Membrane Fouling Mechanism And Control In Treatment Of Pharmaceutical Wastewater By Integrated Membrane System And Impact Of Reagents On Membrane Structure And Performance

The antibiotic wastewater contains high organic content and toxic antibiotic residues. Most existing physical and chemical treatment methods are not effective in treating the wastewater, and the effluent is difficult in compliance with industrial emission standards. Integrated membrane systems could sufficiently remove the pollutants in the wastewater and enable the wastewater to be discharged into the receiving water or even be reused for industrial purposes. In this work, the key problems that membrane fouling mechanism and control during the process of treating antibiotic wastewater by integrated membrane system and impacts of adding reagents in the process on membranes'structure and performance were studied.For ultrafiltration-nanofiltration integrated membrane system,membrane fouling mechanism was explored by several advanced analytical methods. Various influencing factors of membrane fouling and cleaning methods in the process were investigated. The impacts of reagents used in the process on the performances of poly acrylonitrile (PAN) membrane and polysulfone (PS) membrane were studied, respectively. Typical reagents citric acid and sodium bisulfite were chosen to investigate in detail. The impacts of various factors including concentration of reagents, contacting ways with membranes, contacting time, contacting temperature etc. on the membranes'structures and performances were investigated.The water quality was analyzed by Inductively Coupled Plasma Atomic Emission Spectrometer (ICP) and Ion Chromatography (IC). Fouled membrane samples under different conditions were analyzed by Energy Dispersive X-Ray (EDX), Scanning Electron Microscope (SEM), and Attenuated Total Reflection Spectra-Fourier Transform Infrared Spectrometry (ATR-FTIR). It can be deduced from the analytical results that the membrane fouling in the process is caused by adsorption and deposition of inorganic compounds of calcium and iron, and complex organic compounds which may contain phenolic, etheric and carboxylic functional groups etc. on membrane surface and inside membrane pores. In the nanofiltration process, with increasing COD and conductivity of feed water, the membrane fouling becomes more severe. Under the conditions of lower applied pressure, higher crossflow velocity and proper temperature, the membrane fouling can be efficiently suppressed. In addition, the decline of nanofiltration membrane permeate flux becomes slight after adding citric acid (0.03wt%), HCl (pH=3) or ethylene diamine tetraacetic acid diasodium salt (EDTA, 0.5mM) to feed water, and adding EDTA reaches the best result. The fouled membrane was cleaned with pure water for 100 minutes under conditions of TMP 0.2MPa, crossflow velocity 23.6cm·s-1 and temperature 35.0℃. The flux recovery is 98.2%. The severe fouled membrane was cleaned by immerging it in pure water or washing it with different chemicals under room temperature. The results indicate that immerging severe fouled membrane in pure water for 10~40 hours can recover membrane permeate flux. Chemical cleaning with NaOH (pH=11), HCl (pH=3), citric acid (0.3wt%) and EDTA (1.0mM) were carried out, respectively. The results show that the cleaning efficiency increases in the sequence of solutions mentioned above and it is the highest with EDTA which costs only 10 minutes to recover membrane permeate flux completely. The membrane surfaces were analyzed by SEM and EDX after chemical cleaning. The SEM photos show that the smooth and cleanly degree of membrane surface increases in the above sequence and the SEM photo after cleaning with EDTA is close to that of unfouled membrane. EDX analytical results indicate that the containing percent of element Ca on the membrane surfaces decreases in the above sequence and it is null after cleaning with EDTA.The impacts of citric acid and sodium bisulfite on PAN membrane and PS membrane under various operational conditions were systematically investigated. The results show that citric acid has minor influence on membranes'performance when it contacts membranes'surface only. And when citric acid contacts membranes'entirety (including surface and pores) sufficiently, the membranes'pure water permeate flux decreases. After citric acid contacting membranes'entirety for a certain time, the decline of membranes'permeate flux decreases during the process of filtering Poly(ethylene oxide) (PEO) solution, and membranes'rejection of PEO increases. The membranes'antifouling performance is improved. When citric acid contacts membranes'entirety, with increasing concentration of citric acid, the decreasing percent of membranes'permeate flux increases and the contact angle increases first and then decreases. With increasing contacting time or contacting temperature, the decreasing percent of membranes'permeate flux increases first and then becomes stable. With increasing crossflow velocity or decreasing applied pressure, the impact of citric acid on membranes'performance decreases, i.e., and the decreasing percent of membranes'permeate flux decreases. In this work, the permeate flux decreasing percents of PAN and PS membranes after contacting citric acid sufficiently are 15.44%~28.30% and 11.38~28.94%, respectively. As far as sodium bisulfite is concerned, when it contacts membranes'surface only, the membranes'pure water permeate flux increases, and when it contacts membranes'entirety sufficiently, the membranes'pure water permeate flux increases too, but the increasing percent decreases. After sodium bisulfite contacting membranes'entirety for a certain time, the decline of membranes'permeate flux decreases during the process of filtering PEO solution, and membranes'rejection of PEO has no obvious changes. The membranes'antifouling performance is improved. When sodium bisulfite contacts membranes'entirety, with increasing concentration of sodium bisulfite, the increasing percent of membrane permeate flux increases for PAN membrane, but decreases for PS membrane, and the contact angle decreases for two kinds of membranes. With increasing contacting time, the increasing percent of membranes'permeate flux increases first and then becomes stable. With increasing contacting temperature, the increasing percent of membranes'permeate flux decreases first and then becomes stable. With increasing crossflow velocity, the impact of sodium bisulfite on membranes'performance decreases, i.e., the increasing percent of membranes'permeate flux decreases. With increasing applied pressure, the impact of sodium bisulfite on PAN membrane performance increases, i.e., and the increasing percent of membrane permeate flux increases, while, the influence of sodium bisulfite on PS membrane performance decreases, i.e., the increasing percent of membrane permeate flux decreases. In this work, the permeate flux increasing percents of PAN and PS membranes after contacting sodium bisulfite sufficiently are -5.28%~5.22% and 26.49%~45.24%, respectively.The membranes'surfaces after adsorbing reagents were inspected by SEM and ATR-FTIR, and their contact angles were measured. According to the results and the chemical structures of citric acid, sodium bisulfite, PAN membrane and PS membrane, it can be concluded that the mechanisms for impacts of citric acid and sodium bisulfite on membranes'performance are different. When the citric acid adsorbs on membranes'surface, carbon hydrogen functional groups are outward to solution, which makes membranes difficultly be wetted, and thus the membranes'permeate flux declines. However, when the sodium bisulfite adsorbs on membranes'surface, the functional groups such as SO32- and HSO3- are on exterior of surface, which helps improve the wetting property of membranes, and thus the membranes'permeate flux increases. With comparisons of the impacts of citric acid and sodium bisulfite on PAN and PS membranes, it can be concluded that the differences of impacts of the reagents on the membranes are not only caused by the hydrophilic/hydrophobic property of the membranes, but also by the chemical characters of the membranes in the reagents.This work provides a good example for the membrane technique applying in advanced treatments of industrial wastewater containing complicated compounds. The understanding of impacts of reagents on membranes'structure and performance provides a good basis for further study.