REVIEW PAPER

Fertiliser drawn forward osmosis desalination: the concept,performance and limitations for fertigation

Sherub Phuntsho • Ho Kyong Shon •

Seungkwan Hong • Sangyoup Lee •

Saravanamuthu Vigneswaran • Jaya Kandasamy

Published online: 11 December 2011

� Springer Science+Business Media B.V. 2011

Abstract With the world’s population growing

rapidly, pressure is increasing on the limited fresh

water resources. Membrane technology could play a

vital role in solving the water scarcity issues through

alternative sources such as saline water sources and

wastewater reclamation. The current generation of

membrane technologies, particularly reverse osmosis

(RO), has significantly improved in performance.

However, RO desalination is still energy intensive and

any effort to improve energy efficiency increases total

cost of the product water. Since energy, environment

and climate change issues are all inter-related, desa-

lination for large-scale irrigation requires new novel

technologies that address the energy issues. Forward

osmosis (FO) is an emerging membrane technology.

However, FO desalination for potable water is still a

challenge because, recovery and regeneration of draw

solutes require additional processes and energy. This

article focuses on the application of FO desalination

for non-potable irrigation where maximum water is

required. In this concept of fertiliser drawn FO

(FDFO) desalination, fertilisers are used as draw

solutions (DS). The diluted draw solution after desa-

lination can be directly applied for fertigation without

the need for recovery and regeneration of DS. FDFO

desalination can make irrigation water available at

comparatively lower energy than the current desali-

nation technologies. As a low energy technology,

FDFO can be easily powered by renewable energy

sources and therefore suitable for inland and remote

applications. This article outlines the concept of FDFO

desalination and critically evaluates the scope and

limitations of this technology for fertigation, including

suggestions on options to overcome some of these

limitations.

Keywords Forward osmosis (FO) � Desalination �Fertiliser draw solution � Fertigation � Irrigation

1 Introduction

The world population, has crossed 7 billion in 2011

(UNFPA 2011) and is projected to reach 9 billion by

2050 (UN 2009). Therefore, one of the most crucial

challenges of the twenty-first century is to meet the

increasing demand for potable water and food supply

to meet this enormous population growth (Ward and

Pulido-Velazquez 2008). However, food availability

may soon be limited by water availability and

S. Phuntsho � H. K. Shon (&) � S. Vigneswaran �J. Kandasamy

School of Civil and Environmental Engineering,

University of Technology, Sydney (UTS), Post Box 129,

Broadway, NSW 2007, Australia

e-mail: Hokyong.Shon-1@uts.edu.au

S. Hong � S. Lee

School of Civil, Environmental and Architectural

Engineering, Korea University, 1, 5-ka, Anam-Dong,

Sungbuk-Gu, Seoul 136-713, Republic of Korea

123

Rev Environ Sci Biotechnol (2012) 11:147–168

DOI 10.1007/s11157-011-9259-2

therefore, optimum management of global water

resources also presents a crucial challenge (Jury and

Vaux 2005; McDonald et al. 2011). Current estimate is

that, more than one-third of the world’s population

lives in water-stressed countries and it may possibly

rise to nearly two-third by 2025 (Service 2006).

Climate change due to anthropogenic activities has

further created uncertainty regarding water availabil-

ity and food productivity by altering the global

hydrological cycle (McDonald et al. 2011).

It has also been estimated that about 60% of the

food needed to feed the increased population will

come from irrigated production (Plusquellec 2002).

Even as water scarcity becomes a significant issue,

water use for agricultural purposes is not sustainable in

many parts of the world (Jury and Vaux 2005). For

example in Australia, river water from the Murray-

Darling Basin has been long over-allocated for

consumptive use seriously impacting on the fragile

river ecosystem (Goss 2003; MDBA 2010; Cosier

et al. 2010). The need to protect the environment while

managing the very limited resources is becoming

increasingly urgent in drought stricken countries such

as Australia. Fresh water resources are therefore

essential for both drinking water supplies and food

production in order to support life on earth.

