Use of a Forward Osmosis–Membrane Distillation Integrated Process in the Treatment of High-Salinity Oily Wastewater

Green and Sustainable Forward Osmosis Process for the Concentration of Apple Juice Using Sodium Lactate as Draw Solution

China is the world's largest producer and exporter of concentrated apple juice (CAJ). However, traditional concentration methods such as vacuum evaporation (VE) and freeze concentration cause the loss of essential nutrients and heat-sensitive components with high energy consumption. A green and effective technique is thus desired for juice concentration to improve product quality and sustainability. In this study, a hybrid forward osmosis-membrane distillation (FO-MD) process was explored for the concentration of apple juice using sodium lactate (L-NaLa) as a renewable draw solute. As a result, commercial apple juice could be concentrated up to 65 °Brix by the FO process with an average flux of 2.5 L·m-2·h-1. Most of the nutritional and volatile compounds were well retained in this process, while a significant deterioration in product quality was observed in products obtained by VE concentration. It was also found that membrane fouling in the FO concentration process was reversible, and a periodical UP water flush could remove most of the contaminants on the membrane surface to achieve a flux restoration of more than 95%. In addition, the L-NaLa draw solution could be regenerated by a vacuum membrane distillation (VMD) process with an average flux of around 7.87 L∙m-2∙h-1 for multiple reuse, which further enhanced the long-term sustainability of the hybrid process.

A Competitive Study Using Electrospinning and Phase Inversion to Prepare Polymeric Membranes for Oil Removal

Polyacrylonitrile (PAN) is a popular polymer that can be made into membranes using various techniques, such as electrospinning and phase inversion. Electrospinning is a novel technique that produces nonwoven nanofiber-based membranes with highly tunable properties. In this research, electrospun PAN nanofiber membranes with various concentrations (10, 12, and 14% PAN/dimethylformamide (DMF)) were prepared and compared to PAN cast membranes prepared by the phase inversion technique. All of the prepared membranes were tested for oil removal in a cross-flow filtration system. A comparison between these membranes' surface morphology, topography, wettability, and porosity was presented and analyzed. The results showed that increasing the concentration of the PAN precursor solution increases surface roughness, hydrophilicity, and porosity and, consequently, enhances the membrane performance. However, the PAN cast membranes showed a lower water flux when the precursor solution concentration increased. In general, the electrospun PAN membranes performed better in terms of water flux and oil rejection than the cast PAN membranes. The electrospun 14% PAN/DMF membrane gave a water flux of 250 LMH and a rejection of 97% compared to the cast 14% PAN/DMF membrane, which showed a water flux of 117 LMH and 94% oil rejection. This is mainly because the nanofibrous membrane showed higher porosity, higher hydrophilicity, and higher surface roughness compared to the cast PAN membranes at the same polymer concentration. The porosity of the electrospun PAN membrane was 96%, while it was 58% for the cast 14% PAN/DMF membrane.

Investigation of flux stability and fouling mechanism during simultaneous treatment of different produced water streams using forward osmosis and membrane distillation

Forward osmosis-membrane distillation (FO-MD) hybrids were recently found suitable for produced water treatment. Exclusion of synthetic chemical draw solutions, typically used for FO, can reduce FO-MD operational costs and ease its onsite application. This study experimentally validates a novel concept for the simultaneous treatment of different produced water streams available at the same industrial site using an FO-MD hybrid system. The water oil separator outlet (WO) stream was selected as FO draw solution and it generated average fluxes ranging between 8.30 LMH and 26.78 LMH with four different feed streams. FO fluxes were found to be governed by the complex composition of the feed streams. On the other hand, with WO stream as MD feed, an average flux of 14.41 LMH was achieved. Calcium ions were found as a main reason for MD flux decline in the form of CaSO₄ scaling and stimulating the interaction between the membrane and humic acid molecules to form scale layer causing reduction in heat transfer and decline in MD flux (6%). Emulsified oil solution was responsible for partial pore clogging resulting in further 2% flux decline. Ethylenediaminetetraaceticacid (EDTA) was able to mask a portion of calcium ions and resulted in a complete recovery of the original MD flux. Under hybrid FO-MD experiments MD fluxes between 5.62 LMH and 11.12 LMH were achieved. Therefore, the novel concept is validated to produce fairly stable FO and MD fluxes, with few streams, without severe fouling and producing excellent product water quality.

The Water Flux Dynamic in a Hybrid Forward Osmosis-Membrane Distillation for Produced Water Treatment

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.

Role of Cellulose Micro and Nano Crystals in Thin Film and Support Layer of Nanocomposite Membranes for Brackish Water Desalination

Reverse osmosis is a major process that produces soft water from saline water, and its output represents the majority of the overall desalination plants production. Developing efficient membranes for this process is the aim of many research groups and companies. In this work, we studied the effect of adding cellulose micro crystals (CMCs) and cellulose nano crystals (CNCs) to the support layer and thin film nanocomposite (TFN) membrane on the desalination performance. SEM, TEM, ATR-FTIR, and contact angle measurements were used to characterize the membrane's properties; and membrane's performance were evaluated by water flux and NaCl rejection. Filling 2% of CNCs gel in the support layer improved the water flux by +40%, while salt rejection maintained almost the same, around 95%. However, no remarkable improvement was gained by adding CNCs gel to m-phenylenediamine (MPD) solution, which was used in TFN membrane preparation. Filling CMCs powder in TFN membrane led to a slight improvement in terms of water flux.

Triple-Layer Nanofiber Membranes for Treating High Salinity Brines Using Direct Contact Membrane Distillation

A composite, three-layered membrane for membrane distillation was prepared from electrospun polyvinylidene fluoride (PVDF) nanofibers supported by commercial polyethersulfone (PES) nanofiber based nonwoven from E.I. duPont de Nemours company (DuPont). The membranes were tested in direct contact membrane distillation (DCMD) using a 5.0 M sodium chloride brine as a feed solution. The triple layer membrane combines the hydrophobicity of PVDF and the robustness of the PES. The triple layer membrane demonstrated excellent performance in DCMD (i.e., relatively high water flux compared to the commercial PVDF membrane and a complete salt rejection of the brine) with mechanical properties imparted by the PES layer. This work is the first to demonstrate the use of a commercially produced nanofiber nonwoven for membrane distillation.