key: cord-0741305-pnfqcqi7
authors: Li, Ping; Liu, Li; Wu, Jiaojiao; Cheng, Rong; Shi, Lei; Zheng, Xiang; Zhang, Zhenxing
title: Identify driving forces of MBR applications in China
date: 2019-01-10
journal: Sci Total Environ
DOI: 10.1016/j.scitotenv.2018.07.412
sha: 1bb0d4af7d7890d86aea152839bf534a931bc4bb
doc_id: 741305
cord_uid: pnfqcqi7

During the last two decades, MBR applications in China grow exponentially with the first pilot test of 10 m(3)/d in 1999 and the first application with capacity of 110,000 m(3)/d commissioned in 2009. It is critical to examine the drivers of MBR applications in China, which can provide sound scientific basis for future development of MBR applications. This study summarized the historical development of MBR applications and analyzed the driving forces by survey, literature review and interviews with MBR suppliers. The results showed that: (1) technical advantages of MBR and public policy related to water resources and environment promoted MBR beyond lab and pilot test into wide commercial applications in China; (2) petrochemical industry needs for wastewater treatment and reuse promoted medium-scale MBRs as public policy and regulation on water resources and environment tightens; (3) when the breakthrough of capacity of a single project above 10 thousand m(3)/d, the Green Olympic Games and Asian Games and tightening effluent regulations in environmentally sensitive areas incentivized MBR applications; and (4) the emergence of 100,000 m(3)/d MBR was mainly stimulated by water resources stress. Water resources stress and public policy related on resources and the environment are the primary driving forces in the last several decades. The future drivers of MBR applications in China appear to be decreasing operation cost.

MBR applications in China started as early as 1990 (Zheng et al., 2003; Zheng et al., 2010; Huang et al., 2010; Abass et al., 2015; Xiao et al., 2014) . In 1999, the first pilot scale membrane bioreactor (MBR) application in China was tested, which was used to treat industrial wastewater with a scale of only 10 m 3 /d (Zheng et al., 2003) . Since then, MBR research and applications in China have been growing dramatically in that the first MRB application with the treatment capacity of 100,000 m 3 /d was commissioned within 8 years in Beijing Huang et al., 2010; Abass et al., 2015) . The total capacity of large-scale MBRs (≥10,000 m 3 /d) reached 1.7 × 10 6 m 3 /d in 2010 (Fig. 1) . Both the capacity and number of MBR applications increased rapidly. A national survey of membrane manufacturers and operators was conducted to characterize MBR applications in China. Based on the survey, the number of large-scale MBRs in China approached 192, with the total capacity exceeding 11.17 × 10 6 m 3 /d by 2017. Taking the under-construction projects into account, the number of largescale MBRs is expected to approach 210 with the total capacity exceeding 14.04 × 10 6 m 3 /d by 2018. The MBR applications with a capacity of 50,000 m 3 /d or more are shown in Appendix A.

Nowadays, MBR has been widely accepted and applied as an efficient wastewater treatment process in China, due to its clean effluent and rapid performance improvement in many aspects, such as membrane lifespan, fouling mitigation, and energy consumption reduction. The scientific and technical perspectives of MBR have received much attention in literature and engineering operations of MBR applications have also been investigated extensively in China (Wang et al., 2013; Liu et al., 2010; Zheng and Liu, 2006; Sun et al., 2014; Shen et al., 2012; Zheng and Liu, 2007; Zheng et al., 2005; Lv et al., 2006) . A number of membrane materials and modules, such as hollow fiber membranes, flat-sheet membranes, tubular membranes, have been employed in municipal and industrial wastewater treatment facilities in China since the 2000s (Huang et al., 2010; Abass et al., 2015; Zhu et al., 2013; Zhang et al., 2008; Zhao et al., 2014; Yu et al., 2015; Wang et al., 2008) . The various anaerobic-anoxic-aerobic MBR (AAO-MBR) processes for municipal wastewater treatment have been extensively investigated and denitrification on these occasions was enhanced since the 2010s, such as Hefei Tangxihe WWTP and Wuxi Chengbei WWTP .

China's research on MBR technology started in 1990. The first paper on MBR technology was published in Chinese journals and introduced the application of MBR in other countries (Li, 1990) . Since then, some universities and research institutes have involved in MBR research including Chinese Academy of Sciences, Tsinghua University, Tongji University, Tianjin University, Zhejiang University and Harbin Institute of Technology, etc.

