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Effect of a nanofiltration combined process on the treatment of high-hardness and micropolluted water
Environmental Research 182 (2020) 109063
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Effect of a nanofiltration combined process on the treatment of highhardness and micropolluted water
T
Yonglei Wanga,∗, Ling Jua, Fei Xub, Liping Tianc, Ruibao Jiad,∗∗, Wuchang Songd, Yanan Lia, Bing Liue a
College of Environmental and Municipal Engineering, Shandong Jianzhu University, 250101, Jinan, People's Republic of China Shandong Province Metallurgical Engineering Co.Ltd, 250101, Jinan, People's Republic of China c Weifang Municipal Public Utilities Service Center, 261041, Weifang, People's Republic of China d Shandong Province City Water Supply and Drainage Water Quality Monitoring Center, 250021, Jinan, People's Republic of China e Resources and Environment Innovation Research Institute, School of Municipal and Environmental Engineering, Shandong Jianzhu University, 250101, Jinan, People's Republic of China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Nanofiltration High hardness micropollution Combination process Pretreatment Membrane fouling
The quality of raw water and the current high level of pollution presents a phenomenon of high hardness and micropollution. An experimental study was conducted of the nanofiltration (NF) pilot-scale process combined with biological contact oxidation precipitation and ultrafiltration (UF) as the pretreatment process to treat this water. The study investigated the removal efficiency and membrane fouling of the NF process under the continuous and stable operating conditions of the combination process and studied the influence of high-hardness water on the membrane pollution of the combination process. The results showed that the combined process had a positive removal effect on conventional pollutants and characteristic pollutants, and the removal rates of conventional pollutants, such as turbidity, UV254 and CODMn, were 95%, 90% and 85%, respectively. The removal efficiency of total hardness, total alkalinity and soluble total solids reached 98%, 86% and 91%, respectively, and that of total desalination was above 95%. The removal rates of fluorescent organic substances, such as tryptophan, tyrosine, soluble microbial products (SMPs), fulvic acid and humus-like substances, as well as the precursors of disinfection byproducts reached over 88% and 50%, respectively. The pollutant removal efficiency of the combined process was mainly concentrated in the NF unit. The pretreatment process had certain removal effects on turbidity and macromolecular organic substances in the raw water, which provided a perfect operating environment for the NF process. Under long-term operation, the main elements of scaling on the surface of the NF membrane included C, O, Na, Mg, Al, Si, S, Cl, Ca, Ti and Fe, which were mainly concentrated at the outlet of the membrane and mainly came from monomers or compounds composed of inorganic salts in the raw water and some organic compounds. High-hardness water accelerated the change in membrane process parameters, and the surface of the membrane had abundant inorganic scaling. The inorganic scale on the surface of the NF membrane increased noticeably when filtering water with high hardness. Regular cleaning of the UF and NF membranes could effectively restore the parameters of the process and prolong the service life of the membrane process.
1. Introduction According to statistics, in recent years, there have been different levels of water source pollution and safety problems in more than 600 cities in China. The problem of water fouling is becoming increasingly
serious (State Environmental Protection Administration, 2016). The raw water quality of natural water is more complicated, the total hardness is higher and the content of various inorganic pollutants exceeds the highest standard (Wang et al., 2018). At present, most cities in North China use a mixed water supply method of underground and
Abbreviations: AOX, absorbable organic halogen; EDS, energy dispersive x-photon spectrometer; NF, nanofiltration; PAFC, polymeric aluminum ferric chloride; PLC, progammadble controller; SEM, scanning electron microscope; SMPs, soluble microbial products; TMP, trans-membrane pressure; UF, ultrafiltration ∗ Corresponding author. No.1000 Fengming Road, Li cheng district, Jinan, 250101, People's Republic of China. ∗∗ Corresponding author. No.5111, Aotizhong Road, Jinan, 250101, People's Republic of China. E-mail addresses: [email protected] (Y. Wang), [email protected] (R. Jia). https://doi.org/10.1016/j.envres.2019.109063 Received 3 October 2019; Received in revised form 5 December 2019; Accepted 18 December 2019 Available online 25 December 2019 0013-9351/ © 2019 Elsevier Inc. All rights reserved.
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polluted water source. The water quality indicators during the test are shown in Table 1. To more significantly investigate the influence of high-hardness water on the fouling characteristics of the NF membrane, in the analysis of membrane fouling characteristics, two kinds of water samples, raw water and standard addition raw water, were used.
surface water. Due to the problem of water pollution, the hardness and inorganic ion concentration in residential drinking water are generally high, which cause much inconvenience to the residents' production and life (Kim et al., 2010; Huang et al., 2019). Traditional water treatment processes often have defects, such as single pollutant removal and vulnerability to water quality conditions. Most of the traditional treatment processes of water supply plants are facing the problem of continual upgrading. Adoption of a reasonable deep-purification treatment process is urgently needed to solve the problem of high hardness and micropollution in water and to further improve the water quality of residential drinking water. At present, the drug softening method, the ion exchange method and the membrane method are generally used to reduce the hardness in water at home and abroad. The traditional drug softening method has the disadvantages of poor effluent quality and unstable operation (Li et al., 2016); the principle of the ion exchange method is simple, has a promising effluent quality, and it won't cause secondary water pollution, but the cost is high and the operating conditions are harsh (Berezina et al., 2008; Apell and Boyer, 2010); nanofiltration (NF), as a membrane treatment technology developed rapidly in recent years, can effectively remove pollutants, such as inorganic colloids, dissolved organic matter and heavy metal ions, in the process of treating micropolluted water with high hardness (Lai et al., 2013; Mohammad et al., 2015), while retaining small molecules of organic matter and minerals that are beneficial to the human body. In addition, the operating pressure of NF is low, so it is gradually being used in various fields of water treatment (Nghiem and Hawkes, 2009; Park et al., 2019; Rezaeian et al., 2020), but the membrane fouling problem in the NF process of removing hardness has been a bottleneck affecting the further development of the NF process, which cannot be ignored (Vrouwenvelder et al., 2003; Al-Amoudi and Farooque, 2005; Aguiar et al., 2016). With the increasing complexity of source water quality components (Wang et al., 2019), conventional NF processes have been unable to effectively remove a wide variety of contaminants while slowing the occurrence of membrane fouling (Ohno et al., 2010). In this study, we construct a new intensive treatment combination process of NF based on the water quality characteristics of high hardness and micropollution in the Yellow River Reservoir. It is a biological contact oxidation precipitation ultrafiltration (UF) process as a pretreatment process that combines enhanced coagulation, physical adsorption, biological oxidation and depth treatment by the membrane method. All of our work is to study the decontamination efficiency and operational stability of the combined process for a slightly polluted water source with high hardness and to explore the impact of the high hardness problem on the process of membrane fouling, which provides a reference to guide water plant renovation for the purification treatment of high-hardness micropolluted water.
2.2. Experimental device and process flow The pilot test device and process of the biological contact oxidation precipitation UF and NF combined process are shown in Fig. 1. The raw water in the combined process entered the aeration flocculation tank directly through the inlet pipe, mixed with the activated recirculated carbon sludge, entered the flocculation zone, and then entered the submerged ultrafiltration membrane tank through the inclined tube sedimentation tank. The precipitated carbon sludge was deposited in the sludge deposition area at the bottom of the system, which was lifted to the activated carbon slime tank by a gas lifting device and then returned to the aerated coagulation area after aeration activation to participate in the cycle process of the whole biological contact oxidation system. The effluent from the biological contact oxidation precipitation system entered the UF membrane tank and was pumped by centrifugal pump. The effluent from the UF process entered the NF water tank through the suction pump and entered the NF membrane group through centrifugal pump suction. After filtration, the produced water was discharged. The return flow and external displacement of the concentrated water could be adjusted through the flowmeter.
2.3. Operation mode and conditions During the stable operation of the biological contact oxidation precipitation system, the carbon sludge concentration was 8.9 × 103 mg/L, the biomass of the carbon sludge was stable at 85 × 107 CFU/ml, and the specific aerobic rate of the carbon sludge per unit mass reached 0.45 mg O2/(min·g). The hydraulic retention time in the coagulation zone and the flocculation zone was set to 0.5 h and that of the sedimentation zone was 1 h. The centrifugal pump of the UF device was operated intermittently and controlled by a PLC. It took 95 min for a cycle, including filtration for 90 min and backwashing for 5 min, and 7 pumping and filtration cycles comprised a drainage cycle. After 7 drainage cycles, air-water backwashing was used for 10 min, and the membrane pool was finally emptied. During the operation, the device was kept at a constant membrane flux of 36 L/(m2·h), and the initial trans-membrane pressure (TMP) was set to 28.92 kPa. The NF device controlled the parameter change of the NF membrane by controlling the concentrated water return flow of the equipment. The initial values were set as follows: inlet flow at 5 m3/h, return flow of concentrated water at 4.5 m3/h, membrane flux at 10 L/(m2·h), and TMP at 0.222 MPa. The information of nanofiltration membranes is as follows: the membrane material is polyamide; the membrane area is 37 m2; the water yield is 7500–8500 GPD; the desalination rate is 85–95% (2000ppmMgSO4).