Although measures such as water conservation,

infrastructure repair, improved catchment and distri-

bution system could help alleviate water stress to a

certain degree nevertheless, these measures only help

improve the existing water sources, not create new

water resource (Elimelech and Phillip 2011). In the

face of climate change and the increasing global water

crisis, the prospects of scientific solutions playing a

crucial role are demanding (Jury and Vaux 2005)

including making water available from nonconven-

tional sources such as saline water. One such area is

through the application of membrane technologies for

water purification (Shannon et al. 2008). The current

generation of membrane technologies, particularly

reverse osmosis (RO), have significantly improved the

scope for the use of saline water and impaired

wastewater effluent as an alternate source of water to

augment fresh water or to reduce pressure on fresh-

water resources. Desalination technologies are there-

fore seen as a promising alternative in alleviating

water scarcity in arid and densely populated regions of

the world (Service 2006; McGinnis and Elimelech

2007).

While the performance of membrane technology in

terms of energy need has significantly improved in the

last decade or so, it still remains energy intensive in

nature, particularly for desalination with the RO

process (McGinnis and Elimelech 2007; Greenlee

et al. 2009). Fouling still proves to be a major

challenge for membrane application (Greenlee et al.

2009; Phuntsho et al. 2011a) and, any attempt to

further reduce energy for desalination also propor-

tionately increases the capital and operational costs of

the plant (Semiat 2008). Moreover, the law of

thermodynamics sets a minimum limit on work energy

required to desalinate water which is equal to

0.75 kWh/m3 of desalted water at zero recovery and

about 1.06 kWh/m3 at 50% recovery for sea water

(Semiat 2008). However, practically, the most effi-

cient RO desalination plant with energy recovery

system has been reported to consume about

2.1–3.2 kWh/m3. The unit energy consumption for

RO desalination increases with recovery rates (Elime-

lech and Phillip 2011; Subramani et al. 2011). The

total energy would of course increase if the energy for

pre-treatment and post-treatment systems are

accounted. Since energy and climate change issues

are inter-related (Semiat 2008), addressing global

water scarcity problems requires an extensive invest-

ment in research to identify robust and innovative

methods of purifying water at lower energy consump-

tion and cost (Shannon et al. 2008). Any low energy

desalination technologies could have a significant

impact for drought stricken countries such as Austra-

lia, where saline water is abundant in the form of

seawater along the coastal areas and brackish ground-

water in the inland areas.

Forward osmosis (FO), based on the principles of a

natural osmosis, is now an emerging technology for

desalination. The FO process works on the principle of

the natural osmotic process, driven by the osmotic

gradient between two solutions of different osmotic

concentrations when they are separated by a semi-

permeable membrane. Figure 1 shows the basic prin-

ciples of various osmotic processes. When saline feed

water and the highly concentrated solution (referred to

as draw solution or DS) are separated by a semi-

permeable membrane, water moves from the saline

water (lower solute concentration) to the concentrated

DS (higher solute concentration), while retaining the

solutes on both sides of the membrane. The main

feature of this process is that the water transport across

148 Rev Environ Sci Biotechnol (2012) 11:147–168

123

a semi-permeable membrane in the FO process does

not require hydraulic pressure, therefore, the energy

consumption is significantly less (unlike the RO

process) (Moody 1977; McCutcheon et al. 2005;

McGinnis and Elimelech 2007; Elimelech and Phillip

2011). Moreover, due to the absence of hydraulic

pressure, the severity of the fouling problem in the FO

process is also less likely to be a major issue. Fouling

in the FO process is observed to be physically

reversible; hence, chemical cleaning may be only

seldom required in the FO process (Lee et al. 2010; Mi

and Elimelech 2010).

Unfortunately, FO technology still suffers from

some major technological barriers, because of which

its commercial application has been limited. The first

such barrier is the lack of membrane suitable for the

FO process, and the second is the lack of suitable DS.

However, significant progress has been made in FO

membrane fabrication recently, with thin film com-

posites having comparatively higher water flux than

the existing commercial FO membranes. The separa-

tion and recovery of the DS require an additional

processing unit, which consumes energy and therefore

still remains a significant challenge for drinking water

applications. The success of FO desalination for

potable purpose will entirely depend on how easily

and efficiently the draw solute can be separated from

the water (Phuntsho et al. 2011b). The limitations of

this technology are discussed later in more detail.

The FO process is more suitable if the diluted draw

solution after desalination by FO process, can be

directly used without further processing. This includes

fertiliser drawn forward osmosis (FDFO) desalination,

where the diluted fertiliser draw solution after desa-

lination can be directly used for beneficial purpose

such as for fertilised irrigation or fertigation. Such

applications offer the advantage of not having to

separate and recover draw solutes; therefore, energy

consumption will be significantly lower than desali-

nation for potable water by the RO process. This

article explains the concept of FDFO desalination and

evaluates the scope and limitations of this technology

for fertigation. The article begins with a brief expla-

nation of the concept of FDFO desalination, followed

by a discussion on the opportunities for applications,

and a brief review on the performance of the fertiliser

draw solutions. The article also critically assesses

some of the limitations of this particular FO desali-

nation technology, and suggests alternatives in over-

coming these specific limitations.

