What is next for forward osmosis (FO)

What is next for forward osmosis (FO) 

 

                                                                                                                  This short review summarizes our understanding and perspectives on FO and PRO processes and meaningful R&D in order to develop effective and sustainable Forward osmosis; 

·         FO membranes; 

·         PRO membranes; 

·         Integrated systems; 

·         Clean water; 

·         Osmotic energy

O and PRO technologies for water reuse and osmotic power generation.

1. Introduction

Technologies to produce clean water and clean energy have received worldwide attention due to water scarcity, highly fluctuating oil prices and global warming. Forward osmosis (FO) and pressure retarded osmosis (PRO) have received extensive attention during the last decade as emerging technologies for water reuse and seawater desalination, and power generation, respectively. The purposes of this short review are to summarize what we have learned in the last decade and to share our understanding and perspectives on FO and PRO in order to conduct meaningful R&D, and develop useful FO and PRO technologies for clean water and clean energy production.

Basically, FO takes advantage of naturally (osmotically) induced water transport across a semi-permeable membrane from a low osmotic pressure solution to a high osmotic pressure solution [1], [2], [3], [4] and [5]. Ideally, the semi-permeable membrane allows water to pass through it but rejects all salts or unwanted elements. The high salinity solution performs as the draw solution, which has a higher osmotic pressure than the feed solution, to induce water flow across the membrane from the feed solution to itself. Thus, FO requires less energy to transport a net water flow across the membrane compared with pressure-driven membrane processes such as reverse osmosis (RO). However, in contrast to RO, the product of FO is not a potable water but a diluted draw solution, a mixture of the respective draw and feed solutions. Therefore, a second step of separation must be utilized to extract clean water and to regenerate the draw solution.

The second step of separation may be energy intensive depending on the draw solutes and the recycle process. Therefore, for clean water production, one must consider the energy consumptions of both the FO process and the draw solute regeneration in order to make a fair comparison between FO and other water production technologies. Otherwise, the conclusion could be biased and misleading [6], [7] and [8]. Nonetheless, FO may be more cost-effective than pressure-driven membrane processes for water reuse if the regeneration of draw solutes is not needed. Thus, R&D on FO should prioritize those processes and applications without recycling draw solutes.

The idea of osmotic energy generation (PRO) was proposed about 70 years ago, but most of the early research studies were suspended owing to the absence of effective membranes [3], [4], [6] and [9], which are the heart of osmotic power systems. The estimated global osmotic energy using ocean and river water as feeds is high [9]. Statkraft of Norway built the first PRO prototype plant in 2009 using seawater and river water as feeds but terminated it in 2014 possibly due to technology immaturity such as membrane limitations, fouling, limited salinity gradient between seawater and river water, and small power output [10].

2. What is next for FO?

2.1. FO membrane development

There are a few comprehensive reviews on the progress of FO membrane development[2], [3] and [6]. Basically, most FO membranes were fabricated by traditional phase inversion [6] and thin-film composites (TFC) via interfacial polymerization methods[11] and [12]. FO membranes made from the layer-by-layer method have been investigated but their reverse salt fluxes tend to be high [13] and [14]. Using hydrophilic materials as substrates for FO membranes is essential to enhance water flux[15] and [16]. Recently, TFC FO membranes synthesized on nano-fiber [17] and multi-bore [18] substrates with good mechanical properties have also been demonstrated. Future R&D should focus on innovation membranes with minimal fouling and internal concentration polarization (ICP). So far, double skinned FO membranes, consisting of a dense RO skin and a loose RO skin, have shown promise with reduced fouling and ICP[19] and [20].

