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Reduction of Reverse Solute Flux Induced Solute Buildup in the Feed Solution of Forward Osmosis
Abstract
Forward osmosis (FO) has shown advancement towards recovery of useful water from various waste streams. A major issue that arises is the accumulation of salts due to reverse solute flux (RSF) from a draw solution into a feed solution that can result in several negative effects such as decreased water flux and inhibiting biological activities. This paper aims to provide a concise discussion and analysis of methods that can help to alleviate the effects of solute build up. New parameters, solute removal/recovery rate (SRR) and removal/recovery ratio (ReR), are proposed to help better define the performance of reducing solute buildup and employed in case studies to evaluate the selected reduction methods. Solute removal can be accomplished by physical separation, chemical precipitation, and biological removal. Recovery of solutes, one step beyond removal, is discussed and demonstrated by using bioelectrochemical systems and electrodialysis as examples. This work has highlighted the concerns associated with solute buildup and encouraged further exploration of effective tools to mitigate solute buildup for improved performance of FO-based water/wastewater systems.
... Recently, there has been increasing interest in a novel forward osmosis membrane bioreactor (FOMBR) process [16][17][18][19][20][21]. In the FOMBR, high rejection forward osmosis (FO) membranes are used instead of microporous membranes. ...
... Water is transported from the mixed liquor into a draw solution (DS) driven by a natural osmotic pressure gradient across the membrane. Compared to traditional MBR, FOMBR offers unprecedented advantages, such as low fouling tendency, high fouling reversibility, high effluent water quality, potentially low energy consumption [18], and facilitated nutrients [17,19,22,23] and bioenergy recovery [21,24]. For these reasons, FOMBR is considered a promising alternative in wastewater treatment and reclamation. ...
... As according to Luo et al. [34], although no discernible impacts of elevated salinity on the removal of hydrophobic compounds were found, the removal of a range of hydrophilic compounds were notably reduced, owing to the salinity stress on the metabolic activity of microorganisms responsible for the degradation of these chemicals. The introduction of a side-stream microfiltration/ultrafiltration unit into an FOMBR to constitute a hybrid microfiltration forward osmosis membrane bioreactor (MF-FOMBR) process has been a key advantage in the development of the FOMBR process, which allows a rational control of the salt accumulation and enables direct and facilitated nutrients recovery [21,[35][36][37][38]. As such, the side-stream MF configuration is considered as a proof of concept which can potentially lead to a full-scale sustainable option for salinity mitigation [17,18,39]. ...
A hybrid microfiltration forward osmosis membrane bioreactor (MF-FOMBR) was operated with raw municipal wastewater as feed. The fate and removal behaviors of 20 commonly used antibiotics were investigated. With the influent concentration of 4.1-716.9 ng/L (the most prevalent antibiotics were enrofloxacin, sulfamethazine and cefalexin, followed by amoxicillin, lomefloxacin and ampicillin), the system showed 58.9-100% overall removal of all the antibiotics. Almost all the antibiotics was rejected efficiently (86.1-100%) by the FO membrane, except for sulfonamides and trimethoprim (as low as 71.0%) due to their low molecular weights (M.W., 250.1-278.1 g/mol). However, relatively low biological removal (8.7-45.7%) was observed for most antibiotics, except for macrolides (70.6-90.1%) which was effectively removed via biosorption, and β-Lactams (up to 64.6%) which is readily biodegradable, resulting in their significant accumulation (up to 3.4 folds, with the maximum concentration of 1831.5, 1056.7 and 626.3 ng/L for sulfamethazine, enrofloxacin, and cefalexin, respectively) in the mixed liquor. Different antibiotics showed distinct affinities for sorption to the activated sludge. The solid/liquid partitioning of the antibiotics is governed by their hydrophobicity. All the antibiotics was detected in the draw solution (DS) (with average concentrations of 3.8-100.4 ng/L, and enrofloxacin, amoxicillin and sulfamethazine being the most predominant ones) except macrolides in certain cases, highlighting the need to improve their overall removal via the development and application of denser membranes or integration of necessary physio-chemical treatment. The side-stream MF unit acted as a major exit (>50.1% of total mass) for the poorly biodegradable and poorly adsorbed antibiotics to leave the system, preventing their further contamination of the DS. The enrichment of the antibiotics in the MF effluent would also facilitate their final elimination via subsequent advanced treatments.
... where Jw is the water flux at time t, Am is the effective membrane area (m 2 ), V is the volume of permeate, and t is the time for permeate. The reverse salt flux was determined by calculating the change of salt content in the feed solution as the following equation [34]: ...
... where J w is the water flux at time t, A m is the effective membrane area (m 2 ), V is the volume of permeate, and t is the time for permeate. The reverse salt flux was determined by calculating the change of salt content in the feed solution as the following equation [34]: ...
The number of chronic renal disease patients has shown a significant increase in recent decades over the globe. Hemodialysis is the most commonly used treatment for renal replacement therapy (RRT) and dominates the global dialysis market. As one of the most water-consuming treatments in medical procedures, hemodialysis has room for improvement in reducing wastewater effluent. In this study, we investigated the technological feasibility of introducing the forward osmosis (FO) process for spent dialysate reuse. A 30 LMH of average water flux has been achieved using a commercial TFC membrane with high water permeability and salt removal. The water flux increased up to 23% with increasing flowrate from 100 mL/min to 500 mL/min. During 1 h spent dialysate treatment, the active layer facing feed solution (AL-FS) mode showed relatively higher flux stability with a 4–6 LMH of water flux reduction while the water flux decreased significantly at the active layer facing draw solution (AL-DS) mode with a 10–12 LMH reduction. In the pressure-assisted forward osmosis (PAFO) condition, high reverse salt flux was observed due to membrane deformation. During the membrane filtration process, scaling occurred due to the influence of polyvalent ions remaining on the membrane surface. Membrane fouling exacerbated the flux and was mainly caused by organic substances such as urea and creatinine. The results of this experiment provide an important basis for future research as a preliminary experiment for the introduction of the FO technique to hemodialysis.
... However, these solutes generally feature high reverse solute fluxes (RSF) due to their high diffusivity (Zou et al., 2019). The RSF from the draw to the feed solution: (i) increases the salinity of the sewage and (ii) increases the draw solution replenishment costs (Ferby et al., 2020). The higher salinity in the pre-concentrated sewage could partially inhibit anaerobic bacteria with a direct impact on the AnMBR biogas production and effluent quality (both permeate and digestate) (Vinardell et al., 2021). ...
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.
... Some solutions for mitigating reverse solute flux were developed for FO in water treatment. For example, physical separation via microfiltration, chemical precipitation, and biological methods was used for removing the solute transferred to the feed due to the reverse solute flux (Ferby et al., 2020). Other researchers suggested methods for alleviating reverse solute flux such as membrane surface functionalization (Zou et al., 2019) or the addition of surfactants to the OA (Chekli et al., 2018). ...
Concentration of milk in the dairy industry is typically achieved by thermal evaporation or reverse osmosis (RO). Heat concentration is energy intensive and leads to cooked flavor and color changes in the final product, and RO is affected by fouling, which limits the final achievable concentration of the product. The main objective of this work was to evaluate forward osmosis (FO) as an alternative method for concentrating milk. The effects of fat content and temperature on the process were evaluated, and the physicochemical properties and sensory qualities of the final product were assessed. Commercially pasteurized skim and whole milk samples were concentrated at 4, 15, and 25°C using a benchtop FO unit. The FO process was assessed by monitoring water flux and product concentration. The color of the milk concentrates was also evaluated. A sensory panel compared the FO concentrated and thermally concentrated milks, diluted to single strength, with high temperature, short time pasteurized milk. The FO experimental runs were conducted in triplicate, and data were analyzed by single-factor ANOVA. Water flux during FO decreased with time under all processing conditions. Higher temperatures led to faster concentration and higher concentration factors for both skim and whole milk. After 5.75 h of FO processing, the concentration factors achieved for skim milk were 2.68 ± 0.08 at 25°C, 2.68 ± 0.09 at 15°C, and 2.36 ± 0.08 at 4°C. For whole milk, after 5.75 h of FO processing, concentration factors of 2.32 ± 0.12 at 25°C, 2.12 ± 0.36 at 15°C, and 1.91 ± 0.15 at 4°C were obtained. Overall, maximum concentration levels of 40.15% total solids for skim milk and 40.94% total solids for whole milk were achieved. Additionally, a triangle sensory test showed no significant differences between regular milk and FO concentrated milk diluted to single strength. This work shows that FO is a viable nonthermal processing method for concentrating milk, but some technical challenges need to be overcome to facilitate commercial utilization.
