Introduction

The demand for energy and clean water is growing at an alarming rate. The global energy demand is forecasted to increase by 50 % within the next 30 years, and it is also projected that the world would face a 40 % global deficit in freshwater resources at the current consumption rate [1].

This looming crisis can be averted by employing water treatment technologies that are low-cost, energy-efficient, and can be integrated with renewable energy sources. These water treatment technologies can be employed for seawater/brackish water desalination and water recovery from industrial/domestic wastewater.

Membrane-based technologies have emerged as viable alternatives to address this growing concern of water scarcity (Fig.1). The membrane market is projected to reach USD 38.34 Billion by 2027, growing at a CAGR  (Compound Annual Growth Rate) of 7.30% from 2020 to 2027 [2].

Reverse osmosis (RO) is the most widely accepted membrane technology for water recovery from various feed streams since its inception in 1962. A selective membrane is the heart of this technology that permits water molecules to pass and retains almost all the colloidal and dissolved matter larger than 0.1 -1 nm.

Seawater desalination requires a pressure of 55-68 bars to be applied to overcome the osmotic pressure and produce acceptable water flux. It can be seen from Figure 2 that RO technology is the most widely implemented desalination technology.

Although RO is widely implemented, the energy requirements (2.5–4.0 kWh/m³) and the average cost of product water (0.5–1.2 USD/m³) are still limiting factors, especially for low-income countries [5]. Moreover, 71 % of the total electrical energy requirement is solely for the RO, and the lifetime of the membranes is limited due to the severe membrane fouling.

The management of the RO-reject also poses additional challenges [6]. Alternate membrane-based technology such as forward osmosis and membrane distillation are discussed below as potential alternative technology for water recovery and sea/brackish water desalination.

Forward Osmosis

Forward osmosis is a membrane filtration technique introduced in the late 1970s for fruit juice concentrates and is now regained interest as a potential alternative to the RO process [7]. The FO process has lower energy requirements (as it utilises the natural osmosis as the driving force), can recover water from high TDS solutions (such as hypersaline waste streams, RO-reject, etc.), and the membrane experiences lower fouling (longer membrane life) [8].

The principle of operation of the FO is as follows- water is transported from a feed solution (lower concentration) to a draw solution (engineered solution with higher concentration) across the semipermeable membrane due to the osmotic pressure gradient. As a result, the process ends with a highly concentrated feed solution and a dilute draw solution, as seen in Figure 3.

There are two approaches to improving the water recovered in the forward osmosis process. Increasing the concentration of the draw solution leads to a more significant osmotic pressure difference (higher driving force). NaCl is the most widely explored draw solute as it is inexpensive, non-toxic and can be easily recovered from the dilute streams by conventional processes (RO).

The membrane must prevent the reverse solute flux (transport) of the draw solutes to the feed solution. The second approach uses efficient membranes with lower structural parameter ‘S’ (thickness × tortuosity/porosity) to reduce the membrane fouling (ICP) due to the build-up of salts in the membrane support layer. The orientation of the active layer of the membrane can also play a role in the FO performance [9].  

The FO process has been evaluated to recover water from various feed streams such as – oily wastewater, NH₄Cl solution, brackish water, seawater, simulated radioactive wastewater, microalgae broth, biological wastewater treatment effluent, fruit juice, sewage, anaerobic digestion effluent, distillery wastewater, antibiotic solution, fresh synthetic urine.

Various solutes have been investigated as draw solutes, and they can be classified based on   (i) The properties of draw solute, (ii) The regeneration method used, (iii) Compatibility with the feed solution to be treated, (iv) Whether the intended product is a clean permeate or a dewatered feed.

The Various classes of draw solutes are (a) Gas and volatile compounds, (b) Inorganic draw solutes, (c) Fertiliser, (d) Organic draw solutes, (e) Switchable polarity solvents (SPS), (f) Hydrogels, (g) Ionic Liquids, (h) Nanoparticles [10].

Interestingly, the final product of the FO separation process is not clean water, rather a diluted draw solution and a highly concentrated feed. So, the FO process is preferred if the dilute draw solution can be used directly (fertiliser driven irrigation) or the process is used solely to dewater the feed (here, inexpensive draw solutes such as seawater can be used).

In order to obtain pure water, a further separation step is required to remove the draw solute, which again adds to the overall cost of the process.

Two large scale FO-RO installations were set up by Modern Water in Oman and Gibraltar in 2010, which led to them bagging the contract to erect a 200m³/day desalination plant at Al Naghdah, Oman.

The polyamide membrane developed by Porifera has proved to be successful in wastewater recovery and liquid food processing applications. Since its foundation in 2010, Trevi Systems have demonstrated its cellulose acetate (CA) FO membranes at five pilot studies in the USA (2013) and in the Middle East (2015). In 2017, Aquaporin entered the FO market, launching a new hollow fiber (HF) module [11].

