1. Introduction
Oily wastewater is produced as a byproduct of various industries such as petroleum refineries. Ceramic membranes have potential in oil-in-water (O/W) emulsion separation applications compared to conventional methods such as electrocoagulation due to their excellent oil removal efficiency, good chemical, thermal, and mechanical stability, and simple operation[1,2]. However, their high fabrication cost due to the material cost and sintering temperature prevent widespread application of ceramic membranes in O/W separation [3,4]. Another disadvantage is the membrane fouling due to the cake formation and blocking of pores. There are different mechanisms to get rid of membrane fouling by lowering the deposition of particles inside the membrane pore. Some of them are Brownian diffusion and diffusion induced by shear motion etc.[5,6]. Compared to O/W separation by polymeric membranes, ceramic membranes have higher chemical, mechanical and most importantly cheaper cost due to the longer lifetime of the membrane. Conventional ceramic membranes are prepared with oxide materials such as Al2O3, SiO2, ZrO2, and TiO2[7]. Recently, natural materials such as fly ash and clay are being used in fabricating ceramic membranes which have a much lower cost than the conventional membrane[5,7-9]. Cost of the raw material are very cheap and they can be processed at suitable conditions to prepare ceramic membrane.
Ceramic membranes using low-cost natural materials such as kaolin and quartz were successfully fabricated with an average pore size of 55 μm and an oil rejection efficiency of 97.3%. Ceramic membranes also successfully separated biodiesel from waste fish oil. Composite ceramic-polymeric membranes had a maximum of 93% rejection in O/W emulsions with concentration under 250 mg/L[10-12]. This review on ceramic is divided into three parts such as silica, alumina and waste ash membrane which is presented in Fig. 1.
2. Ceramic Membrane
Ceramic membrane filtration is an effective oily wastewater treatment process for the oil-in-water (O/W) emulsions produced by various industries. To improve membrane performance, careful selection of filtration layer materials must be made, as they largely influence membrane properties such as permeance. Micropore-sized superhydrophobic surface metallic tubular membranes and hydrophilic nanoporous alumina coated membrane tubes were fabricated and tested for extraction of water-in-oil (W/O) and oil-in-water (O/W) emulsion[13] (Figs. 2 & 3).
The micropore-sized superhydrophobic membranes show 100% selectivity at high flux from the O/W mixtures. The nanopore-sized super hydrophilic membranes can extract up to 90% of the trace amount of dispersed aqueous phase from W/O emulsions.
Porous anorthite membranes were prepared using construction and demolition (C&D) waste with coal cinder and bauxite as additives at sintering temperatures ranging from 900-1300°C[14]. At 878°C, an exothermic reaction was recorded through thermal analysis during anorthite synthesis. While almost all samples showed high porosity (> 35%) at 900-1000°C, as sintering temperature increased, porosity decreased. The porosity of the samples dropped rapidly to almost zero around 1200°C, apart from P2 and P4. The pore structure in the samples expanded due to the sintering neck formed by calcium containing crystals continuing to grow. As sintering temperature increased, pore-channel smoothness increased, which led to the tortuous factor decreasing and permeance increasing from 0.93 m3/m2 h to 1.59 m3/m2 h. The hydroxylation of anorthite improved the hydrophilicity of the membrane after the alkali- solution, which led to increasing separation of an oil-in-water emulsion.
To investigate the effect of filtration layer surface properties on membrane fouling tendency in the treatment of oil-in-water (O/W) emulsion, five different metal oxides (TiO2, Fe2O3, MnO2, CuO, and CeO2) were deposited into ceramic ultrafiltration membranes [15](Figs. 4 & 5).
Four surface properties, surface hydroxyl groups in water, hydrophilicity, surface charge, and adhesion energy for oil droplets were tested. Surface hydroxyl groups showed distinct bond strength of O – D in MeO – D groups, with Fe2O3 showing the highest bond strength. The strength of the O – D bond indicates the electron density of the O – D bond is high. This results in high affinity for D2O molecules, which possibly leads to high hydrophilicity due to the strong hydrogen bond formed. Fe2O3 showed the highest hydrophilicity as hypothesized, and the differences in hydrophilicity affected the adhesion of the metal oxide toward oil droplets. Therefore, Fe2O3 is the most viable option for a filtration-layer material in ceramic membranes due to its low fouling tendency.