2. Experimental 2.1. Raw water quality The pilot device for this test was located in the pilot base of a water plant in Jinan. The original water for the experiment was taken from the Queshan Reservoir, which is a typical high-hardness and slightly Table 1 Test water quality. Detection indicator
Total hardness (mg/L)
pH
Conductivity (μs/ cm)
Turbidity (NTU)
Total dissolved solids (mg/L)
Total alkalinity (mg/L)
UV254 (cm−1)
TOC (mg/L)
raw water standard addition raw water
440–490 590–600
8.11–8.32 8.11–8.32
950–1360 1429–1440
2.2–7.5 2.2–7.5
555–563 1020–1050
138–143 200–210
0.030–0.039 –
2.35–2.88 –
Note: Standard addition raw water means adding calcium chloride, magnesium chloride and sodium sulfate to the raw water to supplement the inorganic salt and total hardness content in the raw water. “—” means that was not tested. 2
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Fig. 1. Test process flows.
was carried out to obtain information on the spatially resolved chemical composition (Pietrodangelo et al., 2014; Đorđević et al., 2016).
2.4. Analytical method 2.4.1. Test indicators and analytical methods The experimental indicators and analysis methods used in the experiment are shown in Table 2.
2.4.3. Membrane washing method NF membrane needs to be chemically cleaned before use to remove pollutants from the membrane (Gao et al., 2011). Then soak in deionized water for 12 h, then pressurize with pure water at 12 bar for at least 1 h to ensure the structure is stable before each test. Adjust the pH value of feed water with 1 mol/L hydrochloric acid and 1 mol/L sodium hydroxide solution. NF system operates in full circulation mode, and the filtered concentrated water and produced water return to the water inlet tank. At the end of each test, NF membrane shall be cleaned in the order of water washing, alkali washing and acid washing to restore the permeability of membrane. During membrane cleaning, 20% hydrochloric acid or 0.02% sodium hypochlorite should be configured in the raw water tank in advance, and the equipment should be operated. After half an hour of normal cleaning, the equipment in operation should be cleaned with tap water, and then the equipment in operation should be cleaned with test water sample.
2.4.2. SEM-EDS analysis The SEM-EDS (Scanning Electron Microscope and Energy Dispersive X-photon Spectrometer) analysis method has been applied in various industries with its advanced analysis concept and efficient and accurate analysis process. SEM has a large depth of field and a continuously variable magnification rate, which is suitable for studying the stereoscopic morphology and surface microstructure of tiny objects. EDS has a wide range of measurable elements and does not cause damage to the analyte (Stewart J D, Adams K R, 2017; Karaca et al., 2019). Scanning electron microscope (SEM) coupled with an energy dispersed spectrometry (EDS) microanalysis can be employed to determine atomic composition, morphology, microstructure, and texture of tiny objects (Đorđević et al., 2016). Electron microscopic imaging of using SEM in order to get information on the primary particle size, particle morphology, and aggregation status. EDX (energy dispersive X-ray) analysis
3. Results and discussion Table 2 Test indicators and analytical methods used in the experiment. Test indicator
Analytical method
pH Conductivity
FE28 pH meter DDS-307 A thunder magnetic conductivity instrument Hash 2100 N turbidimeter Acidic potassium permanganate titration Total organic carbon analyzer, TOC-LCPH TU-1810 ultraviolet–visible spectrophotometer Ion chromatograph, DIONEX-90(America) EDTA titration method Acid-base indicator titration DDSJ-318TDS tester Aqualog absorption and three dimensional fluorescence scanning spectrometer Gas chromatography Microcoulometric method High-performance liquid chromatography Reading calculation of pressure gauge for equipment Readout of equipment flowmeter JSM-6301 scanning electron microscope Link 300 Energy Dispersion Spectrometer (JOEL/Oxford)
Turbidity CODMn TOC UV254 Inorganic ion Total hardness Total alkalinity TDS Three-dimensional fluorescence Trihalomethane precursor AOX Perfluorooctanoic acid Ransmembrane pressure Membrane flux SEM analysis EDS analysis
3.1. Analysis of decontamination efficiency 3.1.1. Conventional indicators Turbidity, UV254, CODMn and other indicators were taken as research objects in the test, and the removal efficiency of conventional pollutants by the combined process was investigated with the results shown in Fig. 2. The combined process had a positive removal effect on the turbidity in the water source as a whole. During the continuous operation of the combined process, the turbidity of the effluent in each process section was recorded as shown in Fig. 2(a). The effluent turbidity of the biological contact oxidation precipitation unit was stable at approximately 1.0 NTU, and the average removal rate of turbidity was over 50%; thus, the UF and NF processes had superior removal effects on turbidity, and the removal efficiency was more than 95%, which was mostly less than 0.1 NTU, and the effluent turbidity of the NF process was slightly lower than that of the UF process. As shown in Fig. 2(a), the removal of turbidity by the two processes was not significantly affected before and after cleaning of the membrane. The effluent turbidity of the UF and NF processes was stable at approximately 0.07 NTU after membrane cleaning. The test results show that the biological contact oxidation precipitation process as a pretreatment process for the membrane process (Mahlangu et al., 2014; Xing et al., 3
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Fig. 2. Removal efficiency of conventional pollutants in various process sections during continuous operation: (a) turbidity, (b) UV254 and (c) CODMn.
biological contact oxidation precipitation and UF treatment process had unfavorable removal effects on CODMn, which were approximately 10% and 25%, respectively. In contrast, the removal effect of NF on CODMn reached 80%–85%. After membrane cleaning, the removal rate of CODMn in the NF treatment process, which remained stable at approximately 85%, was significantly more remarkable than that in the UF treatment process. The main reason for the low removal rate of CODMn by the UF membrane is that the aperture of the UF membrane is large, and the content of particles and colloids in the raw water of the test is lower. Consequently, there are fewer filter cake layers formed in the filtration process, so the adsorption capacity of CODMn is small. The removal effect of the NF membrane on CODMn is mainly determined by the molecular weight. When the molecular weight of pollutants is greater than the interception relative molecular weight, the organic matter is almost completely removed. However, due to the particle size of the interception material, the ionic charge and the hydrophobicity of the membrane, organic matter smaller than the molecular weight of its interception is still partially removed (Luo et al., 2014). Therefore, when there are many small molecular organic substances in the water, the NF membrane still has a good removal effect.
2019) can effectively reduce the turbidity in the raw water and reduce the workload for the subsequent membrane treatment process (Yan et al., 2014). The UF membrane can effectively remove colloids, suspended matter and polymer organic pollutants (Vatankhah et al., 2018) to improve the inlet water quality in the NF process, which is conducive to the long-term stable operation of the NF membrane. As shown in Fig. 2(b), the combined process had a better removal effect on UV254, and the UV254 content in the effluent was extremely low. The removal efficiency of UV254 by biological contact oxidation precipitation and UF was not noticeable, while that by NF was favorable, with the removal rate reaching approximately 90%, and after membrane cleaning the removal rate was 93%. Although the UV254 in the nanofiltration production water is fluctuating from the figure, in fact, it is relatively stable, the volatility is only within the 0.003 cm−1 range. Studies have shown that UV254 can act as a substitute for trihalomethane precursors and has a good correlation with trihalomethane precursors (Shi et al., 2000; Wang et al., 2014). The content of UV254 in the effluent of the combined process is low, so it is concluded that the combined process can reduce the risk of forming disinfection byproducts during the disinfection process while removing organic substances and plays an important role in improving the chemical safety of drinking water. Fig. 2(c) shows that the combined process has an excellent removal effect on CODMn, and the removal rate reaches approximately 80%. The
3.1.2. Indicators of inorganic salts The indicators of inorganic salts in the effluent of each process section of the combined process are shown in Table 3. From the table, 4
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Table 3 Variation in inorganic salts in the effluent of each process section.
