Performance evaluation of cellulose triacetate and cellulose diacetate hybrid membranes with carbon nanotube (CNT) for sustainable slaughterhouse wastewater treatment via forward osmosis.

Forward osmosis (FO) is an emerging membrane separation technology. It is different from the well-studied pressure-driven membrane separation processes. The FO process is based on water transport under an osmotic pressure difference across a semi-permeable membrane. The distinct operating conditions lead to unique technical challenges during the exploitation of FO technology. According to a comprehensive literature investigation, one of the stringent barriers is lacking of effective FO membranes. The objectives of this research were to develop high performance FO membranes, and furthermore, to systematically study the mass transport and the governing mechanisms in FO process. Thin film composite (TFC) FO membranes with a tailored support structure were developed in this study. The membranes consisted of a highly porous substrate with finger-like pore structure, which was prepared via phase inversion, and a polyamide rejection layer synthesized by interfacial polymerization. The TFC FO membranes had small structural parameters due to the thin cross-section, low tortuosity, and high porosity of the substrates. The membrane rejection layers exhibited superior separation properties (higher water permeability and excellent selectivity) relative to commercial FO membranes. Under FO testing conditions, these membranes achieved high water flux while maintaining relatively low solute reverse diffusion. Comparison of the synthesized TFC FO membranes with commercial FO and reverse osmosis (RO) membranes revealed the critical importance of the substrate structure, with a straight finger-like pore structure preferred over a spongy pore structure to minimize internal concentration polarization (ICP), a unique and critical problem resulting in low water flux in the osmotically driven membrane processes. In addition, membranes with high water permeability and excellent selectivity are preferred to achieve both high FO water flux and low solute flux. The results proved that TFC membranes with a tailored porous substrate and rejection layer are promising for FO applications. In the study of polyamide rejection layer synthesis, the influence of monomer concentrations (i.e., m-phenylenediamine (MPD) and trimesoyl chloride (TMC) concentrations) on the membrane separation properties as well as the FO performance was systematically investigated. A strong trade-off between the water permeability and salt rejection of the membranes was observed, where reducing the MPD concentration or increasing the TMC concentration may result in a higher membrane permeability but a lower salt rejection. In FO tests, membranes with poor salt rejection had severe solute reverse diffusion, which enhanced the severity of ICP. It was found that the FO water flux was governed by both the membrane water permeability and solute rejection. For a membrane with higher water permeability but lower solute rejection, the reduced membrane frictional resistance was compensated simultaneously by the more severe solute-reverse-diffusion-induced ICP. The net effect on the FO water flux depends on the competition of these two opposing mechanisms. Under conditions where solute reverse diffusion may cause severe ICP (e.g., high draw solution concentration and high water flux level), membranes need to be optimized to achieve a high salt rejection even if this is at the expense of lower water permeability. In view of the importance of the water permeability and salt permeability on FO performance, a systematic comparison study of prevailing semi-permeable FO membranes with nanofiltration (NF)-like and RO-like separation properties in terms of flux performance and fouling behavior was conducted. Due to the crucial influence of solute reverse diffusion on FO water flux, the high-rejection RO-like FO membranes generally performed better than the NF-like counterparts in sodium chloride based FO tests. On the other hand, the high permeability of NF-like FO membranes could achieve higher water flux, when proper draw solutes were used to minimize draw solute leakage. Fouling tests suggested that the NF-like TFC FO membranes tended to be more fouling resistant due to their relatively smooth membrane surface. This work further elucidated the major mechanisms that govern the FO performance. These mechanisms were summarized as a frictional resistance loss mechanism (MR), solute-reverse-diffusion-induced ICP (MICP-Js), concentration of feed solutes (concentrative ICP or MICP-feed in the active-layer-facing-draw-solution orientation) and dilution of draw solutes (dilutive ICP or MICP-draw in the active-layer-facing-feed-solution orientation). These mechanisms are related to the properties of membrane, draw and feed solutions. This work led to a set of systematic criteria for the selection of FO membranes, draw solution and optimization of other operating conditions, of which the practicability was demonstrated in potential FO applications.

