Desalination 491 (2020) 114559

Contents lists available at ScienceDirect

Desalination
journal homepage: www.elsevier.com/locate/desal

Membrane desalination and water re-use for agriculture: State of the art and T
future outlook
Wafa Suwaileha, Daniel Johnsona, Nidal Hilala,b,
⁎

a
Centre for Water Advanced Technologies and Environmental Research (CWATER), College of Engineering, Swansea University, Swansea SA2 8PP, United Kingdom
b
NYUAD Water Research Center, New York University Abu Dhabi, Abu Dhabi, United Arab Emirates

GRAPHICAL ABSTRACT

ARTICLE INFO ABSTRACT

Keywords: Membrane-based desalination technologies for agricultural applications are widely applied in many countries
Water desalination around the world. Sustainable and cost-effective desalination technologies, such as reverse osmosis (RO),
Membrane technology membrane distillation, forward osmosis, membrane bioreactor, and electrodialysis, are available to provide
Hybrid system treated water, but the pure water product does not contain the required level of nutrients to supply agricultural
Agriculture
fields. This can be overcome by the use of blended water to meet the required quality of irrigation water for crop
Crop production
production, which is expensive in areas lacking in freshwater resources. The adoption of a hybrid system offers
many advantages, such as generating drinking water and water enriched with nutrient at low cost and energy
consumption if natural power is used. This review focusses on summarizing the current and recent trends in
membrane desalination processes used for agricultural purposes. The challenges being faced with desalinating
seawater/brackish water and wastewater are discussed. A specific focus was placed on the viability of hybrid
desalination processes and other advanced recovery systems to obtain valuable irrigation water. A comparison
between various membrane desalination technologies in terms of treatment efficiency and resource recovery
potential is discussed. Lastly, concluding remarks and research opportunities of membrane technologies are
analyzed. We concluded that the ED process can be utilized to minimize the energy requirements of other

⁎
Corresponding author at: Centre for Water Advanced Technologies and Environmental Research (CWATER), College of Engineering, Swansea University, Swansea
SA2 8PP, United Kingdom.
E-mail address: n.hilal@swansea.ac.uk (N. Hilal).

https://doi.org/10.1016/j.desal.2020.114559
Received 10 May 2020; Received in revised form 28 May 2020; Accepted 28 May 2020
0011-9164/ © 2020 Elsevier B.V. All rights reserved.
W. Suwaileh, et al. Desalination 491 (2020) 114559

membrane technologies. The MD coupled with ED system can also be utilized to generate high quality irrigation
water at low energy requirement. The FO-ED hybrid system exhibited excellent performance and very low
energy consumption as compared to other hybrid systems.

1. Introduction leading system for seawater desalination due to minimum energy ex-
penditure relative to other desalination processes [25,26]. When the
The global demand for drinking water, food security concerns, and seawater was replaced by brackish water in a BWRO plant at Almeria
climate change effects on farming have motivated scientific commu- Cuevas de Almanzora, the product water was used for fertigation [18].
nities to search for alternative resource management strategies [1,2]. The most important advantage of this process was the generation of a
Since petroleum resources are being reduced, most countries have variety of water qualities, which could be used as irrigation water and
looked for agriculturally produced materials to be used for manu- for golf land irrigation. The potable water can also be obtained by
facturing and trade, which imposes further demand on crops [3]. The mixing the permeate stream with raw water. Spain and Australia de-
consumption of plant waste is a promising resource for energy extrac- pend on SWRO desalination technology for seawater desalination to
tion and conversion to electricity [3]. The existing demands on these produce irrigation water for agricultural uses. Australia pioneered the
agricultural products are expected to increase in the future, imposing use of reverse osmosis capable sub-surface drip irrigation (ROSDI) for
challenges to developing nations. It has become necessary to explore fertigation [17]. This process does not require high hydraulic pressure
additional water resources to increase agricultural materials production because it operates based on tension on the soil side to draw water into
and support ever-growing requirements [4,5]. There is an intensive use the system. An acceptable amount of water-rich nutrients of around
for irrigated water estimated at 70% of total usage, followed by in- 0.25 and 1.5 L/h.m2 and salt rejection of around 50% were achievable.
dustrial utilization, around 21%, and domestic use around 9% [1]. Some hurdles associated with the RO process hampered its utilization
There has been a renewed interest in the treatment of wastewater to for agricultural aspects. For instance, the desalinated water does not
irrigate crops in greenhouses. Membrane based desalination processes contain an acceptable amount of nutrients or boron or chloride for ir-
used to treat wastewater are reverse osmosis (RO) [6,7], nanofiltration rigation water, a high quantity of brine is discharged to the sea, harmful
(NF) [8], membrane bioreactor [9,10], membrane distillation (MD) gases may be released into the air, the excess sodium affected the soil
[11], and electrodialysis [12]. For example, to remove nitrogen from and productivity and energy consumption and cost are high [18].
wastewater, high energy input is required around 45 MJ per kg nitrogen Moreover, recovery strategies have been suggested to concentrate nu-
to extract nitrogen gas [11]. NF membranes can be used to separate trients and ensure suitable quality of irrigation water. Some of these
various nutrients such as ammonium, phosphate, and potassium from methods are adsorbents such as carbon-based adsorbents [27] and se-
sewage sludge [8], achieving a high rejection rate of these nutrients at piolite [28] along with membrane technologies such as FO [29] and RO
low hydraulic pressure. However, the wastewater feed solution is processes [30].
composed of various chemical species which may result in fouling and This paper is a timely critical review of recent advances in mem-
membrane deterioration. Fouling is created due to the adherence of brane-based desalination technologies for producing agricultural irri-
solutes and particulates on the membrane surface leading to cake layer gation from saline water and wastewater. It addresses the main lim-
formation and pore clogging [13,14]. Another study reported that there itations associated with membrane-based treatment processes
were limited wastewater resources and that its price is high in many development. It discusses the performance of advanced membrane
developing countries. Thus, researchers shifted to desalinate natural technologies during seawater/brackish water desalination and waste-
groundwater or brackish water for crop growth due to availability and water reclamation in terms of treatment efficiency and resource re-
low salinity (5 ≤ S ≤ 5 g/kg) [15]. covery potential. It also highlights the potentiality of the hybrid desa-
To maximize the agricultural output and minimize impacts on lination process and other complementary processes for recovering
natural water resources, many countries are beginning to utilize irri- nutrients. Finally, conclusions and remaining drawbacks that need to be
gated water produced from different saline water sources to cope with further investigated are summarized.
high food production demands [16]. Some potential solutions are to
develop low cost and climate-independent water resources for fertiga- 2. Applicability of membrane desalination technologies for
tion, which are related to desalination technologies. Efficient desali- fertigation
nation technologies for irrigated agriculture depends on water desali-
nation and wastewater reclamation [17]. Many countries have started Membrane technology is the leading process for treating seawater
using desalinated water for agricultural purposes to meet their water and wastewater, providing sustainable development and targeted pro-
needs. For instance, Spain consumed 22% used of desalinated water for cess efficiency [17]. Many countries over the world have begun to use
fertigation from a total desalination capacity of 1.4 million m3/day membrane technology to produce water-rich nutrients for agriculture.
[16], while Kuwait has a desalination capacity higher than 1 million Nutrient concentrations by membrane technology are a powerful
m3/day and 13% for fertigation. Still, only 0.5% of desalinated water treatment option for combined production of crops and potable water
overall is currently being used for fertigation. Italy and Bahrain im- [20]. One of the advantages of membrane desalination in agriculture is
plemented a desalination capacity of 64,700 m3/day and 620,000 m3/ the generation of additional water resources, known as irrigation water.
day while they used only a small proportion of desalinated water of During the late 1950s to the 1980s, asymmetric cellulose acetate
1.5% and 0.4% for agriculture. The USA and Qatar used only 1.3% and membrane was the first membrane used for the RO process [31]. After
0.1% of desalinated water for agricultural purposes. that, the development of RO membranes continued to enhance the
Brackish water desalinated via RO is the most common practice due performance of membrane desalination processes. Although the high
to high purity product water [18,19]. Additionally, brackish water can cost of the RO process remains the major hurdle to the application of
be desalinated by other membrane-based desalination processes such as RO to seawater desalination and reuse, RO membranes are the most
NF [20,21], ion exchange resins [20], forward osmosis system (FO) technically viable membranes for producing irrigation water [32]. For
[22], closed-circuit reverse osmosis (CCRO) [23], and electrodialysis agricultural fields, RO membranes or membranes in the hybrid system
reversal (EDR) system [24]. Monovalent-selective electrodialysis re- can generate a high quantity of drinking water and water suitable for
versal (MS-EDR) has been employed to concentrate sodium chloride irrigated agriculture at relatively low cost and environmental effects
from seawater [15]. Among these desalination technologies, RO is the [17]. RO membrane can also be used to desalinate brackish water, with

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W. Suwaileh, et al. Desalination 491 (2020) 114559

the cost estimated to be a third that of seawater desalination [20]. water. The needed mineral content for agricultural applications is es-
Several industrial seawater and brackish water plants were developed timated at 0.75 g/L.
by TEDAGUA to supply irrigation water for agriculture [33]. In 1987, The essential nutrients for plant growth are N, P, K, Ca, and S [39].
RO was operated in the seawater desalination plant located in Gran Among these elements, Nitrogen (N)/ Phosphorus (P)/ Potassium (K)
Canaria [33]. The salinity of the seawater feed was about 34,000 mg/L. are the most significant nutrients for mineral or artificial fertilizer.
The production capacity of irrigation water was 6900 m3/d, and a Therefore, the water-soluble fertilizer to be added should contain a
further increase in the capacity by 500 m3/d was expected in the future. suitable quantity of N/P/K nutrients. The concentration of these nu-
The water permeate had an acceptable level of salinity of about trients in the fertilizer solution depends on the type of crops, cropping
200 mg/L. seasons, and soil nutrient amounts [40]. The suggested concentration
Electro-dialysis reversal (EDR) technology was installed in Gran for N/K/P in the irrigation water is ranged from 50 to 200 mg/L, 15 and
Canaria to produce pure water for agricultural fields [33]. This process 250 mg/L, and up to 1 mg/L [37,41]. According to the United Nations
is able to desalinate brackish water with a low concentration of around Food and Agricultural Organization (FAO), the recommended con-
3000 mg/L. The predicted energy consumption to treat this brackish centration of calcium and magnesium in irrigation water is around
water was around 1–2 kWh/m3 [33]. 400 mg/L and 61 mg/L, respectively [42]. Besides, the acceptable
Membrane distillation is currently being researched to generate ir- phosphorus concentration in the product water from a wastewater plant
rigation water from seawater. It has been found that the desalinated should be as low as 1.0 mg/L in most countries in which polypho-
water recovery was high, resulting in a decrease in the discharge cost sphates and organic phosphate species derived from orthophosphate
per unit of water distillate [20]. compounds are the wastewater [41]. The acceptable level of Mg+2 is
The membrane bioreactor (MBR) is a widespread technology used to from 48 to 65 mg/L, while it is around 321 mg/L for SO−2 4 constituents
treat municipal wastewater for agricultural purposes [10]. MBR con- [37].
sists of biological processes coupled with membrane filtration to re- The main physicochemical factors for assessing the quality of ef-
move organic and inorganic pollutants and microorganisms from was- fluent wastewater are chemical oxygen demand (COD), biochemical
tewater [9,34]. This system can be used in countries that rely on oxygen demand (BOD), ammonia‑nitrogen, total organic carbon (TOC),
agriculture to grow their economy and can be implemented in rural and total suspended solids (TSS) [43]. It is, however, impossible to use
areas or modern cities. There are many industrial plants around the these physicochemical factors in determining the acute toxicity and
world able to reclaim wastewater for agricultural fields. An example is genotoxic hazards to aquatic organisms present in the effluent. Aquatic
an MBR employed to purify wastewater for irrigating vegetables in organisms are an effective way to assess the toxic impact of the treated
Chania on the island of Crete [10]. The cost of the MBR system was water and evaluate the detoxification efficiencies of many systems [44].
estimated to be a few cents/m3 to 1 or 2 USD/m3 when treating was- Other parameters, such as boron concentration or Sodium Adsorption
tewater to produce irrigation water for food production. This value is Ratio (SAR), should be taken into account. The concentration of boron
assumed to increase based on the water-scarcity factors. The low price in seawater has been recorded between 4.5 and 6.0 mg/L, while ac-
of purified water relative to the traditional freshwater would encourage cording to the World Health Organization, the acceptable level of boron
farmers on the island to utilize the purified water and improve water in irrigation water is below 0.50 mg/L [32]. The potassium adsorption
resource management. Mixing MBR and RO effluents could achieve the ratio (PAR) is also used determine water quality. It demonstrates the
required quality of irrigation water including acceptable amount of adverse impact of potassium on soil permeability properties [42]. The
salts [35]. In this way, the reclaimed wastewater has negligible impact water infiltration issue is known as relative to SAR (Sodium Adsorption
on the soil, and there is no need to dispose of the reclaimed wastewater. Ratio) with reference to electrical conductivity. Sodium toxicity can be
To that end, membrane technologies are regarded as key elements of measured based on RSC (residual sodium carbonate), SSP (soluble so-
providing the feasibility of extracting irrigation water with appropriate dium percentage), and ESP (exchangeable sodium percentage) [38].
salinity for food productivity by either using desalinated water or re- Blending of the treated water with freshwater can minimize the
claimed wastewater. concentration of toxic compounds and make it reusable for fertigation.
This method is successful in reducing the sodium toxicity because its
3. Water quality required for agricultural irrigation adsorption in the soil depends on the proportion of monovalent (Na+)
and divalent (Ca+2) cations [38]. When diluting the treated water, the
Water quality plays an important role in determining the suitability soil would prefer to adsorb the divalent salts like calcium and magne-
of a water supply to be used for agricultural applications. Nowadays, sium ions more than the monovalent sodium.
new resources with lower quality are being used for irrigation projects
because many good quality water supplies have been intensively used 4. Challenges in membrane technology development
[36]. There are some restrictions for using wastewater effluent directly
for vegetation, such as negative impacts on the physio-chemical prop- The most important challenges in the membrane desalination and
erties of the soil, increasing microbial activity in the soil, aggravation of wastewater treatment industries involve the characteristics of the feed
crop production and yield, and contaminating groundwater with un- solution, the standard quality of the treated water, materials develop-
desired elements [37]. The most significant characteristics in the ment, process advancement, brine discharge, energy consumption, op-
treated water used as irrigation water are salinity, sodium content, erational and capital costs of facilities and instruments [11,45].
trace elements, excess chloride, and nutrients [38]. High salinity in the The desalinated water should possess low salinity, meeting the
irrigation water influence plant health and productivity along with quality standard, and the required nutrient levels for irrigation water.
deterioration of the soil structure and properties [38]. This is because the desalinated water or treated wastewater containing
The product water from the desalination process includes total a high concentration of total dissolved solids (TDS) like Sodium (Na+)
dissolved solids (TDS) with very low concentrations of < 20 mg/L, and Chlorine (Cl−) can deteriorate soil properties, inhibit crop pro-
which can be used as drinking water [16]. If the concentration of the ductivity and affect negatively the environment [45,46]. On the other
inlet fed to the desalination unit is low, the final volume of the hand, the desalinated water may miss some important mineral nutrients
permeate could be maximized by blending the permeate with the inlet for plant growth, and hence adding complementary minerals to the
water, thereby decreasing the unit cost of irrigation water [16]. desalinated water is essential [26]. Other very important problems are
In general, the permeate water has a minimum quantity of calcium the product water quality accuracy, the difference in nutrient require-
and magnesium and is slightly acidic [16]. Therefore, it should be re- ments for targeted crops, and demand. In light of this, recovery
mineralized and balanced to reach the required quality for irrigation methods for concentrating nutrients should be utilized to ensure a