Raw water UF effluent NF effluent Removal rate (UF) Removal rate (NF)
Conductivity (μs/ cm)
Total hardness (mg/L)
Total alkalinity (mg/L)
TDS (mg/L)
K (mg/L)
Ca (mg/L)
Na (mg/L)
Mg (mg/L)
SO42− (mg/L)
Cl− (mg/L)
HCO3− (mg/L)
NO3− (mg/L)
956.0 955.0 96.5 0.1
490.0 490.0 9.0 0.0
141.0 133.0 20.0 5.7
561.0 561.0 49.0 0.0
7.8 8.2 1.0 0.0
42.50 47.00 0.65 0.00
94.5 96.6 15.9 0.0
26.000 25.300 0.481 2.692
154.0 153.0 1.0 0.6
84.8 103.0 15.9 0.0
170.0 162.0 24.0 4.7
1.13 1.83 0.70 0.00
89.9
98.2
85.8
91.3
87.2
98.47
83.2
98.150
99.4
81.3
85.9
38.05
Note: The experimental data is the average of three trials.
the NF process had a promising removal effect on inorganic pollutants, and the removal rate of total hardness, total alkalinity, and TDS reached 98.2%, 85.8% and 91.3%, respectively, the conductivity decreased by 89.9%, the removal rate of Ca2+, Mg2+ and SO42− was higher than 98% and that of K+, Na+ and Cl−, HCO3− also reached 80%. However, the removal efficiency of NO3− was only 38%. In general, the biological contact oxidation precipitation UF and NF combined process could effectively remove the inorganic pollutants in the raw water, especially Ca2+, Mg2+, SO42− and other substances related to hardness. It can be seen that the NF membrane plays a leading role in the process of removing hardness and inorganic ions, while the UF membrane was weaker than the NF membrane. This is mainly because the UF membrane filtration mechanism is dominated by mechanical screening (Stoller, 2009; Shen et al., 2010), which has a poor ability to intercept pollutants smaller than its pore size, so the removal effect on soluble and inorganic pollutants in water is poor. The filtration mechanism of the NF membrane is mainly based on its screening and charge action (Baghoth et al., 2011; Agboola et al., 2015). On the one hand, it intercepts substances whose particle size is larger than the membrane aperture. On the other hand, due to the electrostatic effect of charged particles on the membrane surface to charged pollutants in water, substances smaller than the pore diameter of the membrane will be partially trapped. The combined effect of these two factors makes the NF membrane more effective in removing inorganic pollutants (Cheng et al., 2015; Lin et al., 2019). The above results show that the decontamination of the combined process mainly focuses on the NF membrane process, which has a nearly perfect removal effect on both organic and inorganic pollutants. Biological contact oxidation precipitation and the UF process can remove some suspended substances, colloids and macromolecular organic substances and they are the key pretreatment units in the combination process, providing a guarantee for the efficient operation of the subsequent processes.
Fig. 3. Distribution of organic matter in the effluent of each process section.
organic substances in the UF process effluent decreased significantly after NF treatment. The fluorescence intensity of tryptophan, tyrosine, SMPs, fulvic acid, humus-like substances and other organic substances decreased by 95%, 87%, 88%, 94% and 93%, respectively, which indicated that the NF process can effectively remove organic substances such as SMPs, tyrosine, fulvic acid, tryptophan proteins, and humus-like substances. In conclusion, the combined process can effectively remove organic pollutants in water. 3.1.3.2. Trihalomethane precursor and AOX. The removal efficiency of trihalomethane precursor and absorbable organic halogen (AOX) by the combined process was investigated experimentally as shown in Fig. 4. As shown in the figure, the biological contact oxidation precipitation process removed some of the trihalomethane precursors, and the
3.1.3. Organics indicators 3.1.3.1. Fluorescent organics. Three-dimensional fluorescence is a kind of spectral fingerprinting technology, which has the advantages of high sensitivity, more selectivity, an ocean of information, no damage to the structure of the samples and is more intuitive and easier to compare graphically, so it has been currently exploited in broad fields to describe the characteristics of organic compounds in water treatment. Combined with the relevant literature (Guo et al., 2010; Osburn et al., 2012), three-dimensional fluorescence data were analyzed to draw the organic distribution map of each process segment, as shown in Fig. 3. Tryptophan proteins and soluble microbial products (SMPs) are the main organic substances in raw water, as well as tyrosine, fulvic acids and humus-like substances (shown in Fig. 3). After the treatment by the biological contact oxidation precipitation and UF process, the fluorescence intensity of various organic substances decreased slightly (Shen et al., 2019), which shows that the removal effect of the biological contact oxidation precipitation and UF process on various dissolved organic substances is unfavorable. The fluorescence intensity of various
Fig. 4. Removal efficiency of Trihalomethane precursor and AOX in different process sections of the combined process: Precursor-1 represents CHCl3; Precursor-2 represents CHBrCl2; Precursor-3 represents CHBr2Cl; Precursor-4 represents CHBr3; and AOX represents absorbable organic halogen. 5
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precursor removal rates of CHCl3, CHBrCl2, CHBr2Cl and CHBr3 were 42.75%, 31.09%, 20.35% and 0.62%, respectively. The effluent AOX concentration was even higher than that in the raw water, which might be caused by the mixing of new pollutants in the backflow process of carbon sludge. It was found that the actual concentration of the precursor of ultrafiltration effluent was basically the same as that of the precipitated effluent, the UF process effluent had no obvious removal effect on the trihalomethane precursor, but the removal rate of AOX was 31.7%; thus, the NF process apparently had a removal effect on trihalomethane precursors (Golea et al., 2016). Compared with the raw water, the removal rates of trihalomethane precursors were 83.2%, 93.7%, 91.2% and 51.2%, and that of AOX reached 51.2%. In conclusion, the combined process has a perfect removal effect on the precursors of trihalomethane and AOX. The removal characteristics of the trihalomethane precursor are consistent with the removal effect on UV254 mentioned above, which also verifies the claim that UV254 can act as the substitution parameter of the trihalomethane precursor to some extent (Hwang et al., 2000).
Fig. 6. Changes in membrane flux and TMP during long-term operation of the NF process.
observed mainly by the change in the TMP. Due to the low operating pressure set at the initial stage of the raw water treatment test, the operating pressure was increased by adjusting the return flow of concentrated water on the 70th day, resulting in a sudden increase in the TMP from 0.3 MPa to approximately 0.46 MPa (Fig. 6). During the whole process, the TMP of the NF membrane showed a slow upward trend, after pickling (20% HCl) of the NF membrane on the 80th day, the TMP recovered from 0.48 MPa to 0.44 MPa because the operating pressure was adjusted in the middle, so the TMP could not be restored to the original low level. However, it can be seen that pickling can effectively remove the pollution fouling the NF membrane. After the standard addition, the TMP increased from 0.23 MPa to 0.34 MPa, and the rising rate was higher than that of the raw water test. The TMP recovered to 0.27 MPa after membrane cleaning.
3.2. Analysis of membrane fouling characteristics 3.2.1. Stability of process operation In the membrane treatment process, membrane flux and transmembrane pressure (TMP) are the most intuitive indicators of the membrane's operational stability and membrane fouling. Fig. 5 shows the changes in the membrane flux and TMP of the UF equipment during continuous operation after treatment of raw water and standard addition raw water. Control of the membrane flux of the equipment remained constant during operation, and the contamination of the UF membrane was observed by monitoring the change in the TMP. It can be seen from the figure that the TMP showed an upward trend before the 80th day, indicating that the UF membrane had a certain degree of membrane fouling. During this period, the TMP of the raw water increased from 30 KPa to 46 KPa, and after the standard addition, the TMP increased from 32 KPa to 48 KPa, and the upward trend was obvious. After sodium hypochlorite (0.02%) alkaline washing of the ultrafiltration membrane, the TMP of the raw water decreased from 46 KPa to 35 KPa, a decrease of 24%. After the standard addition, the TMP decreased from 48 KPa to 39 KPa, a decrease of 19%. Thus, highhardness water aggravated the pollution of the UF membrane, and after membrane cleaning, the TMP showed a slow upward trend. Fig. 6 shows the changes in the membrane flux and TMP of the NF equipment during continuous treatment of raw water with different water quality. The operating pressure of the NF membrane was controlled by controlling the concentrated water backflow of the NF equipment, and the flux of the membrane was stabilized at approximately 10 L/(m2· h) during the operation. NF membrane fouling was
3.2.2. SEM-EDS analysis of the NF membrane surface To further observe the membrane fouling caused by the continuous operation of the NF membrane, SEM analysis was carried out on two points of the filter element of the NF membrane used for treating two kinds of water sources. The scaling morphology of the membrane surface was observed, and the effect of high hardness on the scaling of the membrane surface was analyzed. The resulting images are shown in Fig. 7 and Fig. 8. Fig. 7 shows the SEM micrographs of the inlet and outlet of the membrane sample magnified 500, 2000 and 8000 times after long-term treatment of raw water by the NF process. The fouling on the surface of the membrane was mainly concentrated at the outlet, and the fouling adsorbed at the entrance was less and irregularly granular with the majority composed of inorganic colloidal particles and some inorganic pollutants in the raw water (Thomas Rinder et al., 2013Rinder et al., 2013). Most of the fouling adsorbed at the outlet of the membrane had the following characteristics: irregularly arranged, relatively compact, and composed of small particles, with a small number of the particles being large. The bulk of the particles were monomers or compounds composed of inorganic components and some organic substances, among others, which were mainly derived from inorganic salts and organic pollutants in the raw water. SEM analysis also could not find macroscopic biofilms on the membrane surface. On the whole, there are few pollutants attached to the surface of the NF membrane, which indicates that the degree of membrane fouling is not obvious. The SEM micrographs of the inlet and outlet of the membrane sample magnified 500, 2000 and 8000 times, respectively, after longterm treatment of standard addition raw water by the NF process is shown in Fig. 8. The surface contamination of the NF membrane when treating the high-hardness water was much more serious than that of the ordinary raw water, but the surface morphology of the membrane at the entrance was basically the same, and the inorganic scaling with
Fig. 5. Changes in membrane flux and TMP during long-term operation of the UF process. 6
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Fig. 7. Scanning electron microscopic image of surface fouling of the NF membrane for raw water: a entrance enlarged 500 times; b entrance enlarged 2000 times; c entrance enlarged 8000 times; d exit enlarged 500 times; e exit enlarged 2000 times; and f exit enlarged 8000 times.
