Membrane distillation: recent technological developments and advancements in membrane materials

Membrane distillation (MD) is a novel desalination technology that has potential to produce distilled quality water from high salinity brine streams. The driving force for MD is the vapor pressure difference across a hydrophobic membrane resulting in transfer of water vapor from hot to cold side. This vapor contacts a cold surface and condenses to produce distillate. This paper reviews recent and/or multi-year research programs that focused on MD pilot or field testing. The various investigations concluded that while MD can produce distilled water quality, the energy efficiency remains the key bottleneck for future deployment of MD. Membrane wetting and fouling also presents key challenges for desalination due to both the high salinity and the presence of organics in the feed water. The authors contacted several MD vendors requesting updates on their latest products and technology developments. MD vendors with innovative module designs, some of which promise a step change in performance, have recently emerged on the market. In addition to water desalination, MD has a wide range of industrial applications such as hydrogen sulfide removal, the treatment of wastewater from the pharmaceutical, metal finishing industries, direct sewer mining, oily wastewater, and water recovery from flue gas. This paper also reviews novel membrane chemistries with emphasis on membranes prepared by phase inversion and electrospinning techniques to which nanomaterials have been added. The primary objectives in adding various nanomaterials (e.g., carbon nanotubes, graphene, silicon dioxide, fluorinated compounds) are to increase hydrophobicity (to reduce wetting) and increase mass transfer rates (to increase flux and lower cost)

Pilot-scale evaluation of forward osmosis membranes for volume reduction of industrial wastewater

Water treatment is a key aspect for the sustainable management of oil & gas operations. Osmotic concentration (OC) is an advanced water treatment process, adapted from forward osmosis (FO), that does not require water recovery from the draw solution. In this study, two commercial hollow fiber FO membranes [Module 1, cellulose triacetate (CTA) and Module 2, thin film composite (TFC)] were evaluated at pilot scale using actual process water obtained from a gas production facility. The evaluation focused on assessing the membrane productivity, fouling potential and chemical cleaning efficiency while normalizing the performance using a theoretical model that account for the variability of the operating conditions. Performance tests showed that Module 2 has a higher flux compared to Module 1, 9.9 L/m2·h vs 1.7 L/m2·h; and lower specific reverse solute flux (RSF) for most of the ions. Additionally, Module 1 benchmark experiment showed a 13% flux loss attributed to inorganic fouling (calcium phosphate precipitation) while the flux loss in Module 2 was <5% possibly due to enhanced module hydrodynamics and variation in membrane chemistry. Chemical cleaning (citric acid) proved to be successful in restoring the flux for Module 1. From the 8.1 mg/L organic carbon present in the feed, advanced organic characterization revealed that certain group of hydrophilic organics are able to pass through Module 1, but not Module 2, translating to a specific forward organic solute flux of 0.9 mg/L and 0.1 mg/L for Module 1 and 2, respectively. Finally, key sustainable and environmental considerations were presented in support of further development of process implementation. The OC process has strong potential for full-scale installation; however, demonstrating its performance in the field would be the next step necessary for successful implementation of the technology

Pilot-scale investigation of flowrate and temperature influence on the performance of hollow fiber forward osmosis membrane in osmotic concentration process

Forward osmosis (FO) relying on the osmotic pressure difference across semi-permeable membrane draws permeate by the effect of saline draw solution (DS) turning diluted and leaving the feed solution (FS) concentrated. However, the energy intensive step of DS recovery makes FO a challenging process. The energy benefit of FO emerges when recovery step is obviated and FO is applied as an osmotic concentration (OC) process. OC implementations for volume reduction are still at bench-scale and the investigation at larger scale is among the breakthroughs. In this paper, the performance of hollow fiber (HF) membrane in pilot-scale OC process for reducing volume of feed was investigated. The impact of operating conditions such as flowrates and temperature was evaluated. FS and DS flowrates of 1.35 and 0.35 L.min-1 respectively are optimum conditions with 75% feed recovery and 1.90 LMH water flux. Reverse solute flux increased at higher flowrates. Results indicated the role of high DS flowrate and temperature in improving the performance. DS flowrate of 0.35 L.min-1 at constant FS flow of 1.10 L.min-1 and 27 °C was most suitable for achieving 84.5% feed recovery and 1.82 LMH water flux. Above all, the long-term performance of OC pilot-plant was demonstrated through 48 h of continuous operation where stable flux trend at an average water flux of 1.66 LMH was successfully achieved. Lastly, the permeability coefficients of HF membrane were enhanced at higher temperature

Industrial wastewater volume reduction through osmotic concentration: Membrane module selection and process modeling