Hybrid membrane system for desalination and wastewater treatment : Integrating forward osmosis and low pressure reverse osmosis

The present work aims to study forward osmosis process using different kinds of draw solutions and membranes. Three types of draw solutions (sodium chloride, sodium formate, and sodium acetate) were used in forward osmosis process to evaluate their effectiveness with respect to water flux and reverse salt flux. Experiments conducted in a laboratory-scale forward osmosis (FO) unit in cross flow flat sheet membrane cell. Three types of membranes (Thin film composite (TFC), Cellulose acetate (CA), and Cellulose triacetate (CTA)) were used to determine the water flux under osmotic pressure as a driving force. The effect of temperature, draw solution concentration, feed and draw solution flow rate, and membrane types, were studied with respect to water flux. The results showed an increase in water flux with increasing feed temperature and draw solution concentrations In addition, the flux increased with increasing feed flow rate while the flux was inversely proportional with the draw solution flow rate. The results showed that reverse osmosis membranes (TFC and CA) are not suitable for using in FO process due to the relatively obtained low water flux when compared with the flux obtained by forward osmosis membrane (CTA). NaCl draw solution gave higher water flux than other draw solutions and at the same time, revealed higher reverse salt flux.

The research aims to use a new technology for industrial water concentrating that contains poisonous metals and recovery quantities from pure water. Therefore, the technology investigated is the forward osmosis process (FO). It is a new process that use membranes available commercial and this process distinguishes by its low cost compared to other process. Sodium chloride (NaCl) was used as draw solution to extract water from poisonous metals solution. The driving force in the FO process is provided by a different in osmotic pressure (concentration) across the membrane between the draw and poisonous metals solution sides. Experimental work was divided into three parts. The first part includes operating the forward osmosis process using TFC membrane as flat sheet for NaCl. The operating parameters studied were: draw solutions concentration (10 – 95 g/l), draw solution flow rate (12-36 I/h), temperature of draw solution (30 and 40°C), feed solution concentration (10 -210 mg/l), feed solution flow rate (10 -50 l/h), temperature of feed solution (30 and 40°C) and Pressure (0.4 bar). The second part includes operating the forward osmosis process using CTA membrane as flat sheet for NaCl. The operating parameters studied were: draw solution concentration (15 – 95 g/l), feed solution concentration (10-210 mg/l). Constant temperature was maintained at 30°C. The last part includes operating the reverse osmosis process using TFC membrane as spiral wound module in order to separate NaCl salt from draw solution and obtain on pure water so as to usefully in different uses and also obtain on solution of NaCl concentrate which was recirculated to forward osmosis process. It is then used as draw solution. The operating parameter studied was: feed solution flow rate (15-55 l/h). The experimental results show that the water flux increases with increasing draw solution concentration, feed solution flow rate, temperature of draw solution and decreases with increasing feed solution concentration, draw solution flow rate and temperature of feed solution. The experiments also show that CTA membrane gives higher water flux than TFC membrane for forward osmosis operation.