the contents of the elements were relatively consistent. The main elements were C and O, accounting for more than 90% of the total atoms. The EDS analysis showed that the C mainly came from carbonates and some organic matter in the raw water. In addition, the content of Ca in the samples was higher than that of other metal cations, indicating that the inorganic salts in the scaling of the NF membranes were mainly calcium salts. The content of Si was also high, mainly from inorganic colloids and suspended substances in the raw water. It can be seen from the figure that the content proportions of C, Al, Si and Cl at the inlet were higher than those at the outlet, and the content percentages of O, Na, Mg, Ca and Fe at the outlet were higher than those at the inlet. The main reasons were as follows: the SEM results show that the fouling by pollutants in the raw water was mainly concentrated at the outlet of the membrane under the impact of the water flow, and the element
small particles in close arrangement and large area was distributed at the outlet. To determine the specific components of the pollutants and find a solution to the membrane surface pollution, EDS was used to analyze the membrane surface sediments, and according to the results of EDS on the membrane surface, a normalized mass percentage diagram of the contents of various elements in the scaling on the membrane surface was made (Fig. 9). According to the results of the measurement and analysis, the composition of the scaling elements on the membrane surface was consistent. The main elements of scaling on the membrane surface were C, O, Na, Mg, Al, Si, S, Cl, K, Ca, Ti and Fe. During operation, some equipment components might produce rust, which makes the EDS results contain more Fe elements. Although the composition of the elements at the inlet and outlet of the NF membranes was different,
Fig. 8. Scanning electron microscopic images of surface fouling of the NF membrane for standard addition raw water: a entrance enlarged 500 times; b entrance enlarged 2000 times; c entrance enlarged 8000 times; d exit enlarged 500 times; e exit enlarged 2000 times; and f exit enlarged 8000 times. 7
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and the combined process can provide a good operating environment for the NF process. Acknowledgements This work was financially supported by the Natural Science Foundation of Shandong Province (ZR2016EEM32), the Science and Technology Plans of the Ministry of Housing and Urban-Rural Development of the People's Republic of China, and the Opening Projects of the Beijing Advanced Innovation Center for Future Urban Design, Beijing University of Civil Engineering and Architecture (Research and application of key technologies for dissolved air flotation based on water purification of algae-contaminated lakes, UDC2017031612), the Science and Technology Development Plan of Weifang (2019ZJ1088), and supported by Science Foundation of Shandong Jianzhu University （Grant No. XNBS1824）and Shandong Key Research and Development Program (No. 2019GSF109064). Fig. 9. Normalized mass percentage of element contents in scaling on the membrane surface.
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contents of the fouling component was less at the inlet. The mass percentages of Si and Ca in the surface scaling of the NF membrane treating the high-hardness water were much higher than those in the surface scaling of the NF membrane treating the raw water. Due to the standard addition raw water is rich in hardness-related elements such as Mg, Ca, Si, etc., the pollution degree of the standard addition raw water is more serious than that of the raw water during the nanofiltration membrane treatment. Due to the long operation time and the influence of concentrated water reflux, the fouling on the membrane surface increases, and most of them are inorganic scaling rich in Si, CA, etc., resulting in the occurrence of O, Si, Ca and other elements in the effluent content higher than that in the influent. 4. Conclusions (1) The combined NF depth treatment process with the biological contact oxidation precipitation-UF process as the pretreatment process has a good effect on the treatment of the high-hardness micropolluted water of the Yellow River Reservoir, and the effluent quality is good, which meets the requirements of “Sanitary Standard for Drinking Water” (GB5749-2006). (2) The removal rates of turbidity, CODMn, total hardness and other conventional indicators by the combined process reached 95%, 85%, and 97%, respectively. The removal rates of tryptophan, tyrosine, SMPs, fulvic acid, humus-like substances and five other kinds of fluorescent organic compounds reached 95%, 87%, 88%, 94% and 93%, respectively. The average removal rates of THMFPs and AOX reached 80% and 51%, respectively. (3) The combined process mainly focuses on the NF membrane process to remove pollutants. The biological contact oxidation precipitation and UF process, which effectively protects the UF membrane and NF membrane, has certain removal effects on the turbidity and macromolecular organics. In the process of operation, the membrane flux and TMP of the UF and NF membranes decreased to a certain extent. Sodium hypochlorite (0.02%) alkaline washing of the UF membrane and pickling (20% HCl) of the NF membrane achieved good results, and the recovery rate of the TMP after membrane cleaning reached 70%, which basically repaired the membrane fouling of the UF and NF membranes. (4) The main elements of fouling on the surface of the NF membrane under long-term operation are C, O, Na, Mg, Al, Si, S, Cl, K, Ca, Ti and Fe, and the elements are mainly concentrated at the outlet of the membrane. The main components are monomers or compounds composed of inorganic salts in the raw water and some organic substances. However, overall, membrane fouling is relatively light, 8
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Stoller, Marco, 2009. On the effect of flocculation as pretreatment process and particle size distribution for membrane fouling reduction. Desalination 240, 209–217. Vatankhah, H., Murray, C.C., Brannum, J.W., Vanneste, J., Bellona, C., 2018. Effect of pre-ozonation on nanofiltration membrane fouling during water reuse applications. Separ. Purif. Technol. 205, 203–211. Vrouwenvelder, J.S., Kappelhof, J.W.N.M., Heijrnan, S.G.J., van der, K.D., 2003. Tools for fouling diagnosis of NF and RO membranes and assessment of the fouling potential of feed water. Desalination 157, 361–365. Wang, Z., Song, W., Xiao, F., Sun, S., Jia, R., 2014. Study on removal characteristics of organic matters by gac-sand dual media deep filter. Technology of Water Treatment 40, 115–118. Wang, Y., Xu, X., Jia, R., Liu, B., Song, W., Jia, J., 2018. Experimental study on the removal of organic pollutants and NH3-N from surface water via an integrated copolymerization air flotation-carbon sand filtration process. J. Water Supply Res. Technol. - Aqua 67, 506–516. Wang, Y., Wang, W., Jia, R., Li, M., Liu, B., Zhang, K., Song, W., Jia, J., 2019. Research on treating algae-polluted reservoir water by the process of pre-oxidation/dissolved air flotation/carbon sand filter. Water Sci. Technol. Water Supply 19, 823–830. Xing, J., Liang, H., Xu, S., Chuah, C.J., Luo, X., Wang, T., Wang, J., Li, G., Snyder, S.A., 2019. Organic matter removal and membrane fouling mitigation during algae-rich surface water treatment by powdered activated carbon adsorption pretreatment: enhanced by UV and UV/chlorine oxidation. Water Res. 159, 283–293. Yan, Z., Fangshu, Q., Liang, H., Zheng, W., Xing, D.U., Dang, M., Guibai, L.I., 2014. A review on the ultrafitration membrane pollution and pretreatment technology. Membr. Sci. Technol. 34, 108–114+127. Đorđević, D., Buha, J., Stortini, A.M., Mihajlidi-Zelić, A., Relić, D., Barbante, C., Gambaro, A., 2016. Mass distributions and morphological and chemical characterization of urban aerosols in the continental Balkan area (Belgrade). Environ. Sci. Pollut. Res. Int. 23, 851–859.