Osmotic concentration (OC), a form of forward osmosis (FO) but without draw solution recovery, can be applied for reducing wastewater disposal volumes in the oil & gas industry. Within this industry, wastewater is often disposed of by injection through disposal wells into deep underground reservoirs. By reducing wastewater disposal volumes, the sustainability of the disposal reservoir is improved. In this application of OC, seawater or brine from a desalination plant serves as the draw solution and the diluted seawater is discharged to the sea. This study compared 3 commercial hollow-fiber FO membranes (CTA, TFC, aquaporin proteins) for reducing the volume of low salinity wastewater generated during liquified natural gas (LNG) production. Additionally, a model was developed to predict the performance of commercial full-scale membranes by identifying optimum operating conditions, taking into consideration the trade-off between feed concentration factor and water flux. Bench-scale tests were conducted using synthetic and actual wastewater from an LNG facility to evaluate OC technology performance and validate model predictions.Based on model results with a feed mimicking the salinity of actual wastewater, a 4x concentration factor produced a reasonable compromise between feed recovery and draw solution dilution and was considered the optimum for future tests. At higher concentration factors, the increased dilution of the draw solution negatively impacted flux. In bench tests with real wastewater, the TFC chemistry had a ≈5x higher water flux (9.7 vs. 1.9 L/m2-h) and a ≈3x lower specific reverse solute flux (192 vs. 551 mg/L) compared to the CTA chemistry. However, both membranes showed less than 5% fouling and a specific forward organic solute flux of less than 0.5 mg/L of total organic carbon (TOC). Pilot testing for >50 h showed stable performance, comparable to bench scale data and model predictions

Evaluation of polymeric adsorbents via fixed-bed columns for emulsified oil removal from industrial wastewater

Polymeric adsorbents (PAs) have been gaining increased attention for application in industrial wastewater (WW) treatment. However, most studies on evaluating PAs specifically for emulsified oil removal are currently limited to performance screening through batch mode testing. Hence, this paper presents a thorough fixed-bed assessment of advanced PAs for the removal of emulsified oil from industrial WWs. A unique custom-built column setup was developed with a continuous test protocol that involves both adsorption and regeneration of media. A robust procedure was also established to automatically prepare a representative synthetic produced water (PW) containing the oil-water emulsions. Four cutting-edge PAs were evaluated, out of which two being tested for the first time targeting emulsified oil removal. Experimental tests were conducted to address resin capacity, regeneration efficiency, and performance reproducibility in repeat cycles. PA2 treated 168 ± 58 bed volumes (BVs) achieving the lowest capacity of 44 ± 14 mg/g. Higher comparable capacities were observed for PA1 and PA3 at ~100 mg/g, yet PA1 was found capable of treating 807 ± 3 BVs against 548 ± 115 BVs measured for PA3. PA4 treated 1219 ± 86 BVs with a capacity of 301 ± 27 mg/g which indicate its strong potential for industrial WW treatment application. This performance data can provide a reference for comparison when testing other novel resins for emulsified oil removal. Future studies will focus on testing PAs using real PW and evaluating their long-term performance via pilot testing

An empirical determination of the whole-life cost of FO-based open-loop wastewater reclamation technologies

Over the past 5–10 years it has become apparent that the significant energy benefit provided by forward osmosis (FO) for desalination arises only when direct recovery of the permeate product from the solution used to transfer the water through the membrane (the draw solution) is obviated. These circumstances occur specifically when wastewater purification is combined with saline water desalination. It has been suggested that, for such an “open loop” system, the FO technology offers a lower-cost water reclamation option than the conventional process based on reverse osmosis (RO). An analysis is presented of the costs incurred by this combined treatment objective. Three process schemes are considered combining the FO or RO technologies with membrane bioreactors (MBRs): MBR-RO, MBR–FO–RO and osmotic MBR (OMBR)-RO. Calculation of the normalised net present value (NPV/permeate flow) proceeded through developing a series of empirical equations based on available individual capital and operating cost data. Cost curves (cost vs. flow capacity) were generated for each option using literature MBR and RO data, making appropriate assumptions regarding the design and operation of the novel FO and OMBR technologies. Calculations revealed the MBR–FO–RO and OMBR-RO schemes to respectively offer a ∼20% and ∼30% NPV benefit over the classical MBR-RO scheme at a permeate flow of 10,000 m3  d−1, provided the respective schemes are applied to high and low salinity wastewaters. Outcomes are highly sensitive to the FO or OMBR flux sustained: the relative NPV benefit (compared to the classical system) of the OMBR-RO scheme declined from 30% to ∼4% on halving the OMBR flux from a value of 6 L m−2. h−1

Evaluation of pretreatment and membrane configuration for pressure-retarded osmosis application to produced water from the petroleum industry