2.1. Silica
Silica sand-based hollow fiber ceramic membranes (SS-HFCMs) were prepared through phase inversion/ sintering techniques from low-cost and widely available silica sand[16]. The membranes were fabricated in four different sintering temperatures (1300, 1350, 1400, and 1450°C) to examine their effects on bending strength, porosity, pure water permeability, and oil-water separation performance. Mechanical strength generally increased as sintering temperature increased, from 78.5 to 86 to 120 MPa at 1300, 1350, and 1400°C. There was a reduction in pore size due to the absence of finger like morphology at 1450°C, and become denser. Porosity decreased as sintering temperature increased, from 17.23% at 1300°C to 8.90% at 1450°C. An increase in sintering temperature caused the densification of the membrane pores, which resulted in a decrease in pure water permeation flux and an increase in oily-wastewater rejection rates. Different ceramic content was also examined, with the samples with 57.5 wt% silica sand content showing the best mechanical strength (210 MPa) but a changed morphology (finger-like voids in the membrane disappearing). Samples with 55 wt% showed symmetrical finger-like voids. Silica sand-based hollow fiber ceramic membranes showed superior mechanical strength and oil rejection efficiency compared to previously reported work of membranes prepared from different materials.
Silica(SiO2) is one of the materials used to create ceramic membranes. Silica provides various advantages to membrane fabrication such as good acidic chemical resistance and high hardness, enhancing the membranes’ mechanical strength.
Green ceramic hollow fiber membranes with superhydrophobic and superoleophilic surfaces were prepared through facile sol-gel process for oil/water separation[ 17]. A modified Stöber method was used to prepare the silica sol. Superhydrophobic and superoleophilic green ceramic hollow fiber membranes (ss-CHFM/WSBA) were fabricated using the dip-coating method, with varying parameters including grafting time (0-90 minutes), grafting cycles (0-4 cycles), and calcination temperatures (400-600°C). At a grafting time of 30 minutes, nano-silica particles covered the membrane without disturbance while at 60 minutes, the particles attached to the walls of the membrane pores. At 90 minutes, it was found that the pores were totally blocked by nano-silica particles. As grafting cycles increased, there was an increase of evenly distributed nano-silica particles across the surface of the membrane. At grafting cycle of 1, there was an obvious uneven distribution of nano-silica particles, whereas at grafting cycles of 3 and 4, most of the membranes’ pores were covered. Oil separation efficiency also increased with increasing grafting times, reaching 99.9% at grafting cycles of 3 and 4. Oil flux showed a decreasing trend with increasing cycles, with an oil flux of 134.2 L/m2 h at 3 cycles. At a calcination temperature of 400°C, nano-silica particles were evenly distributed. However, an increase in calcination temperature from 400 to 500 and 600°C resulted in the nano-silica particles disappearing from the membrane surface. Therefore, the optimal conditions for fabrication were 60 minutes for 3 cycles at a calcination temperature of 400°C.
Ceramic microfiltration membranes were formed using various compositions of kaolin and fly ash, with dolomite as a pore-former. Three kaolin membranes (KA8, KA9, and KA10) and three fly ash membranes (FA8, FA9, and FA10) were fabricated according to three different sintering temperatures (800, 900, and 1000°C)[18]. Three membranes (M25, M50, and M75) with various weight percentages (25, 50, and 75 wt%) of kaolin and fly ash were fabricated and compared with membranes with no kaolin (M0≡FA9) and membranes that were fully kaolin (M100≡KA9). As sintering temperatures increased, porosity decreased, apart from FA8 to FA9, which is due to the incomplete sintering at 800°C of fly ash membranes. As kaolin content increased, porosity increased. Dolomite showed superior pore former performance compared to porosity previously reported work using calcite and soda ash, with KA9 showing 48.2% and FA9 showing 34.0% porosity. This is due to the higher carbonate content and formation of doloma in dolomite. Kaolin membranes showed higher mechanical strength, smaller pore size, and better chemical stability compared to fly ash membranes, while combining kaolin and fly ash resulted in an increase in porosity, strength, stability and a decrease in pore size and permeability. For treatment of oil-in-water emulsions, M75 is the best choice with 46.3% porosity, 0.62 μm pore diameter, and 49.4 MPa strength.