Y., 2010. NF membrane fouling by aluminum and iron coagulant residuals after coagulation–MF pretreatment. Desalination 254, 17–22. Osburn, C.L., Handsel, L.T., Mikan, M.P., Paerl, H.W., Montgomery, M.T., 2012. Fluorescence tracking of dissolved and particulate organic matter quality in a riverdominated estuary. Environ. Science & Technology 46, 8628–8636. Park, S., Nam, T., You, J., Kim, E.S., Choi, I., Park, J., Cho, K.H., 2019. Evaluating membrane fouling potentials of dissolved organic matter in brackish water. Water Res. 149, 65–73. Pietrodangelo, A., Pareti, S., Perrino, C., 2014. Improved identification of transition metals in airborne aerosols by SEM-EDX combined backscattered and secondary electron microanalysis. Environ. Sci. Pollut. Res. Int. 21, 4023–4031. Rezaeian, M.S., Mousavi, S.M., Saljoughi, E., Akhlaghi Amiri, H.A., 2020. Evaluation of thin film composite membrane in production of ionically modified water applied for enhanced oil recovery. Desalination 474, 114194. Rinder, Thomas, Dietzel, Martin, Albrecht, Leis, 2013. Calcium carbonate scaling under alkaline conditions-Case studies and hydrochemical modelling. Appl. Geochem. 35, 132–141. Shen, Y., Zhao, W., Xiao, K., Huang, X., 2010. A systematic insight into fouling propensity of soluble microbial products in membrane bioreactors based on hydrophobic interaction and size exclusion. J. Membr. Sci. 346, 187–193. Shen, X., Gao, B., Guo, K., Yu, C., Yue, Q., 2019. PAC-PDMDAAC pretreatment of typical natural organic matter mixtures: ultrafiltration membrane fouling control and mechanisms. Sci. Total Environ. 694, 133816. Shi, Y., Ma, J., Li, G., 2000. Effect of pH values on enhanced algae removal by pre -oxidation with composite ferrate chemicals. China Water & Wastewater 18–20. State Environmental Protection Administration, 2016. China environmental status bulletin 2016 (Excerpt). Environ. Protect. 45, 35–47. Stewart, J.D., Adams, K.R., 2017. Evaluating visual criteria for identifying carbon- and iron-based pottery paints from the four corners region using SEM-EDS. Am. Antiq. 64, 675–696.
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Effect of a nanofiltration combined process on the treatment of highhardness and micropolluted water
T
Yonglei Wanga,∗, Ling Jua, Fei Xub, Liping Tianc, Ruibao Jiad,∗∗, Wuchang Songd, Yanan Lia, Bing Liue a
College of Environmental and Municipal Engineering, Shandong Jianzhu University, 250101, Jinan, People's Republic of China Shandong Province Metallurgical Engineering Co.Ltd, 250101, Jinan, People's Republic of China c Weifang Municipal Public Utilities Service Center, 261041, Weifang, People's Republic of China d Shandong Province City Water Supply and Drainage Water Quality Monitoring Center, 250021, Jinan, People's Republic of China e Resources and Environment Innovation Research Institute, School of Municipal and Environmental Engineering, Shandong Jianzhu University, 250101, Jinan, People's Republic of China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Nanofiltration High hardness micropollution Combination process Pretreatment Membrane fouling
The quality of raw water and the current high level of pollution presents a phenomenon of high hardness and micropollution. An experimental study was conducted of the nanofiltration (NF) pilot-scale process combined with biological contact oxidation precipitation and ultrafiltration (UF) as the pretreatment process to treat this water. The study investigated the removal efficiency and membrane fouling of the NF process under the continuous and stable operating conditions of the combination process and studied the influence of high-hardness water on the membrane pollution of the combination process. The results showed that the combined process had a positive removal effect on conventional pollutants and characteristic pollutants, and the removal rates of conventional pollutants, such as turbidity, UV254 and CODMn, were 95%, 90% and 85%, respectively. The removal efficiency of total hardness, total alkalinity and soluble total solids reached 98%, 86% and 91%, respectively, and that of total desalination was above 95%. The removal rates of fluorescent organic substances, such as tryptophan, tyrosine, soluble microbial products (SMPs), fulvic acid and humus-like substances, as well as the precursors of disinfection byproducts reached over 88% and 50%, respectively. The pollutant removal efficiency of the combined process was mainly concentrated in the NF unit. The pretreatment process had certain removal effects on turbidity and macromolecular organic substances in the raw water, which provided a perfect operating environment for the NF process. Under long-term operation, the main elements of scaling on the surface of the NF membrane included C, O, Na, Mg, Al, Si, S, Cl, Ca, Ti and Fe, which were mainly concentrated at the outlet of the membrane and mainly came from monomers or compounds composed of inorganic salts in the raw water and some organic compounds. High-hardness water accelerated the change in membrane process parameters, and the surface of the membrane had abundant inorganic scaling. The inorganic scale on the surface of the NF membrane increased noticeably when filtering water with high hardness. Regular cleaning of the UF and NF membranes could effectively restore the parameters of the process and prolong the service life of the membrane process.
1. Introduction According to statistics, in recent years, there have been different levels of water source pollution and safety problems in more than 600 cities in China. The problem of water fouling is becoming increasingly
serious (State Environmental Protection Administration, 2016). The raw water quality of natural water is more complicated, the total hardness is higher and the content of various inorganic pollutants exceeds the highest standard (Wang et al., 2018). At present, most cities in North China use a mixed water supply method of underground and
Abbreviations: AOX, absorbable organic halogen; EDS, energy dispersive x-photon spectrometer; NF, nanofiltration; PAFC, polymeric aluminum ferric chloride; PLC, progammadble controller; SEM, scanning electron microscope; SMPs, soluble microbial products; TMP, trans-membrane pressure; UF, ultrafiltration ∗ Corresponding author. No.1000 Fengming Road, Li cheng district, Jinan, 250101, People's Republic of China. ∗∗ Corresponding author. No.5111, Aotizhong Road, Jinan, 250101, People's Republic of China. E-mail addresses: [email protected] (Y. Wang), [email protected] (R. Jia). https://doi.org/10.1016/j.envres.2019.109063 Received 3 October 2019; Received in revised form 5 December 2019; Accepted 18 December 2019 Available online 25 December 2019 0013-9351/ © 2019 Elsevier Inc. All rights reserved.
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polluted water source. The water quality indicators during the test are shown in Table 1. To more significantly investigate the influence of high-hardness water on the fouling characteristics of the NF membrane, in the analysis of membrane fouling characteristics, two kinds of water samples, raw water and standard addition raw water, were used.
surface water. Due to the problem of water pollution, the hardness and inorganic ion concentration in residential drinking water are generally high, which cause much inconvenience to the residents' production and life (Kim et al., 2010; Huang et al., 2019). Traditional water treatment processes often have defects, such as single pollutant removal and vulnerability to water quality conditions. Most of the traditional treatment processes of water supply plants are facing the problem of continual upgrading. Adoption of a reasonable deep-purification treatment process is urgently needed to solve the problem of high hardness and micropollution in water and to further improve the water quality of residential drinking water. At present, the drug softening method, the ion exchange method and the membrane method are generally used to reduce the hardness in water at home and abroad. The traditional drug softening method has the disadvantages of poor effluent quality and unstable operation (Li et al., 2016); the principle of the ion exchange method is simple, has a promising effluent quality, and it won't cause secondary water pollution, but the cost is high and the operating conditions are harsh (Berezina et al., 2008; Apell and Boyer, 2010); nanofiltration (NF), as a membrane treatment technology developed rapidly in recent years, can effectively remove pollutants, such as inorganic colloids, dissolved organic matter and heavy metal ions, in the process of treating micropolluted water with high hardness (Lai et al., 2013; Mohammad et al., 2015), while retaining small molecules of organic matter and minerals that are beneficial to the human body. In addition, the operating pressure of NF is low, so it is gradually being used in various fields of water treatment (Nghiem and Hawkes, 2009; Park et al., 2019; Rezaeian et al., 2020), but the membrane fouling problem in the NF process of removing hardness has been a bottleneck affecting the further development of the NF process, which cannot be ignored (Vrouwenvelder et al., 2003; Al-Amoudi and Farooque, 2005; Aguiar et al., 2016). With the increasing complexity of source water quality components (Wang et al., 2019), conventional NF processes have been unable to effectively remove a wide variety of contaminants while slowing the occurrence of membrane fouling (Ohno et al., 2010). In this study, we construct a new intensive treatment combination process of NF based on the water quality characteristics of high hardness and micropollution in the Yellow River Reservoir. It is a biological contact oxidation precipitation ultrafiltration (UF) process as a pretreatment process that combines enhanced coagulation, physical adsorption, biological oxidation and depth treatment by the membrane method. All of our work is to study the decontamination efficiency and operational stability of the combined process for a slightly polluted water source with high hardness and to explore the impact of the high hardness problem on the process of membrane fouling, which provides a reference to guide water plant renovation for the purification treatment of high-hardness micropolluted water.