The Chinese economy increased by about 9.7% per year from the Economic Reform in 1978 until 2014 to become the second largest economic body in the world (Lau and Zheng, 2017) . In the last two decades, many historic events occurred in addition to the unprecedented longterm economic growth. In 2003, the severe acute respiratory syndrome (SARS), a contagious and sometimes fatal respiratory illness, occurred in China, which highlighted the importance of public health management and drew the attention of government and public into sanitary safety (Sun and Gu, 2007) . The Olympic Games in 2008 were held in Beijing and the Asian Games in 2010 took place in Guangzhou. Both events were aimed to be green games to improve the environment and promote the games (Zheng et al., 2016) . In 2014, the South-to-North Water Transfer (SNWTP) project started transport water to northern China to relieve water resources stress in North China (Zheng et al., 2016) . The SNWTP project is a mega-project and has far-reaching impact on social economic and environment. Are these historic events related to the rapid growth of MBR applications?

How the scientific and technical advancement of MBR and the social economic factors interact with each other and impact the MBR applications is not well understood. The driving forces and factors which have resulted in the rapid growth of MBR applications are rarely examined. The study is aimed to summarize the history of MBR applications in China since 1999. The driving forces and historic events' impact on MBR applications are examined, which is critical to the rapid growth of MBR applications. The MBR applications have been constantly impacted by the general factors in one way or another. Besides they have been promoted by the specific factors under specific historic context.

To fully analyze the driving factors, the historical development, general social economic development, water resources stress, water pollution, and historic events which may potentially impact on MBR applications are collected and examined. Published literatures are extensively reviewed to understand the research on MBR and a comprehensive national survey based on face-to-face or telephone interviews with senior scientists or executives of benchmark or major MBR applications was conducted to understand the context of those applications. Why MBR applications have been increasing dramatically in the last two decades in China? The study contributes to literature with the new understanding of MRB applications in China and provides insights of the driving forces of MRB applications. The study provides a unique observation and point of view of MBR applications in China.

The initial researches and studies on MBR started in 1960s and the progress in the next several decades provided sound scientific and technical basis for industrial and commercial applications of large scale MBR.

In 1969, Smith first combined bench-scale membrane separation system and aerobic activated sludge process to treat municipal sewage (Smith et al., 1969) . The first membrane bioreactors were developed commercially by Dorr-Oliver in the late 1960s, with application to ship-board sewage treatment (Bemberis et al., 1971 ). The initial designs, specifically the sidestream MBR configuration are tailored towards a direct hydrodynamic control of membrane fouling and offer the advantages of easier membrane replacement and high flux production but at the expense of high energy consumption. The typical energy consumption of sidestream MBR was 6-8 kWh·m −3 , almost ten times higher than conventional activated sludge process (CAS, 0.3-0.4 kWh·m −3 ). The high energy consumption by sidestream MBR limited its applications (Le-Clech et al., 2006; Judd, 2012) .

To expand MBR applications, it is vital to reduce the energy consumption by MBR. In 1989, Yamamoto invented submerged MBR and removed the circulating pump which greatly reduced energy consumption (Yamamoto et al., 1989) . By the early 1990s, Zenon introduced a series of immersed hollow fiber membrane modules (Urbain et al., 1996) . Over this period, Kubota also developed flat-sheet membrane products with improved overall energy efficiency, introducing a double-decker design in 2003. The energy consumption for sidestream systems is usually one or two orders of magnitude higher than that of submerged systems, regardless of whether the system is used for municipal or industrial wastewater treatments (Gander et al., 2000) . Generally, the external configuration provides more direct hydrodynamic control of fouling and offers the advantages of easier membrane replacement and high fluxes at the expense of high energy consumption (of the order 10 kWh·m −3 ) (Le-Clech et al., 2006) . Several distinct advantages of submerged configuration are their much lower energy consumption, only 0.6-2 kWh·m −3 , which is only 10 to 20% of sidestream MBR (Lin et al., 2012) . The invention of submerged MBR remarkably improved MBR efficiency and promote applications of MBR worldwide. The research in China has improved membrane flux capacity from 8 to 12 L/m 2 /h to about 15-25 L/m 2 /h as well.