Global environmental challenges, especially water scarcity due to the rapid development of industrialization and the ever-increasing world population, have become one of the most dangerous threats humankind is currently facing. Compared to the conventional separation technologies, membrane-based separations have gained considerable attention in water purification due to its excellent performance in the removal of contaminants in a low cost and environmentally friendly way. However, polymer-based membranes are still disadvantaged by several challenges such as low fouling resistance toward contaminants and microorganisms present in the polluted water, which eventually affects the membrane performance in terms of permeability and selectivity. Off-late, metal-organic frameworks (MOFs) are materials of extensive interests, because of their fascinating chemistry, for instance, intrinsic and highly designable porous characteristics, high surface area, abundant adsorption functionalities (sites), etc., These above advantages of MOFs are made their attraction in wide range of industrial scale applications, including membrane-based water treatment processes. In this chapter, we plan to examine the usage of MOFs for the membrane-based water treatment, a relatively narrow yet ever more important part of MOFs application. We explored the role of MOFs in the enhancement of porosity and interconnectivity of water treatment membranes which has been reported by worldwide researchers as a potential pathway for effective water purification. Further, the applications of MOFs when incorporated into various membrane-based water treatment processes such as forward osmosis (FO), reverse osmosis (RO), membrane distillation (MD), nanofiltration (NF) and ultrafiltration (UF) using different experimental techniques have been discussed. In conclusion, some future prospects and challenges to be faced and measures that need to be taken into consideration for wide range applicability of MOFs to remove the contaminants effectively from aqueous environments have been discussed.
Reverse solute flux (RSF) is a key issue for operating forward osmosis (FO) systems and can cause the loss of draw solute (DS) and salt accumulation in the feed. Herein, an electrolysis-assisted FO (e-FO) system was developed for simultaneous RSF reduction and recovery of the reverse-fluxed DS. Applying a voltage of 1.5 V led to RSF of 3.34 ± 0.01 mmol m⁻² h⁻¹ (0.47 g m⁻² h⁻¹) in the e-FO system, 67.5 ± 0.5% lower than that of the control system; in addition, the e-FO system recovered ~0.32 g L⁻¹ of the reverse-fluxed DS and this could not be realized in the control. When the e-FO system was examined with three types of the mimicked fertilizer, RSF reduction and DS recovery were largely affected by the individual components of a fertilizer DS. The energy consumption of the e-FO system was reduced by ~90%, from 0.38 ± 0.01 to 0.04 ± 0.01 kWh m⁻³ when the recirculation rate decreased from 60 to 15 mL min⁻¹. Those results have demonstrated the technical feasibility of an e-FO system that is capable of reducing RSF and recovering the lost DS and will encourage further investigation by addressing several identified challenges.
The water content in the recycled alginate solutions from aerobic granular sludge was nearly 100%. Forward osmosis (FO) has become an innovative dewatering technology. In this study, the FO concentration of sodium alginate (SA) was investigated using calcium chloride as a draw solute. The reverse solute flux (RSF) of calcium ions in FO had a beneficial effect, contrary to the findings of previous literature. The properties of the concentrated substances formed on the FO membrane on the feed side were analyzed by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy, verifying that calcium alginate (Ca-Alg), which can be used as a recycled material, was formed on the FO membrane on the feed side owing to the interaction between SA and permeable calcium ions. Water flux increased significantly with the increase in calcium chloride concentration, while the concentration of SA had little influence on the water flux in FO. Based on this discovery, we propose a novel method for the concentration and recovery of alginate, in which the RSF of calcium ions is utilized for recovering Ca-Alg by FO, with calcium chloride as a draw solute.
A suitable draw solute (DS) concentration in bioelectrochemically assisted osmotic membrane bioreactor (BEA-OMBR) can convert the “negative effect” of salinity accumulation into a “beneficial effect” by using the reverse-fluxed DS as a buffer agent or a carbon source supplement. Herein, the effect of DS concentration from acid buffer solution (i.e., ammonium chloride, NH4Cl), alkaline buffer solution (i.e., sodium bicarbonate, NaHCO3), and organic solution (i.e., sodium acetate, NaOAc) on salinity accumulation was systematically investigated. Salinity accumulation with NaHCO3 DS mainly derived from reversal fluxed sodium ion (Na⁺, major contributor with DS concentration ≤0.25 M) and bicarbonate ion (main contributor with DS concentration ≥0.50 M): Na⁺ accumulation could be mitigated by Na⁺ transport dominant by electrically driven migration (i.e., 21.3∼62.1% of reverse-fluxed Na⁺), and bicarbonate accumulation could be reduced by buffer system. A medium-low concentration of 0.25 M NH4Cl DS had a better performance on current density of 165.0±23.0 A m⁻³ and COD removal efficiency of 91.5±3.4 % by taking advantage that 77.7±1.3% of reverse-fluxed ammonium could be removed by biological treatment and ammonium transport. A high NaOAc DS concentration (i.e., ≥0.05 M) exhibited a higher current density of 145.3∼146.0 A m⁻³ but a lower COD removal efficiency due to the limited carbon source utilization capacity of anaerobic bacteria. Both concentration diffusion (20.9∼28.3%) and electrically driven migration (29.5∼39.4%) promote reverse-fluxed Na⁺ transport to catholyte and thus mitigated Na⁺ accumulation in the feed/anolyte. These findings have provided an optimal DS concentration for BEA-OMBR operation and thus encourage its further development.
Hybrid osmotic membrane bioreactor (OMBR) takes advantage of the cooperation of varying biological or desalination processes and can achieve NEWS (nutrient-energy-water-solute) recovery from wastewater. However, a lack of universal parameters hinders our understanding. Herein, system configurations and new parameters are systematically investigated to help better evaluate recovery performance. High-quality water can be produced in reverse osmosis/membrane distillation-based OMBRs, but high operation cost limits their application. Although bioelectrochemical system (BES)/electrodialysis-based OMBRs can effectively achieve solute recovery, operation parameters should be optimized. Nutrients can be recovered from various wastewater by porous membrane-based OMBRs, but additional processes increase operation cost. Electricity recovery can be achieved in BES-based OMBRs, but energy balances are negative. Although anaerobic OMBRs are energy-efficient, salinity accumulation limits methane productions. Additional efforts must be made to alleviate membrane fouling, control salinity accumulation, optimize recovery efficiency, and reduce operation cost. This review will accelerate hybrid OMBR development for real-world applications.
Salinity accumulation in osmotic membrane bioreactors (OMBRs) is one of the key challenges, which can be mitigated in situ by reverse-fluxed solute transport through integration of bioelectrochemical systems (BES). The effects of several key operating parameters on salinity accumulation were investigated. Salinity accumulation depended on balance between reversal solute flux (RSF) and reverse-fluxed ammonium (RFA) transport, which was driven by electrical migration and concentration diffusion. DS concentration was the primary factor influencing RSF, and the lowest DS concentration exhibited the minimum solute leakage. Aeration played a vital role in RFA transport, and a higher aeration helped to enhance RFA transport. Increased current generation (i.e., influent flow rate of 0.5 mL min-1 and external resistance of 5.0 Ω) contributed to RFA migration. The lack of electrolyte addition in catholyte contributed to RFA diffusion. These optimal parameters encourage the further development of an effective strategy for salinity mitigation in BES-based OMBR technology.
The rejection of disinfection byproducts (DBPs) is an important consideration for the application of forward osmosis (FO) in wastewater recycling. However, the transport of organic compounds in FO is not well predicted by existing models, partially because these models have not incorporated the effect of reverse salt flux, a phenomenon previously shown to influence the transport of pharmaceutical compounds. In this study, we investigated the effects of reverse salt flux on DBP transport in FO and the corresponding mechanisms. We used a commercial Aquaporin membrane and tested sixteen DBPs relevant to wastewater recycling. Using draw solutions constituted by NaCl, MgSO4, or glucose in a bench-scale FO system, we first confirmed that higher reverse salt flux resulted in lower DBP permeance. By integrating results from the bench-scale FO system and those from diffusion cell tests, we showed that two mechanisms contributed to the hindered DBP transport: the steric hindrance in the active layer caused by the presence of the draw solute and the retarded diffusion of DBPs in the support layer via a “salting-out” effect. Lastly, we developed a modified solution-diffusion model incorporating these two mechanisms by accounting for the free volume occupied by draw solute molecules in the active layer and by introducing the Setschenow constant, respectively. The modified model significantly improved the prediction of permeance for halogenated DBPs, and revealed the relative importance of steric hindrance (dominant for large DBPs) and retarded diffusion (dominant for hydrophobic DBPs). The modified model did not accurately predict the permeance of nitrosamines, attributable to their extremely high hydrophilicity or large size.