2 The concept of the FDFO process for direct

fertigation

Although desalination using the natural osmotic

process is a novel concept, its application for potable

water using seawater or brackish water still remains a

significant challenge. Separation of diluted draw

solutes from desalted water for recovery and regen-

eration is not an easy task, requiring an additional

processing unit and therefore consuming extra energy.

So far, only few draw solutes have been found to be

promising candidates for use as DS for potable water

desalination, such as NH3-CO2 solution and magnetic

nanoparticles. NH3-CO2 can be separated by heating

at approximately 60�C, using low grade heat

(McCutcheon et al. 2005, 2006; McGinnis and

Elimelech 2007). Such processes could be economical

only when low grade heat is available from sources

such as waste heat from thermal power plants.

Magnetic nanoparticles have been reported to generate

high osmotic pressure and the particles could be

separated by a magnetic separation process (Ge et al.

2011). However, none of these processes are yet to be

tested on a large-scale level. For FO desalination to

Fig. 1 The principles of

osmotic processes: forward

osmosis (FO), pressure

retarded osmosis (PRO) and

reverse osmosis (RO).

Source (Cath et al. 2006)

Rev Environ Sci Biotechnol (2012) 11:147–168 149

123

prove more competitive over conventional RO desa-

lination processes for potable water production, it is

essential that the separation process have low capital

cost, low energy consumption, and very low operating

cost. However, finding an ideal DS meeting these

requirements is still a big challenge; therefore, the

commercial application of FO desalination for potable

water still requires more research work.

The FO process is certainly more suitable if the

draw solution after dilution can be used directly as it is,

and does not require separation and recovery of draw

solutes. One such case is for desalination or purifying

water for emergency relief supplies during disasters,

using nutrients such as concentrated sucrose (Kravath

and Davis 1975; Cath et al. 2006). It has also been used

for emergency potable water supplies in situations

where there is little storage capacity available, such as

in life boats or small crafts. FO also has been

investigated for application in the concentration of

industrial wastewater using seawater as DS (Anderson

1977), concentration of anaerobic digester concentrate

using RO concentrate as DS (Holloway et al. 2007),

sucrose concentration using NaCl as DS (Garcia-

Castello et al. 2009), dewatering of pres liquor derived

from orange production using NaCl as DS (Garcia-

Castello and McCutcheon 2011) and, drinking water

augmentation with a hybrid FO system using seawater

as DS and impaired water as feed water (Cath et al.

2010).

Another promising area of application is desalina-

tion for irrigation using fertilisers as DS (Moody

1977). When fertilisers are used as DS, the diluted DS

after desalination can be directly used for fertigation;

therefore, there is no need to worry about its separation

and recovery (Phuntsho et al. 2011b). Fertigation is the

application of fertiliser nutrients (dissolved form or

suspended form) to the crops with irrigation water

instead of broadcast application separately. Since

fertilisers are extensively used in agricultural produc-

tion, such a process would provide nutrient-rich water

for direct fertigation from any saline water source.

The concept of using fertiliser as an osmotic

extractor for agricultural water reclamation was first

reported by Moody and Kessler (1976). Although the

potential for such application is immense, works on

this particular concept did not receive attention until

recently, mainly due to the lack of suitable membrane

for the FO process. Figure 2 provides the general

process layout of the FDFO desalination for

fertigation. In the FO process, two different solutions

are used: saline water as the feed water on one side of

the membrane, and highly concentrated fertiliser

solution as the DS on the other side of the membrane.

The two solutions are continuously kept in contact

with the membrane through a crossflow system in

order to minimise the influence of concentration

polarisation (CP) effects. Due to the osmotic gradient

across the semi-permeable membrane, water flows

from the feed solution with lower concentration

towards the highly concentrated fertiliser draw solu-

tion, in the process desalting the saline feed water.

Depending on the osmotic pressure of the concen-

trated DS, it is possible to achieve high recovery rates

of the feed water (McCutcheon et al. 2005; Martinetti

et al. 2009). After extracting the water by the FO

process, the fertiliser DS becomes diluted, with the

extent of dilution depending on the feed water salinity.