The advancement of scientific and engineering research on MBR promote its commercial applications. MBR was employed to treat water and wastewater worldwide. Before 2000, MBR was primarily applied in disposal of a broad range of wastewater, such as landfill leachate, municipal sewage, fecal sewage, and industrial sewage. The largest MBR application before 2000 could treat 15,500 m 3 /d municipal sewage (Brindle, 1997) .

The research of MBR in China started in the 1990s which was much later than the first study on MBR abroad, but it developed dramatically. In 1990, Li (1990) firstly published a review of MBR and introduced MBR to China (Li, 1990) . Fan et al. (1998) later used MBR to treat petrochemical wastewater. In 1999, Zheng et al. (2003) conducted a pilot test of MBR to treat textile wastewater and laid a foundation for the commercial applications of MBR in China (Zheng et al., 2003) . The study of sidestream MBRs was dominant before 2001 while submerged MBRs were more popular since 2001 because of its lower operational cost. Since 2003, a modified sidestream MBR (also called as airlift external circulation MBR), replacing the recirculation pump by H-type recycling pipe, has been well studied for the treatment of toilet wastewater and municipal wastewater (Fan et al., 2006; Li et al., 2007; Fan et al., 2005) . Another innovative submerged anaerobic membrane bioreactor (SAMBR) was developed in 2004. The membrane fouling was controlled by recirculating the biogas and inducing the mixed liquor turbulence (Wang et al., 2008) . Since 2004, many efforts have been dedicated to the study of this SAMBR for the treatment of high-strength alcoholdistillery wastewater. In order for broader applications of MBR, selfforming dynamic membrane bioreactor (SFDMBR), a process even without manufactured membrane, has been proposed, with cheap coarse pore-sized materials such as Dacron mesh, stainless steel mesh, glass fiber, nonwovens, etc., employed as filtration media (Fan and Huang, 2002; Wu et al., 2005; Chu et al., 2008; Yuan et al., 2010) . In general, among these studies submerged MBRs were mostly utilized for municipal wastewater treatment while the sidestream MBRs were commonly applied for particular industrial wastewater treatment. The objectives of research on MBR may lie in two aspects: utilization and optimization. Research on the utilization of MBR includes the exploration of new functions, pragmatization, scaling up and operation maintenance. The optimization aims at higher operational efficiency better performance for target pollutants removal, lower constructional and operational cost and lower energy consumption during operation. While the scientific research and studies in China are progressing continuously, the much more prominent phenomenon since commercial applications of MBR started in China is the remarkably growth in a short period. The summary of the MBR applications history is presented in the following section.

To understand the driving forces of MBR applications, it is vital to examine the development history of MBR applications. The first commercially developed MBR was by Dorr-Oliver in the late 1960s (Smith et al., 1969; Bemberis et al., 1971) . Although MBR systems did not gain wide attention in North America, it was considerably successful in Japan in the 1970s and 1980s. In 1991, the first large-scale sidestream MBR system for treating industrial wastewater was constructed in North America at the GM Mansfield plant (Judd, 2012) . Following this development, numerous MBR systems for treating industrial wastewater were installed in the United States and Canada. From the late 1980s to early 1990s, the government incentivized water recycling program prompted pioneering work by Yamamoto to develop an immersed MBR process in Japan. By 1996, 60 immersed flat-sheet MBR plants had been installed in Japan providing a total capacity of 5500 m 3 /d (Zheng et al., 2016; Judd, 2012) .

In Europe, MBR was used for industrial wastewater treatment in the early 1990s and the first full-scale MBR plant for treatment of municipal wastewater was constructed in Porlock of United Kingdom in 1998 with a capacity of 1900 m 3 /d (Judd, 2012) . In 2002, one MBR line was commissioned in Brescia, Italy with a nominal capacity of 38,000 m 3 /d. In 2004, Nordkanal MBR plant was commissioned with a design capacity of 45,000 m 3 /d in Kaarst, Germany, which, at the time, was the largest MBR plant in the world (Frechen et al., 2008) .