Forward osmosis (FO) has great potential for low energy consumption wastewater reuse provided there is no requirement for draw solutes (DS) regeneration. Reverse solute flux (RSF) can lead to DS build-up in the feed solution. This remains a key challenge because it can cause significant water flux reduction and lead to additional water quality problems. Herein, an osmotic photobioreactor (OsPBR) system was developed to employ fast-growing microalgae to consume the RSF nutrients. Diammonium phosphate (DAP) was used as a fertilizer DS, and algal biomass was a byproduct. The addition of microalgae into the OsPBR proved to maintain water flux while reducing the concentrations of NH4⁺-N, PO4³⁻-P and chemical oxygen demand (COD) in the OsPBR feed solution by 44.4%, 85.6%, and 77.5%. Due to the forward cation flux and precipitation, intermittent supplements of K⁺, Mg²⁺, Ca²⁺, and SO4²⁻ salts further stimulated algal growth and culture densities by 58.7%. With an optimal hydraulic retention time (HRT) of 3.33 d, the OsPBR overcame NH4⁺-N overloading and stabilized key nutrients NH4⁺-N at ∼ 2.0 mg L⁻¹, PO4³⁻-P < 0.6 mg L⁻¹, and COD < 30 mg L⁻¹. A moderate nitrogen reduction stress resulted in a high carbohydrate content (51.3 ± 0.1%) among microalgal cells. A solids retention time (SRT) of 17.82 d was found to increase high-density microalgae by 3-fold with a high yield of both lipids (9.07 g m⁻³ d⁻¹) and carbohydrates (16.66 g m⁻³ d⁻¹). This study encourages further exploration of the OsPBR technology for simultaneous recovery of high-quality water and production of algal biomass for value-added products.
The great potential of bioelectrochemical systems (BESs) in pollution control combined with energy recovery has attracted increasing attention. Classified by their functions in the BES, microorganisms including degraders, electricigens, and element cycle-related microbes play key roles in pollutant degradation and electricity generation, and the functions of these microbes are affected by various environmental and operating conditions. This review systematically summarizes the effects of crucial conditions on the efficiency of the process of contaminant removal combined with electricity generation in BESs, with particular focus on the pH, temperature, conductivity, substrates, inoculums, magnetic field and reactor design parameters, such as architecture, electrode material, and electrode potential. The aim of this review is to help reveal the microbial functions during the bioelectrochemical remediation of environmental media and to optimize the system by determining the appropriate conditions for functional microorganisms, thus better promoting the transition of BESs from the laboratory to actual applications.
The influences of inorganic draw solutes on the performance of the thin-film composite forward osmosis (TFC–FO) membrane in microfiltration (MF) assisted anaerobic osmotic membrane bioreactors (AnMF–OMBRs) were investigated in this study. The results indicated that compared to sodium chloride (NaCl) at the same osmotic pressure, magnesium chloride (MgCl2) led to a higher flux decline of the TFC–FO membrane, induced by more severe membrane fouling. In addition, the NaCl and MgCl2 had no impacts on the rejection for organic matters by the TFC–FO membrane. However, the NH4⁺–N rejection of TFC–FO membrane was neglected in the AnMF–OMBR with NaCl as draw solute, while it was enhanced to a range of 57.5–87.6% for the draw solute MgCl2. The different NH4⁺–N rejection and membrane fouling of TFC–FO membrane with draw solutes NaCl and MgCl2 could be attributed to the Donnan potential. As for NaCl, more Na⁺ diffused into the mixed liquor resulting in NH4⁺–N passing through the TFC–FO membrane to the draw solution for keeping a charge balance. With regard to MgCl2, more Cl⁻ passing through the FO membrane to the mixed liquor led to an accumulation of NH4⁺–N in the reactor. Moreover, Mg²⁺ passing from the draw solution to the mixed liquor enhanced the biofouling on the active layer of the FO membrane, and in the meanwhile more anions passing to the draw solution aggravated the inorganic fouling of the support layer.
In this paper, three different fertilizer draw solutions were tested in a novel forward osmosis-microfiltration aerobic membrane bioreactor (MF-FDFO-MBR) hybrid system and their performance were evaluated in terms of water flux and reverse salt diffusion. Results were also compared with a standard solution. Results showed that ammonium sulfate is the most suitable fertilizer for this hybrid system since it has a relatively high water flux (6.85 LMH) with a comparatively low reverse salt flux (3.02 gMH). The performance of the process was also studied by investigating different process parameters: draw solution concentration, FO draw solution flow rate and MF imposed flux. It was found that the optimal conditions for this hybrid system were: draw solution concentration of 1 M, FO draw solution flow rate of 200 mL/min and MF imposed flux of 10 LMH. The salt accumulation increased from 834 to 5400 μS/cm during the first four weeks but after integrating MF, the salinity dropped significantly from 5400 to 1100 μS/cm suggesting that MF is efficient in mitigating the salinity build up inside the reactor. This study demonstrated that the integration of the MF membrane could effectively control the salinity and enhance the stable FO flux in the OMBR.
Microbial fuel cells (MFCs) offer the possibility to convert the chemical energy contained in low-cost organic matter directly into electrical energy. Nevertheless, in the current state of the art, microbial electrocatalysis imposes the use of electrolytes of low ionic conductivity, at around neutral pH and with complex chemical compositions, which are far from being ideal electrolytes for an electrochemical process. In this context, ion transport through the electrolyte is a key step, which strongly conditions the electrode kinetics and the global cell performance.
The development of suitable draw solution in forward osmosis (FO) process has attracted the growing attention for water treatment purpose. In this study, a series of organic phosphonate salts (OPSs) are synthesized by one-step Mannich-like reaction, confirmed by FTIR and NMR characterizations, and applied as novel draw solutes in FO applications. Their solution properties including osmotic pressures and viscosities, as well as their FO performance as a function of the solution concentration are investigated systematically. In FO process, a higher water flux of 47~54 LMH and a negligible reverse solute flux can be achieved in the PRO (AL-DS) mode (skin layer faces the draw solution) using a home-made thin-film composite membrane (PSF-TFC) and deionized water as the feed solution. Among all OPS draw solutes, the tetraethylenepentamine heptakis(methylphosphonic) sodium salt (TPHMP-Na) exhibits the best FO flux at 0.5 mol/kg concentration, which is further applied for the separation of emulsified oil-water mixture. The recovery of diluted OPS solutions is carried out via a nanofiltration (NF) system with a rejection above 92%. The aforementioned features show the great potential of OPS compounds as a novel class of draw solutes for FO applications.
A green and sustainable technique is desired for juice concentration to increase its shelf life and save the transportation cost. In this study, we investigated an integrated forward osmosis-membrane distillation (FO-MD) process for juice concentration and developed a food preservative potassium sorbate as a renewable draw solute. The upstream FO process was used to concentrate apple juice in ambient operation conditions for preserving juice nutrition and flavor. Potassium sorbate preservative was developed as draw solute to minimize the accumulation of a draw solute in the concentrated juice without interfering juice flavor, and the slow diffusion of potassium sorbate preservative across the FO membrane to the feed juice concentrate can also prevent the bacterial growth during the concentration process by taking advantage of reverse salt flux. The downstream MD process was used to recover potassium sorbate draw solutes from the FO effluent and maintain the constant draw solute concentration for achieving stable water flux of FO membranes. Thin-film composite (TFC) polyamide membranes and poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) electrospun nanofibrous membranes were fabricated and employed in the FO and MD processes, respectively. Results illustrate that the integrated FO–MD process presents a synergistic flux balancing behavior and achieves a constant water flux for both FO and MD membranes. The FO–MD process was able to continuously concentrate apple juice over long-term bench-scale operation. Importantly, the concentrated apple juice has almost no loss in nutrition and also has very low amount of potassium sorbate (0.45 g/L) far below the required maximum level in food industry (1.00 g/L). Our work provides a food preservative potassium sorbate draw solute facilitated FO-MD process for juice concentration, which may have practical application potentials in the food processing.
Bioelectrochemical systems (BESs) are versatile electrochemical technologies that use microbial catalysts for simultaneously harvesting energy and treating wastewater. However, there is a consensus that practical energy applications and clean water production remain technically challenging for stand-alone BESs. To address these technological challenges, membrane-based technologies for water/wastewater treatment and energy production, such as electrodialysis, forward osmosis, reverse electrodialysis, and pressurized filtration (e.g., ultrafiltration), have been integrated into BESs. This integration has created new systems including microbial desalination cells, osmotic microbial fuel cells, pressurized filtration-microbial fuel cells, and microbial reverse-electrodialysis cells. This article aims to provide a comprehensive review on the recent progress in BESs integrated with membrane-based technologies, discuss advantages and limitations, and present outlooks toward further development of these technologies.
Forward osmosis (FO) has emerged as a potentially energy-efficient membrane treatment technology to yield high-quality reusable water from various wastewater/saline water sources. A key challenge remained to be solved for FO is reverse solute flux (RSF), which can cause issues like reduced concentration gradient and loss of draw solutes. Yet no universal parameters have been developed to compare RSF control performance among various studies, making it difficult to position us in this “battle” against RSF. In this paper, we have conducted a concise review of existing RSF reduction approaches, including operational strategies (e.g., pressure-, electrolysis-, and ultrasound-assisted osmosis) and advanced membrane development (e.g., new membrane fabrication and existing membrane modification). We have also analyzed the literature data to reveal the current status of RSF reduction. A new parameter, mitigation ratio (MR), was proposed and used together with specific RSF (SRSF) to evaluate RSF reduction performance. Potential research directions have been discussed to help with future RSF control. This review intends to shed more light on how to effectively tackle solute leakage towards a more cost-effective and environmental-friendly FO treatment process.
