Membranes: A Solution for Our Water Woes

In May 2014, the United States Drought Monitor reported that 100% of Californians experienced “severe, extreme, or exceptional drought conditions” (1). The drought will cost US$2 billion and 17,000 agricultural jobs. Despite a series of fines and awareness campaign efforts to cut water consumption by 20%, water usage still increased in the subsequent month (2). This incident is neither isolated nor exaggerated; water scarcity is a reality in today’s world where the global population is expected to increase 50% by 2050 (3). As water is a sine qua non for virtually every industry for purposes of irrigation, industrial cooling and domestic consumption, any growth in water supply is largely overshadowed by the increase in water demand if a business-as-usual approach, assuming a current trend of efficiency gains, is adopted for the coming decades (4). Therefore, there are only two ways out of this conundrum – significantly lessen the demand for water or hugely increase the supply of water in the decades ahead.

Unfortunately, since water has no viable substitute in our everyday lives, it remains unlikely that water demands can be drastically reduced. On the other hand, Earth’s freshwater resources are limited at approximately 0.01%, or around 200,000 km³ (5) of total global water resources, insufficient to meet even today’s demands, and it seems that the only way to increase global water supply is to recycle water from wastewater or obtain a “limitless” supply from desalination.

In a typical water reclamation seawater desalination setup, the feed solution (seawater of higher solute concentration) and draw solution (pure water of low solute concentration) are separated by a semi-permeable membrane. The membrane contains a thin dense active layer that selects against bacteria, viruses, heavy metals and dissolved salts from entering the permeate, producing water of top quality. Today, Reverse Osmosis (RO) is arguably the leading water desalination technology and up to 80% of global desalination plants employ the RO process (6).

Broadly defined, membranes are selective barriers that allow the passage of certain constituents while retaining others (7), and have had a history of more than a century. The father of membrane science is Thomas Graham, who invented dialysis in 1861 and investigated the diffusion of gases across polymeric membranes (8). These ground-breaking experiments set the groundwork for future advances in membrane science, such as RO in the 1920s (9). Since the 1980s, membrane processes such as ultrafiltration, microfiltration, electrodialysis and RO, have been used in the medical, petrochemical, food processing, drug manufacturing and water treatment industries. As membrane processes have high flexibility, do not require heating, and are able to physically separate a wide variety of compounds, membranes are inextricably linked to our society today. As such, due to the increasing global demand, new innovations and applications of membranes, the membrane technology market is projected to grow 8% annually, from US$15.6 billion in 2012 to US$25 billion in 2018 (10).

Though membranes have a wide variety of applications, this essay will zoom in on the potential of membranes to solve one of mankind’s most imminent problem in the 21^st century – the global shortage of water. Unlike other limited natural resources like fossil fuels, water does not have any feasible alternatives and water shortage is therefore an urgent issue to be resolved.

In water desalination, RO has served us well for the past three decades. Since the 1980s, multiple innovations involving the modification and production of the membrane’s composition and module, as well as energy-recovery devices, have significantly reduced the energy requirements of RO from 16 kWh/m³ in the 1970s to 3 kWh/m³ today (11-14). In the energy-water nexus, the energy demand of water production is a principal consideration because water presents not only a problem per se, but also contributes significantly to our energy constraints. However, despite all these energy-saving measures, RO today is still fairly energy-intensive and expensive. Energy consumption alone accounts for 20-35% of the total cost of RO desalination (15). Moreover, its thermodynamic efficiency is low since a significant 36.2% of the total destruction of exergy (i.e. the maximum useful work that can be done by a device) is lost in the separation units (16). Additionally, 8.4% of the total exergy destruction is also lost as a result via the discharged brine from the RO system.

A further reduction in the energy requirements of desalination in the near-future is imperative. As aforementioned, the energy requirements of RO have been largely decreased. However, if we wish to further reduce the energy demands significantly, it would be extremely difficult to do so if we are confined to improving current membranes and fail to seek the possibilities of synergies with other membrane processes (17). In fact, there are three technologies that have been heralded as having the ability to reduce energy consumption of water production by 30% – forward osmosis (FO), carbon nanotubes, and biomimetics (18). Among these three, FO is the closest to commercialisation and does not require extensive redesigning of the membrane module. FO uses the basic process of osmosis, in which water flows from the seawater feed solution to a more concentrated brine draw solution. However, the claim that FO can alleviate our water woes might seem counter-intuitive and presents a paradox since FO is essentially the reverse process of RO, which has been the main technology for desalination for the past few decades. This is because water flows down a water potential gradient in FO, while conversely, water flows against the water potential gradient in RO, which has an applied pressure on the draw side. (For the benefit of readers, Figures 1 and 2 in the Appendix illustrate schematics of FO and RO).