By 2005, N2200 MBR plants were in operation or under construction worldwide. Although plant capacities of MBR systems have been increasing, most of the plants in operation in the world are mediumscale or small-scale in terms of capacity. There are 219 MBR plants for treatment of municipal wastewater in North America, with only 17 plants exceeding a capacity of 10,000 m 3 /d . By 2008, 37 MBR plants having a capacity higher than 5000 m 3 /d were operating, while N800 commercial MBR applications were in use in Europe (Lesjean and Huisjes, 2008) . The Henriksdal wastewater treatment plant (WWTP) in Stockholm will be upgraded to an MBR that will treat 864,000 m 3 /d of wastewater, making it the largest MBR plant in the world, when it is commissioned in 2018 (Krzeminski et al., 2017) .

Based on the wastewater treatment capacity of MBR applications, roughly five phases are identified , which are demonstrated in Fig. 2 and summarized below. The starting point of each phase is marked by benchmark tests or MBR applications.

Phase 1: laboratory and pilot test (1990) (1991) (1992) (1993) (1994) (1995) (1996) (1997) (1998) (1999) . Phase I started in 1990 when Li published the first review of MBR in China and then laboratory tests on MBR developed quickly. The first pilot test of MBR with capacity of 10 m 3 /d wastewater was completed in 1999 by Zheng who examined the performance of MBR treating dyeing wastewater (Zheng et al., 2016) .

Phase 2: the treatment capacity of hundreds m 3 /d wastewater (2000) (2001) (2002) (2003) . In 2000, MBR was applied in printing and dyeing wastewater of Kangda Gloves Co., LTD with the capacity only 11 m 3 /d and this marked the beginning of the commercial MBR application China. In the same year, a MBR application was completed in Tianjing with the treatment capacity of 25 m 3 /d. In 2002, a hospital in Guangdong used MBR to treat the hospital wastewater with the capacity of 100 m 3 /d. In this period, MBR were mainly applied to treat hospital wastewater with the treatment capacity of hundreds m 3 /d .

Phase 3: the treatment capacity of thousands m 3 /d wastewater (2004) (2005) . The first MBR applications with treatment capacity of thousand m 3 /d were completed in 2004 which treated 5000 m 3 /d petrochemical wastewater. During this phase, the MBR applications with capacity of thousand m 3 /d were primarily applied in petrochemical industry, such as Luoyang Petrochemical Engineering Corporation 

The driving forces of MBR in China could be classified to public policy, petrochemical industry, green Olympic Games, water environmental stress, water resources stress, and advantage of MBR technology. The petrochemical industry and green Olympic Games were also derived from water environmental and resources stresses. Timing of the forcing is critical as well. Public policy, water environmental stress and water resources stress is the general forcing which continuously impact MBR applications and will continue to be the driving forces. It should be noted advantage of MBR has been discussed in previous section. It also interacts with every other driving force to affect MBR applications and thus is not discussed separately. The public health concerns caused by the SARS event in 2003 warranted better treatment of hospital wastewater which was a potential field for commercial MBR applications. The petrochemical industry need and green Olympic Games each increased the MBR application capacity by roughly a magnitude. Public policy, water environment and water resources stress drive the magnitude of MBR applications up to one more magnitude. The five driving forces are discussed in detail in this section, sequentially. which promote environment protection and resources conservation practices. Appendix A provides a list of national plans and policies that has promoted MBR applications in the last decade. According to the Water Pollution Prevention and Control Plan, the wastewater reclamation rate in China should reach 20% by 2020, which provided a great opportunity for MBR technology to be widely used in the upgrading WWTPs due to MBR's distinct technical advantages of high treatment efficiency (Zheng et al., 2016) . Additionally, the development of membrane materials has been outlined in the 12th National Five-Year Plan (2011 to 2015), such as Strategy of Accelerating Cultivation and Development of Emerging Strategic Industries, from both aspects of environmental protection and technological development. These national policies provided enormous opportunity for MBR applications (Huang et al., 2010; Abass et al., 2015; Xiao et al., 2014; Wang et al., 2013) . In general, public policy in China increase the demand on MBR application as it often Table 1 Representative MBR applications in hospitals promotes wastewater treatment and reclamation. Public policy also promotes the supply of MBR as it incentivizes the development of MBR technology and applications. The SARS event increased public awareness of public health concern and lead Chinese government to tighten regulation on hospital sewage which contains many pathogens such as bacteria, viruses, etc. . Therefore, the demand on advanced technology to treat hospital sewage is increasing. In addition, as large hospitals in China are often located in the city centers, where land prices and office rentals are prohibitively high, the ideal hospital sewage treatment systems should use minimal land and space. Generally, hospital wastewater is treated with primary treatment and disinfection process (often by chlorine, ozone and ultraviolet light) and infectious hospitals must use secondary treatment . MBR is an efficient technology in removing pathogenic microorganism. Its low SS of effluent is helpful to improve the efficiency of disinfection processes, minimize disinfectant, and reduce disinfection by-products (Zheng and Liu, 2007; Zheng et al., 2005) . It improves disinfection effects and reduces the toxicity of effluent. In addition, it does not require secondary sedimentation tank and thus reduce land and space usage for wastewater treatment, which made it an ideal potential technology for treating hospital wastewater (Lv et al., 2006) . The SARS event promoted MBR applications for treating hospital wastewater (Zheng et al., 2016) . Table 1 demonstrates some typical MBR applications for treating hospital wastewater. It is noted that MBR treatment capacity for hospital wastewater is a small portion of those for industrial wastewater.