Reverse solute flux (RSF) is a key challenge for operating osmotic membrane bioreactors (OMBRs) and can severely affect the OMBR performance due to solute accumulation in the feed side. Herein, biodegradable polyelectrolyte (polyacrylic acid sodium salt, PAA) was investigated as a draw solute (DS) for the OMBR application with a focus on the mitigation of solute build-up via biodegradation of PAA. Both activated sludge and OMBR tests confirmed that PAA can be biologically degraded. In the OMBR, a residue PAA concentration of 14.1 mg L-1 was observed in the feed solution due to a dynamic balance between PAA removal/biodegradation and continuing RSF. Compared to a non-biodegradable DS – NaCl that had the water recovery of 3.4 ± 0.1 L of water recovery and 9.46 ± 0.21 gMH of reverse solute flus (RSF), the PAA DS achieved a higher water recovery at 17.9 ± 0.5 L over 30-days operation (the average water flux, 4.97± 0.14 LMH), much lower RSF at 0.12 ± 0.01 gMH, and higher nitrogen removal at 58.4 ± 1.5%. The recovery and reuse of the PAA DS was investigated with a long-term operation (150 days), which demonstrated the feasibility of reusing PAA DS, despite some decrease in water recovery and biological treatment performance. Those results have collectively demonstrated the advantages of PAA-based draw solute for reducing reverse-fluxed solutes towards optimized OMBR operation and applications.
Anaerobic digesters (AD) and bioelectrochemical systems (BES) are becoming increasingly popular technologies for the generation of renewable energy from wastes. Synergies between these technologies exist, however, configurations to couple them have been insufficiently investigated. This study compares the theoretical energy efficiencies of converting waste directly into electricity, using AD and BES alone and in various combinations. This study reviews the experimentally demonstrated energy efficiencies reported in the literature with comparisons to the maximum theoretical efficiencies, considering thermodynamic limits. Acetate is used as an ideal substrate for theoretical calculations, whereas complex wastes are used for extended analyses of practical efficiencies. In addition, to evaluate the economic potential of this technology, a brief case study was conducted using the Oak Ridge National Laboratory (ORNL) water resource recovery facility (WRRF). Sensitivity analysis was performed on several parameters in the economic model. The results of this study indicate the combined Anaerobic Digester/Microbial Electrolysis Cell (ADMEC) process may be the best path forward due to the high energy efficiency, combined with potential economic benefits, but is not at commercial readiness. We estimate energy efficiencies of 52.9% and 45.6% for the ADMEC process, using current state-of-the-technology, for converting food waste and sewage sludge to a CH4/H2 mix, respectively. This study concludes with a discussion of new strategies to improve the energy efficiency of AD and BES processes.
Significance
The analysis performed in this study supports the implementation of anaerobic digestion with bioelectrochemical systems for the production of energy from complex wastes. The energy efficiency analysis alludes to research areas that should be pursued to maximize the performance of these technologies in large-scale installation, based on the performance gaps between theoretical and practical energy efficiencies determined in previous studies.
High hydrostatic pressure (HHP) and electrodialysis with ultrafiltration membranes (EDUF) are two efficient technologies used respectively to improve protein enzymatic hydrolysis and recovery of bioactive peptides, but they have never been tested together. Hence, in this study, HHP pre-treatment was performed on defatted flaxseed protein isolate prior to enzymatic hydrolysis and the resulting peptides were separated by EDUF. HHP pretreatment influenced particle size, protein conformation, and degree of hydrolysis. After EDUF separation, peptide fractions (generated after enzymatic hydrolysis of control and pressure-treated protein isolate) recovered in KCl fraction were enriched in arginine and associated with a decrease in systolic blood pressure (SBP) in spontaneously hypertensive rats. Additionally, the final EDUF hydrolysate generated from pretreated protein and the initial EDUF hydrolysate from native protein were also associated with lower SBP. However, only the control KCl fraction obtained from native protein hydrolysate was associated with anti-diabetic activity.
Wastewater phosphorus (P) released into natural water bodies such as lakes and rivers, can cause water pollution as a result of eutrophication. If this P is effectively removed from wastewaters and economically recovered for use as fertilisers, not only can the water pollution be controlled, but also reduce the anticipated global shortage of P. This scarcity will result from the natural phosphate rock reserve being exhausted. Three experiments were conducted using membrane-bioreactor effluent (MBR, 35 mg PO4/L) and reverse osmosis concentrate (ROC, 10 mg PO4/L) waters to supply phosphate, and sea water (1530 mg Mg/L) to supply Mg for the production of struvite. The phosphate in the MBR and ROC was concentrated approximately 15 times by adsorption onto an ion exchange resin column followed by desorption. Struvite was precipitated by mixing the desorbed solution with seawater and NH4Cl. The chemical composition and mineral structure of the precipitates agreed with those of the reference struvite. When Ca in seawater (300 mg Ca/L) was removed before mixing the water with MBR or ROC, the purity of the struvite improved.
A key challenge for osmotic membrane bioreactors (OMBRs) application is reverse solute flux and consequent salt accumulation in the feed side. Herein, a bioelectrochemical system (BES) was employed to drive reverse-fluxed solutes from the feed of an OMBR into a cathode compartment for recovery and subsequent reuse as a draw solute (DS). Compared to an OMBR without BES function, the present OMBR system enhanced water recovery from 925 to 1688 mL and increased the chemical oxygen demand (COD) removal efficiency from 40.2 ± 8.1 to 75.2 ± 3.3%, benefited from its lower anolyte conductivity of 9.0 mS cm-1 than that of the control system (24.1 mS cm-1). The CO2 addition significantly improved the ammonia recovery rate to 93.3-116.7 g N m-3 h-1 (or 248.0-307.4 g N m-2 d-1), 12.1-14.5 times higher than that without CO2 addition. The recovered DS was successfully applied to accomplish water extraction in the reuse test, and such a recovery/reuse process could result in a normalized water recovery of 3870 mL mol DS-1 or a DS usage of 0.26 mol L-1 (of the recovered water). The energy consumption of the system might be compensated by the production of bioenergy, and the net specific energy consumption was estimated to be 0.004-0.112 kWh m-3 wastewater, 0.007-0.179 kWh kg-1 removed COD, or 0.001-0.020 kWh kg-1 recovered NH4+-N. Those results have demonstrated that bioelectrochemical processes can be an effective approach for in situ mitigation of reverse-fluxed solute in OMBR and recovering "the lost DS" towards both reuse and reduced operational expense.
Sulfate-reducing bioprocesses can be used in the treatment of sulfate-containing industrial effluents including mining, metallurgical and food industries. Although membrane bioreactors (MBRs) have been studied for anaerobic wastewater treatment, sulfidogenic anaerobic MBRs (AnMBRs) received little or no attention. The aim of this study was to evaluate the performance of sulfate-reducing AnMBR and investigate the membrane foulants and filtration characteristics. Sulfate reduction and COD oxidation efficiencies exceeded 90% when the reactor was fed with a synthetic sulfate-rich (2000 mg/L) wastewater with COD/sulfate ratio of 0.75. Long-term filtration performance was evaluated using several filterability tests. Deposition of high molecular weight soluble organics and S, Si, Fe, Cu, Na, and Mg were detected in the cake layer. The formation of metal-sulfide precipitates was the main reason for heavy metal deposition on the membrane surface. Desulfovibrio-like sulfate reducing bacteria was detected in the bioreactor. Results showed that sulfate-reducing AnMBR offers potential for real scale applications.
Fluoride and nitrate ions are the main pollutants in the photovoltaic cells manufacturing effluent. Their presence in the effluent is due to the extensive use of HF and HNO3 acids during the silicon wafers production process. This work deals with the electrodialysis application to remove these ions. Synthetic solutions were used for the investigation of the main operational factors affecting the treatment performance; such as current intensity, initial pollutants concentration and pH. Significant fluoride removal was obtained in an acidic or neutral medium after a relatively short treatment time of 6 minutes. Nitrate ions were also removed showing an efficiency of 98% after 18 minutes of treatment using a current intensity of 0.1 A and for an initial concentration of 1000 ppm. The study of competition between these two ions during their eliminations showed that the fluoride ions presence does not affect the nitrate removal, while; the nitrate ions presence in the solution delays the fluoride removal from 7 to 20 minutes, and makes the removal less effective. The results obtained can be used to optimize the pollutant removal recovery at large scale.
Phosphorus recovery attracts increasing attentions because phosphorus is a non-renewable and exhausted resource. This study for the first time remarkably improves the efficiency of phosphorus recovery in the form of struvite using the mixture of two kinds of phosphorus-rich wastewater (i.e., anaerobic sludge supernatant and human urine). An optimal volumetric ratio (VRs/f) of 1:9 of fresh and 10-d stale human urine could yield phosphorus recovery of 90.8% at [Mg²⁺]:[NH4⁺-N]:[PO4³⁻-P] = 1.2:1.05:1, pH 10, and reacting time 15 min. The SEM and XRD analyses confirmed the precipitates were struvite. Struvite precipitation was determined by relative supersaturation in both nucleation and crystal growth phases, with homogenous nucleation predominated in the latter phase. One liter of mixed urine could recover the PO4³⁻-P content in 41.9 L of anaerobic sludge supernatant to form 182.3 g of struvite.
