Application of ultrafiltration technology in drinking water industry of China: A comprehensive assessment of hybrid membrane processes

In recent decades, the emergence of novel drinking water safety crises and the imposition of increasingly stringent water quality regulations have stimulated the innovation of traditional treatment processes (coagulation + sedimentation + filtration + disinfection) (Madaeni 1999). The latter faces challenges in effectively removing Cyanobacteria and protozoan parasites, especially Cryptosporidium and Giardia (Betancourt & Rose 2004). An alternative approach, utilizing biological granular activated carbon filters preceded by the ozonation process (O₃-GAC), also displays restricted ability in terms of removing these pathogens (Stoquart et al. 2012). Conventional disinfection methods can exterminate microorganisms but do not eliminate them, enabling their persistence in the water (Madaeni 1999). In contrast, ultrafiltration (UF) showcases the capability to completely remove particulate contaminants, encompassing protozoan parasites, thereby rendering it capable of replacing the disinfection step (Guo et al. 2010). Beyond addressing concerns regarding drinking water biosecurity, UF has been extensively applied worldwide in the drinking water industry due to its notable attributes, such as compactness, easy automation, and minimal staffing requisites (Yu et al. 2022).

Nevertheless, the inherent sieving retention mechanism of UF membranes dictates their limited removal of dissolved organic matter, particularly those of low molecular weight (Moreira et al. 2021a). Furthermore, there is a problem of membrane fouling during long-term operation. Consequently, UF is coupled with other pretreatment processes, encompassing coagulation, adsorption, and ozonation (Huang et al. 2009). Coagulation is so far the most efficacious pretreatment strategy for controlling UF membrane fouling. Coagulation for membrane filtration can be operated with or without sedimentation (Ma et al. 2020). For the coagulation-UF system, membrane fouling mainly depends on the removal of aqueous foulants and the characteristics of the cake layer, including porosity and thickness (Wang et al. 2019; Yu et al. 2019; Lu et al. 2020). Activated carbon has notably large specific surface areas, facilitating the adsorption of small substances that are difficult to remove by UF membrane from water (Huang et al. 2009). The impact of adding powdered activated carbon (PAC) to mitigate UF membrane fouling depends on the properties of organics, the dosage of PAC, and the membrane materials (Mozia et al. 2005; Lee & Walker 2006; Zhang et al. 2019). Generally, PAC has a limited ability to mitigate membrane fouling, and in certain instances, it may even have adverse effects. Oxidation pretreatment has been identified as modifying the physicochemical properties of effluent organic matter, including molecular weight, distribution, hydrophilicity, charges, etc., to enhance the removal of pollutants and mitigate membrane fouling (Lin et al. 2012, 2013; Lu et al. 2015; Wei et al. 2016; Winter et al. 2016; Huang et al. 2019). Distinct pretreatment technologies have varied impacts on membrane fouling. Thus, pretreatments employed in scaled UF applications frequently entail the integration of multiple processes, thereby amalgamating the advantages of each pretreatment technology. Corresponding hybrid membrane processes include PAC-UF, coagulation–sedimentation–UF, O₃-GAC-UF, etc. Numerous bench-scale and pilot-scale studies have demonstrated the role of these hybrid membrane processes in enhancing the removal of contaminants and mitigating membrane fouling (Siembida-Loesch et al. 2015; Huang et al. 2019; Zhang et al. 2019; Ma et al. 2020; Long et al. 2021).

The application of UF in China started in the 1990s. In 2004, the first drinking water treatment plant (DWTP) with a capacity of more than 5,000 m³/day in China was constructed in Cixi City, becoming a milestone event in the development of UF applications in China (Fan et al. 2013). Since then, UF applications in China have been growing dramatically. This growth can be attributed, in part, to favorable socioeconomic trends and the reduced cost of membrane materials. The cumulative capacity of UF DWTPs (with individual capacities ≥ 5,000 m³/day) reached 1.4 million m³/day by 2011 (Zheng et al. 2012), and remarkably surged to 10 million m³/day by 2020, constituting around 5.8% of China's overall urban water supply capacity (Chang et al. 2022). Against this backdrop, it is significant to undertake a comprehensive review of China's UF drinking water treatment industry's evolution from the point of process and engineering. While certain previous works have touched on the application and evolution of UF technology within China's drinking water industry (Xia et al. 2004; Zheng et al. 2012; Chang et al. 2022), a gap exists in the exploration of various hybrid membrane processes employed in China. An intensified analysis in this domain would be invaluable to advance our understanding of the continually evolving landscape of UF technology in China.