Nowadays, inadequate access to clean water has become one of the most pervasive problems due to the rapidly expanding global population and thus the exponentially growing demand in water and food supply, industry and social life (Shannon et al. 2008). Problems with water have called out for a large number of researchers to pay more attention to water sustainability and put forth effort to explore more robust technologies for wastewater treatment and desalination in addition to improving the efficiency of the current water production and distribution systems (Sikdar 2011). Among many potential solutions, membrane processes such as reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF), and microfiltration (MF) have found their overwhelming applications in water industry. However, these technologies are either chemically or energetically intensive, thus are castigated for high cost due to substantial chemical and energy consumptions as well as high fouling propensity which requires frequent backwash or cleaning. Forward osmosis (FO), utilizing the natural phenomenon of osmosis, is an emerging membrane process driven by the osmotic pressure gradient created across a semipermeable membrane by two flowing streams of varying concentration (i.e., the draw solution and the feed). Hence, the energy required to transport water across the membrane is almost negligible. Far from being so, FO creates much less problem of fouling and cleaning (Mi and Elimelech 2010). By virtue of these unique features, FO distinguishes itself from other membrane processes for sustainable supply of clean water. An example of the FO unit for wastewater treatment is shown in Fig. 1. In the FO process as illustrated, the draw solution (an aqueous solution of magnetic nanoparticles covered with thermosensitive polymer) (Ling et al. 2011) and the feed (wastewater) partitioned by the membrane flow co-currently through corresponding channels. The draw solution, having a higher osmotic pressure than the feed, draws water from the feed and flows back to the reservoir. As it continuously takes clean water from the feed, the draw solution in the reservoir becomes diluted. A regeneration process is connected to the reservoir to re-concentrate the draw solution as well as to produce clean water. A portion of the diluted draw solution is pre-heated with the aid of solar panel or waste heat and traverses a magnetic field. Upon heating, the magnetic nanoparticles covered with thermosensitive polymers change their surface property from hydrophilic to hydrophobic and are easily seized by the magnetic field or other filtration processes. As a result, clean water freely passes through and is collected as the product. The trapped magnetic nanoparticles are then sent back to the reservoir to replenish the draw solution. The 1st key component of the FO unit is the membrane material which should be semipermeable, i.e., allowing water to permeate through while blocking all the solutes in the draw and feed solutions. A tremendous amount of research has been conducted on the molecular design of new membrane materials with superior FO performance and great progress has been achieved in the past 5 years. To date, several types of FO membranes have been reported such as (1) flat sheet membranes made of cellulose esters (Wang et al. 2010a; Zhang et al. 2010); (2) J. Su M. M. Ling T.-S. Chung (&) Department of Chemical & Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117576, Singapore e-mail: chencts@nus.edu.sg

This paper was aimed to study the efficiency of forward osmosis (FO) process as a new application for the treatment of wastewater from textile effluent and the factors affecting the performance of forward osmosis process.The draw solutions used were magnesium chloride (MgCl2), and aluminum sulphate (Al2 ( SO4)3 .18 H2O), and the feed solutions used were reactive red, and disperse blue dyes.Experimental work were includes operating the forward osmosis process using thin film composite (TFC) membrane as flat sheet for different draw solutions and feed solutions. The operating parameters studied were : draw solutions concentration (10 – 90 g/l), feed solutions concentration (5 – 30 mg/l), draw solutions flow rate (10 – 50 l/hr), feed solutions flow rate (20-60 l/hr), constant pressure and temperature were maintained at 0.5 bar and 30ºC respectively. And includes operating the forward osmosis process using cellulose triacetate (CTA) membrane as flat sheet for different draw solutions and feed solutions. The operating parameters studied were : draw solutions concentration (10 – 90 g/l), and feed solutions concentration (5 – 30 mg/l), constant temperature at 30ºC. It was found that water flux increases with increasing draw solution concentration, and feed solution flow rate and decreases with increasing draw solution flow rate and feed solution concentration for TFC and CTA. It was found MgCl2 given water flux larger than Alum. And also found that reactive red given water flux larger than disperse blue.The experiments also show that CTA membrane gives higher water flux than TFC membrane for forward osmosis operation. The increase in water flux for CTA is about 12.85% than TFC.