2.2. Experimental device and process flow The pilot test device and process of the biological contact oxidation precipitation UF and NF combined process are shown in Fig. 1. The raw water in the combined process entered the aeration flocculation tank directly through the inlet pipe, mixed with the activated recirculated carbon sludge, entered the flocculation zone, and then entered the submerged ultrafiltration membrane tank through the inclined tube sedimentation tank. The precipitated carbon sludge was deposited in the sludge deposition area at the bottom of the system, which was lifted to the activated carbon slime tank by a gas lifting device and then returned to the aerated coagulation area after aeration activation to participate in the cycle process of the whole biological contact oxidation system. The effluent from the biological contact oxidation precipitation system entered the UF membrane tank and was pumped by centrifugal pump. The effluent from the UF process entered the NF water tank through the suction pump and entered the NF membrane group through centrifugal pump suction. After filtration, the produced water was discharged. The return flow and external displacement of the concentrated water could be adjusted through the flowmeter.
2.3. Operation mode and conditions During the stable operation of the biological contact oxidation precipitation system, the carbon sludge concentration was 8.9 × 103 mg/L, the biomass of the carbon sludge was stable at 85 × 107 CFU/ml, and the specific aerobic rate of the carbon sludge per unit mass reached 0.45 mg O2/(min·g). The hydraulic retention time in the coagulation zone and the flocculation zone was set to 0.5 h and that of the sedimentation zone was 1 h. The centrifugal pump of the UF device was operated intermittently and controlled by a PLC. It took 95 min for a cycle, including filtration for 90 min and backwashing for 5 min, and 7 pumping and filtration cycles comprised a drainage cycle. After 7 drainage cycles, air-water backwashing was used for 10 min, and the membrane pool was finally emptied. During the operation, the device was kept at a constant membrane flux of 36 L/(m2·h), and the initial trans-membrane pressure (TMP) was set to 28.92 kPa. The NF device controlled the parameter change of the NF membrane by controlling the concentrated water return flow of the equipment. The initial values were set as follows: inlet flow at 5 m3/h, return flow of concentrated water at 4.5 m3/h, membrane flux at 10 L/(m2·h), and TMP at 0.222 MPa. The information of nanofiltration membranes is as follows: the membrane material is polyamide; the membrane area is 37 m2; the water yield is 7500–8500 GPD; the desalination rate is 85–95% (2000ppmMgSO4).
2. Experimental 2.1. Raw water quality The pilot device for this test was located in the pilot base of a water plant in Jinan. The original water for the experiment was taken from the Queshan Reservoir, which is a typical high-hardness and slightly Table 1 Test water quality. Detection indicator
Total hardness (mg/L)
pH
Conductivity (μs/ cm)
Turbidity (NTU)
Total dissolved solids (mg/L)
Total alkalinity (mg/L)
UV254 (cm−1)
TOC (mg/L)
raw water standard addition raw water
440–490 590–600
8.11–8.32 8.11–8.32
950–1360 1429–1440
2.2–7.5 2.2–7.5
555–563 1020–1050
138–143 200–210
0.030–0.039 –
2.35–2.88 –
Note: Standard addition raw water means adding calcium chloride, magnesium chloride and sodium sulfate to the raw water to supplement the inorganic salt and total hardness content in the raw water. “—” means that was not tested. 2
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Fig. 1. Test process flows.
was carried out to obtain information on the spatially resolved chemical composition (Pietrodangelo et al., 2014; Đorđević et al., 2016).
2.4. Analytical method 2.4.1. Test indicators and analytical methods The experimental indicators and analysis methods used in the experiment are shown in Table 2.
2.4.3. Membrane washing method NF membrane needs to be chemically cleaned before use to remove pollutants from the membrane (Gao et al., 2011). Then soak in deionized water for 12 h, then pressurize with pure water at 12 bar for at least 1 h to ensure the structure is stable before each test. Adjust the pH value of feed water with 1 mol/L hydrochloric acid and 1 mol/L sodium hydroxide solution. NF system operates in full circulation mode, and the filtered concentrated water and produced water return to the water inlet tank. At the end of each test, NF membrane shall be cleaned in the order of water washing, alkali washing and acid washing to restore the permeability of membrane. During membrane cleaning, 20% hydrochloric acid or 0.02% sodium hypochlorite should be configured in the raw water tank in advance, and the equipment should be operated. After half an hour of normal cleaning, the equipment in operation should be cleaned with tap water, and then the equipment in operation should be cleaned with test water sample.
2.4.2. SEM-EDS analysis The SEM-EDS (Scanning Electron Microscope and Energy Dispersive X-photon Spectrometer) analysis method has been applied in various industries with its advanced analysis concept and efficient and accurate analysis process. SEM has a large depth of field and a continuously variable magnification rate, which is suitable for studying the stereoscopic morphology and surface microstructure of tiny objects. EDS has a wide range of measurable elements and does not cause damage to the analyte (Stewart J D, Adams K R, 2017; Karaca et al., 2019). Scanning electron microscope (SEM) coupled with an energy dispersed spectrometry (EDS) microanalysis can be employed to determine atomic composition, morphology, microstructure, and texture of tiny objects (Đorđević et al., 2016). Electron microscopic imaging of using SEM in order to get information on the primary particle size, particle morphology, and aggregation status. EDX (energy dispersive X-ray) analysis
3. Results and discussion Table 2 Test indicators and analytical methods used in the experiment. Test indicator
Analytical method
pH Conductivity
FE28 pH meter DDS-307 A thunder magnetic conductivity instrument Hash 2100 N turbidimeter Acidic potassium permanganate titration Total organic carbon analyzer, TOC-LCPH TU-1810 ultraviolet–visible spectrophotometer Ion chromatograph, DIONEX-90(America) EDTA titration method Acid-base indicator titration DDSJ-318TDS tester Aqualog absorption and three dimensional fluorescence scanning spectrometer Gas chromatography Microcoulometric method High-performance liquid chromatography Reading calculation of pressure gauge for equipment Readout of equipment flowmeter JSM-6301 scanning electron microscope Link 300 Energy Dispersion Spectrometer (JOEL/Oxford)
Turbidity CODMn TOC UV254 Inorganic ion Total hardness Total alkalinity TDS Three-dimensional fluorescence Trihalomethane precursor AOX Perfluorooctanoic acid Ransmembrane pressure Membrane flux SEM analysis EDS analysis
3.1. Analysis of decontamination efficiency 3.1.1. Conventional indicators Turbidity, UV254, CODMn and other indicators were taken as research objects in the test, and the removal efficiency of conventional pollutants by the combined process was investigated with the results shown in Fig. 2. The combined process had a positive removal effect on the turbidity in the water source as a whole. During the continuous operation of the combined process, the turbidity of the effluent in each process section was recorded as shown in Fig. 2(a). The effluent turbidity of the biological contact oxidation precipitation unit was stable at approximately 1.0 NTU, and the average removal rate of turbidity was over 50%; thus, the UF and NF processes had superior removal effects on turbidity, and the removal efficiency was more than 95%, which was mostly less than 0.1 NTU, and the effluent turbidity of the NF process was slightly lower than that of the UF process. As shown in Fig. 2(a), the removal of turbidity by the two processes was not significantly affected before and after cleaning of the membrane. The effluent turbidity of the UF and NF processes was stable at approximately 0.07 NTU after membrane cleaning. The test results show that the biological contact oxidation precipitation process as a pretreatment process for the membrane process (Mahlangu et al., 2014; Xing et al., 3
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Fig. 2. Removal efficiency of conventional pollutants in various process sections during continuous operation: (a) turbidity, (b) UV254 and (c) CODMn.
biological contact oxidation precipitation and UF treatment process had unfavorable removal effects on CODMn, which were approximately 10% and 25%, respectively. In contrast, the removal effect of NF on CODMn reached 80%–85%. After membrane cleaning, the removal rate of CODMn in the NF treatment process, which remained stable at approximately 85%, was significantly more remarkable than that in the UF treatment process. The main reason for the low removal rate of CODMn by the UF membrane is that the aperture of the UF membrane is large, and the content of particles and colloids in the raw water of the test is lower. Consequently, there are fewer filter cake layers formed in the filtration process, so the adsorption capacity of CODMn is small. The removal effect of the NF membrane on CODMn is mainly determined by the molecular weight. When the molecular weight of pollutants is greater than the interception relative molecular weight, the organic matter is almost completely removed. However, due to the particle size of the interception material, the ionic charge and the hydrophobicity of the membrane, organic matter smaller than the molecular weight of its interception is still partially removed (Luo et al., 2014). Therefore, when there are many small molecular organic substances in the water, the NF membrane still has a good removal effect.