Therefore, a comprehensive survey of UF applications in China's drinking water treatment is conducted in this work. The data are sourced from municipal design institutes, operators, membrane technologies and water treatment websites, field surveys, and the literatures. Notably, only water plants with a capacity exceeding 5,000 m³/day are considered in scope, as this sector essentially represents the market. The database included 171 UF DWTPs that are operational or under construction between 2004 and the conclusion of 2022 in China. It remains imperative to emphasize that the collection of UF applications in China herein presented may not be exhaustive. A comprehensive analysis is conducted regarding the evolution of capacity, geographic distribution, driving forces, hybrid membrane processes, and the membrane market. Finally, the future perspectives on UF application coupled with various types of hybrid membrane processes for China's drinking water treatment are critically outlined. Those insights are expected to provide valuable implications for the prospective deployment of UF drinking water treatment technology.

The increasingly strict drinking water hygiene standards and source water pollution play critical roles in favoring the application of UF for drinking water treatment in China. The latest Drinking Water Sanitation Standard (GB5749-2022) puts forward higher requirements for disinfection byproducts. To improve drinking water quality, regional authorities have introduced localized standards, surpassing national standards in terms of stringency. Those provided a great opportunity for UF application due to the high quality of produced water. Parallelly, the pollution and subsequent degradation of localized surface and groundwater that can be used to produce drinking water has prompted the development of advanced treatment technologies. The general driving force is the reduction of cost. Using the difference in elevation between the submerged membrane tanks and the downstream processes, the necessary vacuum for UF filtration is created, obviating the need for expensive and energy-consuming osmotic pumps. This strategy dramatically reduces capital and operating costs, such as the Jinan Nankang Water Plant (60,000 m³/day, 2019), Ningbo Taoyuan Water Plant (500,000 m³/day, 2020), Hangzhou Xianlin Water Plant (600,000 m³/day, 2021), Nanjing Qiaolin Water Plant (200,000 m³/day, 2022), and similar cases. Concurrently, benefiting from the development of the social economy and the cost reduction of membranes, compact UF technology is becoming more and more competitive in the renovation and expansion of ageing DWTPs. Especially when the area is limited. The strict standards and the pollution of source water have promoted the UF application number to increase exponentially, while the reduction of cost has increased the capacity of UF applications up to one more magnitude. The super large-scale UF DWTPs (with an individual capacity of more than 100,000 m³/day) are shown in the Supplementary material, Table S1. The three driving forces interact with the advantages of UF to affect the UF application. With the accumulation of UF application experience and the gradual establishment of a comprehensive UF membrane-based drinking water treatment system, UF technology is poised to provide a more reliable water quality safeguard for regions (e.g., Anhui, Jiangxi, and Hunan) with less application of UF.

The utility of UF membranes in the removal of suspended solids is well established. This capability inherently enables UF to replace the conventional steps of coagulation, flocculation, sedimentation, and granular media filtration (Jutaporn et al. 2021; Rho et al. 2022). This gives rise to four distinct UF membrane-based filtration technologies: direct UF, short-flow process, medium-flow process, and long-flow process. In the context of direct UF, the raw water directly enters the UF system without pretreatments and no or less chemical additives. The short-flow process refers to substitute UF for the conventional precipitation-filtration process. The medium-flow process involves the UF membrane replacing the sand filter in the traditional process. The long-flow process employs UF as an advanced treatment process and also includes a variety of advanced treatment combinations.

Currently, there are 14 UF DWTPs undergoing construction utilizing a long-flow process, with a total capacity of 4.1 million m³/day (Figure 3(d)). It can be predicted that the primary focus of construction is persistently centered on super-large UF DWTPs that employ the long-flow filtration process. In contrast, medium-flow and short-flow processes demonstrate favorable applicability, primarily targeted for upgrading water plants and addressing the demands of small or medium-scale plants. Despite the applications of direct UF being comparatively smaller scale, they bear considerable potential in rural drinking water supply and hold valuable prospects for widespread implementation. The characteristics and application status of the four processes are discussed in the following sections.

