Application of Membrane Distillation for desalting brines from thermal desalination plants

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

Membrane Distillation (MD) is a hybrid thermal-membrane process that produces a high quality distillate from concentrated brine. It utilizes either low grade waste heat or solar energy to generate a vapor pressure difference across a hydrophobic membrane. A consortium composed of ConocoPhillips - Global Water Sustainability Center (GWSC), Qatar University (QU) and Qatar Electricity & Water Company (QEWC), was formed to conduct a pilot testing program to assess the suitability of MD at pilot scale to treat brines from thermal desalination plants. This research project aims toward ensuring a sustainable water supply in Qatar, one of the twelve grand research challenges identified recently by Qatar National Research Strategy (QNRS). This presentation summarizes the outcomes of the project including the most relevant conclusions and findings from the field testing program. Bids from the five leading MD technology providers were evaluated and the two most suitable technologies for Qatar environment were selected: a vacuum multi-effect MD unit from memsys in Germany and a single stage air gap MD unit from Xzero in Sweden. Initially, hydraulic testing was conducted at QU and then the pilots were relocated to the QEWC power/desalination plant at Ras Abu Fontas to operate under field conditions. Globally, this is the first study that benchmark MD technologies side by side at pilot scale. The performance of the MD pilot units was evaluated under different operating conditions (temperature, flowrate and feed salinity up to 70,000 mg/L TDS) with the objective of maximizing water production and lowering operational costs. The product stream was of distilled water quality (TDS < 10 ppm), independent of feed salinity. Typical flux values of 5 - 6 LMH were obtained at feed / cooling temperatures of 70oC / 20oC. Results also showed that pretreatment plays an important role on system performance. An important outcome of the project was related to the energy efficiency of the systems. Detailed energy balance showed that a multi-effect design significantly improves energy efficiency. The multi effect system has a performance ratio of 3.0, which was 3 times higher than the single stage system. The projected outcome of the testing program includes: * Sustainable augmentation of water production in Qatar * Reduction of environmental impact * Capacity building in Qatar

Membrane distillation (MD) is a hybrid thermal-membrane desalination process that can use either low-grade waste heat and/or solar energy with hydrophobic membranes to desalinate high-salinity brines and produce high quality distillate. A research consortium was launched to investigate the application of the MD process, at lab and pilot scale, for desalination of concentrated brines. Bench scale results showed the presence of antiscalants in the concentrated brines minimized the scale precipitation potential and offered stable membrane permeability performance. Various MD technologies were screened, and two suitable technologies were selected for field-testing. Pilot unit A was based on multi-effect vacuum showed a stable flux of 6.2 LMH with excellent salt rejection (> 99.9%) from the concentrated brine discharged from thermal desalination plant in Qatar. That pilot unit was also field tested on hypersaline groundwater in Texas (USA) to generate fresh water for reservoir fracking in unconventional oil production operations. The MD unit was coupled with humidification/dehumidification (HDH) unit to achieve zero liquid discharge (ZLD) for inland applications. The MD unit was operated at 40% recovery producing distillate of < 20 mg/L total dissolved solids (TDS) and observed a stable flux of 5 LMH. Key challenges that are critical for large-scale deployment of MD technology were identified at the end of the field-testing program. Finally, a review of active MD technologies was conducted to highlight recent promising developments for full-scale applications.

In order to ensure long-term sustainability of the reservoir, the gas industry in Qatar is faced with the challenge of reducing the volume of produced and process water (PPW) sent to disposal wells by 50% [1-3]. Recently, Qatargas initiated a project to recycle process water and thus, reduce disposal volumes using commercial advanced water treatment technologies [4]. One emerging technology, “osmotic concentration” (OC) has been identified that offers a low-energy alternative to conventional thermal or membrane volume reduction methods. Osmotic concentration is a membrane filtration process that mimics first step in a forward osmosis (FO) system. It requires a high salinity draw solution (DS) which passes on one side of a semi-permeable FO membrane while the feed passes on the other side. Water from the feed is drawn through the membrane, via natural osmosis, reducing the feed volume and increasing the volume of the draw solution. This paper summarizes the results of bench-scale volume reduction tests wit...

Fouling and wetting in the membrane distillation driven wastewater reclamation process – A review

Background: In Qatar, the production of freshwater is achieved by thermal desalination combined with power plants. This means that both hot concentrated brines and low grade waste heat are potentially available. Membrane distillation (MD) is a hybrid thermal membrane process that can use low grade waste heat to generate a vapor pressure difference across a membrane and therefore produce a high quality distillate from high salinity brines. The unique features of the MD process make it an ideal technical solution to increase freshwater production in Qatar. Objectives: The main objectives of the project are to investigate the suitability of the MD technology under conditions in Qatar and address scale-up and process optimization issues using real brines from a local thermal desalination plant. The project is a partnership of ConocoPhillips Global Water Sustainability Center at Qatar Science & Technology Park, Qatar University, and Qatar Electricity and Water Company. Results: Phase 1 - bench scale testing: T...

