A pilot study on the dual membrane process of ultrafiltration and nanofiltration to treat algae-laden raw water compared to O₃-BAC

The frequent outbreaks of algal blooms due to eutrophication in drinking water resources are a grave threat to the water supply (Maeng et al. 2021; Xu et al. 2021). Treating high algae-laden water is a problem in drinking water treatment in China. Raw water with an algal density of 1 million algae per litre or more is defined as high algae-laden water (Chen et al. 2009). During algal blooming, the increase in algae cells may penetrate the filter tank into the water supply network. It will increase assimilable organic matter, promote microbial growth, and reduce the microbiological safety of drinking water (Zhu et al. 2022). In addition, algae can release dissolved organic matter (DOM) during their metabolism. This process will produce disinfection by-products with potential carcinogenic effects during chlorine disinfection (Fang et al. 2010; Xiao et al. 2020; Du et al. 2022; Liu et al. 2022; Pandian et al. 2022). As a source of freshwater in China, East Tai Lake has received widespread attention. Its pH is stable, but the turbidity, organic matter, ammonia nitrogen, and algae vary with the seasons. To be specific, the pH of the raw water in Lake Taihu was relatively stable in the range of 7.76–8.2. On the contrary, the difference between the lowest and highest turbidity is nearly 100 NTU, COD_Mn fluctuates in the range of 3.47–4.74 mg/L and the concentration of ammonia nitrogen varies from 0.0645 to 0.2382 mg/L. The number of algae is low in winter (96.32 × 10⁴ per litre) and increases rapidly in summer, reaching a maximum of 573.44 × 10⁴ per litre. The average density of algae in Taihu Lake can reach 241.83 × 10⁴ per litre. The density of algae in reservoirs as a source of drinking water is generally 1 × 10⁴ per litre. The density of algae in Taihu Lake water is 200 times higher, which satisfies the characteristics of high algae-laden water.

Treating high algae-laden water is a great challenge for large water supply plants. In general, traditional treatment methods for high algae-laden water can be divided into physical, chemical, and biological methods (Su et al. 2016; Pompei et al. 2022; Ren et al. 2022). Physical methods are simple and environmentally friendly but inefficient. Chemical methods are cheap and convenient, but they may cause secondary pollution and cause microorganisms to generate drug resistance. The advantages of biological methods are environmental friendliness and minimal sludge production, but the disadvantages are long-running periods. In addition, pre-oxidation-enhanced coagulation is widely used in drinking water treatment. The main principle is to deactivate and remove algae cells by coagulation and sedimentation (Lin et al. 2021), but cell deactivation is difficult to control. Traditional pre-oxidation or ultrasound and ultraviolet treatment may damage algal cells, so this method has certain limits. The ozone-biological activated carbon process (O₃-BAC) has been applied widely in drinking water treatment. It can remove a proportion of the hydrophobic organic matter. But it may produce ozone by-products that have an impact on drinking water. Therefore, there is still a need to develop a better drinking water treatment (Liu et al. 2021; Sun & Chu 2021).

As an emerging water treatment technology, membranes are receiving increasing attention. The dual membrane process is used in desalination and decontamination (Touati et al. 2018; Ağtaş et al. 2020; Kamp et al. 2021) due to its high efficiency, good stability, small footprint, and easy management (Wang et al. 2020). The first step in the dual membrane process is usually microfiltration or ultrafiltration (UF), which is used primarily to remove larger particulate matter (Sangrola et al. 2020). Then, nanofiltration (NF) or reverse osmosis (RO) is used as a second step to effectively remove various contaminants (Guo et al. 2019), including DOM (Ersan et al. 2016) and heavy metals (Zhang et al. 2016). The dual membrane process has been applied extensively in desalination and decontamination, but it is rarely, if ever, designed as the core part of drinking water treatment, and there is minimal research related to the dual membrane process. It is noteworthy that RO basically removes minerals that are good for human health. So, this study uses a combination of UF and NF membranes to remove algae, to ensure substances that are beneficial to people are not completely removed from the water. Theoretically, the membrane removes pollutants mainly relying on size exclusion, and it can completely remove algae cells without breaking them (Cheng et al. 2021). Therefore, the membrane is suitable for treating high algae-laden water. Moreover, UF–NF could not only remove the target contaminants but also retain the minerals beneficial to human health in drinking water. Thus, UF–NF is considered an effective method to prepare high-quality drinking water. Nevertheless, compared with O₃-BAC, the application experience of a full-scale dual membrane process has not been well established. As a result, a pilot study on the dual membrane process is an important research subject that merits further investigation of the dual membrane process efficiency and membrane fouling in treating high algae-laden water.

