An ultrafiltration (UF) membrane process was employed to treat the secondary effluent discharged from a manufacturing of thin film transistor-liquid crystal display (TFT-LCD) in this study. A bench-scale system was performed to evaluate the fouling removal of a UF membrane with coated titanium dioxide (TiO2) nanoparticles under UV irradiation. The operating pressure and feed temperature were controlled at 300 KN/m2 and 25°C, respectively. It was found that the optimum operating conditions were attained with TiO2 concentrations of 10 wt% for both 5 KD and 10 KD MWCO. Continuous UV irradiation of 5 KD MWCO improved the permeate flux rate from 45.0% to 59.5% after 4 hours of operation. SEM-EDS analysis also showed that the photocatalytic effect had reduced the average thickness of cake fouling on the membrane from 6.40
Plants in Taiwan are usually required to increase the recovery rate for their effluents, but the effluents usually contain many materials that need to be removed further, including suspended solids, colloid matter, and other trace elements; otherwise, the water quality cannot meet the requirements for reuse. In order to improve the quality, the upgrading method is needed. Membrane technology offers the greatest potential due to its relatively high removal rate, ease of setup, and relatively small requirements. However, membrane treatments have operating problems such as concentration polarization and membrane fouling [
Membrane fouling cannot be easily removed; however, there are many methods that prevent membrane fouling, such as the pretreatment of feed water and the cleaning of membranes. Pretreatment methods include sand filtering, coagulation followed by sand filtering, activated carbon adsorption, and dosing with oxidants or antifouling agents [
UV/TiO2 can generate various free radicals, such as hydrated electron (Eaq−), hydroxyl radical (
As described by the previous works, most processes could increase the permeate flux and even attain the self-cleaning effect. However, whether it could be applied to real wastewater is questionable and worthy of further study. This study applied an UF membrane coated with TiO2 nanoparticles under UV irradiation to pretreat the secondary effluent from the wastewater of a TFT-LCD manufacturing plant and investigated the removal of fouling from ultrafiltration membranes in order to improve the recovery rate. We evaluated the affection of the three parameters, namely, concentration of TiO2 nanoparticles, molecular weight cutoff (MWCO), and UV irradiation time. At the end of the experiments, the characteristic changes of dry membrane surface were observed.
In the experiments in this study, we randomly sampled the secondary effluent of the biological treatment system from a TFT-LCD manufacturing plant in Chunan Science Park in Taiwan. The influent flow to the biological treatment system contained little or no toxic organic waste, coming primarily from the production line and the sewerage wastewater. Before entering the biological treatment system, the fluoride-containing wastewater had been treated by chemical precipitation with CaCl2 and coagulation sedimentation. Table
Water quality of secondary effluent.
pH | 6.74~7.28 |
Conductivity (mS/cm) | 2.47~2.97 |
Turbidity (NTU) | 1.57~2.02 |
TOC (mg/L) | 2.16~2.58 |
Figure
Diagram of the UF and UV/TiO2 system.
Prior to the experiments, the new membrane was soaked in pure water overnight and then pretreated with pure water to achieve a more stable permeate flux. The water pressure was fixed at 300 KN/m2, the water temperature was maintained at 25°C, and the velocity of the crossflow was about 0.20 m/s. The permeate flow was directly monitored by an electric balance. The system was kept running under these operating conditions for at least 9 hours. During the pretreatment, a more stable initial permeate flux
The test membranes were coated with TiO2 of different concentrations. After the previous 9-hour pure water feed experiment, distilled water was then continuously pumped into the system. The water pressure during the experiment was still fixed at 300 KN/m2, the water temperature was also maintained at 25°C, the velocity of the crossflow was 0.20 m/s, and the time for UV radiation was 4 hours. At the end of the experiment, microscopic observations of the dried membrane were made by using Fourier-transform infrared spectroscopy (FTIR-ATR, Thermo Nicolet Nexus-470), Contact Angle (Kruss DSA10), Atomic Force Microscopy (Digit Ins NanoScope SPM), and Scanning Electron Microscopy and Energy Dispersive Spectroscopy (SEM-EDS, HITACHI S-800), respectively. Contact Angle was used to observe the hydrophilicity of the membrane surface, and Atomic Force Microscopy was used to measure the membrane surface roughness. FTIR-ATR and SEM-EDS were used to observe the appearance and the chemical composition of the membrane surface.