UF boasts a small footprint, flexible scale, and ease of installation, operation, and maintenance. This renders UF fitting for the demands of rural water supply, characterized by dispersion, limited scale, and operational challenges (Wu et al. 2022). Several studies have demonstrated the viability of a long-term direct UF process, even without pretreatment, in supplying clean water to rural regions (Mierzwa et al. 2008, 2012; Galvan et al. 2014; Ferrer et al. 2015; Wu et al. 2022). An on-site and online investigation is conducted on UF water supply stations situated in rural regions of Zhejiang province. Each station serves a village or several nearby villages, with capacities ranging from 50 to 900 m³/day. The main facilities include integrated UF water purification equipment, an automated control system, and a clear water tank. The integrated UF water purification equipment includes components such as the submerged UF system and backwashing system. The raw water is directly entered into the integrated submerged UF system for filtration, while the coagulant is added during rainstorms and flooding when the turbidity of the raw water is high. Besides, the system does not need a pump to obtain permeate, instead using the differential liquid levels between the immersion membrane tank and the clear water tank to achieve siphon-gravity water production, thereby greatly reducing energy consumption. This integrated UF water purification equipment offers rapid installation, a short construction cycle, and a small footprint. Additionally, it enables fully automated remote control and operation, truly unattended. Simultaneously, it has the advantages of greenness, energy efficiency, simplicity, and easy process control. These make direct UF have broad application prospects in rural water supply engineering.

In recent years, researchers have developed a novel UF process, gravity-driven membrane (GDM), a dead-end operated UF system without any chemical or hydraulic fouling control at remarkably low and constant pressure (40–200 mbar) for decentralized treatment of surface water, stormwater, gray water, and household water (Peter-Varbanets et al. 2010; Peter-Varbanets et al. 2011; Ding et al. 2017), focusing around issues such as water quality assurance, energy saving, and simplifying operation and maintenance. The driving force is hydrostatic pressure. During the dead-end filtration, the microorganisms, organic matter, and inorganic material in the water accumulate on the membrane surface to gradually form a biofilm layer (Pronk et al. 2019), its thickness and resistance with the increase of filtration volume and foulants concentration increase (Peter-Varbanets et al. 2011; Chomiak et al. 2015). Additionally, the biodegradation process and predation activities induced structural changes in the biofilm layer, leading to the formation of a loose layer with stable hydraulic resistance (Peter-Varbanets et al. 2011; Klein et al. 2016; Chen et al. 2021). Consequently, the flux of the GDM system achieves stability after a long-term operation without any physical or chemical cleaning (Shi et al. 2020), and the stability level is influenced by the development of a permeable, heterogeneous biofilm layer. The biofilm layer developed on the GDM is considered a ‘mini ecological system’, enhancing the separation and biodegradation of natural organic matter (NOM) (Peter-Varbanets et al. 2011), assimilable organic carbon (Derlon et al. 2014), dissolved organic carbon (Lee et al. 2019), and micro-pollutants (Chen et al. 2022). Furthermore, research showed that the introduction of pro-coated/pro-added manganese oxides onto the membrane surface or within the system confers efficient manganese and iron performance to the GDM (Tang et al. 2020, 2021a, 2021b). The pro-coated/pro-added manganese oxides prove beneficial in improving the porosities and heterogeneities of the biofilm layer, leading to stabilized flux and improvements.

The GDM system offers a dual guarantee through the biofilm layer and UF membrane. As a result, GDM has better validity and universal applicability for purifying diverse types of surface and underground water when compared to direct UF. Moreover, GDM runs without any flushing or cleaning during long-term operation, endowing it with higher energy efficiency than direct UF. However, GDM usually operates under ultralow pressure with a stable flux of 4–10 L/(m²·h) (Pronk et al. 2019). This is one of the main obstacles to the application of GDM within rural water supply projects (Chomiak et al. 2015). Overall, both direct UF and GDM exhibit distinct advantages and disadvantages. Each has confirmed its efficacy as a decentralized water supply solution in China's rural regions.

