mainly in the northwestern part of China, such as Xinjiang, Ningxia, Qinghai, and Inner Mongolia. There were also some scattered provinces that belonged to the LL category; among them were Shanxi, Heilongjiang, Jilin, Shaanxi, Guizhou, and Hainan, which were the main ones. These provinces and their neighboring provinces all had low urban wastewater discharge. The HL category included Beijing, Liaoning, Hebei, Shandong, Henan, and Guangdong. The remaining provinces belonged to the LH category and are scattered throughout the country.
All provinces in China should strengthen cooperation. Especially in the regions that contain neighboring provinces that belong to the HH category, inter-provincial linkage mechanisms [40
] should be put in place for sharing responsibility and technology transfer among neighboring provinces. This mechanism can serve as a trans-jurisdictional urban wastewater treatment information exchange platform.
3.2. The Urban WWTPs Efficiency with Super-Efficiency DEA Model
The above analysis shows the spatial distribution and evolution pattern of urban wastewater discharge, but whether the current urban wastewater treatment plants are enough to control the pollution, needs further study. Even though the WWTP efficiencies were assessed by previous studies [41
], these studies are about specific aspects of WWTPs. Hence, it is not possible to use them as a base for countrywide policy formulation. Since our study was on a provincial scale, it avoids this limitation.
The efficiency of urban wastewater treatment plants in China was calculated using Equation (2). The results are shown in Figure 3
, and each province’s urban WWTP efficiency from 2011 to 2015 are presented in the Appendix Table A1
From Figure 4
, the efficiencies of the urban WWTPs of only Shandong in 2011, and Beijing and Shanghai in 2015 were bigger than 1, which means they were the most efficient provinces in terms of wastewater treatment. For Beijing, Tianjin, Shanghai, Zhejiang, Shandong, Guangdong, Anhui, Chongqing, and Guizhou, the average efficiency for the five years was above the overall mean. This is because these regions are economically developed and have advanced urban wastewater treatment technology. Generally, the efficiencies of the urban WWTPs decreased between 2011 and 2013 and then increased between 2013 and 2015. The country’s overall average efficiency for the five years was 0.608 (Appendix Table A1
). This indicates that China’s overall urban WWTP efficiency has a great potential to improve since the loss of urban WWTP efficiency was as high as 39.2%. The reason for this low efficiency is the fact that the resources allocated to controlling water pollution were relatively insignificant.
The efficiencies of urban WWTPs of the eastern part of the country were greater than the central part of the country and western part of the country for the study period (Figure 3
). The efficiency of the central part of the country was greater than the western region of the country. East China is the more developed region with a high level of urbanization, industrial structure, and high technological advancement. This is the main factor that made WWTP efficiency in the eastern region better.
However, the efficiency of the eastern part of the country in 2015 was 0.705 (Appendix Table A1
), lower than its efficiency in 2011, which was 0.791 (Appendix Table A1
). This indicates urban WWTP efficiency declined in provinces located in the eastern part of the country, and the technological upgrade did not keep up with the development of socio-economic systems. The efficiencies of urban WWTPs in cities located in the central and western parts of the country were higher in 2015 than in 2011. If this trend were to continue, the superiority of the eastern part of the country in terms of urban WWTP efficiency would disappear.
According to the analysis above, more and more attention should be paid to water pollution remediation, especially by investing capital that can keep up with the economic and population growth. The eastern regions should play a technological spillover effect so that the central and western provinces improve their wastewater remediation efficiency. Provincial governments should, according to their urbanization process, reasonably adjust the scale and financial investment allocated to urban wastewater treatment.
3.3. The Results of the Malmquist Index
Based on the urban WWTPs’ input and output data of China’s 31 provinces and using Equations (3) and (4), the average urban WWTP efficiency and efficiency change for the period from 2011 to 2015 was calculated. The results are shown in Figure 5
and the results of each province are shown in Appendix Table A2
. The total factor productivity change (TFPch) is divided into technological change rates (TEch) and technical efficiency change rate (TEC). If the value of TFPch is greater than 1, there was technological progress. The technical efficiency changes rate (TEC) >1 indicates that technical efficiency was improved.
According to Appendix Table A2
, the average urban WWTP efficiency change (TFPch) for China in the period from 2011 to 2015 was 0.996, which means that urban WWTP efficiency had a negative growth. This was mainly due to the negative growth of the technological change rate (TEch), which was −4.8%, and the scale efficiency change (SEch), which was −0.2%.
As it can be seen from Figure 5
, the technological change rate (TEch) in the eastern, central, and western regions was obviously insufficient for urban WWTPs. Only Shanghai, Hainan, and Guizhou’s TEch improved between 2011 and 2015. For the other provinces, the contrary happened. These provinces should focus on improving the efficiency of their WWTPs by increasing capital investment.
The scale efficiency change (SEch) was on a decline in the recent years, especially in the eastern part of the country (Figure 5
). As a result, the number of WWTPs in the region has not been increasing at a rate that can keep up with the wastewater discharge. Provinces such as Hebei, Liaoning, Jiangsu, Shandong, Hainan, Sichuan, Shaanxi, Gansu, Qinghai, Ningxia, and Xinjiang also have the same problem.
Urban WWTP efficiency with regards to TEC and PTEch increased in the central and western parts of the country between 2011 and 2015. This was mainly due to technological improvements. In addition, improved management of wastewater treatment plants also contributed to the increase in efficiency. Hainan’s, Shandong’s and Ningxia’s technical efficiency change rate (TEC) declined; hence, these three provinces should focus on improving the efficiency of their municipal wastewater treatment plants.
Generally, efficient urban wastewater remediation protects water bodies and promotes sustainable development. Hence, the following measures could help in building efficient wastewater remediation systems. First, provinces located in the eastern part of the country should form a water pollution control alliance with provinces in the central and western parts of China. This would allow for the joint development of new technologies and cost sharing. Second, the government should increase investment in new wastewater treatment plants and also pay attention to the renewal of old treatment facilities to maintain efficiency. Each province should increase the technical and scale of their urban sewage treatment plants according to their different situations. Third, the government should establish a reasonable urbanization speed and plan the scale of their urban WWTP facilities accordingly. Fourth, the government needs to organize technical exchanges and seminars and strengthen the technical training for operational management personnel to improve the overall level of wastewater remediation. Last, the public should vigorously advocate for water conservation, social supervision, and participation, as well as for the reduction of urban wastewater discharge at the source.
Our proposed framework could be modified and improved by addressing the following research gaps through further study. First, to increase the resolution of the assessment, the model needs to be applied on a smaller spatial scale, such as a county. Second, additional factors that influence urban WWTP efficiency should be taken into account. Third, the time span for the study period should be increased.