Desalination of geothermal brackish water by membrane distillation (MD) provides a low recovery rate, but integrating MD with reverse osmosis (RO) can maximize the production rate. In this study, different design configurations of a hybrid system involving brine recycling and cascading are studied via simulations, and the performance improvement due to the process integration is substantiated via the increased recovery rate and reduced specific energy consumption. Brine recycling is also found to improve the recovery rate considerably to 40% at an energy cost of 0.9 $/m3. However, this achievement is only valid when the final brine is recycled to the RO feed: when the final brine is recycled to the MD feed, the overall performance degrades because the recycled brine cools the feed and causes a serious reduction in the driving force and the consequent production rate. Configuring the hybrid system in multiple stages connected in series increases the recovery rate to 90% and reduces the specific energy consumption to 0.9 MJ/kg. Although the specific energy cost increases dramatically because external inter-stage heating is implemented, using a free energy source (such as a geothermal or waste-energy source) for inter-stage heating could provide the optimum configuration.
Citation: Ali E, Orfi J, Najib A, Saleh J (2018) Enhancement of brackish water desalination using hybrid membrane distillation and reverse osmosis systems. PLoS ONE 13(10): e0205012. https://doi.org/10.1371/journal.pone.0205012
Editor: Nicolas Roche, Aix-Marseille Universite, FRANCE
Received: February 16, 2018; Accepted: September 18, 2018; Published: October 9, 2018
Copyright: © 2018 Ali et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Data Availability: All relevant data are within the paper and its Supporting Information files.
Funding: The research is funded by the deanship of scientific research at King Saud University through the research group program number 1438-93. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
Geothermal energy is derived from hot water or steam drawn from sub-soil and is mainly used for generating electricity, producing heat, and cooling. Lund et al. [1] reviewed direct global applications of geothermal energy worldwide and determined that it was used for bathing and swimming, heating spaces and districts, and acted as a ground source heat pump. Specific data and information on the use of geothermal energy in 82 countries up to year 2015 were gathered, presented, and discussed. The report showed that the amount of thermal power installed globally for direct utilization at the end of 2014 was approximately 71,000 MW, and the electrical capacity of the European Union reached 993.6 MW in 2015 (with 915.5 MW in Italy) [2]. However, geothermal energy use is limited in Saudi Arabia, where there is an installed capacity of only 40 MW for bathing and swimming and 4 MW for animal farming, providing a total direct use application of 152.89 TJ/year [1].
In a recent paper, Gude [3] noted that geothermal energy sources are clean, sustainable, and act as both heat sources and energy storage systems. He discussed the current status and future prospects of geothermal desalination, and also stressed the merits and potential use of geothermal energy sources to drive thermal desalination processes, as the energy is available in large quantities (which is required by thermal desalination). Gude [3] also analyzed case studies from various countries (including Saudi Arabia, Costa Rica, and Australia) to determine the progress of technological developments made in this field.
Several studies relating to Saudi Arabia have reported relatively low enthalpy sources at temperatures lower than 100 °C [3–5]. In their study of potential geothermal energy in the kingdom, Demirbas et al. [4] compiled the characteristics of main hot spring locations in the country and discussed the potential utilization of such sources, and AlHarbi [5] conducted a survey on available geothermal energy resources in the country and classified them based on exergy using a specific exergy index (SEI). The SEI values of all identified geothermal wells were found to be very low, which means they are classified as very low-grade energy sources. Their potential uses are therefore limited to low-enthalpy applications, including heating and low-temperature desalination methods, such as LT-MED, humidification and dehumidification (HDH), and membrane distillation (MD). These low-grade geothermal energy sources are thus good candidates for driving conventional thermal desalination processes, and in this respect, low-temperature multiple effect distillation (LT-MED) using geothermal energy has been proposed in several studies. In addition, Davies and Orfi [6] proposed a framework study showing the technical feasibility of self-powered geothermal desalination of groundwater sources at temperatures lower than 100°C. Additionally, desalination processes (including MD) driven by renewable energy sources (such as solar or geothermal sources) has become an attractive concept, as reflected in the increasing number of studies conducted on such integrations [7,8].
In their review of main studies focusing on MD powered by solar energy, Qtaishat and Banat [9] reported that although it has been proven that the combination of solar energy and MD is technically feasible, the cost of water production is relatively high compared to the use of commercial photovoltaic RO. Guillen et al. [10] presented results obtained from two pre-commercial MD modules driven by solar systems under the same weather and operating conditions and discussed data on energy consumption, efficiency, and production rates. The authors noted that although the multistage concept for MD can reduce energy consumption, the production of fresh water is still low. In addition, Manna et al. [11] used cross-flow flat-plate modules to conduct experimental work on the removal of arsenic from contaminated groundwater used for drinking water by employing solar-powered MD; results showed that almost 100% of the arsenic was removed from the water. This confirms one of the major advantages of the MD process: its ability to treat various types of feed waters, even those with high salt concentrations.