The final fertiliser solution can be used directly for

fertigation if it meets the water quality standards for

irrigation in terms of salinity and fertiliser/nutrient

concentration. If the final fertiliser concentration

exceeds the nutrient limit, then further dilution may

be necessary before applying it for fertigation.

3 Advantages of FDFO and opportunities

for specific applications

3.1 Low energy desalination process

The FO process is solely based on the difference in

concentration gradient between the two solutions, with

no hydraulic pressure necessary for driving the water

through the membrane. The only energy required in

the FO process is for maintaining the crossflow of the

feed and draw solutions in contact with the membrane

surfaces and providing adequate shear force to min-

imise the CP effects that are intrinsic to any membrane

filtration process.

Table 1 shows the comparative energy requirement

for different desalination technologies (as available in

the literature). Different figures have been reported on

the total energy requirement for each desalination

technology and some of the selected figures are shown

in Table 1. It is clear from this table that the FO

desalination process using NH3-CO2 as DS requires

comparatively lower energy than any other existing

150 Rev Environ Sci Biotechnol (2012) 11:147–168

123

desalination technologies even after considering the

recovery process for the draw solutes from the diluted

DS. The total energy required has been estimated at

0.84 kWh/m3 which includes 0.5 kWh/m3 of energy

for NH3-CO2 recovery and 0.24 kWh/m3 electrical

energy for running the pumps (including the pumps for

the distillation process). This total energy when

compared to other current desalination technologies

on an equivalent work basis, can be saved between 72

and 85% of energy (McGinnis and Elimelech 2007).

Although, the performance of NH3-CO2 as DS could

vary from the fertiliser draw solutions, nevertheless

the figures in Table 1 provide a fair indication that, the

energy required for FDFO will also be substantially

lower given the fact that the recovery of draw solutes

from the diluted draw solution is not necessary. The

only energy required will be to keep the fluid in

contact with the membrane, using pumps and the

crossflow system. The energy for FDFO desalination

for irrigation is also lower than the theoretical energy

required based on limiting energy in thermodynamics

for separating salt and water from seawater. Energy

consumption by the RO process increases with an

increase in recovery rates, whereas in the FO process,

recovery rates depend on the highest osmotic pressure

a draw solute can generate in solution, and therefore

has no significant relation to external energy input.

The existing desalination technologies are no doubt

energy intensive in nature. This is the main reason why

desalination is still limited to drinking water supplies

and other commercial/industrial uses, rather than for

irrigation purposes where the water requirement is

comparatively large. However, if low energy desali-

nation technologies are made available, it would have

a significant impact on the agriculture sector espe-

cially for those countries where drought is frequent but

have abundant saline water in the form of seawater in

the coastal areas and brackish groundwater in the

inland areas (Phuntsho et al. 2011b). Since FO

desalination requires low energy, this technology

could be easily powered by renewable energy, such

as wind and solar energy, making FO technology with

no carbon foot print. Renewable energy, especially

solar energy, is abundant in most remote communities

in Australia, therefore can be easily tapped for such

uses.

Fig. 2 The conceptual

process layout diagram of

the fertiliser drawn forward

osmosis desalination for

direct fertigation. Modified

from Phuntsho et al. (2011b)

Rev Environ Sci Biotechnol (2012) 11:147–168 151

123

3.2 Fertigation or fertilised irrigation

Fertilisers and water for irrigation are essential

components for improving agricultural productivity.

Agriculture is by far the largest consumer of potable

water, accounting for about 80% of water consump-

tion worldwide, and substantially more in the USA

(Jury and Vaux 2005). In Australia, irrigation usage is

72% of the total water consumption (Khan 2008).

Therefore, even small savings from agricultural water

use through improved efficiency might make sub-

stantial quantities of water available for the commu-

nity and the environment (Ward and Pulido-

Velazquez 2008; McDonald et al. 2011). Since,

freshwater sources are becoming scarcer every year,

low energy desalination processes (such as FO) could

be an effective method for augmenting water for

irrigation. Besides making irrigation water available at

lower energy from saline water sources, FDFO

desalination provides nutrient-rich water for fertiga-

tion. Fertigation is the application of irrigation water

with fertilisers, either in dissolved solution or in

suspended form. Fertigation has several advantages in

comparison to the application of water and fertilisers

separately, such as minimum loss due to leaching,

optimising the nutrient balance by supplying the

nutrients directly to the root zone, control of nutrient

concentration in the soil solution, savings in labour

and energy, and flexibility in timing fertiliser appli-

cation in relation to crop demand (Papadopoulos and

Eliades 1987). Such technology is also handy for those

farms already adopting fertigation, as it can be easily

integrated within their existing fertigation system.