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W. Suwaileh, et al. Desalination 491 (2020) 114559

product of acceptable quality for agricultural fields. Another drawback concentration can be further reduced from 1.5 to 0.5 mg/L in the nu-
is the emission of CO2 into the atmosphere, which estimated to be trient water through multistage RO, electrodialysis and adsorption-
0.9 kg CO2 per cubic L of purified wastewater [11]. membrane filtration hybrid systems [54]. The Ashkelon and Palmahim
Membrane technology based on electricity and thermal energy, such seawater desalination plants in Israel produced high quality desalinated
as electrodialysis/electrodialysis reversal, reverse osmosis, and mem- water with boron concentration lower than 0.4 mg/L [55]. Municipal
brane distillation, are energy-intensive processes and very expensive wastewater includes a high quantity of colloidal particles, suspended
[46–48]. The thermal desalination process is not cost-effective, and solids and dissolved organics, which induces membrane fouling [38]. In
hence it is rarely used for brackish water desalination. The cost of ion this respect, a pre-treatment process is needed to decrease the con-
exchange membranes in the voltage-driven membrane process is higher centration of these species. Another significant concern is brine disposal
than for RO [49]. In parallel, the salt separation efficiency is low when which contains high concentration of different salt species. This causes
using seawater as the feed solution compared to the RO process. adverse impacts on the aquatic ecosystem.
Therefore, some developing countries cannot afford these desalination Osmotic gradient processes, such as FO, have potential for agri-
technologies for irrigated agriculture. Additional issues are the high cultural irrigation. Although the individual FO process requires lower
electrical resistance of the membrane causes a reduction in the non- energy input and less influenced by fouling, it has some disadvantages,
Ohmic voltage [49]. This occurs when voltages move across the like the separation of the draw solution and loss of nutrients [56,57]. To
membrane, thereby influencing the energy expenditure of the system. separate the draw solution effectively, a post-treatment strategy is re-
This electrical resistance is strongly correlated with the solution con- quired, which increases energy consumption. The solute leakage allows
centration. The membrane perm-selectivity can be reduced due to se- accumulation of solute in the feed solution leading to reduced effective
vere concentration polarization phenomena arising from the solute osmotic pressure gradient and fouling/scaling on the membrane sur-
leakage. Since this process is operated using two electrodes, a large size face, which reduces the productivity and lifetime of the membrane
and quantity of the electrodes are required for industrial plants [50]. [58–60]. When the draw solution is being diluted through the support
This increases the operating and investment costs, and therefore, it is layer as a result of the convective flow of water across the selective
difficult to be commercially acceptable for water desalination. layer, a severe dilutive internal concentration polarization occurs
The MD process is not practical for brackish water due to high en- [61,62]. Thus, there is a drop in the osmotic pressure gradient leading
ergy consumption [20]. However, it might be effective for desalinating to low water permeation. If using fertilizer as draw solute, the draw
high salinity brackish water (up to 15,000 mg/L) or seawater. In com- solution will require further dilution to meet the quality standard of
parison, anaerobic membrane bioreactors (An-MBRs) combined with irrigation water [29,56,63].
low-pressure microfiltration (MF) or ultrafiltration (UF) has shown low
rejection towards dissolved organic carbon [51]. The treated water has
5. Water nutrient production from seawater/brackish water
quality like that for effluent generated through aerobic treatment [52].
However, membrane fouling causes high energy demands and therefore
5.1. Pressure-driven membrane process
this technology is not suitable for energy recovery.
Pressure driven membrane processes, especially RO, suffer from
5.1.1. RO process
fouling due to complex feed streams (such as municipal wastewater)
Over the years, pressure-driven membranes, such as RO and NF
impacting the long-term performance of the membrane and the man-
membranes, have been used for desalinating saline water for agri-
agement of brine discharge [48,53]. This can cause the accumulation of
cultural purposes and drinking water consumption [19]. The common
various constitutents on the membrane surface. This leads to low water
characteristics of pressure driven membrane applications is outlined in
permeation and poor water quality, thereby increasing energy input.
Table.1.
However, if the feed pressure is raised to ensure consistency of the
RO has the greatest total capacity worldwide relative to other
water flux, this imposes an additional energy requirement [38,53]. It
membrane technologies. RO membranes have a high rejection rate to-
has been suggested that the energy expenditure and overall cost could
wards salt, high water permeation, and good tolerance at very high
be reduced if the membrane pore size is increased. Therefore, when
hydraulic pressure. Improvement in membrane materials and fabrica-
operating a brackish water feed with a salinity of 15,000 mg/L, the
tion of membrane modules with a large surface area per unit volume
estimated total cost to generate irrigation water approached 0.13 $/m3
has leaded to a reduced price of membrane and water production cost
along with an investment cost of $17.54 million.
[64]. In parallel, the recovery ratio was improved from 35% in the
On the other hand, the salt rejection was decreased from 97% to
1990s to around 45% now, and it can be further increased to 60% when
88% resulting in irrigation water of unacceptable quality. Even though
using the second pass RO process. RO membrane can be utilized to
the RO membrane achieves good quality desalination water when uti-
desalinate seawater with salinity in the range of 2.5 to 35 g/L for
lizing seawater/brackish water membranes, some of the removed mi-
agricultural irrigation and drinking water extraction at a cost of US
neral nutrients (calcium, magnesium and sulfate) are necessary for
$0.50/m3 to US$1.00/m3 [65]. Seawater desalination plants in Israel,
plant growth [17]. As boron, which can retard plant growth, can
such as Sorek, Hadera, and Ashkelon, were the top seawater desalina-
transmit easily through the RO membrane, a second RO cycle in many
tion globally due to high water capacity of around 540,000, 456,000,
industrial plants is needed. It has been highlighted that boron
and 392,000 m3/day respectively [17]. Another plant located in

Table 1
The important properties of pressure driven membrane processes which is classified into reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF), and mi-
crofiltration (MF). Reproduced with permission from Pangarkar et al. [95].
Membrane technology Applied pressure (kPa) Minimum particle size Pollutant removal (type, average removal efficiency%)
removed

Microfiltration 30–500 0.1–3 μm Turbidity (> 99%); bacteria (> 99.99%)

Ultrafiltration 30–500 0.01–0.1 μm Turbidity (> 99%); bacteria (> 99.99%); TOC (20%)
Nanofiltration 500–1000 200–400 Da Turbidity (> 99%); color (0.98%); TOC (> 95%); hardness (> 90%); sulfate (> 97%); virus
(> 95%)
Reverse osmosis 1000–5000 50–200 Da Salinity (> 99%); color and DOC (> 97%); nitrate (85–95%); pesticide (0–100%); As, Cd, Cr,
Pb, F removal (40–98%)

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W. Suwaileh, et al. Desalination 491 (2020) 114559

Australia, operated through a two-pass reverse osmosis membrane (groundwater) with various salinities (1000–3000 mg/L). Desalinated
system, provided 17% of potable water to 1.6 million users in Perth water from feed with a salinity of 500 mg/L contained a sufficient
[20,66]. The seawater plant required energy input between 4 and 12 concentration of nutrients for crop production. Therefore, the RO
kWh/m3. All these factors contribute to high operating costs as the permeate caused an increase of 56% and 73% in crop yield. The yield
energy is responsible for 30–50% of the operation cost. The Australian and profit of crops were maximum when using the treated water with
RO plant produced a high amount of concentrated brine, as much as this feed.
55–60% of the total feed stream [20].
Owing to the above restrictions, brackish water with lower salinity
5.1.2. NF process
has replaced seawater to obtain irrigation water. The first commercia-
In comparison with RO membranes, the NF membrane can be op-
lized brackish water desalination plant was first operated in 1979 [65].
erated under lower hydraulic pressure leading to lower energy con-
The total water capacity was about 20–21 m3/h when using water with
sumption and cost [70]. Birnhack et al. [71] utilized TFC NF mem-
salinity in the range of 4–15 g/L. Earlier, PA TFC RO membranes were
branes in a pilot-scale seawater desalination unit to concentrate Mg+2
used in six brackish water desalination plants, and the performance of
ions while reducing the addition of unnecessary seawater ions such as
this membrane was investigated in terms of permeate water quality
Cl−, Na+, B, Br− in the treated water for crop production. The prin-
[67]. All plants achieved similar productivity with little variation in the
ciple of this NF desalination process involved circulating seawater
water capacity and cost per cubic meter of treated water. The water
across the NF membrane, while Mg+2-rich brine was added into the
recovery was adjusted at 83% for plant-D and at 70% for plant B. Ex-
treated water. It was observed that the highest salt rejection rate ap-
cellent performance of the RO membrane was observed, providing
proached 97% when raising the hydraulic pressure to 28 bar at a re-
water permeate at the required standard for irrigation water. The re-
covery ratio of 40%. However, the rejection rate declined to 90%, 94%,
sults revealed that the membrane was effective in removing nitrate
95% when increasing the recovery ratio at varying hydraulic pressure
reaching 50 mg/L in the purified water, and the fluoride concentration
of 10, 18, 28 bar respectively. The concentration ratio between Mg+2:
was at an acceptable level according to WHO and PS standards. The
Na+1 was decreased upon increasing the recovery ratio, but there was a
chloride, sulfate, sodium, magnesium and potassium concentrations in
negligible change at high hydraulic pressure.
the purified water of all plants met the quality standard for potable
Ghermandi et al. [70] investigated the viability of the NF membrane
water. Production capacity approached 640 m3/day upon raising the
in purifying brackish groundwater with salinity of 1577 mg/L for
flow rate to 80 m3/h.
agricultural farms. A comparison between NF and RO membranes was
Garcia et al. [68] used Polyamide Thin-Film Composite (PA TFC)
also carried out. According to simulation data, the NF permeate had
(BW30-400 Filmtec™) membrane to treat groundwater well brackish
higher concentrations of the required nutrients such as calcium
water with a salinity of about 3.1 and 7.8 g/L to generate irrigation
(14.1 mg/L), magnesium (7.9 mg/L), and sulfate (33.5) than RO
water. The design of the RO system is provided in Fig. 1. The membrane
permeate, which were within the quality standard for irrigation water.
generated product water with acceptable salinity for fertigation. It was
It was suggested that when using the NF membrane, lower brackish
found that membrane scaling and frequent chemical cleaning affected
water volume by 34% was needed compared to the RO membrane.
the water recovery and energy consumption. The fractional water re-
However, using NF permeate was assumed to increase the biomass
covery decreased to 0.6 due to scaling. Another problem was an in-
activity by 18% while the RO permeate had an insignificant impact.
crease in the feed pressure by 980.67 kPa after 40,000 h running time.
Lew et al. [72] examined the performance of various membranes,
The specific energy consumption was relatively high at around 1.4 and
such as NF with 86% rejection and high flux, NF membrane with 91%
1.7 kWh/m3 after 5 years, along with the specific cost of water.
rejection and medium flux, RO membrane with 99.7% rejection and
Ismail et al. [69] investigate RO to desalinate brackish water
high flux, RO membrane with 99.2% rejection and very high flux. An

Fig. 1. A diagram of the BWRO desalination plant located in the island of Gran Canaria [68].