The petrochemical industry needs a huge amount of water and discharges lots wastewater containing a great deal of toxic pollutants and hard biodegradation. About 90% of the raw water used in the petrochemical industry is used for circulating cooling system and chemical system (Hansen et al., 2018) . Water consumption is limited and thus most of the used water is discharged as wastewater. To make it worse, N60% of the Sinopec subordinate enterprises are in regions with serious water shortage issues (Zheng et al., 2016) . Therefore, the water resources stress and wastewater discharge stress are pressing for the petrochemical industry.

According to the requirement of energy conservation and emissions reduction in China, the petrochemical industry enterprises has to treat wastewater to meet effluent quality standard and increase wastewater reuse rate (Yu et al., 2017) . Therefore, recycling and reusing effluent is important for the industry. MBR is an effective technology for oil refineries, saline, phthalate, caprolactam wastewater, etc. (Li, 2008) , because effluent from MBR can meet the 1A effluent standard and be reused for cooling or landscape irrigation purposes (Qin et al., 2007; Liu, 2013a) . Therefore, MBR have been widely employed to treat wastewater for the petrochemical industry since 2004. Table 2 shows the representative MBR applications in petrochemical industry in the period of 2004 to 2009. It can be seen that MBR application capacities increased from 5 thousand m 3 /d to 25 thousand m 3 /d in this field.

The 2008 Olympic Games was hold in Beijing, which is a mega population center and faces severe water resources stress. The average water resource was about 200 m 3 per capita in Beijing. Therefore, water supply and water security were a challenge that Beijing has to address to successful host the 2008 Olympic Games. Increasing wastewater reuse rate to 50% was one of the key policy to address water supply issue in Beijing (Zheng et al., 2016; Zhang, 2001) . The Olympic Park, the National Stadium (Bird's Nest), and the Olympic Forest Park were all designed with various water-saving devices and measures such as rainwater collection systems, wastewater reuse projects, etc. After the 2008 Beijing Olympic Games, the Asian Games was held in 2010 in Guangzhou where water pollution was a big issue to address. Both the Beijing Olympic Games and Guangzhou Asian Games were aimed to be green games based on national and local government goals.

In the meantime, MBR applications in China to treat hospital wastewater and petrochemical industry wastewater spread widely through China and MBR applications reached thousands m 3 /d already. It was a feasible option for achieving the green games goal. For instance, Beixiaohe WWTP used MBR to treat the wastewater. Beixiaohe WWTP Table 2 Representative MBR applications in petrochemical industry Li, 2008 

Water environment stress in China has been increasing ever since 1978 when the economy started to grow rapidly. It is one of the latent but constant driving forces for wastewater treatment technology including MBR. Dianchi Lake has seen many massive blue-green algae outbreaks since early 1990s (Wu et al., 2014) . In 2003, Chaohu Lake had cyanobacteria break-out (Cai et al., 2012) . In the late 1990s, frequent blue algal blooms occurred in the northern and western portion of Taihu Lake (Yang and Liu, 2010) . In 2007, the algae blooms in Taihu Lake caused severe contamination of drinking water for millions of people and resulted in water crisis in Wuxi . After the cyanobacteria break-out in 2007, the regulation on wastewater discharge became increasingly strict and WWTPs in Taihu basin had to meet 1A effluent standard of "Discharge standard of pollutants for municipal wastewater treatment plant (GB18918-2002)" (Anon., 2002) . The provincial government required 169 WWTPs near the Taihu Lake be upgraded and rebuilt to meet the new wastewater discharge standard (Liu, 2013b) .