2.2. Alumina
Alumina-spinel composite ceramic hollow fiber membranes were prepared from aluminum dross waste via phase inversion and sintering techniques to investigate applications in the pretreatment and microfiltration of oily saline produced water[19]. The hollow fiber was sintered at increasing temperatures from 1125 to 1300°C. The average pore diameter of the hollow fiber was found to be between 0.49 and 0.55 μm and the fractured surface porosity was 23.77% while the outer surface porosity was 20.01%. Flexural strength increased as sintering temperatures increased, from 17.6 (± 2.18) to 25.7 (± 1.05), and 84 (± 2.15) MPa at 1250, 1270, 1300°C respectively. However, hollow fiber sintered at temperatures below 1225°C were to brittle measure flexural strength. Microfiltration was tested with a produced water feed of 200 mg/L at 1 bar. Hollow fiber membranes sintered at 1275°C had the maximum oil rejection rate, at 92.41% after 50 minutes. This is attributed to the decreased membrane pore size according to increasing sintering temperature. Moreover, removal efficiencies of turbidity were 92% and 94% following filtration time of 2 hours, with turbidity decreasing from 378 to 28.5 NTU for hollow fiber sintered at 1250°C, and from 378 to 22.5 NTU for hollow fiber sintered at 1275°C. The low-cost alumina- spinel composite ceramic hollow fiber membrane shows promising potential in the pretreatment of oily saline produced water.
A novel ultrarobust and biomimetric hierarchically macroporous ceramic membrane was prepared via surface hydrophobic coating and emulsion-assisted template self-assembly approach[20] (Figs. 6 & 7).
The fabricated membranes showed high oil-water separation efficiency, with a maximum of 99.98% and high recyclability, maintaining the efficiency after 10 separation cycles and up to 4 months of storage. The self-assembly approach led to high compressive strength (120.97 ± 11.69 MPa) and high porosity (74.28% ± 0.17%), which was significantly higher than the traditionally prepared membrane. The increased strength leads to better resistance to sand impingement by the membranes.
A co-sintering process for fabricating a low-cost fly ash based ceramic microfiltration (MF) membrane was proposed[21]. The ceramic support was fabricated with fly ash and mullite fibers, and the alumina MF layer was sprayed on the ceramic support and co-sintered. In the co-sintering process, two issues were addressed: the difference in sintering temperature of the alumina support and MF layer and difference in shrinkage of the fly ash support layer and membrane layer. The low-cost fly ash acts as a sintering aid for the ceramic support in decreasing the sintering temperature. With the incorporation of low-cost fly ash, the optimal sintering temperature of the membrane was 1050°C, where densification had not occurred yet. The fly ash support layer had a greater shrinkage rate than the MF layer. To resolve this issue, low-cost mullite fibers, which have enhanced heat resistance and stability in air atmospheres were used. Due to the “bridging effect”, doping mullite fibers caused an increase in porosity (30.1 to 34.5%), bending strength (20.88 to 30.30 MPa), and permeability (8600 to 10,500 Lm−2 h −1 bar−1). The prepared ceramic MF membranes showed enhanced permeability and a decrease in pore size, as well as high total organic carbon (TOC) removal efficiency of approximately 99% at an O/W emulsion with an oil content of 200mg/L. Thus, the novel co-sintering process was successful in fabricating high-quality, low-cost MF ceramic membranes.