The coagulation-UF process is an effective combination of improving the removal of contaminants such as large-size biopolymers (the fraction with high fouling potentials) (Kimura et al. 2018), extracellular organic matter of algal (Zhang et al. 2017), and humic acid (Liang et al. 2021), all contributors to UF membrane fouling. The deposition of coagulated flocs on the membrane surface enhances the removal of low molecular weight organic compounds by adsorption (Kimura & Kume 2020). The degree of membrane fouling in the system depends on the removal of aqueous foulants and the characteristics of the cake layer, including porosity and thickness (Wang et al. 2019). These characteristics are susceptible to various factors, including the quality of the feed water (Su et al. 2017), the coagulant type and dosage (Dong et al. 2014; Ma et al. 2014), and the membrane material (Huang et al. 2009). Studies have shown that aquatic contaminants, including sodium alginate molecules, biopolymers, and calcium ions, can change the cake layer structure (Su et al. 2017; Zhao et al. 2020; Long et al. 2021). These contaminants act as supportive skeletons within the cake layer, thereby averting compaction during operation. This dynamic, therefore, retards the decline in membrane resistance. Hence, the principal objective of pre-coagulation is to form an ideal cake layer, rather than the larger the flocs, the better.

The coagulation-UF process boasts a compact footprint and exhibits low energy consumption. The coagulation and UF units can be conveniently co-located, thereby optimizing land utilization. By directly introducing coagulated water into the membrane tank, the system efficiently capitalizes on raw water head, enabling gravity-driven water production in most instances. Furthermore, this process does not require sludge discharge operations. Sequential backwashing is applied to all membrane tanks, each undergoing variable flux filtration across the entire filtration cycle. This variable flux operation mode yields significant benefits in mitigating deep membrane fouling. The representative DWTPs employing this process are shown in Table 1. Presently, large-scale domestic UF applications primarily adopt long-flow and medium-flow processes, with fewer instances of short-flow UF applications. Notably, several foreign cases showcase large-scale utilization of short-flow process, including the Chestnut Avenue water plant in Singapore (273,000 m³/day, 2003) (Janson et al. 2006), and the City of Kamloops water plant in Canada (160,000 m³/day, 2013).

The UF membrane has limited efficacy in removing small-molecule-weight contaminants. In contrast, activated carbon exhibits effectiveness in removing fractions with molecular weights ranging between 300 and 17,000 Da (Lin et al. 1999). Therefore, a modified UF process, integrating activated carbon adsorption, has demonstrated potency in enhancing permeate quality. This process is classified into two categories, the granular-activated carbon-UF process (GAC-UF) and the powder-activated carbon-UF (PAC-UF) process. Research has shown that PAC could enhance the removal of organic matter, including cyanotoxins, odor, taste, color, and microcystins (Tomaszewska & Mozia 2002; Stoquart et al. 2012). Many researchers believe that the PAC-UF process could replace the conventional water treatment process for mildly polluted raw water. However, PAC would inevitably deposit on the membrane surface during operation (Shao et al. 2017), requiring frequent backwashing or chemical cleaning. Thus, the current application of the PAC-UF process requires individual reactors and PAC separators, leading to additional costs and procedures, thereby limiting its comprehensive application (Yu et al. 2022). To this end, the utilization of GAC has emerged as a preferred strategy for mitigating membrane fouling. In addition, GAC facilitates recycling and reuse compared to PAC.

The medium-flow process, denoted as the coagulation/sedimentation-UF process, holds significant relevance in drinking water treatment as a pivotal UF combination process. Compared with the short-flow process, the medium-flow process has a comprehensive flocculation and sedimentation stage after coagulation. This removes most of the flocs before the membrane, thereby improving the quality of effluent water and alleviating membrane fouling (Malkoske et al. 2020). Consequently, this process has been widely adopted in water plants encompassing diverse source water characteristics and production scales. The successful operation of several large-scale applications proves the viability of this process in large-scale UF DWTPs. For example, in Ningbo Taoyuan Water Plant (500,000 m³/day, 2020), the raw water is taken from the Qincun reservoir, and the treatment process is coagulation/sedimentation-UF. Located on a hill, this contemporary plant astutely utilizes the elevation difference of the terrain to induce UF filtration via the siphoning effect. Subsequently, clean water is conveyed to the urban water distribution network through gravitational forces, obviating the necessity for secondary booster pumps. Comparative to the conventional pump-driven submerged UF systems, this configuration effectuates nearly 90% power savings. After three years of stable operation, the effluent turbidity remains below 0.1 NTU. Similar gravity-driven large-scale plants employing the medium-flow process include the Nanchang Xiuluoqiao Water Plant (20,000 m³/day, 2017).