Improving environmental protection and finding sustainable and renewable resources of nutrients are core issues in circular bioeconomy. Thus, this study evaluated the efficiency of recovering struvite, MgNH4PO4·6H2O, from different agro-industrial wastewaters (four highly loaded reject waters of anaerobically co-digested agro-industrial waste and a raw swine slurry) and assessed the quality of recovered struvite crystals and their reusability as fertilizer. The efficiency of crystallization (Ec40-80%) and amount of struvite in the precipitate (Pp55-94%) highly varied due to the characteristics of influent wastewaters, particularly to the content of competing elements, such as alkaline and heavy metals and total solids (TS). In particular, Ec(94, 75, 61%) and Pp(76, 66, 48%) decreased at increasing TS (0.57, 0.73, 0.99%), demonstrating the hindering effect of solid content on struvite recovery and quality. According to X-ray diffraction analysis, the structure of all isolated samples corresponded to crystalline, orthorhombic struvite, which exhibited high purity (32-48 g/kgdN, 114-132 g/kgdP, and 99-116 g/kgdMg) containing only a few foreign elements, whose amount depended on the characteristics of the influent wastewater. All struvite contained other plant macronutrients (K, Ca) and many micronutrients (Fe, Na, Cu, Mn, Co, Zn) that further enhance its agronomic value. Therefore, this study showed that struvite can be successfully recovered from a wide range of highly loaded agroindustrial wastewaters, and that the quality of the recovered struvite could be suitable for reuse in agriculture.
Forward osmosis (FO) has increasingly penetrated into traditional process, i.e. microalgae dewatering, to become one of the new energy saving technologies. However, FO membranes can be fouled by soluble algal products (SAP) and fouling behavior of the membrane is not clear due to the variety of SAP composition. In this work, two types of membranes including a self-made TFC and a commercial CTA, and three kinds of draw solutions including NaCl, MgCl2 and CaCl2 were adopted to investigate the dewatering of SAP from microalgae of Chlorella vulgaris. The dependence of permeate flux and foulants composition on the membranes, membrane orientation and reverse salt diffusion were then compared for membrane fouling behavior. The results showed that TFC membrane exhibited higher water permeability but more loss of water flux in comparison with CTA. The SAP was inclined to be adsorbed by TFC membrane while adsorbate was easier to be removed by physical cleaning. By contrast, the pollutants accumulated on the membrane surface of CTA were much more irreversible. The interaction between SAP from feed solution and calcium cations from draw solution induced the formation of cake layer on the surface of TFC membrane while irreversible granular deposits of SAP were found accumulated on CTA membrane surface. Possible membrane fouling mechanism was finally discussed for better fabrication of anti-fouling forward osmosis membranes, thus to propel the integration of forward osmosis with traditional field of microalgae dewatering.
Hybrid forward osmosis (FO) processes such as forward osmosis with membrane bioreactors (FO-MBR), electrodialysis (FO-ED), nanofiltration (FO-NF) or reverse osmosis (FO-RO) present promising technologies for wastewater reuse in agriculture as they meet high effluent quality requirements, especially regarding boron and/or salt content. An FO-NF demonstration plant for this application was built and operated treating 3 m³ h⁻¹ of real wastewater with a salinity of 3-5 mS cm⁻¹ and 1.5 mg L⁻¹ of boron in continuous mode for 480 days. Three draw solutions (DS) were evaluated in different periods of experimentation. Sodium polyacrylate led to reversible fouling on the FO and NF membranes and the permeate was not suitable for irrigation. Magnesium sulphate, used as DS in a second phase, generated severe irreversible fouling on NF membranes and therefore it was discarded. Finally, magnesium chloride showed the best performance, with FO-NF membranes presenting a stable permeability and low membrane fouling during long-term operation. The FO-NF permeate showed high quality for irrigation, achieving a conductivity value of 1 mS/cm, a boron concentration below 0.4 mg L⁻¹ and an average SAR of 1.98 (mequ L⁻¹)0.5. DS replacement costs were reduced by working with high rejection NF membranes. However, energy consumption costs associated with the NF step make the global process more energy intensive than conventional technology.
In this work, the dynamic changes in the composition of fouling layer as a function of operating time were systematically investigated and compared in forward osmosis (FO) based membrane processes for municipal wastewater treatment. Fouled membranes were collected from four different operational cycles (3, 8, 16 and 30 days) and prepared for quantitative analyses of organic and inorganic foulants and microscopic observation by confocal laser scanning microscopy (CLSM). Moreover, the dynamic changes of bacterial concentration and community structure were characterized by the plate count method and Illumina MiSeq sequencing, respectively. The results showed that with the extension of operation time, the amount of organic foulants, inorganic species and bacteria deposited on the membrane surface exhibited a gradual increase trend in direct FO, resulting in a corresponding increase of fouling resistance and decline of water flux. As for OMBR, apart from the organic foulants such as polysaccharides and proteins, both the inorganic species and membrane surface bacterial concentration reached to a plateau phase after 8 days of operation, leading to a much lower fouling resistance and flux reduction. The bacterial community analysis indicated that two dominant genus were detected in the biofilm of OMBR; however, the bacterial community on the membrane surface of direct FO exhibited a significantly higher diversity but with a lower abundance.
The present study examined the influence of metal ions (Cu²⁺, Pb²⁺, and Zn²⁺) on kinetics and morphology of struvite precipitated from aqueous solutions containing equimolar ratios of struvite components: Mg²⁺, NH4⁺, PO4⁻³. Kinetics of the struvite precipitation in the presence of metal ions [0,1, 10, 50 and 100 ppm] was evaluated through the change of pH of the precipitating solution. The kinetic evaluation demonstrated that the precipitation satisfactorily followed the first-order kinetic with respect to Mg²⁺. It was found that for the three metal ions tested, the higher the concentrations of the metal ions: 0, 1, 10, 50, 100 ppm, the less the crystals obtained and the lower the rate constants. Depending on the concentrations of the metal ions added into the solution, the rate constants varied from 4.344 to 1.056 h⁻¹ which agree with most published values. It was postulated that the metal ions were adsorbed onto the surface of the crystals and hence retarded the growth. The crystals obtained were characterized using SEM-EDX and XRPD Rietveld. The characterization revealed that the precipitates were mainly struvite of various sizes (between 10 and 60 μm) with sylvite as impurities. It is envisaged that the present study would add to the understanding of the removal of metal ions from industrial wastewater through struvite precipitation.
Forward osmosis (FO) draws attention due to its advantages compares to traditional pressure-driven membrane processes. In this study, a FO membrane concentrating system was built for sewage concentration to investigate membrane rejection, concentrating effect, membrane fouling behavior. Sewage could be concentrated to 1/10 original volume by FO membrane, while pollutants concentrating multiple could not reach 10. The FO membrane had excellent rejecting effect, with effluent COD, ammonia nitrogen, total nitrogen, total phosphorus concentration of 18, 2.5, 2.8, 0.4 mg/L, respectively. The FO membrane flux was mainly associated with the draw solution (DS) concentration, which increased with DS concentration but more severe membrane fouling engendered in the meantime. Scanning electronic microscope and fourier transform infrared spectroscopy analysis indicated the formation and constitution of the fouling layer, which included humic acid, protein, and polysaccharide. After concentration, fouled FO membrane was remitted by physical and chemical cleaning, with recovery of 90% and 96%.
The membrane fouling formation during electrodialysis of complex solutions is one of the main issues affecting the process performance and costs. This work was focused on the investigation of electrochemical behavior of membrane systems containing a cation-exchange membrane whose surface was affected by mineral fouling of different composition. Together with the scaled membranes, the pristine membrane was studied for comparison. For membrane without scaling on its surface, it was found that the limiting current value exceeded the one theoretically calculated by the convection-diffusion model. It is most likely related to equilibrium electroconvection developed at the membrane surface. The presence of magnesium and calcium hydroxides on the membrane surface leads to an intensification of water splitting at the depleted membrane surface, resulting in suppression of electroconvection and reduction of the overlimiting current. The presence of calcium carbonate on top of magnesium and calcium hydroxides prevents their contact with water molecules. The current-voltage characteristic of such membrane system was almost identical to the characteristics of the membrane system containing the pristine membrane. To our knowledge, it was the first time that the impact of scaling nature on the electrochemical behavior of membrane system was revealed and the relative mechanisms identified and explained.
