July 2019

Water Management

Minimize energy consumption in water treatment with forward osmosis technology—Part 1

Heavy industries like refining, petrochemicals and mining have concerns about the availability of fresh water, as these are affected by local water scarcities and stringent water discharge regulations.

Attarde, D., Das, A. K., Racha, S. M., Ghosh, S., Reliance Industries Ltd.

Heavy industries like refining, petrochemicals and mining have concerns about the availability of fresh water, as these are affected by local water scarcities and stringent water discharge regulations. The oil and gas industry needs relatively huge volumes of water, as the water/oil ratio averages 8:1.

FIG. 1. Percentage distribution of water use in refinery.
FIG. 1. Percentage distribution of water use in refinery.

The preferred conventional water treatment technology, reverse osmosis (RO), is still energy-intensive even after an upgrade of RO process technology. Industries are examining alternative water sources and treatment technologies, and implementing water recycling or reuse practices.

Forward osmosis (FO) technology, an emerging method, can be used to mitigate the aforementioned problems. FO uses the natural osmotic pressure difference between two solutions of different concentrations as a driving force to permeate freshwater through the semi-permeable barrier. Due to freely available renewable osmotic energy, projected energy savings from the FO systems compared with conventional technologies has been realized in the range of 30%–70%, depending on the product (freshwater) recovery.

FO can energetically outperform conventional technologies with much lower fouling propensity. This article provides information on this state-of-the-art process and the physical principles and applications of FO, as well as their strengths, limitations, economics, pilot/commercial-scale status and major challenges. Two different types of FO approaches—direct and indirect FO desalination—are discussed.

Introduction

Industrial water demand has been growing with the pace of industrial development.1–5 The World Bank has projected that approximately $700 B1 will be needed worldwide in the next decade to meet freshwater demand. Progress in some water-intensive industries has been significant, placing further pressure on industrial demand for water.2

Crude oil refining is also a water-intensive industry; around 1.5 bbl of freshwater is essential to process 1 bbl of crude oil. Poor-quality, “price-advantaged” crude needs relatively higher volumes of water to remove salt and impurities from crude, adding heat to the processes (as steam), removing heat from the processes (as cooling water), and equipment cleaning and maintenance purposes. Fig. 1 illustrates the percentage distribution of water use in a refinery.2,3

FIG. 2. Illustration of reverse osmosis (RO) and forward osmosis (FO).
FIG. 2. Illustration of reverse osmosis (RO) and forward osmosis (FO).

Seawater desalination and wastewater reuse are the most feasible means for the world’s biggest industries to mitigate scarcity of freshwater. RO is the preferred technology.6 RO is a process in which water permeates through the membrane from high- to low-solute concentrated solution due to applying higher hydraulic pressure (P) than osmotic pressure (π) on the high-solute concentrated solution, as shown in Fig. 2a. The osmotic pressure of the solution is the minimum pressure required to permeate water in a solution through a semi-permeable membrane.

After many developments, RO is still an energy-intensive technology.5,6 Therefore, the need exists to utilize alternate energy-efficient technology to meet freshwater demand. FO is emerging as an energy-efficient membrane technology for seawater desalination and wastewater reuse.6

The main difference between the FO and RO processes is the direction of water permeation, as shown in Fig. 2. In FO, the water (solvent) permeates, in the opposite direction of RO, from low- to high-solute concentrated solution due to the higher osmotic pressure difference (π2 – π1) than the hydraulic pressure difference (P2 – P1). The low concentrated solution is usually considered as feed solution, while the high concentrated solution is considered a draw solution. Recent developments of FO are mostly focused on seawater desalination and wastewater treatment.

FIG. 3. Potential benefits of FO as compared to the more preferable conventional RO technology.
FIG. 3. Potential benefits of FO as compared to the more preferable conventional RO technology.