Despite the acclaimed prospects of FO, research papers such as McGovern’s and Lienhard’s have shown that FO, when used for direct desalination, is far from energetically comparable to RO due to the need to re-concentrate the draw solution after being diluted in FO (13).
They calculated that the theoretical minimum energy required for FO is approximately 20% higher than that of RO. However, analysing FO and RO as two separate processes restricts our mind. On the contrary, when they are viewed from a different angle and the two processes are combined, a new system for desalination is obtained. This could bring about potential benefits in terms of energy reduction and overall waste reduction.

This hybrid system of RO coupled with FO could be inspired by endosymbiosis, which postulates that the mitochondria of eukaryotes evolved from aerobic bacteria that once lived inside a host cell. Initially, such bacteria would be harmful to the cell and enter it either as an antagonistic prey or parasite. Avoiding detection, it manages to live inside the cell without being digested. The aerobe’s ability to use oxygen to produce energy is beneficial to the host cell as it allows the cell to thrive in an increasingly oxygen-rich environment. Over time, a symbiotic relationship develops, and the original bacteria is assimilated, eventually evolving to form the energy-producing mitochondria in cells today. In a similar manner, FO and RO can be combined in a hybrid two-step process in which the impaired brine from a water reuse system (e.g. the concentrated feed solution from a wastewater treatment plant), which would otherwise be treated and disposed into the ocean, is used to dilute seawater via the naturally occurring FO system. This diluted seawater is then used as the feed solution for the RO desalination process (details of the process can be inferred from Figure 3 in the Appendix).

Fundamentally, the FO process spontaneously dilutes the seawater, reducing the osmotic pressure gradient that the applied force on the feed solution has to overcome in RO. This can significantly reduce the energy demands for the RO desalination process by reducing the total exergy destruction during the RO membrane process, as previously mentioned (16). This illustrates the “symbiotic” relationship between the two processes.

Initially proposed in 2008, this hybrid system has gained considerable attention in recent years (19-21). A subsequent study found that up to 71% of the impaired water from a water reuse system could be recovered economically just by the savings from lower osmotic pressure through the dilution of seawater through an FO process before producing desalinated water (22). In fact, another study also assessed that the hybrid process offers a potential energy reduction of 20%-23% and a total capital cost reduction of 8.7%-20% (23).

Using a theoretical thermodynamic analysis for the energy consumption of a single-stage RO using a model based on the minimum work input for partial separation of the feed solution (16), the minimum theoretical energy needed to produce pure water from seawater (of concentration 35000 ppm) is 0.54 kWh/m³ of product water, given a recovery ratio of 50%. Compared to the hybrid system, setting the dilution factor parameter at 50%, the minimum theoretical energy is 30% less than that needed for a conventional RO system, at only 0.38 kWh/m³, (calculations and graph in Figures 4 and 5 in the Appendix). These calculations show that the hybrid system is effective in lowering the cost of desalination when combining the seemingly antagonistic processes of FO and RO through creative manipulation and capitalising on the different strengths of the two processes.

Moreover, such a hybrid system would also present many other benefits. Due to the usage of impaired water from a water reuse process to dilute the seawater via FO, some of the 8.4% exergy lost as a result of the discharged brine (16) can be recovered to do useful work. This reduces the need for additional energy input for proper management and disposal of brine back into the ocean, and also mitigates the environmental impacts of discharging concentrated brine into the ocean. In fact, waste should not be seen as an unwanted ‘side-effect’, but as a potential resource that can be tapped into, resolving the issues of the water-energy-waste nexus. The impaired brine and seawater brine that were once considered waste are now recycled. Recycling waste is not a new idea as one might initially think; we have used organic waste to produce compost and recycled products. Therefore, coupling FO with RO for seawater desalination not only lowers energy requirements, but also serves the national interests of water self-sufficiency, reducing the likelihood of cross-boundary conflicts for water in water-depleted nations like Israel and Jordan..

A decrease in the energy requirements of desalination would have huge potential in alleviating the woes of water shortages in the coming decades, demonstrating the potential of membranes to further serve us in the 21^st century. The hybrid system also underscores the importance of thinking outside of the box such as capitalising on seemingly opposite processes in the case of FO with RO to inspire elegant solutions in the realm of inventions and science. In this age of quantum computing and 3-dimensional printing, we must also not forget the usefulness and future possibilities of the humble paper-thin membrane.

Appendix

Figure 1: Schematic of a FO process

Figure 2: Schematic of a RO process

Figure 3: Schematic of the hybrid process

Figure 4: Graph relating the minimum theoretical energy, E_t, to the mass fraction of salt, mf_s, in feed solution. Without dilution, typical seawater has a mf_s value of 0.035 (0% dilution) and a E_t value of 0.54 kWh m^-3; The hybrid feed that is seawater diluted by 50% has a mf_s value of 0.019 (50% dilution) and a E_t, value of 0.38 kWh m^-3.

Figure 5: Equation to calculate E_t /kJ kg^-1 of product water from set parameters (15),
R_u – universal gas constant, T₀ – feed temperature, M_product – molar mass of product (pure water), x_{i, j} – mole fraction of i in j, s – salt, w- water

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