2.2. FO for water reuse

Because of no hydraulic pressure involved and low fouling propensity [21], [22] and [23], FO may be more cost-effective and superior in direct fertigation [24] and [25] and produced water reuse [16], [26], [27], [28], [29], [30] and [31] if the recycled water is for industrial reuse. Using fertilizers as draw solutions, directly drawing water from brackish or sea water for agriculture purposes, can significantly simplify fertigation processes with lower costs. It has a great potential for water-scarcity countries to farm salt-tolerant agricultural crops. Recently, oil–water separation has received special attention due to the large amounts of discharged oily wastewater from hydraulic fracturing and petrochemical industries. So far there is no effective method to treat stable emulsified oily wastewater. Promising results with reasonable fluxes, high oil rejections of >99% and low fouling characteristics have been demonstrated using single- and double-skinned FO membranes with sulfonated polymers facing the oil–water feed [20] and [30]. This may provide new insight into how to treat the oily-wastewater. Besides, since the wastewater from hydraulic fracturing contains surfactants and other chemicals, a hybrid forward osmosis–membrane distillation (FO–MD) system with a high water recovery has also been demonstrated to treat oily wastewater containing petroleum, surfactant, NaCl and acetic acid [31].

So far, FO still has difficulties in being a cost-effective technology for direct seawater desalination because of its high energy consumption and lack of effective draw solutes with minimal reverse fluxes. Despite many advances in draw solutes made recently [32],[33], [34] and [35], challenges still exist to (1) minimize the reverse flux of draw solutes, (2) alleviate ICP and (3) find facile regeneration methods. However, FO exhibits potential for impactful environmental applications and enrichment of high value-added pharmaceutical products.

2.3. FO for the removal of toxic ions and concentration of pharmaceutical products

Heavy metal contamination is a severe environmental issue because of an exponential increase of heavy metal compound usage in various industries. Since heavy metals cannot be metabolized by the body or decomposed naturally, they accumulate inside the body and cause severe body dysfunction. Hence, the removal of toxic heavy metal ions from wastewater is a top priority for many countries. Nano-filtration (NF) has been used for heavy metal removal, but it suffers from high fouling tendency and insufficient rejections.

FO has been proposed to remove boron and arsenic [36], [37], [38], [39], [40] and [41]. By using a novel bulky hydroacid complex as the draw solute to minimize reverse solute flux, FO has been demonstrated to effectively remove heavy metal compounds such as Na₂Cr₂O₇, Na₂HAsO₄, Pb(NO₃)₂, CdCl₂, CuSO₄, Hg(NO₃)₂ from wastewater [38]. High water fluxes were harvested with heavy metals rejections of more than 99.5%. In addition, the rejections were maintained at 99.5% when a more concentrated draw solution (1.5 M) or feed solution (5000 ppm) was utilized. Interestingly, rejections greater than 99.7% were still achieved by operating the FO process at 60 °C. These remarkable performances may create new FO applications to treat heavy metals-laden wastewater. However, one must find a disposable draw solute, such as RO brine or an energetic and economic favorable method to recycle draw solutes for this application to minimize the overall process cost.

The demands for pharmaceuticals and proteins are steadily increasing. Athermal enrichment methods are preferred because these products are labile and heat sensitive. Membrane technology has gained importance in biotechnology due to its mild operational conditions and superior separation abilities [42]. However, pressure-driven membrane processes are usually energy intensive, and severe membrane fouling is often encountered. In contrast, FO not only consumes less energy but also has much more reversible fouling. Nevertheless, the high reverse salt fluxes during FO processes using conventional draw solutes such as NaCl may denature the feed proteins. To overcome this, using dual-FO systems and bulky draw solutes with minimal reverse fluxes is recommended for pharmaceutical and protein enrichments [43] and [44].

2.4. System integration

Although FO may not as cost-effective as RO for seawater desalination, an integration of FO and RO, as shown in Fig. 1, may offer a better alternative for seawater desalination with a lower energy consumption and a higher water recovery [45], [46], [47] and [48]. By integrating FO and RO, additional feed water can be drawn from wastewater to lower the concentration of seawater before it enters the seawater reverse osmosis (SWRO) plant. As a result, SWRO can be operated at a lower pressure. In addition, the SWRO retentate can be re-diluted with the aid of another FO process and directly discharged.

 

 

 

 

 

FO and MD integrations are worthy of further studies with the aid of waste heat or solar energy because high water recovery and water purity can be obtained simultaneously. However, up to the present, only limited studies have focused on FO–MD systems for wastewater recycling.