Resistance of Polypropylene Membrane to Oil Fouling during Membrane Distillation

Preparation, characterization, and applications of kaolin/kaolin-based composite membranes in oily wastewater treatment: Recent developments, challenges, and opportunities

Rapid industrialization and the accompanying generation of significant amounts of oily wastewater pose major environmental challenges, which necessitate efficient treatment technologies. Kaolin-based membranes have emerged as a promising option due to their availability, affordability, and effective filtration performance. This review comprehensively analyzes the potential of kaolin for the treatment of oily wastewater. The focus is on the main manufacturing methods such as uniaxial pressing, slip casting, tape casting, phase inversion, and extrusion, as well as modifications that improve the structural integrity of the membrane, and its surface wettability to enhance separation performance. In addition, characterization techniques to evaluate membrane properties such as mechanical strength, thermal and chemical stability, antifouling behavior etc. are discussed. Basically, kaolin can be used as a supporting substrate, as a primary separation layer or as an additive in non-kaolin membranes. In evaluating the oil-water separation performance of kaolin-based membranes, this review highlights important metrics such as oil separation rate, flux, flux recovery, antifouling behavior, and resistance to harsh physical and chemical environments. Articles selected for the review were retrieved from major databases such as EBSCOhost, Scopus, ScienceDirect, Web of Science, and Google Scholar using relevant keywords. Based on the data from these studies, the main advances, challenges, and prospects are highlighted. Although still in the early stages of commercial application, kaolin membranes show significant potential to improve filtration efficiency, mechanical and chemical resistance, and reduce operating costs in industrial oily wastewater treatment systems.

Long-Term Performance of Ultrafiltration Membranes: Corrosion Fouling Aspect

The past decade has seen a rise in the importance of the ultrafiltration (UF) technique in the separation of various complex solutions. However, the fouling phenomenon is the main limitation to faster process development. To the best of the authors' knowledge, the present paper is the first to aim to identify the role of corrosion fouling in long-term UF. For this purpose, polyvinylidene fluoride (PVDF) and polyethersulfone (PES) membranes were used. The investigations were carried out with the use of both pilot-scale and laboratory-scale units. Results obtained in the present study have clearly demonstrated that the oil concentration has a significant impact on the process performance. Indeed, it has been noted that a reduction in oil concentration from 160 to 100 mg/L resulted in an increase in the PVDF membrane flux from 57 to 77 L/m²h. In addition, it has been shown that the feed temperature has a significant influence on the UF performance. Importantly, it has been shown that corrosion fouling is of vital importance in UF membranes. Indeed, corrosion products such as iron oxides contaminated the membrane surface leading to an irreversible decrease in the UF process performance. In addition, it has been found that repeating the chemical cleaning of the membrane units significantly reduced the intensity of the fouling phenomenon. However, the complete elimination of its effects was not achieved. Therefore, it has been indicated that cleaning agents recommended by membrane manufacturers do not remove corrosion products deposited on the membrane surface. Undoubtedly, the obtained results can be used in the design of UF units leading to the extension of membrane installation lifetime.

Janus Distillation Membrane via Mussel-Inspired Inkjet Printing Modification for Anti-Oil Fouling Membrane Distillation

In this work, inkjet printing technology was used to print a thin layer of a hydrophilic solution containing polydopamine as a binder and polyethyleneimine as a strong hydrophilic agent on a commercial hydrophobic membrane to produce a Janus membrane for membrane distillation. The pristine and modified membranes were tested in a direct-contact membrane distillation system with mineral oil-containing feedwater. The results revealed that an integrated and homogenous hydrophilic layer was printed on the membrane with small intrusions in the pores. The membrane, which contained three layers of inkjet-printed hydrophilic layers, showed a high underwater oil contact angle and a low in-air water contact angle. One-layer inkjet printing was not robust enough, but the triple-layer coated modified membrane maintained its anti-oil fouling performance even for a feed solution containing 70 g/L NaCl and 0.01 v/v% mineral oil concentration with a flux of around 20 L/m²h. This study implies the high potential of the inkjet printing technique as a facile surface modification strategy to improve membrane performance.

Ag-CuO-Decorated Ceramic Membranes for Effective Treatment of Oily Wastewater

Although ultrafiltration is a reliable method for separating oily wastewater, the process is limited by problems of low flux and membrane fouling. In this study, for the first time, commercial TiO₂/ZrO₂ ceramic membranes modified with silver-functionalized copper oxide (Ag-CuO) nanoparticles are reported for the improved separation performance of emulsified oil. Ag-CuO nanoparticles were synthesized via hydrothermal technique and dip-coated onto commercial membranes at varying concentrations (0.1, 0.5, and 1.0 wt.%). The prepared membranes were further examined to understand the improvements in oil-water separation due to Ag-CuO coating. All modified ceramic membranes exhibited higher hydrophilicity and decreased porosity. Additionally, the permeate flux, oil rejection, and antifouling performance of the Ag-CuO-coated membranes were more significantly improved than the pristine commercial membrane. The 0.5 wt.% modified membrane exhibited a 30% higher water flux (303.63 L m^-2 h^-1) and better oil rejection efficiency (97.8%) for oil/water separation among the modified membranes. After several separation cycles, the 0.5 wt.% Ag-CuO-modified membranes showed a constant permeate flux with an excellent oil rejection of >95% compared with the unmodified membrane. Moreover, the corrosion resistance of the coated membrane against acid, alkali, actual seawater, and oily wastewater was remarkable. Thus, the Ag-CuO-modified ceramic membranes are promising for oil separation applications due to their high flux, enhanced oil rejection, better antifouling characteristics, and good stability.