FO has a range of potential benefits compared to RO, as shown in Fig. 3. In seawater desalination, a hydraulic pressure in the range of 60 bar–90 bar is required in RO to overcome the osmotic pressure of seawater and obtain sufficient recovery; in FO, negligible hydraulic pressure in the range of 1 bar–2 bar is needed to enter the feed into the system. Due to the lower hydraulic pressure requirement, the fouling tendency (deposition of natural organic matter and polymerized silica on the membrane surface) is relatively less in FO. Fouling is reversible in FO, but irreversible in RO. In other words, the deposition of foulants on the membrane in FO is temporary and, therefore, the reduced flux can be completely recovered after simple cleaning processes that improve the total average water flux, quality of product and membrane life.6,7

Water flux and recovery in FO can be easily increased by raising the osmotic pressure difference across the membrane.8 These aforementioned benefits reduce OPEX against RO. Apart from low OPEX, additional advantages of using FO systems compared to RO include:

  1. Chemical storage and feed systems may be compact for capital, operational and maintenance costs
  2. Lesser process piping
  3. More flexible treatment units
  4. Greater total sustainability of the desalination process.5,6,7

Many scientists and economists are attracted to idea of driving desalination units using solar or wind energy. However, these technologies are restricted to small scales and may be practical only for “off-the-grid” locations.4 The main disadvantage of these technologies is that energy sources are not available 24 hr/d.

Several advances have been seen in graphene membrane, a thin layer of sp2 hybridized carbon, due to its peculiar mechanical, thermal and electrical properties. In such a membrane, water or selected solutes passes through straight pores; at present, however, the pores cannot be made small enough to reject salt.4,9 Furthermore, graphene-based RO membrane would be expensive compared with commercially available membranes.9

Two ways of FO desalination exist: direct and indirect FO desalination.5 The former takes seawater as a feed solution and a solution of relatively higher osmotic pressure as a draw solution, which is again treated to reuse. The latter uses impaired water, like municipal or industrial wastewaters, to dilute seawater through FO; the diluted seawater is then treated by the low-pressure RO process to produce freshwater. Both ways of FO desalination are discussed in detail in the following sections.

Direct FO desalination systems

In a direct FO desalination system (Fig. 4), saline water (like seawater) as feed solution and an osmotic reagent solution of relatively higher osmotic pressure as draw solution are taken on either side of a semipermeable membrane. Freshwater is extracted through the membrane from the feed solution into the draw solution due to the osmotic pressure difference across the membrane. Diluted draw solution is then sent to an additional stage to recover freshwater and regenerate the draw solution. Osmotic reagent in the draw solution can be a volatile or non-volatile salt.5

FIG. 4. Direct FO desalination system.<sup>5</sup>
FIG. 4. Direct FO desalination system.5

The most widely studied direct FO desalination process is with ammonium bicarbonate osmotic reagent due to its easily separable and regenerable characteristics.6 Ammonium bicarbonate converts into ammonia, carbon dioxide and water at very low temperature (around 40°C) in an endothermic process, and again can be recovered in a crystalline structure at 30°C.

It has been reported that energy savings of around 70% may be realized with the use of this osmotic reagent, compared with conventional technologies. However, challenges to making the process commercial include the reduction of loss of ammonium bicarbonate due to its flow into the feed solution and complete transformation of ammonia, carbon dioxide and water into ammonium bicarbonate for regenerating the draw solution.

The studies are being directed toward developing a novel osmotic reagent, which has favorable abilities like high solubility, high osmotic pressure, low cost, nontoxicity, easy separation, reusability and eco-friendliness.5,6 Hydrophilic nanoparticles have also been considered as an osmotic reagent solution, with synthetic seawater as a feed solution.

Ultrafiltration membrane-based processes have been used for the regeneration of draw solutions. Lower recovery of hydrophilic nanoparticles and poorer water flux are the main concerns with this osmotic reagent draw solution. Another study on divalent salts, such as Na2SO4, used an osmotic reagent draw solution for the FO process. The study showed adequate fluxes when desalinating brackish water; the draw solution regeneration process was nanofiltration.9 Most of the draw solutes studied so far are less realistic due to their high cost, poorer water flux and inefficient regeneration.6,7

Indirect FO desalination systems

In indirect FO desalination, FO is not directly involved in the desalination process, but rather used as pretreatment for the conventional desalination unit,7 as shown in Fig. 5.