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W. Suwaileh, et al. Desalination 491 (2020) 114559

analytic hierarchy process (AHP) model and the multi-dimension draw solution, which when diluted can be used in irrigation water [74].
scaling (MDS) models were used to find out the optimal design of the Because the high amount of nutrients in the diluted draw exceeds the
membrane process for brackish water desalination. The theoretical quality standard of irrigation water it requires further dilution. This
outcomes indicated that the NF membrane with low rejection and high FDFO process needs a perfect membrane to separate different types of
flux was likely to have the best performance and produce irrigation nutrients effectively. However, most of the developed membranes are
water with sufficient nutrients concentration. This water product not yet commercialized [22,59]. For example, Lotfi et al. [75] used a
showed a low sodium absorption ratio (SAR). Both the NF membranes TFC hollow fiber membrane and brackish water feed to generate irri-
consumed low energy of 0.26 and 0.20 kWh/m3, respectively, and gation water as demonstrated in Fig. 2. The draw solutions were in-
hence low investment cost. organic fertilizers including ammonium sulfate (SOA) (NH4)2SO4, cal-
NF membranes were also used in a desalination plant in Saudi cium nitrate (CAN) Ca(NO3)2, mono-ammonium phosphate (MAP)
Arabia because they are less prone to fouling relative to PA TFC RO NH4H2PO4, diammonium hydrogen phosphate (DAP) (NH4)2HPO4.
membranes [65,73]. It was reported that the salinity of the desalinated Since the polyamide selective layer is negatively charged, the divalent
water decreased from 45,460 to 28,260 mg/L, and the chloride con- salts like Ca+2 and Mg+2 were efficiently separated and accumulated
centration was lowered from 21,587 to 16,438 mg/L. The NF mem- on the membrane surface, causing scaling. Also, Ca+2 could be trans-
brane achieved maximum rejection rate of sulfate (SO−2₄) of up to 99% ferred to the feed solution due to the reverse solute flux and interaction
while it was lowered to 98%, 92%, and 44% for magnesium (Mg+2), with nutrients such as SO+2 4, creating gypsum scaling (CaSO4) on the
calcium (Ca+2), and bicarbonate (HCO− 3 ), respectively. The hardness of membrane surface. Other nutrients with small hydrated ionic radii, like
the desalinated water was lowered from 7500 to 220 mg/L. The desa- NO−3 and NH+4, were poorly rejected and permeated rapidly through
linated water contained < 2 mg/L of SO−2₄, 29 mg/L of Mg+2, 40 mg/L the membrane to the feed solution. The forward diffusion of nutrients
of Ca+2, and 17 mg/L of HCO− 3 , which is lower than the recommended such as Ca+2 or Mg+2 to the draw solution which interacted with
concentration level for drinking water. phosphate resulted in calcium phosphate scaling. This adversely af-
Although the NF membrane generates high water permeation under fected the membrane performance and the quality of the water
low hydraulic pressure, the membrane can separate divalent ions only, permeate. The SOA fertilizer draw solution achieved the highest water
while allowing the permeation of monovalent ions. Thus, the irrigation flux around 11.2 LMH While CAN and DAP solutions had the lowest
water ends up with a low concentration of required nutrients such as water flux of 10.4 and 8.7 LMH.
SO−2₄ and Mg+2 and a high concentration of unwanted monovalent Phuntsho et al. [63] used a cellulose triacetate (CTA) FO membrane
ions such as Na+ and Cl−. and eleven commercial fertilizer draw solutions such as urea, ammo-
nium nitrate (NH4NO3), (NH4) 2SO4, Monoammonium phosphate
(MAP), potassium chloride (KCl), potassium nitrate (KNO3), Mono-
5.1.3. FO process potassium phosphate (KH2PO4), calcium nitrate Ca(NO3)2, sodium ni-
Fertilizer drawn FO processes for fertigation has been given much trate (NaNO3), Diammonium phosphate (NH4)2HPO4), ammonium
attention. A diverse range of commercial fertilizers can be utilized as a

Fig. 2. A schematic diagram of the semi-pilot scale fertilizer drawn FO system (FDFO) utilizing hollow fiber membrane module. The lumen side of the hollow fiber
membrane made of PA TFC active layer on top of the polyethersulfone (PES) support layer on the outer shell of the fiber. Adapted with permission from Lotfi et al.
[75].

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W. Suwaileh, et al. Desalination 491 (2020) 114559

nitrate (NH4Cl) for brackish water desalination including blended so- and potassium nutrients while the nitrogen nutrient concentration
lutions. It was highlighted that when blending two or three fertilizers in meets the recommended standard for irrigation water. The FO mem-
the draw solution, the product water contained a lower concentration brane was effectively cleaned using 5% citric acid yielding a complete
of nitrogen, phosphorus, potassium (NPK) nutrients relative to the in- recovery of the initial water flux.
dividual fertilizer draw solution. KCl and NH4H2PO4 draw solution Sahebi et al. [77] evaluated the performance of pressure-assisted
included only a small quantity of N nutrient (0.61 g/L), P nutrient FDFO using a flat sheet cellulose triacetate (CTA) FO membrane,
(1.35 g/L), and K nutrient (1.70 g/L) as compared to which individual brackish water feed (10,000 mg/L) and four fertilizer draw solutions
fertilizer draw solution having a high concentration of the single nu- ((NH4)2SO4, NH4H2PO and KCl) for fertigation. It was revealed that the
trient. However, it was observed that there was a significant nutrient membrane achieved higher water permeation corresponding 7.38, 8.62,
loss due to reverse solute flux. For example, the urea draw solution and 9.42 LMH for 0.1 mol/L NH4H2PO4, KCl, and NH4H2PO4, respec-
experienced a high drop by 65% in the amount of N nutrient relative to tively at a hydraulic feed pressure of 10 bar. This was related to 1928%,
other draw solutions. The membrane performance was also influenced 345%, and 237% growth in the water permeation upon using 0.1 mol/L
by mixing two fertilizer draw solutions as the osmotic pressure, and draw solutions as compared to 38%, 29%, and 69% at a draw solution
water permeation was decreased compared to that of individual draw concentration of 3 mol/L. This additional water flux produced when
solutions. using a low concentration of the draw solution at high hydraulic
Kim et al. [76] evaluated the performance of PA (TFC) FO mem- pressure, improved the draw solution dilution beyond the osmotic
brane in an FDFO system using RO brine as a feed solution and am- equilibrium point. A small reduction in the specific reverse solute flux
monium sulfate (SOA), calcium nitrate (CAN), di-ammonium phosphate was noticeable when increasing the hydraulic pressure to 10 bar. For
(DAP), potassium nitrate (KNO3) as draw solutions. The membrane instance, the specific reverse solute flux was reduced from 0.77 g/L and
separation performance was affected by scaling and reverse solute flux 0.60 g/L for NaCl and KCl, respectively, at 0 bar to 0.49 g/L and 0.45 g/
at a varying rate. For example, the lowest water flux, along with reverse L at 10 bar. Therefore, the final water product contained acceptable
solute flux, was assigned to the KNO3 draw solution. The fast transfer of nutrient concentrations for direct irrigation without the need for a post-
calcium ions and accumulation in the feed solution lead to the most treatment stage to lower the fertilizer concentrations.
significant membrane scaling (calcium nitrate). The solute leakage of Recently, Lima et al. [78] proposed a new principle of FO desali-
nutrients ordered from the lowest to highest as follows, SOA (2%), DAP nation that depends on a subsurface irrigation procedure for fertigation.
(5%), CAN (4%), and KNO3 (21%). Interestingly, KNO3 showed the It involves using irrigation pipes made of the BW30 RO membrane and
highest nutrient loss due to its high extraction capacity, which ac- FO 8040 FO membrane. The brackish water feed rich-nutrients passed
celerated the reverse solute flux. In terms of water recovery rate, a through the pipes to the soil and crops, which decreases soil dete-
maximum recovery rate was observed for the DAP draw solution (95%), rioration and yield. It was found that the FO membrane supplied the
followed by SOA (80%), KNO3 (79%), and CAN (70%). The draw so- soil with a higher amount of water permeate than that for the RO
lution with low concentration and high osmotic pressure had the membrane after six days. For instance, the FO membrane produced 11
highest extraction capacity according to the osmotic equilibrium. As a times higher water balance leading to efficient soil hydration as com-
result, the total recovery rate grew significantly. In term of N/P/K pared to that for the RO membrane. The soil treated with RO permeate
nutrients, the final product water contained higher concentrations of N was dried after the third day and remained dry throughout the ex-
(268.40 mg/L) from CAN, N (201.19 mg/L) and P (222.45 mg/L) from periment. To that end, the FO membrane performed better, and its
DAP, N (230.63 mg/L) from SOA, N (114.76 mg/L) and K (320.33 mg/ productivity is complying with the control membrane for the duration
L) from KNO3. This indicated that the nutrient solution needs further of the experiment.
dilution by potable water to lower the concentration of phosphorous

Fig. 3. The common stack unit consisted of cation exchange membranes (CEMs) and anion exchange membranes (AEMs) arranged in alternating sequences. The
electrochemical potential is produced when passing each high concentration compartment (HCC) and low concentration compartment (LCC) generated by aligning
alternatively both membranes. The salinity difference between both solutions allowed the transfer of ions from the membrane to electrodes. This resulted in a redox
reaction to extract electricity. Adapted with permission from Tufa et al. [137].

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W. Suwaileh, et al. Desalination 491 (2020) 114559

5.2. Chemical-driven membrane processes dialysis membrane to separate ions having the same charge signs. He
attempted to separate divalent ions such as SO−2 4 from monovalent,
5.2.1. Electrodialysis (ED) such as Cl−, via the same membrane. The feed was composed of a
A new membrane-based technology rarely used for seawater/ mixture saline solution (NaCl/Na2SO4) with initial concentrations of
brackish water desalination is electrodialysis. There are two types of 7.61, 0.32, 4.48, and 0.43 mmol L−1 for all ions, respectively. The
electro-membrane processes, reverse electrodialysis (EDR) and electro- membrane achieved excellent selectivity at the highest pH. When in-
deionization (EDI) for desalinating low salinity streams. creasing the pH value, the current efficiency of the selector-dialysis
The ED system is operated based on converting the salinity gradient system was also increased. There was a strong correlation between
between the concentrated solution (i.e., seawater) and diluted solution sulfate concentration and pH value. The membrane was capable of
(i.e., river water) into voltages using ion-exchange membranes [49,79]. concentrating sulfate to 4 and 3.5 mmol L−1 at the optimal conditions
In this system, cation and anion exchange membranes are arranged of current densities (31.2 and 46.8 A m−2) and a pH of 10. The purity of
alternately and isolated from each other by spacers to make channels. sulfate in the product water was higher than 85% at a current efficiency
Fig. 3 shows the normal EDR stack model where the ion flux transports of > 50%. This indicated that the selector-dialysis system was viable for
from the concentrated stream to the diluted stream, the selective separating monovalent ions (Cl−) from multivalent ions (SO−2 4 ), and
membrane allows the penetration of cations across a cation exchange therefore, the final product water can be used for agricultural irrigation.
membrane (CEMs) and the anions across an ion-selective anion mem- A new approach for brackish water desalination is using monovalent
brane (AEMs). This leads to the generation of an ionic current through selective cation exchange membranes in the ED process. This special
the multi-membranes in which can be converted into voltage due to membrane can be fabricated by adding a poly-cation layer on the
reactions occurring on the electrode [79]. The electricity can be col- membrane surface to reject monovalent salts such as Na and Cl while
lected using an electrical conversion device. In 2015–2016, the first ED/ retaining divalent salts such as Ca, Mg, and SO4 ions. A recent work
EDR and EDI electro-membrane process plants were operated using described the use of this membrane for desalinating brackish water to
saline water as a feed solution. An EDR plant implemented in South obtain irrigation water containing the required amount of mineral nu-
Africa produced water capacity in the range of few tens of m3/day up to trients [82]. To select the best performing commercial monovalent se-
10,000 m3/day from the brackish water inlet. lective ion exchange membranes (MIEM) in removing the monovalent
Eberhard et al. explored the feasibility of the electrodialysis process ions, the process conditions were optimized, and the effect of mem-
for separating micronutrients such as copper chloride and copper sul- brane selectivity was investigated. All MIEM membranes exhibited su-
fate from brackish water and coal seam gas water [80]. The electro- perior selectivity for sulfate than chloride. The performance of these
membrane had an active area of 207 cm2, and 20 cell pairs, including membranes was more efficient than monovalent selective cation ex-
the cation/anion membranes in alternated series, were employed. One change membranes when using brackish water with low conductivity. It
of the important findings is that the rejection rate of the copper and the was noticed that the anionic membranes purchased from MVK and CMS
sulfate reached 98% and 100%, respectively, after three hours of op- exhibited the good perm-selectivity for Ca+2 and Mg+2. The removal
eration time at 23 °C. In comparison, the removal efficiency of both the ratio of these cations was about 80% and 70%, respectively, while it
copper and sulfate was faster than that for NaCl with a rejection rate of was only 37–48% for Na ions. An anionic membrane manufactured by
around 72%. The water content in the diluted solution was reduced by CSO produced superior monovalent perm-selectivity of < 1 upon using
only 10%, which minimized brine disposal. The theoretical work sug- brackish water with conductivities of < 4.5 dS/m. The removal ratio of
gested that the mass/charge ratio of sulfur ion with large ionic radii Na+1, Ca+2 and Mg+2 amounted to 52%, 44%, and 24%, respectively.
could reveal the separation efficiency. For instance, the ions with small To achieve the best selectivity of monovalent ions, the current densities
ionic radii can be removed rapidly as compared to that with larger ionic should be maintained lower than the limiting current corresponding to
radii. The diluted solution contained 3.0 mg/L of copper nutrients and sodium concentration. When the total salinity of the product water
2.7 g TDS/L, which can be used directly for fertigation. decreased by 50%, the removal efficiency of Cl and SO−2 4 was as high as
Zhang et al. [81] studied the possibility of using a novel selector- around 90% and 12% for CSO membranes-modified with

Fig. 4. The design of the capacitive de-ionisation system. A circulation pump is used to drive the solution to the cell and the effluent return to the inlet tank with a
volume of 25 l. The cell is supplied with the required voltage via a power supply. The temperature of the solution was kept constant at 25 °C and the flow rate was
fixed at 0.5 L/min. Reproduced with permission from Mossad et al. [86].