Abstract Landfill leachate contains substances that can be potentially recovered as valuable resources. In this study, magnesium in a landfill leachate was recovered as struvite with calcium pretreatment; meanwhile, the leachate volume was reduced by using a submerged forward osmosis (FO) process, thereby enabling significant reduction of further treatment footprint and cost. Without pretreatment, calcium exhibited strong competition for phosphate with magnesium. The pretreatment with a Ca2 +: CO32– molar ratio of 1:1.4 achieved a relatively low loss rate of Mg2 + (24.1 ± 2.0%) and high Ca2 + removal efficiency (89.5 ± 1.7%). During struvite recovery, 98.6 ± 0.1% of magnesium could be recovered with a significantly lower residual PO43 −-P concentration (< 25 mg L− 1) under the condition of (Mg + Caresidual): P molar ratio of 1:1.5 and pH 9.5. The obtained struvite had a similar crystal structure and composition (19.3% Mg and 29.8% P) to that of standard struvite. The FO process successfully recovered water from the leachate and reduced its volume by 37%. The configuration of calcium pretreatment - FO - struvite recovery was found to be the optimal arrangement in terms of FO performance. These results have demonstrated the feasibility of magnesium recovery from landfill leachate and the importance of the calcium pretreatment, and will encourage further efforts to assess the value and purity of struvite for commercial use and to develop new methods for resource recovery from leachate.
Osmosis membrane bioreactors (OMBRs), which integrate forward osmosis (FO) and a biological process, followed by reverse osmosis or membrane distillation (MD) have been receiving increasing attention for wastewater treatment and reuse. However, OMBR application in wastewater treatment is still hindered by the accumulation of inorganic and organic salts, which affects microbial activity in the OMBR. Therefore, in this study, a novel hybrid OMBR–MD system integrated with periodic microfiltration (MF) extraction was developed for simultaneous salinity reduction and phosphorus recovery using a magnesium-based draw solute by taking advantage of magnesium salt reversal. In the OMBR system, MgCl2 was used as the draw solution to withdraw clean water passing through the FO membrane, whereby all contaminants and mineral salts, including phosphate, ammonia and magnesium reversed from the draw solution, were retained in the bioreactor. The MF membrane was used to bleed the water out of the bioreactor for salinity reduction and subsequent phosphorus recovery. The pH of the effluent from the MF containing high phosphorus was adjusted to 10 to precipitate struvite, and the amount of the produced struvite was quantitatively determined to be 41 mg per liter of the MF permeate. The MD process was used to recover the diluted MgCl2 draw solution with an initial flux of 8.2 L/m² h under a temperature difference of 30 °C (55 °C in the feed and 20 °C in the distillate). Subsequently, the flux slightly decreased to 6.3 L/m² h after 6 h because of the decreasing vapor pressure in the salt solution based on Raoult's law.
Biofouling in fertilizer-drawn forward osmosis (FDFO) for water reuse was investigated by spiking pure bacteria species Pseudomonas aeruginosa PAO1+GFP and using three different fertilizers KNO3, KCl and KH2PO4 as draw solutions. The performance of FO process for treating synthetic wastewater was assessed and their influence on the membrane fouling and in particular biofouling was evaluated relative to the type of different fertilizers used and their rates of reverse diffusion. FO performances using KNO3 as draw solute exhibited severer flux decline (63%) than when using KCl (45%) and KH2PO4 (30%). Membrane autopsy indicated that the mass of organic foulants and biomass on fouled membrane surface using KNO3 as draw solute (947.5 mg/m² biopolymers, 72 µm biofilm thickness and 53.3 mg/m² adenosine triphosphate) were significantly higher than that using KCl (450 mg/m² biopolymers, 33 µm biofilm thickness and 28.2 mg/m² ATP) and KH2PO4 (440 mg/m² biopolymers, 35 µm biofilm thickness and 33.5 mg/m² ATP). This higher flux decline is likely related to the higher reverse diffusion of KNO3 (19.8 g/m²/h) than KCl (5.1 g/m²/h) and KH2PO4 (3.7 g/m²/h). The reverse diffused potassium could promote the organics and bacterial adhesion on FO membrane via charge screening effect and compression of electrical double layer. Moreover, reverse diffused nitrate provided increased N:P nutrient ratio was favorable for the bacteria to grow on the feed side of the FO membrane.
Nanofiltration was thought to be a good option for the recovery of perfluorohexanoic acid (PFHxA) from industrial wastewater. In this study, two commercially available nanofiltration (NF) membranes (NF 270 and NTR-7450) were tested to concentrate the PFHxA in aqueous solution. Filtration test was conducted in crossflow filtration mode. Membrane flux and PFHxA rejection rate were monitored throughout the filtration test. The impact of initial feed water pH on membrane performance was investigated. Results demonstrated that the two NF membranes showed different response to the change of initial feed water pH, which was caused by the intrinsic properties of membrane material. The flux performance of NF 270 was stable, while its rejection rate of PFHxA was very sensitive to the change of initial feed water pH. Opposite result was obtained with NTR-7450. It has a very good stability on rejection rate, while its flux is very sensitive to the change of initial feed water pH. The mechanisms behind these phenomena were also discussed. The results obtained in this study should be very useful for the process design in practical engineering.
An intensive evaluation of draw solutions (DS) was performed by focusing on the wastewater reuse applications of hybrid forward osmosis (FO) processes. The substances studied were potassium formate, potassium phosphate, magnesium sulphate, sodium chloride, sodium polyacrylate and polyethylene glycol, and their osmotic pressure, conductivity, pH, thermostability, sunlight exposure, toxicity, FO filtration performance and replenishment costs were determined. Additionally, commercially available FO membrane modules were evaluated at pilot scale. The results revealed that the most relevant DS properties for wastewater reuse under the studied conditions were the DS regeneration method, DS replacement price, pH adjustment and toxicity. These properties were shown to be more relevant than filtration flux when a maximum DS osmotic pressure value of 10 bar was used. This was the limit for efficient DS recovery. When the different FO membranes were compared, thin-film composite (TFC) flat-sheet membranes showed the highest flux and the highest salt rejection, and the lowest permeability and salt rejection values were presented by cellulose triacetate (CTA) hollow fibre membranes. Based on the information obtained, a TFC-FO/nanofiltration (NF) demonstration plant will be constructed next to the wastewater treatment plant (WWTP) in San Pedro del Pinatar, in the region of Murcia (Spain). This represents the world’s first FO demonstration plant for municipal wastewater reclamation and its results will allow this technology to be evaluated for wastewater reuse for agricultural purposes.
This study investigated the impact of reverse salt flux (RSF) on microbe community and bio-methane production in a simulated fertilizer driven FO-AnMBR system using KCl, KNO3 and KH2PO4 as draw solutes. Results showed that KH2PO4 exhibited the lowest RSF in terms of molar concentration 19.1 mM/(m².h), while for KCl and KNO3 it was 32.2 and 120.8 mM/(m².h), respectively. Interestingly, bio-methane production displayed an opposite order with KH2PO4, followed by KCl and KNO3. Pyrosequencing results revealed the presence of different bacterial communities among the tested fertilizers. Bacterial community of sludge exposed to KH2PO4 was very similar to that of DI-water and KCl. However, results with KNO3 were different since the denitrifying bacteria were found to have a higher percentage than the sludge with other fertilizers. This study demonstrated that RSF has a negative effect on bio-methane production, probably by influencing the sludge bacterial community via environment modification.
Environmental and economic impacts of the fertilizer drawn forward osmosis (FDFO) and nanofiltration (NF) hybrid system were conducted and compared with conventional reverse osmosis (RO) hybrid scenarios using microfiltration (MF) or ultrafiltration (UF) as a pre-treatment process. The results showed that the FDFO-NF hybrid system using thin film composite forward osmosis (TFC) FO membrane has less environmental impact than conventional RO hybrid systems due to lower consumption of energy and cleaning chemicals. The energy requirement for the treatment of mine impaired water by the FDFO-NF hybrid system was 1.08 kWh/m³, which is 13.6% less energy than an MF-RO and 21% less than UF-RO under similar initial feed solution. In a closed-loop system, the FDFO-NF hybrid system using a TFC FO membrane with an optimum NF recovery rate of 84% had the lowest unit operating expenditure of AUD $0.41/m³. Besides, given the current relatively high price and low flux performance of the cellulose triacetate and TFC FO membranes, the FDFO-NF hybrid system still holds opportunities to reduce operating expenditure further. Optimizing NF recovery rates and improving the water flux of the membrane would decrease the unit OPEX costs, although the TFC FO membrane would be less sensitive to this effect.
The fertilizer-drawn forward osmosis (FDFO) was investigated for treating coal seam gas (CSG) produced water to generate nutrient rich solution for irrigation. Its performance was evaluated and compared with reverse osmosis (RO) in terms of specific energy consumption (SEC) and nutrient concentrations in the final product water. The RO-FDFO hybrid process was developed to further improve FDFO. The results showed that FDFO has the lowest SEC followed by the RO-FDFO and RO processes. The final nutrient concentration simulation demonstrated that the RO-FDFO hybrid process has lower final concentration, higher maximum recovery and lower nutrient loss than the stand alone FDFO process. Therefore, it was suggested that the RO-FDFO is the most effective treatment option for CSG produced water as well as favorable nutrient supply. Lastly, membrane fouling mechanism was examined in CSG RO brine treatment by FDFO, and the strategies for controlling fouling were critically evaluated. KNO3 exhibited the highest flux decline corresponding to the highest reverse salt flux, while the most severe membrane scaling was observed with calcium nitrate, primarily due to the reverse transport of calcium ions. To control membrane fouling in FDFO process, both physical flushing and chemical cleaning were examined. Membrane cleaning with citric acid of 5% resulted in a complete flux recovery.