FIG. 5. Indirect FO desalination system.<sup>5</sup>
FIG. 5. Indirect FO desalination system.5

Unlike direct desalination, an additional regeneration step is not required in this type of desalination. Seawater as the draw solution and any impaired water source, such as industrial or municipal wastewater, is taken as the feed solution. The freshwater is permeated through the FO membrane into seawater from impaired water using the free osmotic energy, leading to partially desalinated water. This water is then sent to a relatively low-pressure RO unit for further desalination.

The coupling of seawater desalination and simultaneous wastewater treatment by integrating FO and RO lessen overall energy consumption, offering a viable solution for the water-energy nexus for coastal locations. The supplementary benefits of this integration are providing cost-effective wastewater treatment and mitigating expensive treatment management.

The concentrated wastewater has further worth, which can be used to produce biogas or other valuable compounds. Furthermore, freshwater in wastewater permeates through both FO and RO membranes. Due to these double barriers, the rejection of most of the micro-pollutant impurities in wastewaters is relatively high7 in such a desalination system vs. traditional wastewater treatment systems.

Like traditional RO technologies, fouling on the surface of the FO membrane is also seen in indirect desalination. The various methods of FO membrane cleaning, like chemical cleaning, air scouring and osmotic backwashing (which reverses the permeate flow by generating the opposite osmotic pressure gradient across the membrane) have been extensively investigated. It is reported that osmotic backwashing does not help recover flux, but air scouring and chemical cleaning using commercial chemical agents can increase the flux recovery to 90%–95%.5,7,10,11

This offset of flux restore of 5%–10% may be ascribed to the permanent deposition of biopolymers on the surface of the membrane. However, it can also be noted that the percentage of flux recovery depends on the type of foulants present in wastewater. The spacers (mesh-like structures) in membrane modules generally mitigate the fouling rate in membrane processes.

Part 2 of this article, to be published in the August issue, will look at membrane development and manufacture, as well as the integration of FO with an existing multi-stage-flash-distillation unit. HP

References

  1. Doble, M. and V. Geetha, “Understand the fundamentals of wastewater treatment,” American Institute of Chemical Engineering, 2011.
  2. Jenkins, M., “Refinery tackles water issues,” Chemical Processing, 2016
  3. Guernsey, C. H., “Optimization of water usage at petroleum refineries,” Water/Energy Sustainability Forum, Ground Water Protection Council, 2009.
  4. Kumar, M., T. Culp and Y. Shen, “Sustainability in water desalination,” Penn State University, 2015.
  5. Linares, R. V., Z. Li, S. Sarp, Sz. S. Bucs, G. Amy and J. S. Vrouwenvelder, “Forward osmosis niches in seawater desalination and wastewater reuse,” Water Research Vol. 66, 2014.
  6. Cath, T. Y., A. E. Childress and M. Elimelech, “Forward osmosis: Principles, applications, and recent developments,” Journal of Membrane Science, Vol. 281, 2006.
  7. Akther, N., A. Sodiq, A. Giwa, S. Daer, H. A. Arafat and S. W. Hasan, “Recent advancements in forward osmosis desalination: A review,” Chemical Engineering, Vol. 281, 2015.
  8. Iyer, S., “Hybrid FO-EED system for high salinity water treatment,” US Patent 20170326499.
  9. Wang, E. N. and R. Karnik, “Water desalination: Graphene cleans up water,” Nature Nanotechnology, 2012.
  10. Linares, R. V., Z. Li, V. Y. Quintanilla, Q. Li and G. Amy, “Cleaning protocol for a FO membrane fouled in wastewater reuse,” Desalination Water Treatment, Vol. 51, 2013.Linares, R. V., Z. Li, V. Y. Quintanilla, Q. Li and G. Amy, “Rejection of micropollutants by clean and fouled forward osmosis membrane,” Water Resources, Vol. 45, 2011.

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