This study explores the pollutant removal capacity and membrane fouling of the dual membrane process consisting of UF and NF to treat high algae-laden water. A pilot-scale experiment was built at the No. 1 Water Treatment Plant in Wujiang District, Suzhou, China. To better explain the feasibility of the dual membrane process, an O₃-BAC (the drinking water treatment used at the plant) was set up for comparison. The feasibility of the dual membrane process for treating high algae-laden water was explained in terms of pollutant removal efficiency, stability, and technical economy. Furthermore, this pilot study aims to provide a reference for large water plants to produce high-quality drinking water.

A pilot-scale experiment was set up at the first Water Treatment Plant in Wujiang District, Suzhou, China. The raw water was taken from East Tai Lake, which is the main source of drinking water in Suzhou, Jiangsu Province. The features of the raw water quality are summarized in Table 1.

UF–NF and O₃-BAC were simultaneously carried out in water plants. The membranes in the dual membrane process are UF and NF. Ultrafiltration (INGE, Germany) is a seven-hole UF membrane with an average molecular-weight cut-off (MWCO) of 150,000 Da. The hydrophilic UF membrane is made of polyethersulfone (PES). The UF that contains seven membranes with a total effective filtration area of 6.5 m² adopts internal dead-end filtration with a constant flow, which is shown in Figure 1. The hydrophilic nanofiltration membrane (Toray, Japan) with an average MWCO of 200 Da is made of polyamide (PA). The spiral-wound NF membrane that contains two NF membranes with a total effective filtration area of 8 m² adopts cross-flow filtration with a constant flow.

For O₃-BAC, the raw water from East Tai Lake was sent to a feed tank. Then, the effluent was pumped into a mechanical agitation tank. The coagulant was added to the mechanical agitation tank using a peristaltic pump with a dosing rate of 20 mg/L. The water subsequently flowed to the sloping pipe settling tank with an effective area of 2 m³. After 1.5 h of precipitation, the supernatant was sent to a sand filtration column with a flux of 0.002 m/s. Then, the effluent was pumped into an ozone contact column with a reaction time of 15 min. Finally, the water entered the activated carbon column with a flux of 0.002 m/s. The contact time in the empty bed of the activated carbon column is 12 min. The backwash of the sand filtration and activated carbon was periodically performed with the effluent of the reservoir.

An online conductivity detector (CR 300) was used to measure the total dissolved solids (TDS) and conductivity. Turbidity was measured with a portable turbidity meter (HACH 2100P, USA), and pH was measured with a pH meter (PHS-25, Shanghai). Chemical oxygen demand (COD_Mn) and ammonia nitrogen (NH₃-N) were determined by the methods of the National Environmental Protection Standard of the People's Republic of China (GB 11892-89 and HJ 535-2009). Three-dimensional excitation-emission matrix (3D-EEM) spectra were investigated on a 3D fluorescence spectrophotometer (VARIAN Cary Eclipse, USA). The UV₂₅₄ absorption spectra were obtained by a DR 6000 UV spectrophotometer (Hash, USA). Total organic carbon (TOC) was measured using a TOC analyser (TOC-VCPH, Japan), and algae were determined by a water quality analyser (YSIEXO2, USA). The molecular-weight (MW) distribution was determined using a high-performance liquid chromatography (Waters 2695, USA) system with a detector (Sievers 900).

