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城市污水处理工程毕业设计（含图纸及说明）,城市,污水处理,工程,毕业设计,图纸,说明
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Impact of Membrane Surface Modificationon the Treatment of Surface WaterDaniella B. Mosqueda-Jimenez1; Roberto M. Narbaitz2; and Takeshi Matsuura3IntroductionThe removal of natural organic matter (NOM) from water is a critical issue since chemical disinfection of these compounds produces a number of potentially harmful disinfection byproducts (Rook 1977). Pilot studies and full-scale operation (Taylor et al. 1987; Thorsen 1999) have demonstrated that nanofiltration membranes may achieve NOM removals greater than 90%. The possibility of replacing them by ultrafiltration (UF) membranes seems very attractive because UF membranes are likely to require less pretreatment, to have higher water production rates and lower operating pressures. However, published results of pilot studies with UF membranes have generally shown NOM removals of less than 50%, when they are not combined with some kind of pretreatment (Taylor et al. 1987; Jacangelo et al. 1994).Another obstacle that hinders more widespread application of low-pressure membrane processes is the decline of the permeateflux with time, generally known as membrane fouling (Song 1998). The term fouling includes all the phenomena responsible for permeate flux reduction with time, except those linked to membrane compaction and mechanical characteristics modification (Anselme and Jacobs 1996). In several studies of drinking water treatment, NOM has been reported to be a major membrane foulant (Braghetta et al. 1997; Cho et al. 2000). Fouling increases the costs and complexity of membrane filtration operations (Song 1998) and may also affect the quality of the water that is produced from the membrane system (Tracey 1996). There are several approaches used to reduce membrane fouling, for example: Feed pretreatment, or adjustment of the operating conditions, especially cross-flow velocity, together with pressure and temperature (Cheryan 1986). It is well documented that fouling depends on the affinity between the solutes and the polymer that constitutes the membrane (Cheryan 1986; Zeman and Zydney 1996). Then, another way to reduce fouling may be by modifying the surface of the membrane. According to Zeman and Zydney (1996), almost 50% of all microfiltration and ultrafiltration membranes marketed by 1996 were surface modified. The additives used and procedures followed in commercial membrane manufacture are, however,industrial secrets.Surface modification employing tailor-made surface modifying macromolecules (SMMs) is an approach that has been used during the last decade in an attempt to improve polyethersulfone (PES) membranes for environmental applications. The surface characteristics of the membranes are altered by the addition of these SMMs. The SMM additives were specially developed to be hydrophobic (via their fluorinated groups) and to be compatible with PES so that they can be simply blended into the polymer casting solution (Pham et al. 1999). The SMMs migrate to the membrane surface during a single-step casting process and the surface of the membrane becomes more hydrophobic while keeping its bulk properties essentially the same as that of an unmodified membrane. The surface-modified membranes have demonstrated to have a somewhat better performance in the separation of volatile organic compounds (such as chloroform) from aqueous solutions via pervaporation (Pham et al. 1999); UF of oil/water emulsions (Hamza et al. 1997), and biomedical UF and microfiltration applications (Ho et al. 2000). One of the main advantages of this new technology is that the PES/SMM membranes have superior mechanical strength compared to PES membranes (Suket al. 2002). Because of the characteristics associated with the fluorinated-based compounds used in the SMM synthesis, such as surface lubrication and low surface free energy, the PES/SMM membranes are expected to exhibit low adhesive energy and consequently have more fouling resistant characteristics. DiGiano etal. (2001) followed a similar approach in the quest for better membrane materials. Several studies (Cabasso et al. 1976; Lafrenire et al. 1987; Torrestiana-Sanchez et al. 1999, among others) have demonstrated that the use of polymeric additives, such as polyvinylpyrrolidone (PVP) and polyethylene glycol (PEG), improve the performance of PES membranes by increasing the productivity.The aim of this work was to investigate the effects of the surface modification via the addition of SMM to the casting solution (with and without PVP), and the nature of the feed water on the performance of UF membranes and the extent of fouling. Membrane performance in the treatment of a surface water was evaluated in terms of total organic carbon (TOC) removal, the amount of foulants deposited on top of the membrane after filtration, the flux reduction, and