Recovery of nutrients, water, and energy from high-strength sidestream centrate offers benefits such as reusable resource, minimized discharge and cost-savings in mainstream treatment. Herein, a microbial electrolysis cell - forward osmosis (MEC-FO) hybrid system has been investigated for integrated nutrient-energy-water (NEW) recovery with emphasis on quantified mass balance and energy evaluation. In a closed-loop mode, the hybrid system achieved recovery of 54.2 ± 1.9% of water (70.4 ± 2.4 mL), 99.7 ± 13.0% of net ammonium nitrogen (8.99 ± 0.75 mmol, with extended N2 stripping), and 79.5 ± 0.5% of phosphorus (as struvite, 0.16 ± 0.01 mmol). Ammonium loss primarily from reverse solute flux was fully compensated by the reclaimed ammonium under 6-h extended N2 stripping to achieve self-sustained FO process. The generated hydrogen gas could potentially cover up to 28.7 ± 1.5% of total energy input, rendering a specific energy consumption rate of 1.73 ± 0.08 kWh m−3 treated centrate, 0.57 ± 0.04 kWh kg−1 COD, 1.10 ± 0.05 kWh kg−1 removed NH4+-N, 1.17 ± 0.06 kWh kg−1 recovered NH4+-N, or 5.75 ± 0.54 kWh kg−1 struvite. Recycling of excess Mg2+ reduced its dosage to 0.08 kg Mg2+/kg struvite. These results have demonstrated the successful synergy between MEC and FO to achieve multi-resource recovery, and encouraged further investigation to address the challenges such as enhanced hydrogen production, reducing nutrient loss, and optimizing MEC-FO coordination towards an energy-efficient NEW recovery process.
In this study, the behavior of organic micro-pollutants (OMPs) transport including membrane fouling was assessed in fertilizer-drawn forward osmosis (FDFO) during treatment of the anaerobic membrane bioreactor (AnMBR) effluent. The flux decline was negligible when the FO membrane was oriented with active layer facing feed solution (AL-FS) while severe flux decline was observed with active layer facing draw solution (AL-DS) with di-ammonium phosphate (DAP) fertilizer as DS due to struvite scaling inside the membrane support layer. DAP DS however exhibited the lowest OMPs forward flux or higher OMPs rejection rate compared to other two fertilizers (i.e., mono-ammonium phosphate (MAP) and KCl). MAP and KCl fertilizer DS had higher water fluxes that induced higher external concentration polarization (ECP) and enhanced OMPs flux through the FO membrane. Under the AL-DS mode of membrane orientation, OMPs transport was further increased with MAP and KCl as DS due to enhanced concentrative internal concentration polarization while with DAP the internal scaling enhanced mass transfer resistance thereby lowering OMPs flux. Physical or hydraulic cleaning could successfully recover water flux for FO membranes operated under the AL-FS mode but only partial flux recovery was observed for membranes operated under AL-DS mode because of internal scaling and fouling in the support layer. Osmotic backwashing could however significantly improve the cleaning efficiency.
This study investigated the use of multiple-solute salts as potential draw solution (DS) for forward osmosis (FO) process. The novel concept of applying readily available waste byproducts as DS is briefly described in this paper. Two organic salts (sodium acetate and sodium formate) were used as draw solutes. The results indicated that the water flux performance was consistent with the osmotic pressure of the DS and that the binary-solute DS (each with 0.5 M concentration) was capable of achieving comparable water flux compared to the 1.0 M single-solute DS. Considering the variation of byproduct concentration during fermentation, a newly developed ternary-solute DS is introduced by employing NaCl additive. The physicochemical properties including osmotic pressure and viscosity were calculated by OLI Stream Analyzer software. Coupling these organic salts with NaCl demonstrated improvement of the DS including osmotic pressure and water flux. Succinate rejections of greater than 99% were obtained indicating insignificant succinate loss to the DS compartment. Although an increase of the reverse chloride flux was observed at increasing NaCl additive concentration, the acetate and formate ions demonstrated reduction of specific reverse solute flux attributed to higher water flux shown by ternary-solute DS.
Ethylenediaminetetraacetic acid disodium (EDTA-2Na) has been demonstrated as an excellent draw solution in the forward osmosis (FO) process because of its high osmotic pressure together with low reverse salt flux but its application is hindered by difficulties in the recovery of draw solution. Hence, in this study, microporous hydrophobic membranes were used in direct contact membrane distillation (DCMD) to concentrate the diluted EDTA-2Na draw solution. The MD was found to require lower operating pressures than do all other widely applied pressure-driven membrane processes, particularly in RO. This study systematically investigated the effect of different polytetrafluoroethylene membranes under various cross flow velocities of 2.67-14.67cm/s, feed temperatures of 35-60°C, and distillate temperatures of 10-20°C in DCMD process for regeneration of diluted EDTA-2Na. The results revealed that DCMD system could achieve a salinity rejection rate exceeding 99.99%; furthermore, the conductivity of the permeate distillate was consistently below 6.4μS/cm for all of the EDTA-2Na feed concentrations. More importantly, the water flux slightly decreased from 8.27 to 7.04L/m² h when the concentration of the EDTA-2Na feed increased from 0.1 to 0.5M, corresponding to increased osmolality from 300 to 1411mOsm/kg, indicating that water flux in DCMD is not significantly influenced by the osmotic pressure gradient across the membrane. This study demonstrated that MD could be an effective method for EDTA-2Na recovery in FO-MD systems and could economically utilize the wasted heat from industrial sources.
Anaerobic osmotic membrane bioreactor (AnOMBR) has aroused growing interests for its low energy demand, ability to efficiently process low ionic strength wastewater and high effluent quality. However, salt accumulation remains a main obstacle for causing severe water flux decline, fouling aggravation and inhibitory on the microbial activity. Here, we report a novel microfiltration (MF) assisted AnOMBR (AnMF-OMBR) for mitigating salt accumulation. The results indicated that the MF membrane effectively prevented salt accumulation in the bioreactor. The stable salinity level (within the range of 2.5–4.0 mS/cm) enabled the AnMF-OMBR to achieve a long-term continuous operation together with a higher methane production in comparison with a conventional AnOMBR. The forward osmosis (FO) permeate from the AnMF-OMBR had excellent water quality, while the MF permeate required further treatment (e.g., phosphorus precipitation and activated carbon adsorption) before its beneficial reuse. A thick fouling layer combining biofouling and inorganic scaling was existed on the FO membrane. Further confocal laser scanning microscopy (CLSM) revealed the dominance of polysaccharides and microorganisms over proteins. The current study demonstrated that the AnMF-OMBR can be a promising and sustainable wastewater treatment technology for its simultaneous energy recovery (in the form of biogas) and water reuse (from both FO and MF membranes).
As the volume of oil sands process-affected water (OSPW) stored in tailings ponds increases, it is urgent to seek for water management approaches to alleviate the environmental impact caused by large quantity of toxic water. Forward osmosis (FO) utilizes osmotic pressure difference between two solutions, thereby giving a potential to manage two wastewaters. In this study, FO was proposed to manage OSPW, using on-site waste basal depressurization water (BDW) as draw solution. To investigate its feasibility, both short and long-term OSPW desalination experiments were carried out. By applying this process, the volume of OSPW was decreased>40% and high rejections were achieved, especially, the major organic toxicity source - naphthenic acids (NAs). Although comparative low water flux (≤3L/m(2)h) was obtained, water flux caused by membrane fouling can be completely recovered using water physical cleaning. Moreover, calcium carbonate precipitation was observed on the OSPW-oriented membrane side. With respect to flux decline, the active layer facing the feed solution (FO mode) and active layer facing draw solution (PRO mode) did not demonstrate a significant difference on anti-fouling performance. The advantages provided by this approach include zero draw solution cost, less reversible membrane fouling and beneficial reuse/recycle of diluted BDW.