Ageing water plants located in central urban areas confront multifaceted challenges, including the threat of biosecurity, algae contamination, bacterial viruses, and other problems in water. Additionally, these plants also face difficulties associated with stringent water quality requirements and spatial constraints, impeding the adoption of advanced treatment processes. In this context, the selection of an appropriate process is crucial for upgrading ageing water plants. The medium-flow process provides a feasible modification solution. By retaining the original coagulation and sedimentation facilities and integrating UF to replace conventional filters, the upgrade of the ageing water plant is realized. This strategy aligns with the principles of sustainable upgrading.

The Jiangdong Water Plant in Ningbo City was employed with the conventional treatment process before 2016. The raw water is taken from the Baixi and the Tingxia Reservoirs, where the turbidity is typically maintained below 15 NTU. However, inherent limitations of the siphon filter structure resulted in compromised interception capacity, frequent incomplete flushing, and escalated effluent turbidity. To improve water quality and safety, the existing coagulation and sedimentation processes were retained. The transformation involved converting two 50,000 m³/day siphon filters into a single 200,000 m³/day submerged UF tank. After upgrading, the effluent water quality surpassed the Sanitary Standard for Drinking Water (GB 5749-2006), turbidity consistently remained below 0.1 NTU, and oxygen consumption maintained below 1 mg/L. Other successful cases include Jinan Fenshuiling Water Plant (70,000 m³/day, 2020), Tangshan Water Supply Company Water Plant (125,000 m³/day, 2019), Jinan Xueshan Water Plant (30,000 m³/day, 2018), Ningbo Jiangdong Water Plant (200,000 m³/day, 2016), and Zhaoqing High-tech Zone Water Plant (2,000 m³/day, 2012).

Small flocs escaping from the sedimentation have been identified as a primary contributor to UF membrane fouling in the medium-flow process (Kimura & Kume 2020). The sand filter could effectively control microbial growth and reduce the release of extracellular polymeric substances by intercepting unseparated coagulation flocs, thus reducing the membrane fouling rate (Yu & Graham 2015). As a multi-tiered process, the long-flow process includes complete conventional treatment processes and advanced UF treatment, often combined with diverse advanced treatment technologies like O₃-GAC, NF, and reverse osmosis (RO). This combination endows the process with adaptable capabilities concerning source water quality and seasonal variations. Efficiently addressing source water challenges, the Guogongzhuang Water Plant (500,000 m³/day, 2021), Shijingshan Water Plant (200,000 m³/day, 2021), and Chengzi Water Plant (43,000 m³/day, 2015) in Beijing employ the long-flow process. This process effectively mitigates issues related to high summer algae levels, winter low temperatures and turbidity, as well as challenges associated with organic matter, color, and odor.

The long-flow process is predominantly used in new water plant constructions and the upgrading of ageing water plants where space conditions permit. Process components are reasonably selected based on the source water quality, and the process can be flexibly matched to achieve process optimization. Remarkably, long-flow UF DWTPs generally have considerable scale. Among the super large-scale UF DWTPs either operational or in progress, 29 installations employ the long-flow process, with a total capacity of 8.685 million m³/day. These large-scale long-flow UF DWTPs usually play primary roles in the local water supply. For example, the super large-scale UF DWTPs that are under construction include Dongguan Songshan Lake Water Plant (1,100,000 m³/day), Shenzhen Changliupo Water Plant (550,000 m³/day), Dongguan Luhuakeng Water Plant (500,000 m³/day), Jiangmen Xinyuan Water Plant (300,000 m³/day), and so on. Large-scale UF DWTPs based on long-flow processes will become the focus of future water plant development and renovation.