Nitrogen and phosphorus are the main pollutants of eutrophication. MBR is a potential technology in removing nitrogen and phosphorus and naturally is used to treat wastewater for meeting the new wastewater discharge standard (Dong et al., 2010) . Table 4 shows representative MBR applications in Wuxi.

Throughout China, there are 2759 lakes (area ≥ 1 km 2 ) with the total area about 91,019 km 2 , while about 900 of which are facing eutrophication issues . The tightening effluent regulations continue to be the driving force of MBR applications.

The average water resources per capita in China are only about a quarter of the world average, which makes it the 108th in the world and a severe water shortage country (Li et al., 2017) . The uneven geographic distribution of precipitation and water resources increases water resources stress for arid and semi-arid regions in China. The average water resources per person each year in Beijing, Tianjin, Hebei, Shanghai, Shandong and Ningxia was under 300, which is considered extremely low water resources (Li et al., 2017) . Over 400 cities among about 600 cities in China are confronted with serious water shortage (Song et al., 2018) . Although water resources are relatively rich in southern China, water pollution results in water shortage for many cities in southern China.

To address the long-term water resources stress, wastewater reuse is one of effective and efficient ways. MBR technology can provide high quality reclaimed water for landscape irrigation, agricultural irrigation, industry, etc. The increasing need of reused water will promote MBR applications.

Another way to relieve water resources stress in northern China is the South-to-North Water Transfer Project (SNWTP) (Ma et al., 2016) . On December 12, 2014, the water system of the middle line of SNWTP begun to convey 9.5 billion m 3 water per year. The conveyed water was used for public water supply, ecosystem, and other purposes (Sheng and Webber, 2018) . The water source of SNWTP includes many rivers and reservoirs whose water quality is critical for the success of the SNWTP (Xiong et al., 2006) . For example, Shiyan City is the core region of water source and water conservation area of SNWTP. The municipal wastewater of Shiyan is discharged into Shending River and thus effective and efficient treat of the municipal wastewater in Shiyan is warranted. A large MBR WWTP was constructed in Shiyan in 2009 for this purpose. Table 5 listed representative MBR applications with capacity of 100 thousand m 3 /d, which primarily were driven by local water resources stress. All these large-scale MBR applications were used to treat municipal wastewater except Wenyu River Water Plant which was designed to treat polluted river water.

The five driving forces discussed in previous section have resulted in the status of MBR applications in China. While water environment and water resources stresses will continue to promote MBR applications, the return of invest and operation cost will be the driving forces in the future that expand MBR applications into more fields and regions.

The total capital investment cost of MBR and conventional WWTP are shown in Table 6 . The total capital investment of MBR WWTP for municipal wastewater is higher than that of conventional WWTP. However, if the effluent must meet 1A of national standard (Anon., 2002) , the conventional municipal WWTPs have to include additional facilities, Table 5 Large scale MBR applications with capacity of 100 thousand m 3 /d since 2010 Dong, 2017 (Dong et al., 2012; Yang and Chen, 2012 such as layout cloth-media filter or sand leach for advanced treatment, its total investment cost will be similar with that of MBR applications. MBR process becomes more and more attractive for municipal/domestic wastewater treatment when a compact technology is required due to a lack of space or the high cost of additional land in urban areas. The average footprint areas of a MBR municipal WWTP (with a capacity of 50,000-200,000 m 3 /d) are 0.4-0.6 m 2 ·m −3 ·d −1 , respectively. In contrast, the standard footprint area of a conventional municipal WWTP (including tertiary treatment) is 1.2-1.6 m 2 ·m −3 ·d −1 . Land price in many Chinese cities is as high as 1500 RMB/m 2 nowadays. When cost for land is considered, the MBR invent will closer to that of conventional WWTPs. The available locations for new WWTPs in big cities are extremely limited because drainage systems are often completed and could not be changed easily. In this situation, MBR has an advantage over conventional WWTPs as MBR need less space and has small footprint. From Table 6 , the average footprint of MBR municipal WWTPs is 0.9 m 2 ·m −3 ·d −1 , less than that of conventional municipal WWTPs. In addition, underground MBR WWTPs have been constructed with the total investment of 3000-6000 RMB/m 3 to save space and reduce environmental impact in big cities (Cao et al., 2012) . Table 7 shows are representative underground MBR WWTPs. When land prices in big cities are considered, MBR applications may be preferred in big cities. With the rapid urbanization, MBR applications will be stimulated by the shortage of land resources and high price of land.