Forward osmosis (FO) membrane fouling and performance were systematically studied for extended-time during treatment of produced water using cellulose triacetate (CTA) and polyamide thin film composite (TFC) FO membranes. Performance was evaluated with integrity tests that measured water flux, reverse salt flux (RSF), and specific reverse salt flux (SRSF). The CTA membrane reached steady performance after one week and exhibited decreased water flux, RSF, and SRSF. The TFC membrane did not reach steady performance—water flux drastically decreased, and both RSF and SRSF increased. Streaming potential analyses was used to derive membrane zeta potential—the polyamide membrane zeta potential became increasingly negative over three weeks of continuous testing, while the CTA zeta potential was stable. The negative zeta potential reflected foulant deposition on the membrane surface and may have contributed to high RSF through the TFC membrane. The TFC membrane experienced a higher fouling propensity despite smoother, more hydrophilic, and more neutrally charged virgin membrane surfaces. Fouling layers on both membranes consisted of hydrocarbons, iron, and silica. Chemically enhanced osmotic backwashing was performed weekly, which removed calcium, sodium, and chloride from the membrane surface but only marginally improved water flux. Gas chromatography-mass spectroscopy was used to measure hydrocarbon concentrations in the feed and draw solution. The results showed that while both membranes had over 90% rejection of neutral hydrophobic compounds, the TFC membrane exhibited a higher rejection of small organic molecules. Compounds with carbonyl functional groups were not well rejected compared to all other aliphatic and polycyclic aromatic hydrocarbons of the same molecular size, and CTA membrane had lower rejection of these compounds than the TFC membrane.
Forward osmosis (FO) is one of the emerging membrane technologies which has gained interest in the last decade for being a low energy desalination process. The most important factors controlling FO processes are performance, recyclability and cost of the draw solution (DS) used together with the FO membrane itself because they play a crucial role on the feasibility of this technology. Consequently, the selection of an appropriate DS is vital for the process efficiency, besides the required selectivity and permeance of the membrane and the efficient DS regeneration process. A wide variety of DS have been tested so far and this paper aims to review recent advances in the synthesis and selection of an appropriate DS. It provides valuable information on a new type of draw solutes based on hybrid organic-inorganic nanosystems which, at a certain extent, show synergistic properties that face some of the technology shortcomings. Magnetic nanoparticles (MNPs) are the most promising nanosystems intended for desalination because they can be readily recovered applying a magnetic field or by conventional membrane processes. This review also deals with the most important characteristics of DS based on nanoparticles (NPs) and how they affect the performance of the overall processes. Finally, this review also highlights future research directions, where nanosystems will mitigate inverse diffusion and concentration polarization phenomena widely reported as limiting factors in FO processes.
Extensive research in recent years has explored numerous new features in the forward osmosis membrane bioreactor (FOMBR) process. However, there is an aspect in the process, which is revolutionary but not yet been investigated. In FOMBR, FO membrane shows high rejection for a wide range of soluble contaminants. As a result, hydraulic retention time (HRT) does not correctly reflect the nominal retention of these dissolved contaminants in the bioreactor. This decoupling of contaminants retention time (CRT, i.e. the nominal retention of the dissolved contaminants) from HRT endows FOMBR a potential in significantly reducing the HRT for wastewater treatment. In this work, we report our results in this unexplored treatment potential. Using real municipal wastewater as feed, both a hybrid microfiltration-forward osmosis membrane bioreactor (MF-FOMBR) and a newly developed hybrid biofilm-forward osmosis membrane bioreactor (BF-FOMBR) achieved high removal of organic matter and nitrogen under HRT of down to 2.0 h, with significantly enhanced phosphorus recovery capacities. In the BF-FOMBR, the used of fixed bed biofilm not only obviated the need of additional solid/liquid separation (e.g. MF) to extract the side-stream for salt accumulation control and phosphorus recovery, but effectively quarantined the biomass from the FO membrane. The absence of MF in the side-stream further allowed suspended growth to be continuously removed from the system, which produced a selection pressure for the predominance of attached growth. As a result, a significant reduction in FO membrane fouling (by 24.7–54.5%) was achieved in the BF-FOMBR due to substantially reduced bacteria deposition and colonization.
This work uncovers an important feature of the forward osmosis membrane bioreactor (FOMBR) process: the decoupling of contaminants retention time (CRT) and hydraulic retention time (HRT). Based on this concept, the capability of hybrid microfiltration-forward osmosis membrane bioreactor (MF-FOMBR) in achieving high through-put treatment of municipal wastewater with enhanced phosphorus recovery was explored. High removal of TOC and NH4+-N (90% and 99%, respectively) was achieved with HRTs down to 47 min, with the treatment capacity increased by an order of magnitude. Reduced HRT did not affect phosphorus removal and recovery. As a result, the phosphorus recovery capacity was also increased by the same order. Reduced HRT resulted in increased system loading rates and thus elevated concentrations of mixed liquor suspended solids and increased membrane fouling. 454-pyrosequecing suggested the thriving of Bacteroidetes and Proteobacteria (especially Sphingobacteriales Flavobacteriales and Thiothrix members), as well as the community succession and dynamics of ammonium oxidizing and nitrite oxidizing bacteria.
Fundamental understanding of pH dynamics in Forward Osmosis (FO) processes is integral for further development of this emerging technology in desalination and wastewater reclamation. In this study, the pH changes during FO membrane filtration were investigated. CaCl2.2H2O was used as the draw stream and NaCl as the feed stream. During the FO process, water would flow from the feed to draw so that the feed stream would become concentrated while the draw stream being diluted. It was found that the pH of the feed stream increased with time. Assuming that the pH or the concentration of hydrogen ions to be constant in the FO system, the pH of the draw stream should decrease. Surprisingly, the pH of the draw stream did not increase. To explain this unexpected phenomenon, standard curves for both CaCl2.2H2O and NaCl were constructed. In contrast to conventional belief, the standard curves showed that pH of NaCl increased with the increase in concentration while pH of CaCl2.2H2O decreased with the increase in concentration. As such, the standard curves explain the reason for pH increase in both feed and draw solutions. Furthermore, the curves showed that the pH to be dependent on salt concentration, where charges on ions have the ability to influence the pH of the solution.
A novel osmotic anammox (OsAMX) system coupling nitritation-anammox with forward osmosis (FO) has been developed for removal of reverse-fluxed ammonium when using NH4HCO3 as a draw solute. In this study, long-term performance and microbial community structure were investigated. The nitritation-anammox reactor maintained an ammonium concentration of 7.0±5.0mgNL(-1) (DO=0.9±0.2mgO2L(-1)), while the FO achieved a water flux of 2.3±0.4LMH (0.5M NH4HCO3 draw). The low water flux was obtained likely due to concentration polarization, reverse salt flux (RSF) and membrane fouling. Sequencing analyses reveled that Candidatus Jettenia was the dominant anammox genus, while Candidatus Brocadia was most abundant in biofilm. The shift of anammox bacterial population indicated possible higher tolerance of Ca. Brocadia for DO or elevated RSF. These results encourage further investigation of OsAMX system optimization, membrane fouling migration strategies, and application with actual wastewater.
Water management is an integral part of coal mining operations. Due to the constraints on releasing saline water, coal mines require additional water storage facilities and therefore seek to minimise their inventory of saline water. Adopting efficient treatment technologies on-site would minimise the risk of wet season run-offs, freshwater contamination and allow segregation of different qualities of water to enable greater water recycling. This study aims to evaluate the application of an integrated forward osmosis (FO) and reverse osmosis (RO) system with three different actual coal mine waters, containing various concentrations of sulphates and silica that are generally associated with scaling and fouling of membrane systems. Three different FO draw solutions, di-sodium hydrogen phosphate (DHSP), sodium hexametaphosphate (SHMP) and sodium lignosulphonate (SLS) were evaluated. Two different modes of integrating the FO and RO systems were identified. The integrated system was able to concentrate the brackish mine waters, recovering more than 80% of the volume of mine water and obtaining dischargeable quality treated water. Simple physical cleaning with clean water circulation was found to be effective in restoring the FO water flux. The osmotic gradient between two mine waters was also utilised to adopt mine water as a draw solution. The effect of solution temperature on stand-alone and integrated FO and RO systems was also evaluated. The combination of FO with RO provided a better performance than individual FO or RO in treating coal mine wastewater. The FO unit served as an effective pre-treatment system prior to RO and the integrated FO-RO systems has a strong potential to successfully eliminate conventional pre-treatment processes for RO.
In this study, we demonstrate a novel seawater-driven forward osmosis (FO) process to recover phosphorus in the form of calcium phosphate precipitates from digested sludge centrate without any chemical addition and draw solute regeneration. The FO process effectively pre-concentrated phosphate and calcium in the digested sludge centrate. Spontaneous precipitation of calcium phosphate minerals in the digested sludge centrate was achieved by the sustained concentrative action of the FO process and the gradual pH increase due to the diffusion of protons to the draw solution. Pre-concentrating digested sludge centrate by three-fold resulted in a 92% recovery of phosphate via precipitation. The phosphate precipitate only constituted 3% of the total inorganic solids recovered, therefore subsequent treatment steps would be required to recover phosphorus in a useable form. A water flux decline of 30% from the initial value was observed as the digested sludge was concentrated by three-fold. This observed water flux decline was mostly attributed to the decrease in the effective osmotic driving force due to the increasingly concentrated feed solution and diluted draw solution. It is also noteworthy that membrane fouling was readily reversible. By flushing the membrane with deionised water and subjecting the membrane to feed and draw solutions with the same osmotic pressure as the initial conditions, complete water flux recovery could be achieved.