After the previous 9-hour pure water feed experiment, the secondary effluent was pumped into the system continuously. The water pressure during the experiments, the water temperature, the velocity of the crossflow, and the time for UV radiation were controlled as same as in the membrane property test. The permeate flow was also monitored directly by an electric balance and then stored in the permeate tank. The reject flow was recycled into the feed water tank. The experiment continued until the permeate flux
Additionally, in order to study the effects of different UV irradiation types on permeate flux, the experiment contained three different processes for the UF membranes (5 KD, 10% TiO2). In the first process, prior to ultrafiltration, the membrane had been irradiated about 30 minutes. In the second process, the membrane was irradiated continuously after one hour of ultrafiltration. The third process was with continuous UV irradiation and ultrafiltration.
The quality of feed water and permeate was measured using Standard Methods for the Examination of Water and Wastewater before and after changing the operational mode, including the analysis of pH, conductivity, turbidity, and TOC measurements. Laser Particle Size Analyzer (ASYS HIAC ROYCO 8000A) was used to measure the size distribution of particles in the water.
Table
Contact angle between deion water drop and the surface of the PES membrane.
TiO2 concentration (%) | Contact angle (°) | |
---|---|---|
0 | 61 | |
Without UV radiation | 5 | 56 |
10 | 54 | |
| ||
0 | 61 | |
With UV radiation | 5 | 47 |
10 | 44 |
AFM analysis for different wt% of TiO2 on the PES membrane.
0 wt% TiO2
2 wt% TiO2
10 wt% TiO2
Figure
Effects of different UV irradiation types on permeate flux (5 KD, 10% TiO2).
Figure
Effects of different TiO2 concentration UV radiation on permeate flux during continuous UV radiation (5 KD).
Figure
Effects of different TiO2 concentration UV radiation on permeate flux during continuous UV radiation (10 KD).
Size distribution of TFT-LCD effluent.
Figures
Surface of the UF membrane after ultrafiltration (5 KD, magnified 10000x), (a) without TiO2 and (b) with 10 wt% TiO2.
Surface of the UF membrane after ultrafiltration (10 KD, magnified 10000x), (a) with 0 wt% TiO2 and (b) with 10 wt% TiO2.
Profile of the UF membrane after ultrafiltration (5 KD, magnified 1200x), (a) with 0 wt% TiO2 and (b) with 10 wt% TiO2.
Profile of the UF membrane after ultrafiltration (10 KD, magnified 1200x), (a) with 0 wt% TiO2 and (b) with 10 wt% TiO2.
This study used FTIR-ATR analysis to observe the chemical adsorption of TiO2 on the membrane surface. The characteristic peak for new membrane was observed in the range of 3400–3600 cm−1, as shown in Figure
FTIR-ATR analysis for observing chemical adsorption of TiO2 on the membrane surface: (a) new membrane without TiO2 coating, (b) before ultrafiltration with TiO2 coating, and (c) after ultrafiltration with TiO2 coating.
Figures
Permeate quality when treated with different TiO2 concentrations (5 KD).
Permeate quality when treated with different TiO2 concentrations (10 KD).
The UV/TiO2 photocatalysis primarily destroyed the organic matter screened on the membrane surface, turning it into dissolved organic matter by
This study was carried out within three parameters, namely, TiO2 concentration, MWCO, and UV irradiation time. Among these parameters, MWCO affected the permeate flux mostly, because the size distribution of feed water particles was close to the pore size of the membrane, allowing it to pass through the pore of membrane and deposit onto the membrane pore, thereby reducing the permeate flux. Therefore, when selecting the MWCO of membrane, it is necessary to analyze the size distribution of feed water.
Photocatalysis increased the hydrophilicity and the UV radiation area of the membrane surface, leading to the increase in permeation flux. The UV/TiO2 photocatalysis also destroyed organic matter of the cake deposit into small organic matter by
Although discontinuous photocatalysis can increase membrane hydrophilicity, self-cleaning effect is not good for discontinuous UV irradiation in this study. Therefore, continuous UV irradiation is necessary to destroy the organic fouling on the membrane for self-cleaning effect.
The authors would like to thank the National Science Council of Taiwan, for financially supporting this research under Contract no. NSC NSC 97-2622-E-239-009-CC2.