The essential focus points in full scale MBR design and operation include operation issues such as membrane fouling, energy consumption and associated cost. The comparison of operation cost and energy consumption of MBR and conventional process are shown in Table 8 . Energy consumption accounts for 50%-70% of MBR operation cost (Zheng et al., 2016; Meng et al., 2012; Gabarrón et al., 2014) . For municipal wastewater, both operation cost and energy consumption of MBR are higher than those of conventional WWTPs.

For high COD concentration wastewater such as high concentration organic industrial wastewater and landfill leachate, energy consumption, COD removal energy consumption and operation cost are lower than those of conventional WWTPs. Thus, MBR is preferred when treating high concentration organic wastewater or landfill leachate.

Given that energy demand contributes significantly to the running cost of an MBR system, it is important to optimize the process energy consumption to make the technology more competitive. The high energy consumption of MBR plants is due to membrane fouling and clogging and high DO concentrations for a large amount of biomass. Energy cost of aeration plays a key role in the total energy consumption in MBR processes. Optimization of aeration is consequently believed as the most effective way to cut down energy requirement and eventually lower operating cost. Energy consumption was reduced in China by elevating the aeration efficiency for membrane scouring by bettering the configuration of membrane module, aeration system, and tank geometry. Much of the MBR plants development over the past 12 years has focused on maximizing the mixing imparted by membrane air scouring while minimizing the amount of air required for this, resulting in significant improvements in energy efficiency. According to some surveys, the energy consumptive could be as low as 0.5 kWh/m 3 for large MBR plants. Table 9 lists the specific electrical consumption of integrated/ submerged-MBR. It indicates that the specific electrical consumption of integrated/submerged-MBR ranges from 0.31 to 0.70 kWh·m −3 , and a little higher than that of average electrical consumption of CAS (0.34-0.38 kWh·m −3 ) in China.

Membrane fouling is a critical operation issue as it impacts membrane lifespan and energy consumption. Membrane fouling could be reduced in these major ways: gas sparging, sustainable flux operation, Table 7 Representative underground MBR applications Jia et al., 2016 Table 6 The footprint and average capital investment of MBR and conventional municipal WWTP .

Municipal wastewater 3800 2200 0.9 1.2-1.6 Industrial wastewater 6100 1.7 Polluted surface water 2300 0.5 a MBR capacity is larger than 10 thousand m 3 /d. b Conventional municipal WWTP (10-50 thousand m 3 /d) includes tertiary treatment systems.

Energy consumption and operation cost of MBR and conventional WWT Wang and Fan, 2012 (Abass et al., 2015) .

The other operation issue that impacts MBR applications is the influent water quality. Natural and social conditions greatly affect influent features. For example, the low organics concentration often limits TN removal in South China. The design and operation of a MBR process are often influence by the influent quality. In China, simultaneous removal of both N and P, AAO-MBR has become the most prevailing process in MBR applications in China . Compared with Europe or North America, the low C/N ratio of the influent in China impeded the denitrification process and increase the operation cost. Another unique feature of influent in China is the high concentration of fat and oil in influent which request pretreatment of the influent.

The study summarizes the development history of MBR applications in China and the associated driving forces which resulted in the rapid growth of MBR applications in the last two decades. The following conclusions are reached.

The SARS event increased demand on hospital wastewater treatment and the technical advantages of MBR made MBR an ideal technology to treat hospital wastewater in China. The need of petrochemical industry in mid-2000s promoted MBR applications and increased the scale to thousands m 3 /d. Olympic Games in 2008 and Asian Games in 2010 took place when MBR applications needed further stimulation and the MBR application scale expanded to 10 thousand m 3 /d.

Public policy of water resources and environment and membrane industry promoted MBR applications continuously. The other driving forces are water environment and water resources stresses that China has been facing for a long time. Water stresses promoted MBR applications to a much larger scale (100 thousand m 3 /d) and to broader geographic regions.