Evaluation of citrate-coated magnetic nanoparticles as draw solute for forward osmosis

Mathematical approach for improved performance of flat-sheet forward osmosis membrane

The research aims to apply the novel forward osmosis (FO) process to recover pure waterfrom contaminated water. Phenol was used as organic substance in the feed solution, while sodiumchloride salt was used as draw solution. Membranes used in the FO process is the cellulosetriacetate (CTA) and polyamide (thin film composite (TFC)) membrane. Reverse osmosis processwas used to treatment the draw solution, the exterior from the forward osmosis process. In the FOprocess the active layer of the membrane faces the feed solution and the porous support layer facesthe draw solution and this will show the effect of dilutive internal concentration polarization andconcentrative external concentration polarization.In the FO process was a run-time for five hours, and the concentration of phenol 100 and1000 mg/l, and for the NaCl the concentration was 10000 and 30000 mg/l. It was found thatrecovery percent increases with increasing time, while water flux through membrane decreases withincreasing time. Also, it was found that recovery and water flux increases with increasing drawsolution concentration, on the contrary, water flux and the percentage of recovery decreases withincreasing the concentration of phenol (feed solution). Increase in draw solute (NaCl) concentrationhas more effect on the water flux in FO process compared with increase in the concentration ofphenol. Outlet phenol concentration increases with time, while the outlet salt concentrationdecreases with increasing the time. The results showed that the cellulose triacetate membrane gavethe highest recovery ratio from the thin film composite membrane. The highest recovery wasreached in five hours is 51.33%, while using CTA membrane recovery rate increase, by 23%compared with TFC membrane. The value of the resistance to solute diffusion within the membraneporous support layer is 36.83 h/m. Reverse osmosis is perfect method for removal of dissolved saltsfrom water, thus its suitable process for reducing the content of NaCl in draw solution; therefore thesodium chloride rejection percentage was 91.6 – 96 % for polyamide membrane (TFC). Within twohours of work of the reverse osmosis system the recovery percentage of pure water is 58%.

Standalone membrane distillation (MD) and forward osmosis (FO) have been considered as promising technologies for produced water treatment. However, standalone MD is still vulnerable to membrane-wetting and scaling problems, while the standalone FO is energy-intensive, since it requires the recovery of the draw solution (DS). Thus, the idea of coupling FO and MD is proposed as a promising combination in which the MD facilitate DS recovery for FO—and FO acts as pretreatment to enhance fouling and wetting-resistance of the MD. This study was therefore conducted to investigate the effect of DS temperature on the dynamic of water flux of a hybrid FO–MD. First, the effect of the DS temperature on the standalone FO and MD was evaluated. Later, the flux dynamics of both units were evaluated when the FO and DS recovery (via MD) was run simultaneously. Results show that an increase in the temperature difference (from 20 to 60 °C) resulted in an increase of the FO and MD fluxes from 11.17 ± 3.85 to 30.17 ± 5.51 L m−2 h−1, and from 0.5 ± 0.75 to 16.08 L m−2 h−1, respectively. For the hybrid FO–MD, either MD or FO could act as the limiting process that dictates the equilibrium flux. Both the concentration and the temperature of DS affected the flux dynamic. When the FO flux was higher than MD flux, DS was diluted, and its temperature decreased; both then lowered the FO flux until reaching an equilibrium (equal FO and MD flux). When FO flux was lower than MD flux, the DS was concentrated which increased the FO flux until reaching the equilibrium. The overall results suggest the importance of temperature and concentration of solutes in the DS in affecting the water flux dynamic hybrid process.