Treatment of produced water from oil & gas operations by Membrane Distillation

Unconventional resources (Shale gas/oil) use significant volumes of water for hydraulic fracturing (fracking). While some of the water used is fresh groundwater, there are more environmental pressures to use brackish water sources for fracking. This brackish water may need to be treated to lower the saturation levels and to allow mixing of field chemicals. Unconventional resources also produced high volume of flow-back water (produced water). This produced water (PW) contains high levels of total dissolved solids (TDS) and desalination may be needed to allow recycling or reuse of this water source. Membrane Distillation (MD) is an innovative process that can desalinate highly saline waters (30,000–100,000 mg/L TDS) more effectively than reverse osmosis. As a proof of concept, bench-scale MD testing were performed on brackish and produced water samples (30,000 mg/L-60,000 mg/L TDS) obtained from Texas. Results have shown excellent TDS rejection (99.9 %) on all the water samples that were tested without impacting membrane's flux performance. To evaluate the O&amp;M and scale up issues, two one m3/day MD pilot units are currently operating side by side at a local desalination plant in Doha. Brine from the thermal desalination plant was used as representative high salinity water (70,000 mg/L), similar salinity levels could be found in brackish groundwater and/or flow-back water. It was assumed that all other contaminants that could cause membrane fouling (such as suspended oil, solids, organics, microorganisms) will be removed in a pretreatment step prior to MD. Preliminary results showed that the pilot units were successful in completely removing salt. Flux was very stable for more than 2 weeks. However, it was concluded that pretreatment is critical for stable performance of the MD units. This presentation will provide up to date data on MD bench and pilot-scale performance with O&amp;M issues and projected cost estimates.

Membrane desalination processes for water recovery from pre-treated brewery wastewater: Performance and fouling

Membrane processes with outstanding advantages have been developed over the past three decades and could be used for oily wastewater treatment applications, effectively. Nowadays, forward osmosis (FO) and membrane distillation (MD) processes, as two essential membrane processes have received considerable attention, for oily wastewater treatment applications. The excellence of MD and FO processes compared to other membrane processes is their cost-effectiveness and capability to remove smaller oil drops with lower energy consumption. Membrane fouling as a challenge with decreasing separation efficiency has hindered the commercialization of FO and MD processes. Therefore, developing new FO and MD membranes with lower fouling tendency is very important for water treatment applications especially for oily wastewater treatment. In this chapter, after bibliographic analysis, the conventional methods developed for treating various types of oily wastewaters are reviewed, in brief. Then, various membrane processes with particular emphasis on their challenges and advances in oily wastewater treatment applications are presented. Afterward, focusing on FO and MD processes, the current materials used to fabricate proper membranes for FO and MD processes are discussed.

Membrane distillation (MD) is an emerging desalination technology that uses low-grade heat to drive water vapor across a microporous hydrophobic membrane. Currently, little is known about the biofilms that grow on MD membranes. In this study, we use estuarine water collected from Long Island Sound in a bench-scale direct contact MD system to investigate the initial stages of biofilm formation. For comparison, we studied biofilm formation in a bench-scale reverse osmosis (RO) system using the same feedwater. These two membrane desalination systems expose the natural microbial community to vastly different environmental conditions: high temperatures with no hydraulic pressure in MD and low temperature with hydraulic pressure in RO. Over the course of 4 days, we observed a steady decline in bacteria concentration (nearly 2 orders of magnitude) in the MD feed reservoir. Even with this drop in planktonic bacteria, significant biofilm formation was observed. Biofilm morphologies on MD and RO membranes were markedly different. MD membrane biofilms were heterogeneous and contained several colonies, while RO membrane biofilms, although thicker, were a homogeneous mat. Phylogenetic analysis using next-generation sequencing of 16S rDNA showed significant shifts in the microbial communities. Bacteria representing the orders Burkholderiales, Rhodobacterales, and Flavobacteriales were most abundant in the MD biofilms. On the basis of the results, we propose two different regimes for microbial community shifts and biofilm development in RO and MD systems.