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W. Suwaileh, et al. Desalination 491 (2020) 114559

polyethyleneimine. Lastly, the SAR in the final product water was 2.3 strongly to the electrodes leading to lower co-ion expulsion impact
making it suitable as irrigation water for crop production. It was con- compared to the commercial MCDI system.
cluded that this novel procedure facilitated the generation of irrigation More recently, Bales et al. [85] developed a simulation model to
water, which provides another water resource for fertigation and predict the performance of the MCDI process and combined it into an
eliminates negative effects on the environment. agricultural economics model. In this model, the environmental con-
ditions in Australia and a crop-water-salinity function were used to
5.2.2. Capacitive deionization (CDI) process estimate crop yield and profits. The MCDI consisted of an ion exchange
Capacitive Deionization is a desalination technology that depends membrane attached to each carbon electrode to eliminate the passage
on an electrical capacitance to separate or release charged ions from/ of ions during the recharge cycle. The current adsorption remained
into solutions [47,83]. Both CDI and ED had a similar operating prin- constant at zero-volt desorption leading to reduced energy consumption
ciple, especially the ions, transfer through the solution and across the relative to commercial CDI. According to the theoretical information,
membrane. However, CDI does not need a membrane and is considered this system can be utilized to irrigate many valuable crops, and it can be
a low-pressure process. This means that the CDI process is competing optimized based on the environmental conditions of any agricultural
with the pressure-driven processes (RO) and temperature-driven pro- area. Different salinity limits were used according to thresholds for
cesses (MD), which is capable of producing pure water at a lower op- different crops of 4.2 dS/m, 5.5 dS/m, 4.4 dS/m, 14 dS/m, and 8.5 dS/
erating cost [50]. The principle of the CDI can be explained as follows m for grapes, oranges, almonds, apples, and tomatoes for a 60 ha crop
[49,84]. We can see in Fig. 4 that a saline solution passes into a channel and investment period of 10 years. The cost of the treated water was
between capacitive electrodes that are separated by an ion-selective varied in each scenario, and it was estimated to be less than AUD$ 1/kL.
layer. This selective layer is used to increase the voltage efficiency and Therefore, this cost-effective MCDI system is feasible to desalinate
improve the performance of the system. The transfer of ions towards the brackish water providing irrigation water after further dilution by
capacitive electrodes is induced by applying an electrical potential freshwater.
difference between the electrodes. Thereafter, the ions are adsorbed on
this electrode, and hence the ions from the feed solution are removed. 6. Water nutrient production from industrial wastewater
As a result, the feed solution becomes almost free of salt ions providing
pure water. It should be mentioned that, at the saturation point of the 6.1. Pressure-driven membrane process
electrode, the salt ions are released from the electrode and transported
through a purge stream to the channel. This causes the accumulation of An alternative source of water for many agricultural applications is
ions in the solution generating a concentrated brine. The most widely treating different types of wastewater. Pressure driven membrane pro-
used applications are seawater desalination, brackish water desalina- cesses are effective methods for wastewater treatment due to high
tion, wastewater reclamation, and water softening [47,83]. Industrial productivity and selectivity towards organic and inorganic con-
plants for numerous applications are operated in the Netherlands and taminants [88]. Bunani et al. [38] used brackish water reverse osmosis
China, achieving water capacity around up to 2000 m3/h [85]. (AK-BWRO) and seawater reverse osmosis (AD-SWRO) membranes in
The CDI process for brackish water desalination has been evaluated an RO system to generate irrigation water from mixed secondary
in two stages [86]. In the first stage, the electro-sorption capacity of the treated urban effluent. The performance of this membrane was tested
lab-scale CDI rig was assessed. In the second stage, the salinity removal under a hydraulic pressure of 10 bar. It was observed that both the
efficiency and energy consumption were investigated for the prototype membranes exhibited good rejection, and adjusting the pressure
CDI system in the Wilora area, Australia. The possibility of im- showed an insignificant impact on the rejection efficiency. At 10 bar,
plementing this system in this field with a temperature of 45 °C and the BWRO membrane achieved rejection of 94.6%, 95.2%, 85.8%,
humidity of 80% was explored along with, the separation efficiency of 76.4%, and 91.3%, respectively against conductivity, salinity, chemical
the system. The theoretical data indicated that there was an increase in oxygen demand (COD), total organic carbon (TOC) and color whereas
the electro-sorption capacity and adsorption rate constant upon in- these values were 98.3%, 98.3%, 84.6%, 69.7%, and 86.6%, for the
creasing the feed concentration. The electro-sorption rate was 48.29% BWRO membrane. The water permeation was varied for both the
for a salt solution having a concentration of 1500 mg/L. The selectivity membranes as the AK-BWRO membrane permeate approached 38.0
of the system was excellent, and the highest salinity removal was LMH. The AD-SWRO membrane permeate was as low as 3.81 LMH, and
achieved at the lowest flow rate (1.0 L/min). The removal efficiency of it was maximized to 14.8 LMH at 20 bar. The AK-BWRO membrane
metal ions and non-metal ions was roughly 89%, 85%, 73%, 84%, 74%, showed the best water quality with higher water recovery. When
and 80% for Ca+2, Mg+2, Na+, Nitrate, and Arsenic, respectively. adding 20–30% of secondary treated urban effluent to 70–80% of the
Raising the flow rate to 7.0 L/min yielded a minimum energy ex- final product water, acceptable SAR values of around 6.41–7.67 and
penditure of about 1.89 kWh/m3 for the desalinated water. A total 7.36–8.31 with ECw values of 1.62–2.25 dS/m and 1.52 to 2.10 dS/m
water recovery around 75 to 80% was achievable. These findings make for AD-SWRO and AKBWRO membranes were achieved. Therefore, this
the CDI system a potential alternative for desalinating brackish water. mixture solution was suitable for fertigation meeting the standard of
To further improve the removal efficiency, Liu et al. [87] developed irrigation water.
membrane capacitive deionization (m-MCDI). Here, the electrodes were Ranganathan et al. [89] assessed the behavior of RO for purifying
manufactured from carbon nanotubes incorporating a cation exchange tannery wastewater and stated the cost analysis of this process. It was
polymer (Polyethyleneimine (PEI)) and an anion exchange polymer confirmed that the RO membrane was efficient in separating organic
(dimethyl diallyl ammonium chloride (DMDAAC)). It was found that components and the total dissolved salts in the desalinated water. The
the new electrodes achieved high removal efficiency for NaCl of 93%, membrane demonstrated a rejection rate of 93–98%, 92–99%, and
greater than that for other CDI systems. A CDI unit using carbon na- 91–96% for TDS, sodium, and chloride, respectively. It was suggested
notube electrodes and MCDI unit with commercial anion and cation that the wastewater was recovered by 70–85%, and the TDS in the
exchange membranes had a lower removal efficiency of 25% and 74% desalinated water approached 118–438 mg/L, meeting the quality
under the same electrical current of 1.2 V and solution conductivity of standard of potable water. The overall operating and maintenance costs
50 μS/cm. The modified MCDI also achieved superior electro-sorption of the RO unit were low.
of 0.159 mmol/g and charge efficiency of 0.70 at < 2.0 V. At the same UF membrane was also examined for treating wastewater under two
time, the commercial MCDI cell demonstrated an electro-sorption be- different experimental conditions of “stressed operating conditions”
havior around 0.114 mmol/g and 0.53. This enhancement can be at- against “conventional operating conditions” [43]. The stressed oper-
tributed to incorporation ion-exchange polymers, which adhered ating conditions phase consisted of three typical process cycles while

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W. Suwaileh, et al. Desalination 491 (2020) 114559

the conventional operating conditions consisted of one typical process above the discharge limit values while the NF membrane rejected the
cycle. Experimental results showed that the desalinated water from color completely. In terms of fouling impacts, the water flux reduction
both the conditions contained a minimum amount of Total Suspended was dropped by 68% for the FM UP005 membrane, while NF 270 and
Solids (TSS) < 10 mg/L; Chemical Oxygen Demand (COD) < 100 mg/ Desal 5DL membrane achieved the lowest water flux reduction around
L and Escherichia coli < 10 CFU/100 mL. The quality of this desali- 5% only. Similarly, the Desal 5DL membrane had better antifouling
nated water satisfied the Italian guidelines for irrigation water pro- property at operating parameters of pH 7, 12 bar, and 25 °C as com-
duced from wastewater. However, the desalinated water obtained using pared to other membranes. The product water purified by two NF 90
the conventional operating condition satisfied the quality standard of passes met the standard regulations for irrigation water.
irrigation water issued by the State of California. This desalinated water
was free of TSS and turbidity while the total coliforms were < 2.2 CFU/ 6.2. FO process
100 mL. This can be ascribed to a localized membrane pore micro-en-
largement mechanism that controlled the permeability and transmem- In the FO process, the feed water will be converted to nutrient water
brane pressure during the experiment. Consequently, a thin cake layer for agricultural purposes when using a fertilizer draw solution and,
created on the membrane surface contributed insignificantly to the therefore, there is no need for a recovery system to separate the draw
fouling and pore blocking. The treated water from both the conditions solution [91]. Research was conducted using three commercial all-
did not include any E. coli microorganisms. It was suggested that the purpose solid fertilizers with concentrations ranged from 1.0–3.0 mol/L
conventional operating condition is the best option for operating the UF to draw pure water from wastewater [92]. The performance of a
membrane in the process of achieving a good quality of irrigation commercial cellulose triacetate membrane with an active area of
water. 0.0025 m2 was evaluated in terms of water permeation and water re-
Balcıoglu et al. [90] made a study of using different membranes (FM covery. The nutrient concentrations in both the draw and feed solutions
UP020, FM UP005, NF 270, NF 90, and Desal 5DL) to treat baker's yeast and nutrient loss were analyzed and the energy required to operate the
wastewater for agricultural irrigation. The effect of the operating con- FDFO system was optimized. The results revealed that the fertilizer DS-
ditions on fouling, water permeation reduction, and quality of the F1 (N = 24/P = 8/K = 16) was the best performing draw solution
permeate was explored. Membrane separation performance and fouling when using wastewater as the feed solution due to the low concentra-
analysis suggested that the Desal 5DL and NF 270 membranes were tion of urea. Also, water extracted was around 324 mL, which
feasible for treating baker's yeast wastewater due to excellent rejection amounted to 41% of the total water required to dilute irrigation water
rate, reduced flux declines, and low water contact angles. This is be- within 72 h of running time. Likewise, the highest water permeation
cause the Desal 5DL membrane achieved a removal efficiency of 90%, approached 4.2 LMH while the reverse solute flux was estimated at
87%, higher than 88% for the COD, chloride, total dissolved solids, 92%, 98%, 75%, and 81% for NH4-N, TN, K, and P nutrients. The final
respectively. NF 90 membrane demonstrated rejection efficiency diluted draw solution (F-1) included N from NH4 at 12.0 mmol, N from
against total dissolved solids around 88%. The NF membranes showed urea of around 30.6 mmol, P nutrient around 5.9 mmol, K nutrient
total hardness and sulfate removal efficiency in the range of 70–98% around 16.5 mmol. Phosphorus was rejected by the FO membrane
and 97–99%. The removal efficiency of chloride corresponded to 13%, leading to a high amount in the feed solution, but the amount of total
25%, and 87% chloride removal for NF 270, Desal 5DL, and NF 90 nitrogen and potassium increased in the FS due to reverse solute flux.
membranes, respectively. However, the chloride was not rejected by the Finally, reducing the flow rate from 100 to 10 mL min-1 resulted in
FM UP020 membrane. The NF membrane rejected the suspended solids energy consumption reduction from 1.86 to 0.02 kWh m−3. Although
completely, while the UF membrane showed rejection of only 75–81%. the reverse solute flux was challenging, the commercial solid fertilizers
The FM UP020 membrane exhibited poor color rejection, which was as a draw solution showed potential for obtaining irrigation water from

Fig. 5. A schematic diagram of the fertilizer driven FO unit (FDFO). It consists of a membrane cell with dimensions of 2.6 cm width x 7.7 cm length × 0.3 cm depth.
The membrane active area is of about 20.02 cm2. The draw solution container is placed on a digital scale to calculate the permeate volume. Both conductivity and pH
meters were connected to the feed container to measure the pH and conductivity of the feed solution. Reproduced with permission from Volpin et al. [139].