The flux of the membrane is an essential factor that affects the treatment effectiveness, stability, energy consumption, and membrane fouling. The constant permeate fluxes of 45 and 60 L·m⁻²·h⁻¹ (LMH) were, respectively, employed to assess the operation under different fluxes. Figure 3(b) shows that the TMP of NF displayed a linear increasing trend with extending filtration. Besides, the TMP of NF changes from 0.13–0.198 to 0.162–0.201 MPa when the flux increases from 45 to 60 LMH. This is attributed to the fact that NF is more susceptible to contamination at a higher permeate flux. To avoid microbial exposure, a fungicide (HS-301 RO membrane specific fungicide containing DBNPA (2,2-dibromo-3-nitroso-propionamide)) was applied by shock dosing. DBNPA is a broad-spectrum industrial fungicide that rapidly penetrates the cell membranes of microorganisms and causes cell death by suspending normal redox in the cells (Da-Silva-Correa et al. 2022). The fungicide was added at 100 mg/L every 7 days for 30 min. To prevent the effects of residual fungicide and its decomposition products, the effluent of NF was completely discharged during sterilization (Liu et al. 2021). The pressure significantly decreases due to the fungicide because the fungicide can inhibit the growth of microorganisms (Yu et al. 2012; Wang et al. 2018a, 2018b). As a result, UF as a pretreatment can maintain the long-term stability of NF.

The removal of turbidity by the dual membrane process is less affected by raw water and performing conditions, and the removal is extremely high. In contrast, the removal of turbidity by O₃-BAC is largely influenced by raw water. Therefore, the dual membrane process is much more effective than O₃-BAC in treating high-turbidity water. In addition, most of the suspended matter is removed by UF mainly because the size of the suspended matter is larger than the pores of UF. UF retention supplies effluent quality, guarantees biological safety, and ensures subsequent stable operation.

These results show that O₃-BAC can effectively remove organic matter mainly due to a biofilm attached to the surface of the sand filter after a long period (Guo et al. 2018). Rejecting DOMs by the UF membranes remains difficult, since the particle of dissolved organics is much smaller than the pores of the UF membrane (Guo et al. 2018). The NF membrane achieves high retention of DOM. For rejection, NF can retain all organic matter with an MW of 200–1,000 Da. Moreover, ion adsorption and charged groups of the NF membrane can fundamentally affect the removal of DOM (Castaño Osorio et al. 2022). Humic substances and other hydrophobic organic matter in water are usually negatively charged, while the NF membrane is also negatively charged under neutral pH conditions. Therefore, NF can seize negative ions or negatively charged organic matter (Verliefde et al. 2008). In addition, the NF in the experiment is a hydrophilic membrane that plays a critical role in the removal of hydrophobic substances (Nghiem et al. 2005). As a result, the NF membrane directly compensates for the UF membrane, which ensures that the system is effective in removing organic matter.

As shown in the figure in the chart, O₃-BAC is influenced by raw water, whereas the dual membrane process is stable. In O₃-BAC, stirring paddles and oxidation prefer to disintegrate algal cells (Zamyadi et al. 2013), releasing phycotoxins and odour during algal blooming. The secretion from algae can adhere to the surface of filter media and undermine the adsorption capacity and biological effect (Yu et al. 2017), which affects the security of water quality. Additionally, the coagulation and sedimentation in the O₃-BAC are ineffective in removing algae, so most algae are retained and subsequently enter the sand filter (Henderson et al. 2008). The algae can adhere to the surface of the sand filter and decrease the removal effect. The flux cannot be restored with periodic backwashing and even increase the frequency of backwashing, which will increase the long-term operational burden. The reason why the removal rate of chlorophyll-a in O₃-BAC has been rising steadily is due to microbial action and appropriate temperature. Biofilms gradually developed due to the accumulation of microorganisms. Besides, the range of optimum temperature for microbial growth is 16–30 °C. During the experiment, the temperature rose steadily and gradually reached a temperature suitable for microbial growth, which was beneficial to microbial action.

In contrast, most algae are removed by the UF membrane, and the removal rate is up to 90%. The diameter of algae is larger than the pore diameter of the UF membrane (Chiou et al. 2010; Cheng et al. 2018), so it can be completely rejected by the UF membrane. It is noteworthy that the removal effect of UF in the graph suddenly decreased. This may be because UF is not stable enough as a pretreatment. But it also could reduce the burden of the NF membrane and does not affect the effluent quality of the dual membrane process.

A three-dimensional excitation-emission matrix (3D-EEM) was utilized in this experiment to provide information about chemical components of DOM in the water (Liu et al. 2011). Five fluorescence regions were classified using fluorescence regional integration (FRI) methods and were represented by I, II, III, IV, and V (Chen et al. 2003), including tryptophan-like proteins (Ex: 220–250 nm, Em: 280–330 nm); tyrosine-like proteins (Ex: 220–250 nm, Em: 330–380 nm); fulvic acid-like substances (Ex: 220–250 nm, Em: 380–500 nm); soluble microbial product substances (Ex: 250–280 nm, Em: 280–380 nm); and humic acid-like substances (Ex: 250–400 nm, Em: 380–500 nm), respectively.