Wettability Studies of Capillary PTFE Membranes Applied for Membrane Distillation

In the present study, the membrane distillation (MD) process was studied with the use of commercial polytetrafluoroethylene (PTFE) capillary membranes. For this purpose, both solutions with NaCl concentrations up to 300 g/L and brines contaminated with oil (70-120 mg/L) were used as feeds. The membrane's wetting resistance was tested by conducting long-term experiments for over 3500 h. Using detailed studies, it has been shown that increasing the salt concentration from 35 to 300 g/L resulted in a 50% reduction in the permeate flux. Nevertheless, the membranes retained 100% of the salt rejection after 2000 h of the module's operation. Moreover, it has been found that performing the MD process with brines contaminated with oil (120 mg/L) led to the wetting of some membranes pores, which it turn resulted in an increase in the distillate's conductivity to 450 µS/cm after 700 h running the process. The mechanism of pore wetting by oil droplets adsorbed on the membrane's surface was presented. Finally, the proposed method of membrane cleaning with the use of a solvent allowed restoring the initial MD module's performance. Consequently, both the permeate flux and distillate conductivity were stable during the MD of the feed containing 35 g/L of NaCl over the next 280 h.

Membrane Distillation Crystallizer Applied for Separation of NaCl Solutions Contaminated with Oil

In the present study, the membrane crystallizer was used to separate a saturated NaCl solution contaminated with an oil emulsion. The crystallizer was connected via a mesh separator with a feed tank in which capillary submerged modules were assembled. The effect of scaling and oil sorption on the wetting of polypropylene (PP) membranes has been investigated during the long-term studies. It has been found that cooling the solution in the crystallizer by 15 K below the feed temperature resulted in intensive NaCl crystallization in the zone below the mesh separator. A result, the salt crystallization on the membrane surface was eliminated. Contamination of saturated brines with oil in the concentration exceeding 100 mg/L caused the oil penetration into the membrane pores. The application of a PP net assembled on the capillary membranes surface reduced the intensity of wetting phenomenon caused by scaling and the oil sorption, which provides a stable membrane module performance during 1300 h test.

The Impact of Operational Parameters on Polypropylene Membrane Performance during the Separation of Oily Saline Wastewaters by the Membrane Distillation Process

In the present study, membrane distillation (MD) was applied for the treatment of oily saline wastewaters produced on ships sailing the Baltic Sea. For comparison purposes, experiments were also carried out with model NaCl solutions, the Baltic Seawater and oil in water emulsions. The commercial Accurel PP V8/2 membranes (Membrana GmbH, Germany) were used. In order to investigate the impact of the operational parameters on the process performance, the experiments were conducted under various values of the feed flow velocity (from 0.03 to 0.12 m/s) and the feed temperature (from 323 to 343 K). The obtained results highlight the potential of PP membranes application for a stable and reliable long-term treatment of oily wastewater. It was demonstrated that the permeate flux increased significantly with increasing feed temperature. However, the lower temperature ensured the limited scaling phenomenon during the treatment of oily wastewaters. Likewise, increasing the feed flow velocity was beneficial to the increase in the flux. Moreover, it was found that performing a cyclic rinsing of the module with a 3% HCl solution is an effective method to maintain a satisfactory module performance. The present study sheds light on improving the MD for the treatment of oily wastewaters.

Membrane Distillation of Saline Water Contaminated with Oil and Surfactants

Application of the membrane distillation (MD) process for the treatment of high-salinity solutions contaminated with oil and surfactants represents an interesting area of research. Therefore, the aim of this study is to investigate the effect of low-concentration surfactants in oil-contaminated high-salinity solutions on the MD process efficiency. For this purpose, hydrophobic capillary polypropylene (PP) membranes were tested during the long-term MD studies. Baltic Sea water and concentrated NaCl solutions were used as a feed. The feed water was contaminated with oil collected from bilge water and sodium dodecyl sulphate (SDS). It has been demonstrated that PP membranes were non-wetted during the separation of pure NaCl solutions over 960 h of the module exploitation. The presence of oil (100–150 mg/L) in concentrated NaCl solutions caused the adsorption of oil on the membranes surface and a decrease in the permeate flux of 30%. In turn, the presence of SDS (1.5–2.5 mg/L) in the oil-contaminated high-salinity solutions slightly accelerated the phenomenon of membrane wetting. The partial pores’ wetting accelerated the internal scaling and affected degradation of the membrane’s structure. Undoubtedly, the results obtained in the present study may have important implications for understanding the effect of low-concentration SDS on MD process efficiency.