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W. Suwaileh, et al. Desalination 491 (2020) 114559

wastewater. 6.3. Temperature-driven membrane system

Li et al. [44] used the PA TFC FO membrane to treat landfill lea-
chate containing a high amount of undesired species such as dissolved The basic concept of MD lies in the thermally driven transference
organic matter, inorganic components, heavy metals, and other com- process across a hydrophobic membrane [56]. A wide range of com-
pounds. Different concentrations of the NH4HCO3 draw solution and mercial membranes was used as the MD membrane such as poly-
the flow rate were investigated. The water recovery rate corresponded propylene (PP), polytetrafluoroethylene (PTFE), polyvinylidene
to 91.6% within 72 h, which is higher than that for the Changsheng fluoride (PVDF), and polyethylene (PE) [95]. The selection of mem-
Bridge Landfill plant located in Nanan District, Chongqing, China. The brane configuration (flat sheet or capillary) and type depends on the
water permeation increased to 6.7 LMH when increasing the DS con- MD application, if highly pure water or concentration of the ionic so-
centration to 3.0 mol/L. However, it declined after 5 h. When the flow lution is required. The most important characteristics of MD mem-
rate was raised to 8.4 cm/s, the water permeation was increased to 7.5 branes are high hydrophobicity, high porosity; pore size ranged from
LMH due to improved fluid shear stress at the membrane leading to a several nanometers to few micrometers, small pore size distribution,
thin boundary layer. As a result, the accumulation of solute and con- high liquid entry pressure, thick single-layer to produce low thermal
centration polarization was mitigated across the membrane. After 48 conductivity, antifouling material and excellent chemical and thermal
running hours, the product water was free of the metals Hg, As, Cr, Cd, stabilities [96,97]. In comparison with pressure-driven membranes, the
Pb, had no odor and negligible precipitates, pH within the re- MD process needs low hydraulic pressure, produces a high water re-
commended value, minimum TOC (42.2 mg/ L) and chloride (38.5 mg/ covery rate, integrates with natural energy to generate heat, thereby
L). The product water met the standard regulation of commercial liquid reducing energy consumption [95]. The MD process is effective in ex-
fertilizer, and therefore it can be reused for fertigation. tracting nutrients from wastewater effluent. For example, Zarebska
Iskander et al. [93] estimated the energy consumption of the FO et al. [98] made a study on the removal of ammonia from swine manure
system for purifying landfill leachate. Several operating parameters, and examined the membrane anti-fouling resistance. In this study,
such as the draw solution concentration and flow rate, were optimized. tubular polypropylene membrane and a liquid fraction of undigested
The treatment performance and energy consumption were compared manure as feed solution were used in the FO system. The feed solution
when varying the landfill leachate properties. Cellulose triacetate contains valuable nutrients such as potassium, ammoniacal nitrogen,
commercial membrane was tested in the FO process, and the effect of sodium, phosphorus, sulfur, calcium, magnesium and iron for plant
membrane fouling on energy expenditure was also explored. Experi- growth. During the test, there was a fast drop in the ammonia flux from
mental data showed that the water recovery rate increased from 42 kg∙m-2∙h−1 to 3 kg∙m−2∙h−1. This is because of reduction in the
63.8 ± 7.7 mL to 277.3 ± 3.8 mL when using concentrated draw so- ammonia partial pressure after 25 h of operation time. It was found that
lution (3.0 mol/L). The reverse solute flux was slightly increased from the membrane was affected by accumulation of proteins, some in-
4.60 ± 0.59 to 5.37 ± 1.15 gMH. It was observed that raising the organic components such as O, S, Fe, Na, Mg, K and microorganisms
flow rate to 110 mL/min at a draw solution concentration of 1.0 mol/L leading to serious fouling. This resulted in altering the surface char-
resulted in higher energy requirements estimated at acteristics and the membrane surface was converted from hydrophobic
0.276 ± 0.033 kW h m−3. This energy expenditure was minimized to to hydrophilic in nature, thereby hampering the separation of am-
0.005 ± 0.000 kW h m−3 upon decreasing the flow rate to 30 mL/min, monia. The fouling layer was thick (10–15 mm) after a week of running
increasing the water recovery rate, and higher draw solution con- the experiment.
centration of 3.0 mol/L. The fouling was easily removed from the To improve the membrane performance for rejecting ammonia, re-
membrane through osmotic backwashing. Simultaneously, the leachate search was carried with a modified direct contact membrane distillation
with a low amount of pollutants required low energy consumption due (MDCMD) process to remove ammonia from municipal wastewater
to high water recovery. The FO process can be used to lower the volume [99,100]. The PVDF membrane was operated in the system, and its
of leachate while extracting highly pure water for direct reuse. characteristics were 80% porosity, mean pore size of 0.22 μm, liquid
Very recently, Volpin et al. [94] proposed a new concept of the FO entry pressure of 250 kPa, and water contact angle of 87o. This mem-
process as MgSO4 and Mg(NO3)2 fertilizer draw solution was employed brane was tested in three different process modes: a conventional direct
to dewater synthetic human urine. The diagram of the fertilizer driven contact MD, a hollow fiber membrane contactor, and a modified DCMD
FO process is shown in Fig. 5. It was assumed that the reverse solute apparatus. The influence of operating conditions such as feed pH,
flux of Mg+2 would trigger P-recovery through struvite precipitation. temperature, flow rate, and concentration on the ammonia stripping
Next, the leakage and precipitation of urea in the Mg-fertilizer draw was explored. It was observed that the removal efficiency of ammonia
solution will increase the amount of N nutrients. Also, a lower volume for direct contact membrane distillation (DCMD), hollow fiber mem-
of urine at the end of the experiment leading to enhanced productivity brane contactors (HFMCs), and MDCMD modes amounted to 52%, 88%,
in downstream processes for N-recovery. The Mg(NO3)2 draw solution and 99.5% after 105 min of running time, respectively. This meant that
produced higher water flux by 3-fold (31 LMH) as compared to that for MDCMD was the best process for removing ammonia from water/
the MgSO4 draw solution at a concentration of 1.0 mol/L because of wastewater. A negligible effect on the removal efficiency of ammonia
high mass transfer through the FO membrane. Similarly, the reverse and the distillate flux was noticed at an optimal feed pH value of 12.2.
solute flux was higher at 0.89 g/L for Mg(NO3)2 draw solution and However, adjusting the cross-flow velocity from 0.15 to 0.5 m/s caused
0.1 g/L for MgSO4 draw solution. It was reported that the urea was not an increase in the water flux from 5.75 to 11.75 LMH and high diffusion
rejected completely by the PA TFC FO membrane, but it was recovered of ammonia through the membrane leading to improved vapor mass
in the Mg-based draw solution. The volume of urine was decreased to transfer. When raising the feed temperature and feed flow rate, se-
60%, which can promote efficiency in downstream processes. Accord- paration efficiency towards ammonia and the water permeation were
ingly, a high amount of nutrients amounted to 40% of the P as struvite enhanced. However, the separation efficiency was independent of the
fertilizer, and 50% of the N in the urine were recovered when urine was initial feed ammonia concentration in the MDCMD process.
concentrated by 60%. The agricultural companies can be supplied with Macedonio et al. developed a PVDF MD membrane and compared
solid struvite as the diluted fertilizer can be utilized as irrigation water its performance with two polypropylene MD commercial membranes
for green walls, parks, and farms. The new development of the FO for treating saline oily wastewater [101]. The impact of feed tem-
process opened an opportunity to effectively extract nutrients from perature and hydrodynamic conditions on the rejection efficiency was
human urine and reused for sustainable agriculture. studied. Furthermore, an economic analysis was performed to de-
termine the viability of the direct contact MD process for saline oily
wastewater treatment. The experimental results demonstrated that the

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W. Suwaileh, et al. Desalination 491 (2020) 114559

new PVDF2 membrane had the highest water flux of 6.0 LMH and re- efficiency against sewage organic matter as the removal efficiency of
jection rate of TDS and total carbon of 99.8% and 90.6%, respectively. COD and organic carbon removal approached up to 98% and 95%.
Overall, the rejection rate was > 99.0% and 90.0% against TDS and TC Methane as biogas was released around 4.5 L/d, which is beneficial for
for all the fabricated membrane modules. The distillate included TDS, energy recovery and membrane cleaning. The product water contained
Total carbon (TC), total inorganic carbon (TIC), total organic carbon, 95.5% of the cumulative recovery for nitrogen and 93.4% of the cu-
and conductivity of about 415 mg/L, 91.36 mg/L, 48.46 mg/L, mulative recovery for phosphorous after 100 d of running time.
TOC = 42.9 mg/L, and 614 μs/cm respectively. The cost analysis Therefore, the product water included an acceptable amount of nu-
showed that the water production cost of the PVDF-2 membrane trients from sewage organic matter, and it can be used for fertigation
reached 0.72 $/m3 at a recovery rate of 70%, the temperature of the depending on the specific nutritional requirements of the crop.
produced water passed to the unit was 50 °C, and the lifespan was Bolzonella et al. [107] highlighted the results of 10 years of in-
10 years. The cost was higher of about 1.28 $/m3 when the temperature vestigations on the performance and feasibility of the MBR process for
of the produced water passed to the plant reached 20 °C and lifespan of removing various contaminants from industrial wastewater. The MBR
5 years. Thus, these findings proved that the developed membrane is a system was effective in rejecting solids, nutrients, and micropollutants
cost-effective alternative method for industrial wastewater treatment. as the removal efficiency of nitrogen, phosphorus, and heavy metals
was 80%, > 60%, and 10–15%, respectively, while COD was reduced
6.4. Membrane bioreactor (MBRs) process from 100 mg/L to < 40 mg/L. The removal efficiency of the toxic
compound,2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), was superior
Membrane bioreactor (MBR) has been commonly used for waste- as the concentration decreased to < 0.5 pg/L in 60% of the samples
water treatment as an alternative process to traditional treatment sys- while the concentration of other dioxins was < 10 pg/L. The removal
tems due to a simple design and high-quality product effluent [102]. It efficiency of organic pollutants was enhanced when using a high con-
consists of a classical biological sludge process coupled with a micro- or centration in the influent while there were no E. coli bacteria in the
ultrafiltration membrane module. The biological process is used to effluent. The total coliforms in the effluent ranged from 0 to 240 MPN/
decompose the waste species or microorganisms while the membrane 100 mL in the MBR-1 and higher around 13 and 460 MPN/100 mL in
separates the water from the mixed liquor [103,104]. The membrane the MBR-2. Therefore, the treated water had high quality, and was
has pore diameter ranged from 0.01 to 0.1 μm to reject contaminants appropriate to be reused directly or after treatment with another
and bacteria, so it was an alternative method to gravity sedimentation method. The operating and maintenance costs were reduced sig-
system in the biological sludge process. The practicality of the MBR nificantly to between 0.11 and 0.15 USD/m3, which indicated the
process has been shown through lab and pilot plants for wastewater workability of the system in treating wastewater and reuse of the
applications. treated water in the Mediterranean Region.
Matosic et al. [105] studied the performance of a pilot MBR plant
with a hollow fiber membrane for treating wastewater from a soft 7. Application of hybrid systems for agriculture
drinks production facility compared with the performance of a tradi-
tional treatment process (biological activated sludge system). The bio- The development and performance of existing integrated systems in
logical activated sludge process failed to completely remove COD, different agricultural applications are discussed.
leading to a high concentration in the effluent. The MBR performed
better in rejecting organic contents, and the amount of COD and TOC 7.1. Seawater/brackish water desalination
was decreased by 94% in the effluent. This can be attributed to the
higher concentration of activated sludge biomass in the bioreactor 7.1.1. RO integrated system
governed by the rejection of these species by the hollow fiber mem- To reach the required quality of irrigation water, some membrane
brane. The membrane was effective in removing total suspended solids processes need an additional pass to recover nutrients. For instance,
and other contaminants, which improved the quality of the effluent. seawater desalination plants use several passes to polish the desalinated
The initial water permeation was 5.43 LMH but declined due to fouling. water and remove boron and chloride [32]. When using low hydraulic
The fouling was caused by a high amount of total hardness and high pH pressure and neutral pH, the removal efficiency of boron was 83%. The
value in the influent, leading to precipitation of scale precursors. The removal efficiency of boron can be maximized to 99% when using high
most severe fouling was after the first 10 days of the operating period, hydraulic pressure and pH 10.5 [108]. Generally, the second RO pass
and then it decreased slowly. The initial water permeates value was requires high energy consumption of about 0.5 kWh/m3 [32]. A SWRO
restored after chemical cleaning via hypochlorite, acid, and alkaline industrial plant used multiple RO passes to effectively reject boron and
solutions. For instance, the water recovery rate reached 72% when chloride from the treated water [32]. However, energy consumption is
immersing the membrane in a hypochlorite solution. The superiority of a crucial hurdle for the SWRO facility for generating irrigation water.
the MBR treatment proved its feasibility in treating wastewater rather The pre-treatment or post-treatment stage of the RO membrane can
over the traditional treatment process. consume energy of 0.7–0.9 kWh/m3. This accelerated the total energy
Prieto et al. [106] invented a gas-lift anaerobic membrane bior- consumption for the facility reaching 3–7 kWh/m3 of produced water.
eactor (Gl-AnMBR) for household wastewater treatment. The perfor- Altaee et al. [109] explored the efficiency of RO membranes as a
mance of the Gl-AnMBR was evaluated, and a comparison between post-treatment stage coupled to a NF system to recover nutrients from
membrane fouling mitigation strategies was addressed. PVDF UF seawater with a salinity of 35,000 mg/L to improve the quality of ir-
membrane combined suspended-growth bioreactor and synthetic rigation water. The NF/RO process did not efficiently separate the
household wastewater was used for the treatment operation over NO−3 nutrient, and hence it was mixed with KNO− 3 to increase the
100 days. It was found that the highest water flux corresponded to 18 rejection. The solution contained KNO− 3 as the main component, and a
LMH at a constant cross-flow velocity of 0.3 m/s and constant trans- high rejection by the RO membrane was noticeable. When two BWRO
membrane pressure, and the water flux was independent of the higher membrane passes were used to recover nutrients from the Ca (NO3)2
cross-flow velocity. This means that the membrane permeate was draw solution, the membrane achieved high rejection, and another RO
controlled by mass transfer resistance across the membrane during the cycle was also applied to provide an acceptable level of NO− 3 and K
+

process. After 100 days of the operation period, the water permeation nutrients in the product water. It was important to use two RO stages
declined to 10–15 LMH. Fouling was alleviated by backwash cleaning for recovering the nutrients, and the permeate was further recovered to
every week. A further improvement in the water flux was achieved by obtain the required concentration of NO− 3 and K
+
nutrients in the final
to backwash cleaning every 4 h. There was an excellent removal product water. In the first RO pass, a high rejection rate of around