2019) can effectively reduce the turbidity in the raw water and reduce the workload for the subsequent membrane treatment process (Yan et al., 2014). The UF membrane can effectively remove colloids, suspended matter and polymer organic pollutants (Vatankhah et al., 2018) to improve the inlet water quality in the NF process, which is conducive to the long-term stable operation of the NF membrane. As shown in Fig. 2(b), the combined process had a better removal effect on UV254, and the UV254 content in the effluent was extremely low. The removal efficiency of UV254 by biological contact oxidation precipitation and UF was not noticeable, while that by NF was favorable, with the removal rate reaching approximately 90%, and after membrane cleaning the removal rate was 93%. Although the UV254 in the nanofiltration production water is fluctuating from the figure, in fact, it is relatively stable, the volatility is only within the 0.003 cm−1 range. Studies have shown that UV254 can act as a substitute for trihalomethane precursors and has a good correlation with trihalomethane precursors (Shi et al., 2000; Wang et al., 2014). The content of UV254 in the effluent of the combined process is low, so it is concluded that the combined process can reduce the risk of forming disinfection byproducts during the disinfection process while removing organic substances and plays an important role in improving the chemical safety of drinking water. Fig. 2(c) shows that the combined process has an excellent removal effect on CODMn, and the removal rate reaches approximately 80%. The
3.1.2. Indicators of inorganic salts The indicators of inorganic salts in the effluent of each process section of the combined process are shown in Table 3. From the table, 4
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Table 3 Variation in inorganic salts in the effluent of each process section.
Raw water UF effluent NF effluent Removal rate (UF) Removal rate (NF)
Conductivity (μs/ cm)
Total hardness (mg/L)
Total alkalinity (mg/L)
TDS (mg/L)
K (mg/L)
Ca (mg/L)
Na (mg/L)
Mg (mg/L)
SO42− (mg/L)
Cl− (mg/L)
HCO3− (mg/L)
NO3− (mg/L)
956.0 955.0 96.5 0.1
490.0 490.0 9.0 0.0
141.0 133.0 20.0 5.7
561.0 561.0 49.0 0.0
7.8 8.2 1.0 0.0
42.50 47.00 0.65 0.00
94.5 96.6 15.9 0.0
26.000 25.300 0.481 2.692
154.0 153.0 1.0 0.6
84.8 103.0 15.9 0.0
170.0 162.0 24.0 4.7
1.13 1.83 0.70 0.00
89.9
98.2
85.8
91.3
87.2
98.47
83.2
98.150
99.4
81.3
85.9
38.05
Note: The experimental data is the average of three trials.
the NF process had a promising removal effect on inorganic pollutants, and the removal rate of total hardness, total alkalinity, and TDS reached 98.2%, 85.8% and 91.3%, respectively, the conductivity decreased by 89.9%, the removal rate of Ca2+, Mg2+ and SO42− was higher than 98% and that of K+, Na+ and Cl−, HCO3− also reached 80%. However, the removal efficiency of NO3− was only 38%. In general, the biological contact oxidation precipitation UF and NF combined process could effectively remove the inorganic pollutants in the raw water, especially Ca2+, Mg2+, SO42− and other substances related to hardness. It can be seen that the NF membrane plays a leading role in the process of removing hardness and inorganic ions, while the UF membrane was weaker than the NF membrane. This is mainly because the UF membrane filtration mechanism is dominated by mechanical screening (Stoller, 2009; Shen et al., 2010), which has a poor ability to intercept pollutants smaller than its pore size, so the removal effect on soluble and inorganic pollutants in water is poor. The filtration mechanism of the NF membrane is mainly based on its screening and charge action (Baghoth et al., 2011; Agboola et al., 2015). On the one hand, it intercepts substances whose particle size is larger than the membrane aperture. On the other hand, due to the electrostatic effect of charged particles on the membrane surface to charged pollutants in water, substances smaller than the pore diameter of the membrane will be partially trapped. The combined effect of these two factors makes the NF membrane more effective in removing inorganic pollutants (Cheng et al., 2015; Lin et al., 2019). The above results show that the decontamination of the combined process mainly focuses on the NF membrane process, which has a nearly perfect removal effect on both organic and inorganic pollutants. Biological contact oxidation precipitation and the UF process can remove some suspended substances, colloids and macromolecular organic substances and they are the key pretreatment units in the combination process, providing a guarantee for the efficient operation of the subsequent processes.
Fig. 3. Distribution of organic matter in the effluent of each process section.
organic substances in the UF process effluent decreased significantly after NF treatment. The fluorescence intensity of tryptophan, tyrosine, SMPs, fulvic acid, humus-like substances and other organic substances decreased by 95%, 87%, 88%, 94% and 93%, respectively, which indicated that the NF process can effectively remove organic substances such as SMPs, tyrosine, fulvic acid, tryptophan proteins, and humus-like substances. In conclusion, the combined process can effectively remove organic pollutants in water. 3.1.3.2. Trihalomethane precursor and AOX. The removal efficiency of trihalomethane precursor and absorbable organic halogen (AOX) by the combined process was investigated experimentally as shown in Fig. 4. As shown in the figure, the biological contact oxidation precipitation process removed some of the trihalomethane precursors, and the
3.1.3. Organics indicators 3.1.3.1. Fluorescent organics. Three-dimensional fluorescence is a kind of spectral fingerprinting technology, which has the advantages of high sensitivity, more selectivity, an ocean of information, no damage to the structure of the samples and is more intuitive and easier to compare graphically, so it has been currently exploited in broad fields to describe the characteristics of organic compounds in water treatment. Combined with the relevant literature (Guo et al., 2010; Osburn et al., 2012), three-dimensional fluorescence data were analyzed to draw the organic distribution map of each process segment, as shown in Fig. 3. Tryptophan proteins and soluble microbial products (SMPs) are the main organic substances in raw water, as well as tyrosine, fulvic acids and humus-like substances (shown in Fig. 3). After the treatment by the biological contact oxidation precipitation and UF process, the fluorescence intensity of various organic substances decreased slightly (Shen et al., 2019), which shows that the removal effect of the biological contact oxidation precipitation and UF process on various dissolved organic substances is unfavorable. The fluorescence intensity of various
Fig. 4. Removal efficiency of Trihalomethane precursor and AOX in different process sections of the combined process: Precursor-1 represents CHCl3; Precursor-2 represents CHBrCl2; Precursor-3 represents CHBr2Cl; Precursor-4 represents CHBr3; and AOX represents absorbable organic halogen. 5
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precursor removal rates of CHCl3, CHBrCl2, CHBr2Cl and CHBr3 were 42.75%, 31.09%, 20.35% and 0.62%, respectively. The effluent AOX concentration was even higher than that in the raw water, which might be caused by the mixing of new pollutants in the backflow process of carbon sludge. It was found that the actual concentration of the precursor of ultrafiltration effluent was basically the same as that of the precipitated effluent, the UF process effluent had no obvious removal effect on the trihalomethane precursor, but the removal rate of AOX was 31.7%; thus, the NF process apparently had a removal effect on trihalomethane precursors (Golea et al., 2016). Compared with the raw water, the removal rates of trihalomethane precursors were 83.2%, 93.7%, 91.2% and 51.2%, and that of AOX reached 51.2%. In conclusion, the combined process has a perfect removal effect on the precursors of trihalomethane and AOX. The removal characteristics of the trihalomethane precursor are consistent with the removal effect on UV254 mentioned above, which also verifies the claim that UV254 can act as the substitution parameter of the trihalomethane precursor to some extent (Hwang et al., 2000).
Fig. 6. Changes in membrane flux and TMP during long-term operation of the NF process.
observed mainly by the change in the TMP. Due to the low operating pressure set at the initial stage of the raw water treatment test, the operating pressure was increased by adjusting the return flow of concentrated water on the 70th day, resulting in a sudden increase in the TMP from 0.3 MPa to approximately 0.46 MPa (Fig. 6). During the whole process, the TMP of the NF membrane showed a slow upward trend, after pickling (20% HCl) of the NF membrane on the 80th day, the TMP recovered from 0.48 MPa to 0.44 MPa because the operating pressure was adjusted in the middle, so the TMP could not be restored to the original low level. However, it can be seen that pickling can effectively remove the pollution fouling the NF membrane. After the standard addition, the TMP increased from 0.23 MPa to 0.34 MPa, and the rising rate was higher than that of the raw water test. The TMP recovered to 0.27 MPa after membrane cleaning.