2.3. Waste ash
The conventional method of ceramic membrane fabrication through alumina powders was costly due to the material cost and high sintering temperature. Therefore, in the last decade, efforts have been made to utilize alternative materials such as waste ash. Green metakaolin- corn cob ash (h-MCa) ceramic hollow fiber membranes were prepared to enhance properties of pristine metakaolin (MK) membranes including mechanical strength and permeability[22]. The corn cob ash (CCA) was obtained via a controlled combustion process at 700°C for 8 h at a heating rate of 10°C per minute from natural green waste corn cob (CC). KCl derived from CCA (CCA-KCl) acted as a porogen in the membrane. The prepared h-MCa membranes were formulated with five different composition ratios A – 100:0, B – 25: 75, C – 50:50, D 75:25, E – 0:100 at a sintering temperature of 1200°C. As CCA-KCl loading increased, the membrane structures showed less order, with loose particles. Mechanical strength was highest in fiber C, followed by fiber B and fiber A with 82.44, 41.61, and 11.37 MPa respectively. As CCA loading increased, pure water flux (PWF) decreased from 1159.93 to 740.97 L/m2h. Fiber B was determined to have optimal conditions for membrane support, with adequate mechanical strength and PWF, as well as high oil removal efficiency (74.73%) in oil/water emulsions.
Ceramic hollow fiber membranes (CHFM) were prepared from palm oil fuel ash (POFA) using phase inversion- based extrusion/sintering techniques to be used in water filtration applications[23]. Several extrusion process parameters, including suspension viscosity and sintering temperature were investigated to modify the finger-like macrovoid structure of the membrane. Suspension viscosity and the size of the structure were inversely related, meaning the size of the structure decreased as suspension viscosity increased. Air gap distance was affected by the length of the structure, as well as wall thickness and fiber diameter. A higher air gap distance led to longer structures, thinner walls, and smaller fiber diameters. Lower bore fluid flow rate caused the fiber lumen to deform while increasing the bore fluid rate caused the lumen diameter to grow. Finally, lower sintering temperature showed higher porosity and lower tortuosity. Considering these parameters, the optimal CHFM morphology in water filtration applications was fabricated with 55 wt%, air gap distance of 5 cm, bore fluid flow rate of 9 mL/min and sintering temperature of 1050°C. POFA-derived CHFM have shown multiple advantages over alternative CHFM, such as excellent bending strengths of over 75 Mpa and the reduction of energy consumption through lower sintering temperatures.
Palm oil fuel ash (POFA) was used to fabricate ceramic hollow fiber membranes and pretreated at increasing temperatures from 500 to 1000°C (labelled TP500, TP600, TP700, TP800, TP900, and TP1000) to determine its effects on morphology, bending strength, pore size distribution, porosity, and pure water flux (PWF)[24]. Morphology and mechanical strength generally improved with pretreatment and the increase of pretreatment temperature. Untreated POFA (UP) membranes show sponge-like structures with insignificant macrovoids, while pretreated POFA membranes show sponge-like layers and macrovoid layers organized asymmetrically as a result of lower suspension viscosity. UP membranes showed a mechanical strength of 11 MPa and as pretreatment temperatures increased from 500 to 700°C, mechanical strength increased up to 60 MPa. However, there was a decrease in mechanical strength at pretreatment temperatures higher than 700°C, as it was affected by the larger macrovoids of the membranes. UP membranes had the highest porosity of 60.6%, which was the result of the unburned carbon in POFA oxidizing during the sintering. Porosity decreased with when POFA was pretreated, with TP700 membranes showing 33.2% porosity. PWF was highest in UP membranes, at 219.2 L/m2 h, which showed a decrease with the pretreatment of POFA, with PWF dropping to 28.4 L/m2 h for TP700 membranes. However, oil/water separation was improved, with UP membranes showing 32.1% rejection and TP700 membranes showing 88.9% rejection.
3. Conclusions
Ceramic membranes show high removal efficiency in O/W emulsions of up to 99%, making them a potential candidate for oily wastewater separation. Several types of materials can be used to fabricate ceramic membranes for oily wastewater separation processes such as silicon, alumina, and waste ash. Waste ash membranes provide a cheap and environmentally friendly alternative to traditional fabrication materials, making them cost-competitive. Further research on surface material may build on anti-fouling properties, improving membrane performance. This review details different types of ceramic membranes using different fabrication materials and methods and discusses the membrane performance. Natural clay and environmental waste can be used to prepare cheaper membranes as an alternative to conventional ceramic membranes.