12
W. Suwaileh, et al. Desalination 491 (2020) 114559

99.5% was achieved for monovalent ions while it was decreased to 90% water desalination. The heating system was employed to recover a
for MgCl2 and KNO3. However, the BW30-440i RO membrane exhibited volatile fertilizer draw solution, including ammonia and carbon di-
the greatest rejection rate of about 99% against monovalent ions, but oxide. The theoretical results indicated that the hybrid system con-
the power expenditure was relatively high. The final product water sumed power of 0.25 kWh/m3 at a water recovery rate of 64%. Also, the
contained the recommended level of nutrients for agricultural irrigation energy consumption was very high for the heating recovery system
when two RO stages were used as the recovery processes. It was re- approaching 75 kWh/m3, which hindered the feasibility of this re-
ported that when increasing the recovery rate, the energy expenditure covery strategy. Other drawbacks are the high reverse solute flux and
was minimized. The specific energy consumption for the RO recovery the accumulation of ammonia in the product water.
stage was 3.0 kWh/m3, which was lower than for a conventional RO A closed-loop FO-NF hybrid system could be an effective process for
desalination plant. seawater desalination, with the NF stage used to recover the nutrients
To recover nutrients in the desalinated water with minimum energy, from a fertilizer draw solution [112]. Experimental results showed that
Atab et al. [110] applied an adsorption cycle (AD) after the RO mem- the water permeation of the hybridized FO-NF process reached 10 LMH,
brane process. The layout of this hybrid system is presented in Fig. 6. while the solute rejection by the FO membrane was as high as 99.4% for
Although the temperature influenced the performance of the RO all the tested draw solutions. The solute rejection by the NF membrane
membrane, increasing the temperature to 85 °C led to the high water was lower, around 97.9%.
capacity of the AD cycle of about 6.3 m3/day. The salinity can affect Furthermore, dual NF passes were applied to purify the diluted draw
membrane performance, but there was a negligible impact on the AD solution obtaining high-quality potable water. According to Chekli et al.
cycle. It was found that this desalination plant generated 24,000 m3/ [48], the second stage is necessary to remove Na+ and Ca+2 from the
day of irrigation water with salinity < 1600 mg/L. The total water re- diluted draw solution completely. Another negative impact is that the
covery achieved was around 65% for the hybrid system. The estimated passage of salt solution through the membrane may deteriorate the
energy consumption of the hybrid system at a water recovery of 45% lifespan of the membrane. After that, the final product water contained
was about 0.8 kWh/m3. This resulted in a reduced cost of around 0.54 a minimum amount of TDS of about 113.6 mg/L, which is lower than
$/m3 as compared to the stand-alone RO system. The significance of the recommended level for drinking water (500 mg/L).
desalination by combining RO and a post-treatment method is not only Phuntsho et al. [111] reported the effect of operating conditions on
to minimize the energy expenditure but also to enhance the quality of each other in a closed-circle large-scale FDFO-NF hybrid process ac-
irrigation water. cording to the mass balance of the flow rates, the draw and the feed
solutions. The theoretical information suggested that when the capacity
7.1.2. FO integrated system and feed concentration in the FO/NF hybrid system are constant, the
FO coupled with another membrane-based process could be bene- initial flow rate of the draw solution was inversely proportional to the
ficial and comparable to the RO process in terms of operating cost and initial concentration of the draw solution or other way around. The
energy saving. The FO integrated process has potential in treating mass flow rate of the draw solution is correlated to the concentration of
complex impaired water sources from the oil and gas industry, brine the feed solution and the constant capacity of the closed-cycle FO/NF
desalination, and drilling flow back water [111]. Many earlier patents plant. The data shows that when one of the conditions or both got
reported the use of the FO combined heating process to extract a vo- higher, the mass flow rate can be grown, causing an increase in the
latile fertilizer solution from the product water [48]. In this respect, a concentration of the diluted draw solution and the energy requirements
dual-stage of the FO/heating process was used for seawater/brackish of the NF recovery system. Besides, the initial concentration and the

Fig. 6. A schematic diagram of the RO combined adsorption system. The RO rig involves of several vessels containing membrane modules, pressure exchanger that
generate energy from rejected solution to circulation pump. The adsorbent was made of Silica gel type-RD. Adapted with permission from Atab et al. [110].

13
W. Suwaileh, et al. Desalination 491 (2020) 114559

flow rate of the draw solution were crucial conditions. They can in- concentrations ranging from 0.1 to 0.8 mol/L. The water permeate had
fluence the water recovery rate of the NF system, thereby imposing a a conductivity value of < 500 μS/cm, which satisfied the recommended
higher energy input. One of the practical hurdles is the nutrients loss standard of the drinking water. At an optimum temperature of 60 °C,
and accumulation in the concentrated feed solution resulting in highly the MD membrane produced distillate permeate around 5.7 LMH and
concentrated brine exceeding the standard limit for brine disposal. This excellent salt rejection around 99.55%. This can be attributed to low
issue can occur when running the FDFO system at a high recovery rate, membrane fouling as the FO removed most of the salts from the feed
and therefore a highly selective membrane is needed to minimize the solution before fed the MD process. Furthermore, the energy con-
reverse solute flux. Since the electricity requirement and operating cost sumption was minimized significantly from 7.06 KWh/m3 to 1.1 KWh/
are not validated for the closed-loop hybrid system, a quantitative m3, which confirmed the potentiality of the hybrid system in the se-
economic analysis and energy consumption estimation are essential for paration of salts and recovering the fertilizer draw solution. The final
the large-scale plant. product water can be directly used for fertigation.
The MD integrated PRO process is beneficial to recover the draw
7.1.3. MD integrated system solution with low energy expenditure. Through using the osmotic
Widespread studies on MD hybrid processes in various applications power gradient process (PRO), the energy consumption can be reduced,
have been reported for its high contribution to treat complex impaired and the concentration of the draw solution can be maximized to en-
water, high rejection towards different organic and inorganic compo- hance power exploitation [115]. Lin et al. investigated the performance
nents, improve water recovery, recover valuable nutrients, alleviate of an advanced closed-loop system involving PRO coupled MD to re-
fouling/scaling, reduce brine disposal, low energy if natural power generate the low and highly concentrated feed solutions and produce
source used, and cost-effective [113]. It has been utilized in many fields drinking water. The PRO was used to extract useful power via a hydro-
such as seawater desalination, wastewater treatment, agriculture, oily turbine. It was found that the energy efficiency of the system ap-
wastewater, landfill leachate, and pharmaceutical industry [113,114]. proached 9.8%, equivalent to 81.6% of the Carnot efficiency when
Because the commercial membrane provided high quality and stable using 60 °C for the hot stream, 20 °C for the cold stream, and 1.0 mol/L
permeate, thereby improving the efficiency of the processes and pre- NaCl feed solution. However, increasing the concentration of the feed
venting the drawbacks that hampered its realization in large-scale op- solution led to greater theoretical energy efficiency. It should be said
erations [114]. An example is the hybrid MD- crystallizer, which is that the experimental energy could be lower than the theoretical energy
capable of recovering various mineral nutrients from seawater and efficiency due to the impact of various operating conditions. Besides,
wastewater and can improve the production of drinking water by up to operating different concentrations of feed solutions in the range of 1.0,
95% [113]. The function of a crystallizer is to reach the supersaturation 2.0, and 4.0 mol/kg NaCl needed very high hydrostatic applied pressure
level of the saline solution to capture solid salts in a tank through water around 46, 100, and 220 bar in the PRO system. As a result, the PRO
recovery. The most common salt nutrients extracted from seawater and membrane can be deformed at high hydraulic pressure yielding lower
wastewater brine are sodium (NaCl, Na2SO4, and Na2CO3), calcium water flux, poor salt rejection, low energy generation, and high mem-
(CaCO3, CaSO4), and magnesium (MgSO4, MgOH). Ji et al. utilized the brane replacement cost. Thus, the development of membrane with high
hybrid MD- crystallizer to obtain NaCl from RO brines. The perfor- mechanical strength is essential to take advantage of the great power
mance of the hybrid system was investigated in terms of crystallization output at high feed solution concentration.
kinetics, productivity, controlled size and shape distribution of the solid
nutrient salts. A comparison between actual and synthetic RO brine was 7.1.4. ED integrated system
carried out to understand the impact of organic matter dissolved in raw RED is a voltage-driven process that produces electricity from a
seawater on the water permeate, suspension density, and nucleation salinity gradient, and it can be combined with the NF or MD membrane
and growth rates of NaCl. The results revealed that when using syn- process [113]. The ED process has been found to be a potential process
thetic RO brine, the system captured 21 kg/m3 of NaCl crystals having for obtaining concentrated brine, although the monovalent ion-selec-
common cubic shape with size ranged from 20 to 200 μm. The system tive membrane is expensive. Liu et al. [116] employed a novel NF-ED
achieved much higher water recovery factor of 90%. However, when hybrid system in which the NF membrane was to separate divalent ions
using actual brine, the growth rate of NaCl crystals was reduced by like SO−2
4 while the ED was to re-concentrate the water permeates from
15–23% as compared to that for the synthetic brine. The dissolved or- the NF. The applied pressure and feed stream concentration caused a
ganic matter in the real brine influenced the water flux and the quantity reduction in the water flux and salt rejection. A slow increase in the
of salt crystals and the reduction was estimated at 20% and 8.0%, re- water flux at higher applied pressure was noticeable due to membrane
spectively. Consequently, a pre-treatment step before the RO is neces- compaction. When using artificial seawater with a salinity of
sary to remove dissolved organic matter and avoiding their effects on 88,000 mg/L at a hydraulic pressure of 32 bar, water permeation of
the MD membrane. Since the supersaturation of the solution was ef- 57.5 LMH was achieved. It was indicated that the NF membrane almost
fectively controlled along with the polarization issue, nucleation pro- completely rejected SO−2 4 from the brine solution. However, the re-
cess and hydrodynamics, the distillate permeate during 100 h in the MD jection rate was lower of about 40% and 87% for Ca+2 and Mg+2 salt
process was stable. nutrients. The NF membrane showed poor rejection of < 5% against
Recovering nutrients can also be done by MD. The hybrid MD monovalent ions such as Cl−, K+, and Na+. High concentration of Ca+2
process can continuously re-concentrate the diluted solution and, at the were detected in the NF water permeate of 392 mg/L, which minimized
same time generating drinking water in the outlet side. Suwaileh et al. ED membrane fouling when used as a feed solution in the ED process.
[56] explored the efficiency of using the FO-MD hybrid process to treat When the NaCl was concentrated to 160 g/L at 15 V for over 5 h in the
brackish water and recover nutrients from fertilizer draw solution. It ED system, the greatest water recovery was around 70%. This brine
was assumed that the thermal heating for operating the MD system solution contained a total amount of mineral nutrients (K+, Ca+2, and
could be supplied from a renewable power source, such as solar Mg+2) around 5 g/L of the total TDS. The energy consumption was
heating, to reduce the overall energy requirement. The salinity of the approximately 0.6 kW h/m3 for the NF system, and it was higher,
feed solution and concentration polarization had no effect on the re- around 1.4 kW h/kg NaCl for the ED process. To that end, the hybrid
moval efficiency of the MD membrane. It was observed that when using NF-ED system could be a prospective strategy to re-concentrate high
a low salinity feed solution of 0.5 mol/L of KCl fertilizer draw solution, salinity NaCl from seawater desalination brine.
the water permeation reached 7.7 LMH. This flux value dropped to 4.9 The MD coupled RED can generate concentrated brine in the outlet,
LMH when using a high concentration feed solution of 1.4 mol/L. the freshwater product, and power output. Long et al. [117] studied the
average salt rejection was as high as > 99.4% when using feed performance of innovated MD-RED hybrid system using low-grade heat