However, one of the major limitations of MD is its low recovery ratio, and an increasing number of studies focused on solving this limitation have been published in the last several years. Such studies have proposed associated methods and configurations, assessed their respective effectiveness, developed new types of materials and membranes, and conducted appropriate experiments and tests. For example, Summers et al. [12] presented a fundamental study on the energy efficiency of single stage MD under different configurations of brine regeneration and energy recovery. Their results showed that air gap membrane distillation (AGMD) and direct contact membrane distillation (DCMD) have the potential to provide a high gain output ratio (GOR) if properly optimized. Summers et al. [13] also developed theoretical frameworks using various main MD configuration models, which enabled them to conduct a comparative study on the performance of these configurations. The results for single stage MD showed that, in particular, vacuum membrane distillation (VMD) is very limited, as its GOR is always lower than one. From a different perspective, Winter et al. [14] conducted an experimental investigation on flux enhancements using feed water deaeration on spiral wound MD modules. Their results showed that deaerated water can be used to remove air from the volume pores of the membranes, thereby highlighting one of the beneficial effects of such a method.
Several concepts and methods have been used to enhance MD performance, particularly with respect to increasing the recovery ratio and improving product fluxes, such as brine recycling methods, the use of energy recovery devices, and the use of hybrid desalination processes. In addition, the implementation of a multi-stage (or multi-effect) concept, which is commonly used in conventional multiple-effect distillation and multi-stage flash technologies, has received much interest and attention [15,16], as it can increase the recovery ratio and also reduce the specific energy consumption of the desalination process.
Integrating MD with other processes can improve the overall performance of the entire combined system. Macedonio and Drioli [17] designed and studied the performance of a reverse osmosis system followed by a membrane distillation unit; their results are encouraging as they present the possibility of overcoming the limitations of each single unit. Criscuoli and Drioli [18] conducted an energy and exergy analysis of an integrated system coupling RO, MD, and nanofiltration (NF) modules, in which the MD unit operated on the RO brine while the NF unit was used for RO feed pretreatment, and concluded that such an integrated system represented an attractive alternative to RO and to conventional thermal desalination processes. In addition, Mericq et al. [19] proposed an integrated VMD—RO unit (in which VMD was used as a complimentary process to RO) to further concentrate the RO discharge brines and thus increase the overall recovery of the plant. Furthermore, El-Zanati and El-Khatib [20] proposed a hybrid system consisting of NF and RO followed by VMD, where the overall recovery for seawater was increased from 30–35% when using RO to 76.2% when using the hybrid system. Pangarkar et al. [21] reviewed the coupling of RO and MD processes for desalination of groundwater and presented several advantages of using such an integrated system with groundwater in India. Zhang et al. [22] investigated the performance of a membrane distillation crystallization unit operated on brines from a seawater RO unit and obtained a water recovery ratio of 90%. Swaminathan et al. [23] proposed a theoretical analysis of a hybrid mechanical vapor compressor (MVC) and MD to reduce specific energy consumption, and Osman et al. [24] analyzed the performance of hybrid multi-stage flash distillation (MSF)–RO desalination plants for large applications.
To provide further results on integrated systems, this current paper presents a theoretical analysis of an integrated MD—RO driven partially by geothermal energy. Several integration scenarios are proposed, and results are analyzed in terms of the recovery ratio, production rate, product quality, and energy consumption.
Geothermal brackish water is usually distilled using RO units after the treated feedstock has been cooled. However, there is much potential in using the thermal energy of geothermal water and employing MD, which provides the added advantages of producing high-purity water and being insensitive to feed salinity. In this study, different design configurations of MD/RO hybrid systems were investigated, which included differences in brine recycling and the cascading structure. The results substantiated the superiority of the integrated system compared to the conventional one; the recovery ratios ranged from 30% to 40% and the energy costs per m3 ranged from 0.4 to 0.9 $ when using variations in the RO operating pressure between 6 and 40 bar, respectively. It was found that brine recycling also improved the recovery rate and performance ratio only when brine was reused around the RO unit. When the rejected brine was fed back to the MD feed, the performance deteriorated because cold recycled brine quenched the MD feed and led to a reduced driving force inside the MD membrane. It was also revealed that using a multi-stage MD/RO interplay system connected in series enhanced the performance. In fact, a 90% recovery ratio and 0.9 MJ/kg performance ratio were obtained when 8 stages were employed. Although the production cost increased to 9 $/m3 because inter-stage heating was involved, if waste heat was used for inter-stage heating, the specific energy cost would be considerably reduced.
1.
Lee J.W., Boyd T.L. Direct utilization of geothermal energy 2015 worldwide review. Geothermics, 2016; 60: 66–93.2.