the permeate flux at the end of the test.Experimental Methods and AnalysisMembrane PreparationThe membranes were cast using the phase inversion technique described by Matsuura (1994). Four different ingredients were used to prepare the casting solution: PES, PVP, SMM41, and NMP. The concentration of PES in the casting solution was kept constant for all membranes at 18 wt %. PVP was used in three different concentrations 0, 6, and 18 wt %. The PVP/PES ratios in the casting solution were, therefore, 0, 1:3, and 1:1, respectively. For the modified membranes, 1.5 wt % of SMM41 was added to the casting solution. All concentrations are weight-based percentages. Filtered homogeneous solutions were degassed and cast onto a glass plate at room temperature (wet thickness=0.25 mm). Then, the plate was immersed into a gelation bath at 4C where membranes eventually peeled off from the plate. Membraneswere cut from flat sheets into 20 cm2 coupons and stored at 5C in Milli-Q water until they were tested.AnalysisThe general physicochemical characteristics of the test waters were determined using standard methods (APHA 1995). TOC concentrations were measured using a Dohrman Total Carbon Analyzer (Model DC-180, Santa Clara, CA) and UV absorbances were measured at 254 nm using a Beckman DU-40 Spectrophotometer (Beckman Instruments Inc., Irvine, CA) with a 1 cm quartz cell. The apparent molecular weight distribution (AMWD) of the organic matter within the ORW was determined by UF fractionation using a 50 mL Amicon batch stirred UF cell (model 8050, Amicon Corp., Danvers, MA), and regenerated cellulose membranes (Millipore, Bedford MA) with four different nominal molecular weight cutoffs (MWCO): 5,000, 10,000, 30,000, and 300,000 Daltons. The UF fractionation technique followed is detailed by different authors: Amy et al. (1987), Logan and Jiang (1990), among others. A model JSM-6400 JEOL (Japan Electron Optics Limited, Japan) scanning electron microscope (SEM) was used to observe the cross-sectional area of the membranes. An atomic force microscope (AFM) (Nanoscope III from Digital Instruments) operated in the tapping mode was used to get the topological information of the membrane surface measuring roughness. The chemical analysis of the membrane surface was done by using a Kratos Axis X-ray photoelectron spectrometer (Kratos, Manchester, UK).Testing ProtocolThe performance evaluation of the membranes was made using unfractionated CORW, HMW, and LMW. The UF membranes were placed in random order in a six-cell in-series system. This apparatus was selected because it permitted the simultaneous evaluation of six coupons with essentially the same results as those of a SEPA CF membrane cell by Osmonics, recommended by the U.S. Environmental Protection Agency (MosquedaJimenez et al. 2004). The system recirculated the permeate back to the feed reservoir to maintain a constant feed concentration. The existence of a possible cell effect or cell order effect within the cells in-series was discarded using an analysis of variance (ANOVA) test of a greaco-latin square type of design, because statistically no significant effect was observed. Prior to each run, the membranes were subjected to a precompaction protocol filtering Milli-Q water through each new coupon at 620 kPa s90 psigd for 52 h. Next, the pure water permeation (PWP) rate was determined during a period of 50 h under a 345 kPa s50 psigd operating pressure using Milli-Q water. This was considered the second stage of the test. Then, the UF membranes were characterized based on solute transport data of probe solutes (PEG and polyethylene oxides with different molecular weights) (Singh 1998). After the completion of membrane characterization, the membranes were ready for the fourth phase of the test, which involved filtration of CORW or one of its fractions, depending on the type of experiment. The permeation rates of CORW and its fractions were measured at a feed flow rate of 1.1 Lpm and an operating pressure of 345 kPa s50 psigd during six days of continuous operation. The permeation rates were measured at different intervals and feed and permeate samples were collected to assess the TOC removal. The amount of foulants accumulated on top of the individual membranes at the end of the six-day test (referred to as foulant deposition) was measured using the technique described by Hong and Elimelech (1997). Beakers were preconditioned in the oven at 105C overnight. After the temperature was lowered to room temperature in a desiccator, the beakers were weighed. Membrane coupons were removed very carefully from the test cells and placed in the beakers. Then, the layer of foulants was dissolved using 25 mL of0.1 M NaOH solution within the preconditioned beakers. After the foulants were completely removed, the membrane coupons were withdrawn and all the beakers were baked in an aircirculated oven at 105C for 24 h, together with at least two blanks (consisting of just 25 mL of NaOH solution). Afterwards, the beakers were allowed to reach room temperature inside of a desiccator and then were reweighed. The mass of foulants was calculated by the difference between the dry weight of the foulants-NaOH solution and the average dry weight of the blanks.Experimental DesignA multifactor design 33233 was used to evaluate the effect of the concentration of PVP (0, 6, or 18%) and SMM in the casting solution (SMM41 or No SMM), together with the fraction of the NOM used as test water (Unfractionated CORW, LMW, or HMW). This design allowed the study of the interplay of the independent variables in each of the response variables with less experimental runs than a one-factor-at-a-time experiment would require (Devore and Farnum 1999). The response variables, which determine the performance characteristics of every membrane tested, were the level of TOC removal achieved by the membrane, the amount of foulants deposited on the surface of the membrane, the normalized flux reduction, and the permeate flux at the end of the six-day test. It is worth noting that the “repeats” in the multifactor design, which allowed us to measure the experimental error, correspond to a complete replicate of the experimental conditions of each test (Devore and Farnum 1999). This means that each coupon tested came from a different membrane sheet and it was randomly located in one of the six test cells. Statistical calculations were performed using STATGRAPHICS Plus version 7.0 (Statistical Graphics Corporation, Manugistics, Inc., Rockville MD).Results and DiscussionFeed WaterThe water quality characteristics of the three different feed waters used in this study are presented in Table 1. According to the AMWD of the organic matter within ORW as determined by Mosqueda-Jimenez et al. (2004) using UF fractionation, close to 30% of the NOM was smaller than 5,000 Daltons, almost 60% of TOC of the NOMs was in the molecular weight range from 10,000 to 30,000 Daltons, while less than 10% was higher than 30,000 Daltons (Fig. 1). For the LMW and HMW fractions, TOC concentration decreased and increased by nearly 50%, respectively, compared with the value of CORW. Because the MWCO of this membrane is 3,000 Dalton, the size of the NOM in the LMW should be less than 3,000 Daltons, and the size of most of the NOM in the HMW should be greater than 3,000 Daltons. The specific UV absorbance is commonly considered a measure of the relative aromatic content. Therefore, LMW had less aromatics than the CORW and HMW. The color measurements demonstratedthat the brownish color of the water is associated with the larger NOM molecules. In a previous study (Mosqueda-Jimenez et al. 2004), it was found that the concentration process performed to obtain CORW not only modified the molecular weight distribution of the organic matter, but also increased the concentration of mono and divalent ions in the concentrate. On the other hand, the YM3 membrane used in the NOM fractionation was a tight UF membrane, with almost no rejection of inorganic ions, thus, the alkalinity and hardness changed only very slightly.Membrane CharacteristicsMembrane characteristics are presented in Table 2. Numbers in parenthesis represent the range of variation of these parameters. It is believed that the wide intervals were due to the inherent variability of membrane manual casting (Hester and Mayes 2002). The ANOVA performed on the data in Table 2 indicated that the PVP/PES ratio in the casting solution had a significant effect (P value a, as explained later) on the molecular weight cutoff and pure water permeation rate. The only significant difference was between the molecular weight cutoff of the membranes without PVP and that of the membranes with 1:1 PVP/PES ratio. The pure water permeation rate of the membrane without PVP was statistically smaller than the pure water permeation rate of the membranes with PVP. Other researchers have observed the same type of behavior, using different polymer concentrations (Lafrenire et al. 1987; Torrestiana-Sanchez et al. 1999). In the case of the solute separation, the test solute used was PEG with molecular weight of 35 kDaltons. The average PEG separation was higher for the membranes without PVP (96%), which is consistent with the MWCO information. The standard deviations of separations of membranes with PVP were very high. Therefore, statistically neither the PVP/PES nor the use of SMM made a difference in the solute separation. SMM migration was corroborated through measurements of the amount of fluorine at different depths (d1 and d2, where d1,d2). In the three surface-modified membranes, the closer to the surface, the greater the amount of fluorine. The fluorined1 / fluorined2 ratio was 1.45, 1.43, and 1.14 for modified membranes with 0, 6, and 18% PVP, respectively. Thus, the migration rate was inversely
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