14
W. Suwaileh, et al. Desalination 491 (2020) 114559

sources varying from 40 °C to 80 °C and the NaCl feed solution with with minimum dissolved solids, and the lowest SAR value was applied
different salinities of 1.0, 2.0, 3.0, 4.0, and 5.0 mol/kg. The con- directly to a crop field. This type of treated effluent had a negligible
centrated brine from the MD fed to the ED to convert the mixing energy effect on the groundwater salinization and enrichment with undesired
to electricity, thereby minimizing the energy consumption of the hybrid nitrates. Despite that, the permeate from stabilization ponds, including
system. The operating conditions influencing the performance of the high contents of organic matter and a medium level of salinity, led to a
process were optimized. The energy efficiency of this hybrid system was higher crop yield.
also determined to evaluate its viability for the large-scale plant. In the Shanmuganathan et al. [120] integrated the NF process with the RO
analysis, the distribution of the mass flow rate and heat through the MD process to treat biologically treated sewage effluent aiming at produ-
membrane was determined using the mass and heat transfer models. cing irrigation water. The results indicated that the NTR 729HF mem-
The energy efficiency of the MD system depends on the operating brane achieved the greatest rejection rate towards bivalent ions around
temperature and concentration of the feed solution. It was observed 99% for SO−2 4 , 62% for Ca
+2,
and Mg+2. However, a very low rejection
that the energy efficiency approached 1.15% at a temperature of 20 °C was observed for monovalent ions like Na+, Cl−, and NO−3 of about
and 60 °C for the cold and the hot compartments, respectively, and feed 19%, 11%, and 5%, respectively. The NF membrane separated most of
NaCl concentration of 5 mol/kg. In the RED system, the efficiency of the organic matter with a rejection rate around 76–95%, and the
currents to extract low-grade heat was around 1.2%, with the re- permeate contained only 0–0.8 mg/L of DOC. However, the con-
generative efficiency being 50%. This calculated energy efficiency centration of pharmaceuticals and personal care products, Na+
confirmed the feasibility of the system to generate low-grade heat that (202 mg/L), Cl− (110 mg/L), and SAR level, were still higher than the
can be converted to electricity. However, to further maximize the ex- allowable level for irrigation water. Therefore, further treatment using
tractable power, the electrode material should be improved. Since the the RO membrane was conducted, yielding maximum rejection rate
properties of both the MD and RED membranes play an important role reaching > 99%, 99%, 98%, and 88% for Na+, Cl−, SO−2 4 , Ca
+2
,
in determining the total energy efficiency of the process, advanced Mg+2, and NO− 3 , respectively. The RO membrane rejected valuable
conductive materials for the membranes can be used. This hybrid nutrients required for crops, and hence 10% of feed water was blended
technology has potential for harvesting natural power to heat water, with 90% of RO permeate. The final irrigation water included an ac-
which can be utilized for industrial and agricultural applications. ceptable SAR value of 6 and concentrations of Na+ (40 mg/L) and Cl−
Recently, the hybrid MD-RED system was examined by Tufa et al. (15.5 mg/L). The hybrid system has potential for the removal of phar-
[118] for seawater desalination to generate freshwater production and maceutical and personal care products from effluent wastewater to
power output. In this study, high energy efficiency was generated of produce high-quality irrigation water and which will not contaminate
49% when operating the MD system with the temperature of the hot soil and groundwater.
stream around 60 °C and synthetic seawater feed concentration around Later, NF and RO hybrid system was investigated to purify MBR
0.5 mol/L NaCl, while the specific energy consumption was slightly treated wastewater to reuse for agricultural applications [35]. The
reduced at 8%. The resultant brine from the MD with a salinity of about analysis of the water permeates from the NF and RO processes was
5.0 mol/L NaCl was transferred to the RED system to boost the ex- performed based on different international standards. It was found that
tractable power. It was reported that the power density approached the water permeate from NF is not suitable for irrigation water because
2.2 W/m2 membrane pairs. This indicated that increasing the MD brine the SAR level is 25.7, which may hinder the crop growth and affect the
concentration to 5.0 mol/L caused an increase in the attainable energy soil permeability. It is most likely that poor rejection of Na+ and Cl−
from the RED system compared to RO brine (1.0 mol/L NaCl) in com- and high rejection of Ca+2 and Mg+2 by the NF membrane caused great
bination with seawater (0.5 mol/L NaCl). Overall, this reliable and cost- SAR value. A second pass with RO was utilized to reduce the SAR value
effective hybrid system offers several advantages, such as low brine and create irrigation suitable water. The RO permeate showed the
discharge, harvesting low-grade heat to produce electricity, and is lowest concentrations of mineral nutrients, such as Na+ (7.83 mg/L)
useable in various desalination processes where high-power input is Cl− (4.96 mg/L), PO4 (< 0.05 mg/L), Ca+2 (1.56 mg/L), Mg+2
needed. (0.06 mg/L), K+ (0.93 mg/L), salinity of 0.37 g/L, and low SAR value of
12.5. The turbidity of the permeate was reduced from 0.81 to 0.23,
7.2. Wastewater treatment satisfying the acceptable level for irrigation water. As the sodium
concentration was higher than calcium and magnesium, the water in-
7.2.1. RO integrated system filtration problem was low. The RO blended MBR with a ratio of 2:1
Another water resource is secondary effluent wastewater, and achieved the best SAR value of (5.30) and low salinity (0.57 g/L). By
treatment is required to remove pathogens, dissolved solids, and other using this optimum ratio 2:1 of the product water, it can be reused
pollutants to allow the water to be reused in sustainable agriculture. RO directly for fertigation, improved waste management, and is cost-ef-
membrane-based process is frequently utilized for wastewater treat- fective.
ment globally, due to process enhancements, small footprint, un-
complicated maintenance, high water capacity, and workable process 7.2.2. FO integrated system
[6]. Among pressure-driven processes, UF coupled with RO is proven to FO treatment process using fertilizer draw solution is attractive
be an effective hybrid system for wastewater reclamation. In line with because the fertilizer draw solution can be used directly or blended
this, Oron et al. [119] used a pilot plant composed of the UF membrane with potable water to irrigate crops. Several studies have been carried
to separate suspended matter, organic matter, and microorganisms out utilizing the FDFO integrated process to treat wastewater due to
while the complementary RO membrane was used to reject total dis- excess of valuable nutrients for plant growth [48]. However, the diluted
solved solids (TDS). After 681 h of operation, the UF permeate showed draw solution should be mixed with potable water [121]. This is
very low turbidity of < 1.0, low organic matter (BOD = 6.6 mgO2/l, challenging because in many parts of the world freshwater resources
and COD =64 mgO2/l), and was free of fecal coliforms. Next, the UF are limited. Therefore, the FDFO process, combined with another
permeate entered the RO system for further purification resulting in treatment process, can minimize nutrient concentrations in the diluted
water permeate with low organic matter (BOD = 4.8 mgO2/L and draw solution reaching the quality of irrigation water. MBR has been
COD = 16 mgO2/L), lowered salts (TDS = 69.8 mg/L, Cl− = 65.6 mg/ used commonly for wastewater reclamation giving clean water having
L, Na+ = 42 mg/L, K+ = 10.4 mg/L, Ca+2 = 6.6, Mg+2 = 4.4, N- adequate nutrients concentration for fertigation [17]. For example, the
NH+ = 10.8 mg/L, and PO4 = 1.8 mg/L). Treatment by RO membrane combination of FDFO and an anaerobic membrane bioreactor (AnMBR)
produced water permeate that is suitable for agricultural applications was employed to treat wastewater and generate irrigation water for
meeting the quality guidelines for irrigation water. The RO permeate hydroponics [121]. Firstly, the optimum water recovery rate was

15
W. Suwaileh, et al. Desalination 491 (2020) 114559

determined by using Bio-methane potential (BMP) measurements. The the new hybrid system. Firstly, the struvite recovery from landfill lea-
performance of a wide range of fertilizer draw solutions in terms of chate, and the influence of the pretreatment method on recovery rate
water flux, water recovery, reverse salt flux, and final nutrient con- was sought. It was essential to understand how the pre-treatment stage
centrations were evaluated in the FDFO when using synthetic municipal impacted water recovery behavior in the FO system. Lastly, the optimal
wastewater as the feed solution. Biogas generation was increased when arrangement of chemical pretreatment, struvite precipitation, and FO
increasing the water recovery, and the recovery rate of 95% demon- water recovery was also assessed. When adding the calcium into the
strated the greatest cumulative biogas production. It was reported that landfill leachate, the magnesium was precipitated as pure struvite.
the water flux was strongly correlated to the water recovery, and Then, the FO process was used to minimize the volume of wastewater,
therefore the performance of both KCl and NH4Cl draw solutions was which eliminated the use of another post-treatment stage and reduced
similar. Among the tested fertilizer draw solutions, the KCl and NH4Cl the investment cost. After applying the pre-treatment step with a molar
fertilizer draw solutions generated the highest water permeation of 21.1 ratio of 1:1.4 for Ca+2: CO−2 3 , the Mg
+2
leakage was decreased by
LMH followed by KNO3 with 13.2 LMH. The KH2PO4 and ammonium 24.1 ± 2.0% while the rejection efficiency of Ca+2 amounted as
phosphate dibasic (DAP) exhibited lowest water flux of about 13.3 89.5 ± 1.7%. The high amount of Mg+2 can be recovered of about
LMH. Similarly, the highest water recovery achieved for NH4Cl and KCl 98.6 ± 0.1%, and traces of PO−3 4 –P detected in the solution of < 25
reaching 42.2% and 38.6%. The ammonium sulfate showed the highest mg/L under the condition of (Mg + Ca residual): P molar ratio of 1:1.5
water recovery rate around 76% followed by KH2PO4 with water re- and pH 9.5. The struvite product created from the process showed
covery around 75% after hydraulic cleaning. The MAP and SOA ferti- crystal structure and composition mimicking the commercial struvite
lizer draw solutions exhibited the lowest reverse solute flux around 1.0 (19.3% Mg and 29.8% P). When using 4.0 mol/L NaCl draw solution in
and 1.7 gMH, respectively. Although the MAP fertilizer liquid included the FO system, the water extracted was around 621.5 mL over 95 h of
minimum final nutrient concentration (N = 54.1 mg/L/P = 10.8 mg/L/ operational time, meaning 36.6% of recovery efficiency. The FO was
K = 0 mg/L), it still needs further dilution by fresh water to reach ir- capable of lowering the volume of wastewater by 37%. The optimal
rigation water quality. system configuration was chemical pre-treatment-FO- struvite recovery
Another proposed desalination technology for leachate treatment is for the best FO performance.
the combined chemical precipitation method and the FO process. Wu The FO process can also be integrated with the bioelectrical process
et al. [122] proposed using a pre-treatment strategy involving the ad- to control brine production and extract more pure water from waste-
dition of carbonate to improve the struvite precipitation and purity, water. During the FO operation, the wastewater feed gets concentrated,
followed by the FO desalination process as presented in Fig. 7. The and the brine caused more mass transfer resistance for the pure water,
researchers investigated three aspects to evaluate the performance of which is controlled by the osmotic difference through the FO membrane

Fig. 7. The lab-scale unit of chemical precipitation pre-treatment procedure integrated the FO process. The FO process was arranged in three different modes: 1- FO –
calcium pretreatment - struvite precipitation (C1), 2- calcium pretreatment - FO - struvite precipitation (C2) and 3- calcium pretreatment - struvite precipitation - FO
(C3). Adapted with permission from Wu et al. [122].

16
W. Suwaileh, et al. Desalination 491 (2020) 114559

Fig. 8. A diagram showing the microbial desalination cells (MDCs) and forwards osmosis (FO) hybrid system. CEM is the cation exchange membrane while AEM is
the anion exchange membrane. Adapted with permission from Yuan et al. [124].

[123]. A microbial desalination cell (MDC) can be coupled with the FO draw solution was purified in the desalination cell of the MDC. The
system to further desalinate the diluted draw solution from the FO influence of initial COD, salt concentration, and hydraulic retention
system and generate irrigation water. For example, Yuan et al. [124] time were investigated to study the practicality of the hybrid system. In
used the MDC-FO hybrid system to improve the efficiency of the FO to the hybrid system, a synthetic anode solution involving 750 mg/L COD,
treat wastewater over 16 h, as illustrated in Fig. 8. The working prin- 35 g/L NaCl solution at the MDC anode, and HRT of 12 h was utilized. It
ciple depends on the blending the anode effluents together and using was reported that the hybrid system produced a lower wastewater vo-
them as the feed solution for the FO process. Two different solutions lume estimated by 64% due to water permeation in the FO and eva-
were produced from the FO process. The concentrated feed solution is poration on the cathode as compared to the stand-alone MDC system
fed to the cathode of the MDC to remove the COD while the diluted (14%). The conductivity reduction in saline water (HRT) was improved

Fig. 9. The design of the FO–membrane distillation (MD) process composed of FO membrane channel, a direct contact MD membrane compartment, pumps,
temperature monitoring sensors. Adapted with permission from Xie et al. [140].

17
W. Suwaileh, et al. Desalination 491 (2020) 114559

by 2-fold as compared to individual MDC systems. The removal effi- to fouling was observed; however, after the first and second cleaning
ciency towards COD approached 93%, and the conductivity reduction stages, the water recovery was 82% and 68%, respectively. As a result, a
improved to 99.4% when using a low concentration of NaCl. The effi- high amount of water permeate was fed to the MD, which exhibited
ciency of the hybrid system was promising, which makes it an appro- stable water permeation. The hybrid system achieved an excellent re-
priate desalination process for brackish water or as a pre-treatment jection of inorganic salts (ammonium and orthophosphate), organic
method for hypersaline solution and wastewater. matter (TOC and total nitrogen, TN). Because the magnesium trans-
ferred from the draw solution to the concentrated digested sludge and
protons diffused in the forward direction, the struvite crystals were
7.2.3. MD integrated system
created. A decrease in the pH of the feed solution and the accumulation
Wastewater treatment by a MD membrane is an excellent opportu-
of magnesium facilitated the formation of struvite crystals. Thus, the
nity to eliminate the technical barriers of the RO process. It can be
hybrid system was effective in extracting phosphorus nutrients in the
coupled with another membrane process providing fresh water for in-
form of struvite precipitate.
dustrial uses, for fertigation, and for domestic uses. Several studies
A recent study was reported by Volpin et al. [114] using an FO-MD
highlighted that purified municipal wastewater could be reused for ir-
hybrid system to recover nutrients like nitrogen, phosphorous, and
rigation because it contains high quantities of nutrients for crop growth
potassium from human urine. The optimization and performance of the
[17]. A group of researchers assessed the performance of a bench-scale
hybrid system were explored. A novel protocol was developed to
FO-MD system to treat for direct sewer mining [125] as shown in Fig. 9.
minimize the nitrogen transfer to the MD outlet, thereby obtaining
They studied the efficiency of the process based on water permeation
water products for direct irrigation. The operating conditions in the FO,
and the rejection rate of trace organic contaminants (TrOC). Experi-
like urine pH and draw solution concentration, were optimized. The
mental data showed that the water flux was stable upon using natural
feed temperature, nitrogen concentration, and membrane properties
sewage as the feed solution in the hybrid process at water recovery up
were optimized for the MD process. It was noted that the FO water
to 80%. The removal rate of trace organic contaminants was high in the
permeates ranged from 31.5 to 28.7 LMH upon utilizing 2.5 mol/L NaCl
range of 91 to 98%. The high rejection of TrOC can be ascribed to the
as a draw solution while the nitrogen flux was very low at 1.4 g/L. The
solute−membrane interaction of the FO membrane and, in the case of
nitrogen flux as NH3/NH+ 4 /Urea dropped significantly by 33% when
the MD membrane, was due to the volatility of these species. When the
decreasing the hydraulic pressure at the draw solution side to 2.0 bar,
water recovery was increased, there was an increase in the TrOCs
but a decline in the water flux by 42% was noticeable. When the feed
concentration in the draw solution. The TrOCs accumulation in the
solution became acidic (pH = 6–7), the nitrogen rejection by both the
draw solution was probably due to the variation in the removal effi-
FO and MD membranes was improved. The importance of acidification
ciency between the FO and MD membranes. To avoid this issue, acti-
was to maintain a high rejection of nitrogen and to prevent the hy-
vated carbon adsorption or ultraviolet oxidation can be used to separate
drolysis of urine. The MD membrane achieved maximum distillate
these contaminants completely, achieving rejection of > 99.5%. It was
permeate of 16 LMH due to high porosity and hydrophobicity. The
noted that the energy expenditure was high due to operating the MD at
ammonia vapor pressure was raised due to the high concentration of
a temperature between 20 °C and 40 °C. In this respect, it can a pro-
ammonia and inlet temperature of 60 °C. The membrane pore size and
mising process for agricultural purposes in arid areas where renewable
thickness controlled the transport of ammonia through the membrane.
power is available.
It was concluded that this dual separation process was reliable for
Xie et al. [126] employed a similar approach to separate phosphorus
wastewater treatment in space application and nutrient regeneration
nutrient and freshwater from digested sludge centrate using a 1.5 mol/L
for urban applications.
MgCl2 as draw solution. The bidirectional flux of magnesium and pro-
tons induces struvite precipitation. The role of FO was to concentrate
orthophosphate and ammonium for phosphorus recovery when creating 7.2.4. ED integrated system
struvite (MgNH4PO4·6H2O). MD was utilized to regenerate the draw The membrane desalination technology operated based on ther-
solution and obtain fresh water from the digested sludge centrate. A modynamic reaction is an attractive method for converting extractable
reduction in the water permeation obtained from the FO membrane due power to electricity that created with water recovery [127]. It is

Fig. 10. A schematic diagram of the FO–Electrodialysis (ED) hybrid process with a semi-continuous configuration. Adapted with permission from Zou et al. [129].