11 - Reverse osmosis and forward osmosis in integrated systems

Forward osmosis (FO) is a membrane process that occurs when solutions of different osmotic pressures are separated by a membrane which is selectively permeable to water. It is a process that drives water permeation across the membrane spontaneously even in the absence of hydraulic pressure difference across the membrane. FO has attracted lots of attention over the last decade and has been explored as a potential alternative to desalination, wastewater treatment and liquid food processing. Significant progress has been made in the development of high-performance FO membranes with high water flux and low reverse solute flux, particularly cellulosic membranes, thin-film composite (TFC) membranes and polyelectrolyte-based membranes. Yet, a few major challenges continue to hamper the widespread implementation of the process in the industry, mainly internal concentration polarization, reverse solute diffusion, membrane fouling, mechanical durability and draw solution regeneration. Most of these challenges are associated with membrane characteristics, which has significantly limited the efficiency of the FO process. To address these challenges, firstly, hollow fiber ultrafiltration membranes were fabricated from polyethersulfone (PES) via a non-solvent induced phase separation (NIPS) process and were used as substrates to prepare inner-selective TFC hollow fiber membranes via an interfacial polymerization (IP) process. The effect of the hollow fiber substrate fabrication conditions on the properties of the substrate and TFC membranes were briefly investigated. The FO performance of the TFC membranes were characterized by using 0.5 M NaCl and DI water as the draw and feed solutions. when the membrane was operated in the active layer-facing-feed solution (AL-FS) and active layer-facing-draw solution (AL-DS) configurations, water flux as high as 41.2 L/m2/h and 74.9 L/m2/h were achieved, while specific reverse solute flux were 0.11 g/L and 0.10 g/L, respectively. Subsequently, a novel double-skinned hollow fiber TFC FO membrane has been successfully fabricated. The FO membrane consisted of a one-step dual-layer substrate and a thin inner selective layer formed via the IP process. The substrate comprises a dense ultrafiltration (UF) outer layer and a relatively porous UF inner layer, both of which were constructed from PES by using a dual-layer co-extrusion technique. The fouling resistance of the double-skinned hollow fiber membrane was evaluated under various testing conditions to verify the viability of double-skinned hollow fiber membranes as a solution to membrane fouling in the FO process. Compared to the commercial and reported double-skinned FO membranes, the FO membrane developed in this thesis exhibited a higher permeate flux with humic acid solution as a feed solution. Furthermore, the double-skinned FO membrane was applied in concentrating activated sludge using 0.5 M NaCl as a draw solution. A permeate flux at 5.4 L/m2/h was achieved after 5-hour operation, which was higher than, or comparable to, those of the reported FO membranes. Membrane autopsies and foulant analysis suggested that the dense UF skin layer helped to reject larger-sized organic foulants (> 300 Da), which shed light on the importance of fabrication features and promising application of the double-skinned hollow fiber TFC FO membrane in sludge concentration. On the other hand, a series of characterization revealed that TFC hollow fiber membranes may experience significant compaction during the FO process despite the lack of applied pressure. Three TFC hollow fiber membranes were fabricated with varied water permeability to study the effect of the osmotic pressure on the TFC membranes. The TFC membranes were continuously tested in FO experiments for 24 h using DI water as feed and varied concentration of NaCl solutions as draw solutions, and their performances were evaluated again using fresh feed solutions. At the end of the FO experiments, all TFC membranes experienced water and salt flux decline to different extents. Visible changes in the cross-sectional morphology and surface topography of the TFC membranes were observed. These observations suggested that the occurrence of membrane compaction is strongly associated with the characteristics of the hollow fiber substrates that were used to prepare the TFC membranes and may be attributed to “negative pressure” build-up within the support layer of the TFC membranes.

Concentration polarization results in significant reduction in the difference in osmotic pressure between the draw solution and the feed solution in forward osmosis (FO) and is regarded as a major challenge in the FO process. Hence, mitigating concentration polarization is essential for increasing the efficiency of the FO process. In the current study, vibration of hollow fiber (HF) membranes was systematically studied as a method for the mitigation of concentration polarization in submerged FO process. A membrane module consisting of polyether sulfone (PES) HF membranes with inner selective layer was designed and fabricated. Using a water flux model that takes into account the internal concentration polarization (ICP) and external concentration polarization (ECP) on both the lumen and shell sides of the HF membranes, the vibrating frequency and amplitude were evaluated with regards to the change of mass transfer coefficient. Results showed that when low vibration frequency and amplitude of (3 Hz and 1.2 cm) were applied, the mass transfer coefficient increases from 0.7×10-5 m/s (at no vibration) to 1.8×10-5 m/s, which approaches the optimal value as determined from the FO modelling results. The effects of the vibration of membranes were then evaluated in the active layer facing feed solution (AL-FS) and active layer facing draw solution (AL-DS) orientations. Results showed that in the AL-FS orientation, vibration could enhance mass transfer coefficient significantly at low water flux. However, in the AL-DS orientation, the enhancement of mass transfer coefficient was minimal at low water flux level due to minimal concentrative ECP present. Interestingly, there was significant improvement of mass transfer coefficient at high DS concentration/high water flux level attributed to the significant concentration polarization under high water flux condition. In addition, the vibration-assisted FO process was carried out using polyelectrolyte poly (sodium-4-styrenesulfonate) (PSS) as a draw solute. The change of draw solute to PSS has limited effects on the vibration, as the low diffusion coefficient and high viscosity of PSS offsets the benefits of vibration of membranes. In summary, this study demonstrated a facile method for mitigating concentration polarization in submerged FO process, whereby vibration of HF membranes at low frequency and amplitude could enhance FO water flux by improving hydrodynamic mixing and induce boundary layer disturbances at the shell side of submerged HF membranes. The method could possibly be adopted for improving FO efficiency in submerged osmotic processes.