Membrane Distillation

Membrane Distillation (MD) is an emerging technology explored in seawater desalination, sewage treatment, and water recovery from industrial effluents (food, pharmaceutical, healthcare, etc.).

The key advantages of the MD process are relatively lower energy costs (low-pressure operation as compared to RO, lower temperature of operation as compared to multiple-effect distillation), theoretically 100 % rejection of non-volatile solutes (high rejection of partially volatile solutes), and a microporous membrane with significantly lower membrane fouling

This MD process involves a hot saline solution in the feed side of the membrane and a cold pure distillate (water) in the permeate side with the vapour permeating through the membrane (Figure 4).   The distinctive hydrophobic nature of the membrane prevents the entry of liquid and permits only the volatile component to  diffuse across the dry membrane pore [12].

The water vapor flux is proportional to the vapor pressure difference existing between both sides and this driving force is produced by maintaining a transmembrane temperature difference or by lowering the pressure in the permeate side by use of vacuum/sweep gas  [13].

The energy requirement for employing MD for desalination of highly-salty feeds with a latent heat recovery system lies somewhere in between RO and MSF. Further, MD requires minimum/ no pretreatment even for produced water [14], can utilise low-grade energy, is independent of feed concentrations, and operates at lower temperature/pressure [15].

Low-grade energy requirement allows it to utilise renewable energy sources – solar, geothermal, tidal, wind, etc. or even co-locating the MD units close to industrial facilities and power generation systems to take advantage of the waste heat.

Hybrid systems integrating MD with other separation methods are also being explored to improve water recovery [16]. Table 1 shows the different applications of MD and its advantages and limitations.

However, the performance of MD is severely affected by two key factors: (i) wettability as a result of condensation of water vapor inside the pores of the membranes; and (ii) fouling due to the accumulation of biofilm, organic, inorganic, and colloidal substances on the surface or in the internal pore structure of the membranes. These two limiting factors restrict the choice of suitable polymers for the synthesis of MD membranes [17].

For example, the cost of production of water using petroleum or electricity as the energy source in 10.8 USD / m³ using the AGMD configuration. This cost can be significantly decreased to 2.68 USD / m³ if an additional and affordable solar energy can be used, thus rendering the operating expenses equivalent to traditional desalination technologies

Hydrophobic polymers such as Polypropylene (PP), Polytetraflouroethylene (PTFE), Polyethylene (PE), Polyvinylidiene fluoride (PVDF) are some of the most widely explored in MD. Nanoparticles can increase the tensile strength or prevent heat conduction in the membrane which leads to improved mechanical and thermal stability.

Roughness is an important factor that determines the antiwetting capability of the membranes. It promotes the hydrophobicity. Nanomaterials increases the surface roughness of membranes due to the presence of numerous air pockets. Self-cleaning methods inspired from nature can be conferred to membranes via functionalising with nanomaterials  [18].  

Integrated FO-MD process

As mentioned earlier, FO process ends with a diluted draw solution, an additional recovery such as MD, RO, or nanofiltration is required to obtain freshwater [19]. The MD system perfectly complements the FO process due to its ability to withstand high saline concentrations.

FO-MD systems are used for seawater desalination as well as effluent treatment. In a lab-scale study, FO alone could recover only 95 % Hg2+ (i.e. 1- 10 ppb gets transferred to the draw-solution). However, coupling with the MD process could guarantee a 100 % rejection of Hg²⁺. Thus, MD can also serve as an effective secondary process for eliminating organic and inorganic pollutants [21].

Applications of FO-MD include:

• Desalination of brackish water [22] 
• High salinity landfill leachate treatment [23] 
• Recovery of minerals like nitrogen, phosphorous and potassium from wastewater [24] 

Conclusions

Membrane-based technologies for sea/brackish water desalination and wastewater treatment are reliable and economically viable solutions to stem the increasing water scarcity.  Forward osmosis and membrane distillation have emerged as attractive alternatives to reverse osmosis, the gold standard in water treatment.

The lower operating pressure, ability to handle higher concentrations of dissolved solids, and lower membrane fouling make these processes attractive to researchers and technologists alike.  

The application of a standalone FO system is limited to cases where the diluted draw solution can be directly used, there is no need for DS recovery or where the DS can be regenerated easily. However, coupling FO with MD can have several advantages- the FO process acts as a pretreatment step for the MD (reduces membrane fouling/wetting of the hydrophobic membranes), and high purity water can be recovered from the diluted DS.

Membrane distillation can remove ~100 % of the dissolved solutes, and the FO process could remove organic contaminants that could wet the MD membrane. Using waste heat or integrating the process with renewable energy sources (solar energy, geothermal, etc. ) can make the process sustainable and reduce operating costs.

Acknowledgements

The authors acknowledge the support from the SERB Startup Research Grant  titled “ Development of Electrospun Membranes with graded hydrophobicity for Membrane Distillation” (Scientific Social Responsibility) to Dr Noel Jacob Kaleekkal

References