The future driving forces for MBR applications include declining capital investment and decreasing operation cost as well as water environment and water resources stresses. It should be noted that government policy on MRB technology, water and environment has significant impact of MBR applications in the future as well.

The comprehensive understanding of the driving forces of MBR applications is not only valuable for reflecting the past but also for looking forward to the future as it provides insights on how MBR applications may be promoted in the future. Energy consumption level of MBRs in China Hao et al., 2018; Yang et al., 2017; Liu et al., 2016; Xu et al., 2017 

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Saving and consumption reduction of MBR in WWTP

Applications of membrane technology in treating wastewater from the dyeing industry in China: current status and prospect

Virus removal performance and mechanism of a submerged membrane bioreactor

Water loss by evaporation from China's South-North Water Transfer Project

Recent advances in membrane bioreactors: configuration development, pollutant elimination, and sludge reduction. Environ

Feasibility study on petrochemical wastewater treatment and reuse using submerged MBR

Characterization of soluble microbial products in 10 large-scale membrane bioreactors for municipal wastewater treatment in China

Review of the work of Wuxi Taihu governance and suggestion on strengthening basic key research

Using incentives to coordinate responses to a system of payments for watershed services: the middle route of South-North Water Transfer Project

The use of ultrafiltration membrane for activated sludge separation. Proceeding of the 24th Annual Purdue Industrial Waste Conference

Water resources utilization efficiency and influence factors under environmental restrictions

Chlorination disinfection and by-products for the effluent from membrane bioreactor in hospital wastewater treatment

Seasonal characteristics of supernatant organics and its effect on membrane fouling in a full-scale membrane bioreactor

Membrane bioreactor: a new treatment tool

Analysis of energy consumption in membrane bioreactors for wastewater treatment and reuse

Research and applications of membrane bioreactors in China: Progress and prospect

Recent advances in membrane biotechnologies for sludge reduction and treatment

Function of dynamic membrane in self-forming dynamic membrane coupled bioreactor

Seasonal dynamics of water bloom-forming Microcystis morphospecies and the associated extracellular microcystin concentrations in large, shallow, eutrophic Dianchi Lake

Engineering application of membrane bioreactor for wastewater treatment in China: current state and future prospect

Building water resources protection defensive lines to ensure the safety of water source regions of the middle route of south-to-north water diversion, south-to-north water transfers

Design and application of A2/O process and MBR process in the same scale wastewater treatment plants

Direct solid-liquid separation using hollow fiber membrane in an activated-sludge aeration tank

Application of membrane bioreactor process in Wuxi Chengbei sewage treatment plant

Strategy of water pollution prevention in Taihu Lake and its effects analysis

State-of-the-art of membrane bioreactors: worldwide research and commercial applications in North America

Analysis of MBR of 7 years practical operation in the fourth wastewater purification plant of Kunming

Using polyaluminum chloride and polyacrylamide to control membrane fouling in a cross-flow anaerobic membrane bioreactor

A review of treating oily wastewater

Study of a membrane bioreactor with glass fiber flat grille modules and the modules' optimization based on the local critical flux theory

Olympic Games bid committee

Adsorptive fouling of extracellular polymeric substances with polymeric ultrafiltration membranes

The 2007 water crisis in Wuxi, China: analysis of the origin

Highly effective antifouling performance of PVDF/graphene oxide composite membrane in membrane bioreactor (MBR) system

Dyeing and printing wastewater treatment using a membrane bioreactor with gravity drain

Virus rejection with two model human enteric viruses in membrane bioreactor system

A pilot scale anoxic/oxic membrane bioreactor (A/O MBR) for woolen mill dyeing wastewater treatment

Evaluation of virus removal in MBR using coliphages T4

Survey of MBR market trends and perspectives in China

Report for Sustainable Development Strategy of China Water Treatment Industry (Membrane Industry II). Press of Renmin University of China

A novel membrane showing both hydrophilic and oleophobic surface properties and its non-fouling performances for potential water treatment applications

This project is supported by the special fund of Key Laboratory of Drinking Water Science and Technology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences (Project No. 18K02KLDWST). This work was partially supported by the National Natural Science Foundation of China (Grant No. 51778618, 51478460). The authors are grateful to these organizations.Appendix A. Related public policy on membrane industry