According to the above analysis results, O₃-BAC plays a critical role in the removal of medium- and high-MW organics mainly because of the adsorption of sand filtration and the degradation of biofilm attached to the surface of sand filtration (Guo et al. 2018). Additionally, ozone can oxidize medium-MW organics to low-MW organics and subsequently remove them by adsorption of activated carbon (Wei et al. 2021). The dual membrane process is effective in removing almost all organics. The medium- and high-MW organics are almost completely removed by the size exclusion of the UF. NF can remove low- and medium-MW organics, which can be attributed to its size exclusion and adsorption (Teixeira & Sousa 2013). Surface water contains both hydrophilic and hydrophobic substances. Figure 13 shows that the dual membrane process is slightly more effective in removing UV₂₅₄ than TOC because the UF and NF membranes in this test are hydrophilic, so they play less role into removing hydrophilic substances from the water, which is consistent with the research findings of the previous study (Nghiem et al. 2005). A component of hydrophilic substances is also responsible for hydrophilic polysaccharides and neutral hydrophilic matter secreted by algal cells (Gray et al. 2007). In summary, the dual membrane process can effectively remove organic matter, which ensures water safety.

A detailed analysis of the cost for the dual membrane process and O₃-BAC is shown in Table 2. It is well known that the water quality, water quantity, and scale of the treatment system greatly affect the cost. Thus, the economic analysis is performed based on the plant capacity in this study, i.e., 12,000 m³/day. The unit costs ($/m³) for both processes are estimated for the comparison.

O₃-BAC includes the building of coagulation, sedimentation, sand filtration, ozonation, and activated carbon with a total area of 360.08 m². The cost of construction, facilities, and materials is $898,524, the cost of depreciation is $34,150 per year, and the annual operating cost is $324,920. Operating costs are mainly the purchase of quartz sand, activated carbon and electricity, which can be saved by changing the material. According to the calculation, we find that the treatment cost is $0.082/m³, and the payback period is 7 months with an internal rate of return of 178.34%. At present, as the most popular water treatment process, low cost is its major advantage.

The dual membrane process mainly includes the structure of UF and NF with a total area of 200 m². The cost of construction and facilities is $1,223,538, the cost of depreciation is $38,739 per year, and the annual operating cost is $525,766. Operating costs are mainly for the purchase of UF membranes, NF membranes, electricity, chemicals and bactericidal additives. Accounting for the calculation, we find that the treatment cost is $0.13/m³, and the payback period is 11 months with an internal rate of return of 111.44%. As an emerging water purification technology, NF has drawbacks such as high cost and low recovery rate, but the shorter process and smaller footprint create more economic benefits. When the cost of membranes decreases in the future, people may incline to think in terms of footprint. Therefore, the dual membrane process is considered an effective method to prepare high-quality drinking water.

In this paper, a dual membrane process of UF–NF was set up to purify algae-laden water from Tai Lake. The pilot-scale experiment was conducted in a water treatment plant on the southeastern shore of Tai Lake. Compared to O₃-BAC, the test systematically evaluates the removal efficiency, membrane contamination, long-term characteristics, and technical economy of the dual membrane process. The results show that the dual membrane process is effective in removing turbidity, ammonia nitrogen, algae, and organic matter. The dual membrane process is stable, even with high concentrations of fouling, and the effluent satisfies the need for high quality. During operation, the dual membrane process controls biological fouling and reduces the rate of membrane fouling through the application of biocides. UF as pretreatment effectively reduces fouling and prolongs the lifetime of the NF membrane. In addition, O₃-BAC consumes many chemicals and is less effective in removing contaminants, while the dual membrane process is environmentally friendly and highly efficient. In summary, the dual membrane process has high economic and technical feasibility in potable water treatment in the future.

This work was supported by the National Natural Science Foundation of China (Grant No. 51922078) and the Open Project of the State Key Laboratory of Pollution Control and Resource Reuse Foundation (Grant No. PCRRF14011). The authors acknowledge helpful conversations regarding the interpretation of these data with Dr Juncheng Han and we thank Qianqian Xu for providing access to instrumentation.

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

The authors declare there is no conflict.