18
W. Suwaileh, et al. Desalination 491 (2020) 114559

recognized that the accumulation of various nutrients on the feed 0.02% (w/w) of nitrogen around 240 mg NH3/L over 12.5 h when using
stream due to reverse solute flux and salinity build-up from the mem- a low initial concentration of fertilizer feed solution. After that, the ED
brane rejection is one of the key challenges in the FO process. To avoid was capable of concentrating these ammonium salts by a factor of
this technical hurdle, the ED system was coupled to an FO system for 1.6 ± 0.3, which created a liquid fertilizer contained 15.6% (w/w) and
further treatment of the concentrated feed solution and therefore con- 16.2 ± 1.2% nitrogen as NH4NO3 by Fujifilm membranes and a
trolling the salinity build-up on the feed stream [128]. The combination PCCell, respectively. Under constant applied current of 7 V., The esti-
of FO and ED processes delivered a remarkable advantage for waste- mated energy consumption was as low as 0.21 ± 0.08 kWh/kg am-
water treatment. Zou et al. [129] followed this strategy to desalinate monium salt and 93.1 ± 4.2% of faradaic yield. This indicated that this
wastewater using (NH4)2HPO4, fertilizer draw solution. A schematic of novel hybrid system is a promising technology for the valorization and
the hybrid system is demonstrated in Fig. 10. In the FO system, the recovery of ammonia nutrients from wastewater solutions.
influence of draw solution concentration on the water recovery and
reverse solute flux was investigated. In the ED system, the removal 8. Conclusions
efficiency, regeneration of the fertilizer draw solution, and energy
consumption using different applied voltages was also studied. Ex- Water desalination and wastewater treatment could have useful
perimental findings demonstrated that the FO process generated a impacts on fertigation and environment providing additional water
stable water recovery volume around 375.5 mL when utilizing con- resources and regenerating poor-quality water. Membrane technologies
centrated fertilizer draw solution (2.0 mol/L). A minimum specific re- can supply water for agriculture and increasing food production. The
verse solute diffusion of 0.063 g/L and 0.083 g/L for NH4-+N and stand-alone or hybrid RO and FO membrane-based processes are proven
PO−34 –P nutrients, respectively, was observed upon using 1.0 mol/L to be the most effective desalination technologies used in many coun-
draw solution. The negligible concentration of Na+, Cl−, and organic tries around the world. Due to the efficient separation performance, low
constituents was detected in the diluted draw solution, and therefore fouling tendency, reduced energy expenditure, widespread for saline
the diluted draw solution is reusable for fertigation. At the optimum water desalination and wastewater treatment, they can provide ferti-
applied current of 3.0 V, the ED showed excellent water recovery of lizer solution and irrigation water with an acceptable level of nutrients
96.6 ± 3.0% reverse-fluxed draw solution. The specific energy con- for fertigation. The stand-alone RO and electrodialysis are available,
sumption of the hybrid system was very low of about 0.72 kWh m−3 have effective performance, and can provide high quality nutrient
and 0.35 kWh m−3 (55.7% reduction) when applying 2.5 V and 3.0 V, water, but the water production cost is still higher than that for
respectively. The synergistic cooperation of both processes achieved common technologies used for agriculture. A potential approach to
excellent water recovery and consistent performance. desalinate hypersaline feed solution is the MD system. It can produce
Ippersiel et al. [130] integrated an ED system with an air stripping high quality water, but it should be blended with liquid nutrients to
method to concentrate ammonia nutrients followed by direct aeration reach the acceptable standard of irrigation water. Also, the MD system
or vacuum to separate the volatile ammonia from the concentrate so- requires high thermal energy which increases the energy consumption
lution by an acidic trap. The aim was to extract concentrated nitrogen and the operating cost. The MBR process generates water enriched
fertilizer from liquid swine manure through the addition of acids to nutrients from wastewater effluent and can be reused immediately for
eliminate scaling stripping towers. In the ED process, the optimum fertigation. On the other hand, membrane fouling is a serious problem
applied voltage was 17.5 V resulting in efficient energy expenditure. due to high concentration of complex wastewater and the energy con-
The best pH values of the feed solution were ranged from 8.5 to 8.2, sumption of the system is considered high. Thus, the operating and
facilitating electromigration of NH4. It was noted that the maximum maintenance costs are high due to frequent replacement of the mem-
achievable ammonia nitrogen recovered was 21,352 mg/L in the con- brane and high energy demand. Furthermore, hybrid FO process can
centrate solution corresponding 7-folds the concentration in the swine achieve efficient nutrient recovery and low energy consumption only if
manure. This value was greater by 33% than that extracted from the natural energy is available for recycling of the draw solution.
open-to-the-atmosphere system. In this work, 95% of the TAN was re- Integrating MD with another membrane desalination technology is
covered from the swine manure utilizing a closed-to-the-atmosphere promising for nutrient recovery when RO brine is used as the feed so-
system. The increase in concentration of the solution was hindered lution. This is because the concentrated brine is valuable source of
during the process due to the transport of the pure water from the di- mineral nutrients and therefore no need to discharge high volume of
luted stream governed by electroosmosis and osmosis. When the con- brine to the sea. Other technologies such as MD and ED processes will
centrate reservoir was exposed to vacuum, the ammonia recuperated be applied for agricultural purposes where natural energy is abundant
was around 14.5% of the theoretical value of the NH3 in the concentrate and can be utilized to reduce the energy requirements and overall cost.
solution relative to 6.2% only when applying aeration. However, ef- Among stand-alone and hybrid desalination technologies evaluated in
fective energy usage caused a lower concentration gradient between the this review, FO coupled ED processes showed superior performance
concentrate and the diluted solutions by a factor of 10. This caused the efficiency and minimum energy expenditure around 0.35 kWh m−3.
presence of swine manure TAN traces in the diluted solution after Although these membrane technologies for treating saline water or
shutting down the process. The pH of the concentrate solution should wastewater are expensive, they could be cost-effective when producing
be increased to > 8.6 further to improve the volatilization of NH3 to- nutrient water for fertigation, increasing crop production, and enhan-
wards the acid trap. cing the quality of crop yield. To that end, all these membrane-based
Vecino et al. [131] proposed using liquid-liquid membrane con- desalination technologies require advances in irrigation water prac-
tactors (LLMCs) to re-concentrate ammonia from wastewater as am- tices, reducing the need for freshwater supply resources, and max-
monium salts (NH4NO3 and NH4H2PO4). Two different concentrations imizing water reusability efficiency.
of ammonia fertilizer as feed solutions (1.7 g/L, N = 0.33% (w/w)-
4.0 g/L, N = 0.14% (w/w)) were used to create ammonium salts by an 9. Future prospects
acid stripping solution (nitric and phosphoric acid). After that, the ED
system was connected to the LLMCs for further desalination of the The membrane in an individual treatment process has often failed in
LLMCs permeate and obtaining product water depicting the quality of providing the required quality for irrigation water for diverse types of
irrigation water. In this work, over 29.0 h of LLMCs experiment, the feed salinities. The additional purification of the diluted draw solution
ammonia concentration declined to 0.03% (w/w) of nitrogen at 360 mg to reach the quality water nutrient can be achieved using another de-
NH3/L when utilizing high initial concentration of fertilizer feed solu- salination technology process. The recovery system should possess
tion. However, the ammonia concentration was further reduced to minimum energy expenditure and efficient output. The recovery

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W. Suwaileh, et al. Desalination 491 (2020) 114559

process combined with the desalination process is necessary in some improving thermal conductivity. An advanced glass membrane showed
cases to re-concentrate the draw solution, extract valuable nutrients, excellent thermal and chemical efficiencies as compared to polymeric
and produce drinking water. Other merits are accelerating the water membranes [136]. The thermal efficiency can also be enhanced by in-
production, decreasing the energy requirement, recover nutrients from corporating self-healing metal nanoparticles or carbon-based sunlight
hypersaline solution and wastewater, and lowering the volume of brine absorbers into the MD membrane [118], photothermal surface coatings
and wastewater for discharge. The hybrid desalination systems in this like plasmonic nanoparticles [132]. Furthermore, membranes with high
review generated product water with varying qualities depending on hydrophobicity are required to reduce wetting, fouling, scaling, and
the availability of freshwater resources, the type of crops, and soil. purer condensate. Prior research suggested that scaling can be mini-
However, if a perfect draw solution in the FO system provided water mized when exposing the membrane to superhydrophobic fluor-
nutrients suitable for direct irrigation, the recovery method can be ig- osilicone coatings [136], but the stability and separation performance
nored, and minimal power is needed. For instance, the FO integrated in long-term experiments necessitate further investigations.
MD system can potentially generate irrigation water and drinking water For the ion exchange membrane, high water–solute selectivity of
when using a complex wastewater stream or brine containing nutrients higher than 95% and a low resistance material with a price < 4 €/m2
and a thermolytic fertilizer draw solution. The implementation of the are the main elements to promote the membrane separation perfor-
FO-MD process in the industry needs special consideration related to mance [137]. The performance of membranes incorporating polyolefin,
promoting system design and heat recovery. The use of a heat ex- polyaryletherketones, halogenated polyethers, polyethylene, and poly
changer can improve the energy efficiency of the system [132]. Besides, (arylene ether sulfone) opened room for further explorations. More-
this system can be considered energy-wise, cost-effective, and low en- over, researches should be dedicated to optimizing the stack design
vironmental impacts, especially if low-grade heat source or natural involving spacers and electrodes. The design and evaluation of new
power such as effective solar absorber, waste heat, geothermal heating, geometries and shapes of spacers to decrease pressure loss and polar-
is supplied to the recovery system. The production of vapor by solar ization phenomena are necessary [49]. In parallel, a novel stack design
energy can be increased through efficient solar absorptive materials like involving manifolds layout can ameliorate the solution flow distribu-
carbon nanomaterials, plasmonic materials, metal oxide nanomaterials, tion in the feed channels and should be tested in a real application. It is
and non-thermal-conductive material such as wood and foams. Cur- possible to enhance the fluid dynamics, mixing behavior of the feed
rently, research is directed to maximize energy efficiency by de- stream, lower resistance, and pressure drop employing by using an ion-
termining latent heat recovery [132]. The improvement of latent heat exchange membrane with optimum geometry leading to extraordinary
recovery depends on optimizing the system design. power output [137]. To exploit a large amount of natural power from
Another promising technology is the MD coupled ED system to re- the low-grade heat source, a closed-loop RED system is workable,
cover nutrients and convert the thermal potential of MD brine and especially when it is integrated with another desalination technology
energy of mixing to electricity. To fulfill commercial potential for the achieving low overall energy consumption [137]. To achieve com-
MD-ED hybrid process, on-site optimization of membrane-based pro- mercialization of the hybrid system, accurate thermo-economic ana-
cesses through the mobile pilot plant can be an effective suggestion for lysis, and cost assessment for a pilot plant in the field are needed [138].
evaluating the operating parameters. Many works devoted to devel- Also, establishing thermodynamic models to evaluate the performance
oping novel electrode materials like pseudocapacitive and carbon ma- of the membrane and overall process is needed for scaling up the pro-
terials with superior electrical conductivity, fast rapid adsorption, and cess and realization in the agricultural industry.
desorption of salts, and high salt adsorption capacity to promote the Although these membrane-based techniques present several chal-
system efficiency [49,132,133]. Increasing the electrode capacitance is lenges, they could be a viable option to produce irrigation water for
important because a lower amount of applied voltage would be re- agricultural applications. The prospect of implementing industrial
quired, and a certain amount of charges would be stored [132]. plants with optimal operating conditions and system design does not
Another important aspect is developing revolutionary anti-fouling depend only on the important requirements for each desalination pro-
TFC membranes by impregnation of antibacterial nanomaterials like cess but also makes the membrane the most significant factor for water
graphene oxide, carbon nanotubes, catalytic nanoparticles such as ti- generation in the agriculture industry.
tania (TiO2), silver or copper nanoparticles [134]. The long-term per-
formance of the membranes can be further increased by removing Acknowledgements
foulants and their precursors through transparent exopolymer particles
(TEP), and novel modification strategies such as layer-by-layer as- The authors would like to thank The Qatar National Research Fund
sembly, polymer grafting, zwitterionic coating with easy to scale up (QNRF) for funding the PhD student Wafa Suwaileh. The authors also
procedure and multifunctionality [135]. An alternative method to al- acknowledge the support provided by the Royal Society for funding this
leviate fouling is using a pre-treatment stage such as UF or MF mem- work through a Royal Society International Collaboration Award
brane, but this practice can impose an additional energy cost. There- (IC160133).
fore, employing real-time monitoring is a promising option to monitor
fouling in early-stage, and its effectiveness needs to be tested during References
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