This research investigated the impact of draw solute and membrane material on the economic balance of a forward osmosis (FO) system pre-concentrating municipal sewage prior to an anaerobic membrane bioreactor (AnMBR). Eight and three different draw solutes were evaluated for cellulose triacetate (CTA) and polyamide thin film composite (TFC) membranes, respectively. The material of the FO membrane was a key economic driver since the net cost of TFC membrane was substantially lower than the CTA membrane. The draw solute had a moderate impact on the economic balance. The most economically favourable draw solutes were sodium acetate and calcium chloride for the CTA membrane and magnesium chloride for the TFC membrane. The FO + AnMBR performance was modelled for both FO membrane materials and each draw solute considering three FO recoveries (50, 80 and 90%). The estimated COD removal efficiency of the AnMBR was similar regardless of the draw solute and FO membrane material. However, the COD and draw solute concentrations in the permeate and digestate increased as the FO recovery increased. These results highlight that FO membranes with high permselectivity are needed to improve the economic balance of mainstream AnMBR and to ensure the quality of the permeate and digestate.

Novel highly porous cellulose triacetate (CTA)/cellulose acetate (CA) blends were fabricated as flat sheet membranes, for water desalination using the forward osmosis (FO) procedure. Maleic acid (MA) was used as a pore-forming additive and as a polymeric modifier in combined casting. The aluminum oxide nanoparticles (Al2O3) (NPs) were used for the modification of MA/CTA/CA membrane performances. The synthesized FO membranes are characterized by FTIR spectroscope, contact angle measurement, membrane porosity, SEM, AFM, and mechanical properties. The Al2O3/MA/CTA/CA nanocomposite (NC) modified membrane showed a higher water flux of 27.1 L/m2 h, reverse solute flux of 10.3 g/m2 h, and lower salt rejection of 99.15% using 1 M NH4Cl water solution as the draw solution and 0.1 M NaCl as feed solution. The Al2O3/MA/CTA/CA nanocomposite modified membrane shows a higher porosity (60.3 ± 2), a lower contact angle (55°), and its reduced structural parameter (S) to 0.87 mm. The results revealed that the Al2O3/MA/CTA/CA nanocomposite modified membrane showed the highest water flux using 1 M of the KCl and NH4Cl (20 L/m2 h); followed by (NH4)2SO4 (19.7 L/m2 h) and K2HPO4 (17.6 L/m2 h) as draw solutions (DS) under the FO approach and using natural groundwater sample collected from Al-Zafer village, Sidi Barrani Area, north-western coast of Egypt with salinity of 8536 mg/L as feed solution (FS). It was revealed that the synthesized Al2O3/MA/CTA/CA nanocomposite modified membrane has great potential for application of FO process in brackish water desalination. The current cost per m2 of an Al2O3/MA/CTA/CA nanocomposite modified membrane is AU$ 32/m2 when compared with the commercial FO membrane (AU$ 188/m2).