The dual-membrane process is a membrane technology that combines UF with NF or RO technology. This process is primarily tailored for water sources with excessive inorganic salts, such as brackish groundwater or surface water affected by seawater backflow. Several large-scale DWTPs exemplify the success of this process: The Xi'an Wanzi Water Plant (100,000 m³/day, 2017) applies the UF + NF process for brackish groundwater treatment, the Yantai Gongjiadao Water Plant (72,000 m³/day, 2022) adopts the UF + RO advanced treatment process to tackle excessive nitrate in the raw water, the Jinan Donghu Water Plant (200,000 m³/day, 2021) employs the O₃-GAC + UF + RO process for targeted removal of organics, algae, taste, odors, and sulfates. The integration of NF or RO after UF further reduces the concentration of soluble inorganic salts, significantly improving issues related to water hardness and taste. On the other hand, the NF membrane could efficiently remove trace toxic and harmful organics while retaining essential mineral elements beneficial to human health. This transition from providing ‘qualified water’ to supplying ‘healthy water’ aligns with the growing public demand for high-quality drinking water. Consequently, some economically developed regains have embraced the UF + NF process to elevate the supply of water quality beyond national standards. For example, the Zhangjiagang Third and Fourth Water Plants have implemented the UF + NF advanced process to treat Yangtze River water, thereby ensuring the supply of high-quality water to consumers.

There exist two types of UF systems in water plants: the submerged UF membrane system (sUF) and the pressurized UF membrane system (pUF). The two types of UF systems are similar in terms of removal efficiencies of various contaminants (Chae et al. 2009; Lowenberg et al. 2014), but different in operation mode, permeate flux, fouling propensity, and costs. In general, the pUF system operates by depending on a high-pressure pump, whereas the conventional sUF system employs permeate pumps to create the vacuum essential for filtration, thereby limiting the transmembrane pressure (Akhondi et al. 2017). Consequently, the pUF system generally has a higher permeate flux compared to the sUF system. Based on existing domestic UF DWTPs, pUF system designs typically target permeate flux within the range of 50–80 L/(m²·h), whereas the sUF system aims for 20–40 L/(m²·h) (Zheng & Rui 2021). However, the higher permeate flux or higher external positive pressure corresponds to an accelerated fouling rate (Moreira et al. 2021b), thus endowing the sUF system with a longer operational lifespan. Economically speaking, the pUF system boasts a smaller land footprint due to its higher installed density. However, it comes with a higher energy consumption due to the utilization of pressure pumps. Conversely, the sUF system exhibits the opposite characteristics.

This paper provides a comprehensive overview of the advancements in UF applications, the current status of four distinct hydraulic membrane processes, and the distribution of the membrane market in the Chinese drinking water industry. The conclusions and perspectives drawn from the survey of UF DWTPs with an individual capacity of ≥5,000 m³/day are as follows:

• China has emerged as a focal point for UF applications, with a total commissioned capacity of 12.54 million m³/day in December 2022, poised to reach 19.61 million m³/day with ongoing projects. Medium-scale UF DWTPs dominate, while the super large-scale UF DWTPs also contribute significantly, mainly in East and North China.

• Stringent standards and water pollution concerns drive UF application growth exponentially, while cost reduction significantly increases capacity.

• Within UF DWTPs, the long-flow and medium-flow processes are popular, with long-flow expected to maintain dominance. The medium-flow process gains prominence in small to medium-scale projects and plant upgrades. The short-flow process is effective for treating well-quality raw water, and direct UF is suitable for rural water supply. The GDM process is one of the effective solutions for decentralized water supply in rural areas.

• Major Chinese membrane manufacturers, including Litree (20.86%), Motimo (15.96%), OrigionWater (8.36%), Meineng (6.98%), and Zhaojin Motian (4.51%), collectively account for 56.67% of total capacity. Notably, 90.7% of the sUF systems utilize domestic membranes, while pUF systems rely on imported membranes (51.8%).

• With the boost of the sustainable policy stimulus and the domestic development of membrane materials and equipment, UF application in the Chinese drinking water industry is going to be one of the most promising world markets in the foreseeable future. It will contribute to decentralized water supply in economically challenged rural areas. Forecasts indicate a surge in the integration of UF technology into an increasing number of super large-scale DWTPs, and the UF DWTPs will develop toward modernization, intelligence, and energy-efficiency trends.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.