Chapter 8 - MD Membrane Characterization

Fast global population growth, serious environmental pollution and rapid economic developments have resulted in water scarcity around the world. Membrane distillation (MD) processes were considered as an attractive technology to treat waste water, recycle polluted water and provide more freshwater resources. This thesis provides a brief review on the research and developments of MD process, commercial MD membranes and lab-fabricated MD membranes. As the electrospun composite nanofibrous membranes have great potential to be used in MD due to their unique structural features, the complex electrospinning process has also been reviewed, including the materials and operating parameters which could control nanofiber formation, and various designs of electrospun apparatus which can produce nanofiber membranes with different appearances. However, it is found that limited works have been carried out to fabricate MD membranes by electrospinning. &#13;\nIn this work, poly (vinylidene fluoride) (PVDF) nanofiber membranes were fabricated through electrospinning for direct contact membrane distillation (DCMD) as a first trial. The effects of PVDF dope concentration, inorganic salt additives, sprayer’s moving speed, and chamber moisture on the properties of resultant membranes were investigated. It also illustrates the importance of processing parameters and heat-press post-treatment, and demonstrates that the heat-press post-treatment improved membrane integrity significantly and enhanced permeate flux in DCMD process. All the electrospun nanofiber membranes possessed high water contact angles (between 135° to 142°) due to their high surface roughness. The post-treated PVDF nanofiber membranes were able to present a steady water permeation flux of 21 kg m-2h-1 throughout the entire testing period of 15 h, using a 3.5 wt% NaCl solution as the feed under the feed and permeate inlet temperatures of 323 K and 293 K, respectively. &#13;\nHowever, PVDF nanofiber membranes without hydrophobic additives or surface modification do not have sufficient anti-wetting performance. Further treatment of PVDF nanofiber should be carried out to impart them with better wetting resistance and long-term stability. Two types of superhydrophobic PVDF nanofiber membranes, integrally-modified and surface-modified PVDF membranes, have been successfully fabricated by electrospinning followed by surface modification, which includes dopamine surface activation, silver nanoparticle deposition and hydrophobic treatment. The modification is convenient because of mild reactions and wide applicability. The characterizations revealed that the modifications have altered the membrane surface morphology and topology, and made the membrane superhydrophobic due to their hierarchical structures. Compared with unmodified membrane, the integrally-modified membrane (I-PVDF) can achieve a high and stable MD water flux of 31.6 kg m-2h-1 using a 3.5 wt% NaCl as the feed solution while the feed and permeate temperatures were fixed at 333 K and 293 K, respectively. To the best of our knowledge, this result is superior to all other PVDF flat sheet membranes tested under the same or similar conditions, which is believed to be attributed to the open surface pore structure and the thin thickness of the PVDF nanofiber membrane with the aid of electrospinning. The superhydrophobic nature of the membrane surface brought by the integral modification on all nanofibers renders the membrane anti-wetting property while remaining high water flux.&#13;\nMoreover, inspired by the unique structure of lotus leaf, a novel strategy is developed to construct composite nanofiber membranes with robust superhydrophobicity and high porosity suitable for use in MD. The newly developed membrane consists of a superhydrophobic silica-PVDF composite selective skin formed on PVDF porous nanofiber scaffold via electrospinning. This fabrication method could be easily scaled up due to its simple preparing procedures. The effects of silica diameter on membrane contact angle, sliding angle and MD performance were investigated thoroughly. For the first time, the DCMD tests demonstrate that the newly developed membranes are able to present stable high performance over 50 h of testing time, and the superhydrophobic selective layer exhibits excellent durability in ultrasonic treatment and continuous DCMD test. It is believed that this novel design strategy has great potential for MD membrane fabrication.&#13;\nAdditionally, to further improve the wetting repellent property of superhydrophobic membranes, 3-dimensional (3D) superhydrophobic membranes were developed as a possible solution. Moreover, since highly porous nanofiber membranes usually suffer from insufficient mechanical property, which have adverse impact on membrane packing in the module, thus, a dual-layer membrane was fabricated by electrospinning 3D superhydrophobic composite layers on a non-woven support to improve its wetting resistance and enhance mechanical robustness Another type of dual-layer superhydrophobic composite membranes consisting of PVDF nanofibrous support and an ultrathin 3D superhydrophobic selective layer was prepared to compare with the as-fabricated non-woven-supported superhydrophobic dual-layer membrane. All these membranes exhibit superhydrophobicity towards distilled water, salty water, oil-water mixture and beverages, which enables them to be used not only for desalination but also for other concentrating treatments. Compared with nanofiber-supported dual-layer membranes, the non-woven-supported membranes exhibit higher mechanical strength as a result of excellent combination with non-woven support and better long-term performance because of the thicker 3D superhydrophobic structure. The morphology, pore size, porosity, mechanical properties as well as liquid enter pressure of water of these superhydrophobic composite membranes and commercial PVDF membrane are measured and compared. The possible wetting procedures of the as-prepared superhydrophobic dual-layer membranes are also illustrated in this study. &#13;\nFinally, this thesis provides some personal perspectives for the future developments in which the composite nanofiber membranes could be pursued for water research.&#13;\nIn conclusion, this thesis presents the design and development of novel superhydrophobic nanofiber membranes based on the studies of the fundamental mechanisms of electrospinning, surface modification on nanofiber membranes, fabrication of robust superhydrophobic membranes, and preparation of 3D superhydrophobic dual-layer membrane. This work contributes to the development of membrane fabrication technology and facilitates the practical applications of membrane distillation process.

Anti-fouling and anti-wetting membranes for membrane distillation

Integration of forward osmosis and membrane distillation for sustainable wastewater reuse