Pressure-retarded osmosis (PRO) is a promising membrane technology for harnessing the osmotic energy of saline solutions. PRO is typically considered with seawater/river water pairings however greater energy can be recovered from hypersaline solutions including produced water (PW) from the petroleum industry. One of the major challenges facing the utilization of hypersaline PW is its high fouling propensity on membranes. In this unique experimental evaluation, real PW from different sites was pretreated to varying degrees: i) minimal, ii) intermediate, and iii) extensive. The treated effluent was subsequently used for PRO testing and fouling rates were assessed for different membrane configurations over multiple cycles. Commercial grade flat sheet (FLS) coupons and novel hollow fiber (HF) modules were compared to validate the lower fouling propensity of HF membranes in PRO application. When minimally pretreated PW (10-micron cartridge filtration (CF)) was tested in FLS mode, severe membrane fouling occurred and the PRO flux decreased by 60%. In contrast, HF modules showed <1% flux decrease under both minimal and intermediate pretreatment schemes. Extensive pretreatment (1-micron CF, dissolved air flotation (DAF), powdered activated carbon, and microfiltration) reduced FLS PRO flux decline to <1%. These results confirm that PW can be treated to suitable levels for PRO application to avoid membrane fouling. Further validation of these pretreatment methods requires long term pilot testing and techno-economic assessment

Field evaluation of membrane distillation technologies for desalination of highly saline brines

Membrane distillation (MD) is a hybrid thermal-membrane desalination process that uses low-grade waste heat and hydrophobic membrane to produce high quality distillate. The MD process can treat highly saline brines that other conventional desalination processes cannot treat. These unique features of the MD process make it an ideal candidate to desalinate concentrated brines from thermal desalination plants to augment fresh water production from existing facilities. A consortium consisting of ConocoPhillips Global Water Sustainability Center, Qatar University, and Qatar Electricity & Water Company was formed to evaluate the application of MD for the desalination of concentrated brines from thermal plants. Five different MD technologies were evaluated and the two most suitable technologies were selected for field-testing. The pilot units A & B are based on multi-effect vacuum and air gap MD technologies, respectively. These units were tested side-by-side at a full-scale thermal desalination plant in Qatar. Pilot unit A showed a stable flux of 6.2. LMH under optimized conditions with excellent salt rejection (>. 99.9%). Pilot unit B achieved a distillate flux of 2.5. LMH and salt rejection greater than 98.9%. Overall, MD was shown to be a feasible technology to produce potable quality water from the brines discharged from thermal desalination plants.Scopu

Pilot Scale Osmotic Concentration Process for Reducing Wastewater Volumes from Gas Processing Facilities in Qatar

Over the past 10-15 years, there has been increasing attention in the development of forward osmosis (FO) technology as a low-energy technical solution to wastewater treatment through the exploitation of the natural osmosis phenomenon across semi-permeable membrane. The significant energy benefit arises in applications where direct recovery of the permeate product from the draw solution (DS) is obviated such as in osmotic concentration (OC) process. In the current research, an OC FO-based pilot-scale unit was applied for wastewater volume reduction from oil and gas processing facilities in Qatar. The pilot unit uses seawater of 40 g/L salinity as a DS and wastewater generated during oil and gas operations as a feed. This feed water is of comparatively low conductivity (2 g/L salinity), making it unusually suited to treatment by OC. Based on FO technology principles, the feed gets concentrated at lower volume with the water permeation through the membrane, meanwhile the water transfer to DS side dilutes it. The diluted DS could be directly discharged into the ocean; so the energy intensive step of DS recovery is entirely eliminated. Two FO membranes (Toyobo and NTU) of hollow fiber configuration were tested to assess their performance and fouling propensity on both synthetic and real wastewaters. Results demonstrated that the membrane-based process can achieve feed water recoveries up to 90% without any scaling issues. Achieved water flux ranges between 1.5 to 12 LMH for feed recoveries between 60 and 90% using a constant dilution rate of the draw solution. Above all, the pilot unit maintained stable water flux of 1.62 and 6 LMH using at 75% feed recovery for over 48 hours of continuous operation Toyobo and NTU membranes respectively

The status of forward osmosis technology implementation

The current review appraises the status of forward osmosis (FO) technology implementation and prospective commercial exploitation through examination of its energy consumption and other key process attributes compared with classical desalination technologies. The outcomes of 15 studies conducted at pilot scale revealed the energy associated with the draw solution (DS) recovery to present a significant barrier to implementation of the technology, with a 40–50% decrease in energy consumption required for the DS recovery step for the process to successfully compete with classical reverse osmosis (RO) based processes. Against this, FO can be energetically favoured if deployed: (a) without the DS recovery step, i.e. osmotic concentration (OC) or fertilizer-drawn forward osmosis (FDFO), (b) in competition with evaporative desalination processes, or (c) for combined desalination and wastewater purification, when a comparatively low salinity stream is available for providing osmotic dilution (OD) of the saline feed water. Whilst these specific applications show some promise for the technology, there remains a paucity of pilot and demonstration-scale studies corroborating the theoretical energy benefit of the proposed technology configurations.This work was made possible by the support of a National Priorities Research Programme (NPRP) grant from the Qatar National Research Fund (QNRF), grant reference number NPRP10-0118-170191 . The statements made herein are solely the responsibility of the authors. The authors would like to thank Dan Jerry Cortes from Qatar University and Arnold Janson from ConocoPhillips, Qatar for providing useful information for this paper.Scopu