The state of renewable energies in Europe, www.eurobserv-er.org/16th-annual-overview-barometer/3.
Gude V.G. Geothermal source potential for water desalination—Current status and future perspective. Renew Sust Energ Rev. 2016; 57: 1038–1065.4.
Demirbas A., Alidrissi H., Ahmad W., Sheikh M.H. Potential of geothermal energy in the Kingdom of Saudi Arabia. Energ Source Part A, 2016;38(15): 2238–2243.5.
Alharbi M. Geothermal energy in Saudi Arabia and its application in desalination, Master thesis, Department of Mechanical Engineering, King Saud University, KSA, 2014.6.
Davies P., Orfi J. Self-powered desalination of geothermal saline groundwater: Technical feasibility. Water, 2014; 6: 3409–3432.7.
El-Bourawi M.S., Ding Z., Ma R., Khayet M.A. Framework for better understanding membrane distillation separation process. J Membrane Sci. 2006; 285: 4–29.8.
Drioli E., Ali A., Macedonio F. Membrane distillation: Recent developments and perspectives. Desalination, 2015; 356: 56–84.9.
Qtaishat M.R., Banat F. Desalination by solar powered membrane distillation systems. Desalination, 2013; 308: 186–197.10.
Guillén E., Blanco J., Alarcón D., Zaragoza G., Palenzuela P., Ibarra M. Comparative evaluation of two membrane distillation modules. Desalin Water Treat. 2011; 31: 226–234.11.
Manna A.K., Sen M., Martin A.R., Pal P. Removal of arsenic from contaminated groundwater by solar driven membrane distillation. Environ Pollut. 2010; 158(3): 805–811. pmid:1988396112.
Summers E.K., Arafat H.A., Lienhard J.H. Energy efficiency comparison of single stage membrane distillation (MD) desalination cycles in different configurations. Desalination, 2012; 290: 54–66.13.
Ali M.I., Summers E.K., Arafat H.A., Lienhard J.H. Effects of membrane properties on water production cost in small scale membrane distillation systems. Desalination, 2012; 306(15): 60–71.14.
Winter D., Koschikowski J., Ripperger S. Desalination using membrane distillation: Flux enhancement by feed water deaeration on spiral-wound modules. J. Membrane Sci. 2012; 423–424: 215–224.15.
Drioli E., Ali A., Macedonio F. Membrane distillation: Recent developments and perspectives. Desalination 2015; 356: 56–84.16.
Guan G., Yang X., Wang R., Fane A.G. Evaluation of heat utilization in membrane distillation desalination system integrated with heat recovery. Desalination 2015; 366: 80–93.17.
Macedonio F., Drioli E. Pressure-driven membrane operations and membrane distillation technology integration for water purification. Desalination, 2008; 223: 396–409.18.
Criscuoli A., Drioli E. Energetic and exergetic analysis of an integrated membrane desalination system. Desalination, 1999; 124 (1–3): 243–249.19.
Merciq J.P., Laborie S., Cabassud C. Vacuum membrane distillation of seawater reverse osmosis brines. Water Res. 2010; 44(18): 5260–5273. pmid:2065975320.
El-Zanati E., El-Khatib K.M. Integrated membrane based desalination system. Desalination, 2007; 205 (1–3):15–25.21.
Pangarkar B.L., Sane M.G., Guddad M. Reverse osmosis and membrane distillation for desalination of groundwater: A review. International Scholarly Research Network ISRN Materials Science (2011), Article ID 523124, 9 pages.22.
Zhang Y., Peng Y., Ji S., Li Z., Chen P. Review of thermal efficiency and heat recycling in membrane distillation processes. Desalination, 2015; 367: 223–239.23.
Swaminathan J., Nayar K.G., Lienhard J.H. Desalin Water Treat. 2016; 57:1–11.24.
Hamed O.A. Overview of hybrid desalination systems—current status and future prospects. Desalination, 2005; 186, 1–3: 207–214.25.
Ali E., Orfi J., Najib A. Assessing the thermal efficiency of brackish water desalination by membrane distillation using exergy analysis. Arab J Sci Eng. 2018; 43: 2413–2424.26.
Ali E., Orfi J. An experimentally calibrated model for heat and mass transfer in direct contact membrane distillation. Desalin Water Trea 2018; 116: 1–18.27.
Turton R., Bailie R., Whiting W., Shaeiwitz J. Synthesis, Analysis, and Design of Chemical Processes, Prentice Hall, USA, 2008.28.
Choi Y.J., Lee S., Koo J., Kim S.H. Evaluation of economic feasibility of reverse osmosis and membrane distillation hybrid system for desalination. Desalin Water Treat. 2016; 57: 24662–24673.
For more information Nanofiltration Membrane, Reverse Osmosis and Nanofiltration, please get in touch with us!