3.2. Analysis of membrane fouling characteristics 3.2.1. Stability of process operation In the membrane treatment process, membrane flux and transmembrane pressure (TMP) are the most intuitive indicators of the membrane's operational stability and membrane fouling. Fig. 5 shows the changes in the membrane flux and TMP of the UF equipment during continuous operation after treatment of raw water and standard addition raw water. Control of the membrane flux of the equipment remained constant during operation, and the contamination of the UF membrane was observed by monitoring the change in the TMP. It can be seen from the figure that the TMP showed an upward trend before the 80th day, indicating that the UF membrane had a certain degree of membrane fouling. During this period, the TMP of the raw water increased from 30 KPa to 46 KPa, and after the standard addition, the TMP increased from 32 KPa to 48 KPa, and the upward trend was obvious. After sodium hypochlorite (0.02%) alkaline washing of the ultrafiltration membrane, the TMP of the raw water decreased from 46 KPa to 35 KPa, a decrease of 24%. After the standard addition, the TMP decreased from 48 KPa to 39 KPa, a decrease of 19%. Thus, highhardness water aggravated the pollution of the UF membrane, and after membrane cleaning, the TMP showed a slow upward trend. Fig. 6 shows the changes in the membrane flux and TMP of the NF equipment during continuous treatment of raw water with different water quality. The operating pressure of the NF membrane was controlled by controlling the concentrated water backflow of the NF equipment, and the flux of the membrane was stabilized at approximately 10 L/(m2· h) during the operation. NF membrane fouling was
3.2.2. SEM-EDS analysis of the NF membrane surface To further observe the membrane fouling caused by the continuous operation of the NF membrane, SEM analysis was carried out on two points of the filter element of the NF membrane used for treating two kinds of water sources. The scaling morphology of the membrane surface was observed, and the effect of high hardness on the scaling of the membrane surface was analyzed. The resulting images are shown in Fig. 7 and Fig. 8. Fig. 7 shows the SEM micrographs of the inlet and outlet of the membrane sample magnified 500, 2000 and 8000 times after long-term treatment of raw water by the NF process. The fouling on the surface of the membrane was mainly concentrated at the outlet, and the fouling adsorbed at the entrance was less and irregularly granular with the majority composed of inorganic colloidal particles and some inorganic pollutants in the raw water (Thomas Rinder et al., 2013Rinder et al., 2013). Most of the fouling adsorbed at the outlet of the membrane had the following characteristics: irregularly arranged, relatively compact, and composed of small particles, with a small number of the particles being large. The bulk of the particles were monomers or compounds composed of inorganic components and some organic substances, among others, which were mainly derived from inorganic salts and organic pollutants in the raw water. SEM analysis also could not find macroscopic biofilms on the membrane surface. On the whole, there are few pollutants attached to the surface of the NF membrane, which indicates that the degree of membrane fouling is not obvious. The SEM micrographs of the inlet and outlet of the membrane sample magnified 500, 2000 and 8000 times, respectively, after longterm treatment of standard addition raw water by the NF process is shown in Fig. 8. The surface contamination of the NF membrane when treating the high-hardness water was much more serious than that of the ordinary raw water, but the surface morphology of the membrane at the entrance was basically the same, and the inorganic scaling with
Fig. 5. Changes in membrane flux and TMP during long-term operation of the UF process. 6
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Fig. 7. Scanning electron microscopic image of surface fouling of the NF membrane for raw water: a entrance enlarged 500 times; b entrance enlarged 2000 times; c entrance enlarged 8000 times; d exit enlarged 500 times; e exit enlarged 2000 times; and f exit enlarged 8000 times.
the contents of the elements were relatively consistent. The main elements were C and O, accounting for more than 90% of the total atoms. The EDS analysis showed that the C mainly came from carbonates and some organic matter in the raw water. In addition, the content of Ca in the samples was higher than that of other metal cations, indicating that the inorganic salts in the scaling of the NF membranes were mainly calcium salts. The content of Si was also high, mainly from inorganic colloids and suspended substances in the raw water. It can be seen from the figure that the content proportions of C, Al, Si and Cl at the inlet were higher than those at the outlet, and the content percentages of O, Na, Mg, Ca and Fe at the outlet were higher than those at the inlet. The main reasons were as follows: the SEM results show that the fouling by pollutants in the raw water was mainly concentrated at the outlet of the membrane under the impact of the water flow, and the element
small particles in close arrangement and large area was distributed at the outlet. To determine the specific components of the pollutants and find a solution to the membrane surface pollution, EDS was used to analyze the membrane surface sediments, and according to the results of EDS on the membrane surface, a normalized mass percentage diagram of the contents of various elements in the scaling on the membrane surface was made (Fig. 9). According to the results of the measurement and analysis, the composition of the scaling elements on the membrane surface was consistent. The main elements of scaling on the membrane surface were C, O, Na, Mg, Al, Si, S, Cl, K, Ca, Ti and Fe. During operation, some equipment components might produce rust, which makes the EDS results contain more Fe elements. Although the composition of the elements at the inlet and outlet of the NF membranes was different,
Fig. 8. Scanning electron microscopic images of surface fouling of the NF membrane for standard addition raw water: a entrance enlarged 500 times; b entrance enlarged 2000 times; c entrance enlarged 8000 times; d exit enlarged 500 times; e exit enlarged 2000 times; and f exit enlarged 8000 times. 7
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and the combined process can provide a good operating environment for the NF process. Acknowledgements This work was financially supported by the Natural Science Foundation of Shandong Province (ZR2016EEM32), the Science and Technology Plans of the Ministry of Housing and Urban-Rural Development of the People's Republic of China, and the Opening Projects of the Beijing Advanced Innovation Center for Future Urban Design, Beijing University of Civil Engineering and Architecture (Research and application of key technologies for dissolved air flotation based on water purification of algae-contaminated lakes, UDC2017031612), the Science and Technology Development Plan of Weifang (2019ZJ1088), and supported by Science Foundation of Shandong Jianzhu University （Grant No. XNBS1824）and Shandong Key Research and Development Program (No. 2019GSF109064). Fig. 9. Normalized mass percentage of element contents in scaling on the membrane surface.
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contents of the fouling component was less at the inlet. The mass percentages of Si and Ca in the surface scaling of the NF membrane treating the high-hardness water were much higher than those in the surface scaling of the NF membrane treating the raw water. Due to the standard addition raw water is rich in hardness-related elements such as Mg, Ca, Si, etc., the pollution degree of the standard addition raw water is more serious than that of the raw water during the nanofiltration membrane treatment. Due to the long operation time and the influence of concentrated water reflux, the fouling on the membrane surface increases, and most of them are inorganic scaling rich in Si, CA, etc., resulting in the occurrence of O, Si, Ca and other elements in the effluent content higher than that in the influent. 4. Conclusions (1) The combined NF depth treatment process with the biological contact oxidation precipitation-UF process as the pretreatment process has a good effect on the treatment of the high-hardness micropolluted water of the Yellow River Reservoir, and the effluent quality is good, which meets the requirements of “Sanitary Standard for Drinking Water” (GB5749-2006). (2) The removal rates of turbidity, CODMn, total hardness and other conventional indicators by the combined process reached 95%, 85%, and 97%, respectively. The removal rates of tryptophan, tyrosine, SMPs, fulvic acid, humus-like substances and five other kinds of fluorescent organic compounds reached 95%, 87%, 88%, 94% and 93%, respectively. The average removal rates of THMFPs and AOX reached 80% and 51%, respectively. (3) The combined process mainly focuses on the NF membrane process to remove pollutants. The biological contact oxidation precipitation and UF process, which effectively protects the UF membrane and NF membrane, has certain removal effects on the turbidity and macromolecular organics. In the process of operation, the membrane flux and TMP of the UF and NF membranes decreased to a certain extent. Sodium hypochlorite (0.02%) alkaline washing of the UF membrane and pickling (20% HCl) of the NF membrane achieved good results, and the recovery rate of the TMP after membrane cleaning reached 70%, which basically repaired the membrane fouling of the UF and NF membranes. (4) The main elements of fouling on the surface of the NF membrane under long-term operation are C, O, Na, Mg, Al, Si, S, Cl, K, Ca, Ti and Fe, and the elements are mainly concentrated at the outlet of the membrane. The main components are monomers or compounds composed of inorganic salts in the raw water and some organic substances. However, overall, membrane fouling is relatively light, 8
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