Fertigation is more efficient and cost-effective for

supplying water and nutrients to the crops simulta-

neously, instead of conventional broadcast application

(Hanson et al. 2006). Fertigation can also be advan-

tageous for application in mixtures with other micro-

nutrients, chemical pesticides (as in chemigation),

and/or fungicides (by fungigation), all in the correct or

necessary proportions, thereby eliminating separate

application modes for those chemicals (McBeath et al.

2007).

3.3 Potential application in the context

of the Murray-Darling Basin (MDB)

in Australia

Australia is considered as the driest continent on Earth

in terms of overall runoff per unit area and rainfall-

runoff ratio, with one of the highest river flow

variability in the world (Khan 2008). However,

Australia is surrounded by three oceans and four seas

and has one of the longest marine boundaries in the

world; therefore, the sources of saline water are

abundant. Australia also has 25,780 GL of groundwa-

ter, with about 28% having a salinity higher than

1,200 mg/L of TDS (total dissolved solids), and as

such, saline groundwater is abundant within the MDB

(ANRA 2009).

The MDB, consisting of 23 river valleys and an area

of more than one million km2, covers about 14% of the

Australian land mass (MDBA 2010). The basin is a

highly significant factor in Australia’s ecological

health, as it is home to the country’s most diverse

and rich natural environments. The basin is also

critical to the Australian economy and food security,

as it supports 39% of the national agricultural

production and supplies water to three million resi-

dents (MDBA 2010). Although MDB receives 6% of

Australia’s annual rainfall, about 75% of Australia’s

total irrigated land is concentrated here (MDBA

2010). However, the basin suffers from major envi-

ronmental issues considered to be of national signif-

icance (Goss 2003; MDBA 2010; Cosier et al. 2010).

Table 1 Comparison of energy requirements for seawater

desalination with the existing desalination technologies

Desalination technology Total equivalent work

energy (kWh/m3)

Multi stage flash (MSF) distillation 10–58a, 5.66b

Multi effect distillation (MED) 6–58a, 40.05b

MED-low temp/electrical 5–6.5a, 3.21b

Reverse osmosis (RO) 4–6b

RO with energy recovery 3–4a, 3.02b, 2.1–3.2c

Ammonia-carbon dioxide forward

osmosis desalination (low temp,

1.5 M feed) with draw solute

recovery process. It includes

energy for pumping of feed and

draw solutions and pumping

required during the distillation

process

0.84b

Adapted from a (Semiat 2008), b (McGinnis and Elimelech

2007) and c (Elimelech and Phillip 2011; Subramani et al.

2011). The data for FO for direct fertigation has been adopted

from McGinnis and Elimelech (2007) by removing the energy

required for draw solution separation by distillation process

152 Rev Environ Sci Biotechnol (2012) 11:147–168

123

One of the major issues is the reduced volume of

water that flows in the river system due to over-

allocation of water for consumptive use (such as

agricultural and other economic uses) significantly

affecting the fragile river ecosystem within the basin

(Goss 2003). Over-allocation of river water within the

basin for consumptive use has been widely docu-

mented and agreed not only within the scientific

communities but also across the wider community

(Goss 2003; MDBA 2010; Cosier et al. 2010). It is

genuinely believed that there is an urgent need to

lower the water allocation in order to maintain

adequate environmental flows for the sustainable river

ecosystem (MDBA 2010; Cosier et al. 2010). With

further water use restriction in the future becoming

imminent with the Basin Action Plan being proposed

(MDBA 2010), alternative sources of water must be

explored if the agricultural production within the

region is to survive, which Australia significantly

depends upon. The groundwater source within the

basin is plenty; however, this groundwater cannot be

used directly for irrigation because of high salinity.

The other major environmental issue facing the

MDB is the increased river water salinity caused

primarily by the intrusion of saline groundwater from

the basin (Ife and Skelt 2004). The basin is also a

naturally saline environment, a result of the weath-

ering of rocks and cyclic salts deposited over many

years. Salinity in Australia has damaged natural

resources and infrastructure, and is also impacting

terrestrial biodiversity (Goss 2003). The allocation of

river water for extensive consumptive use has signif-

icantly reduced the river flow volume and further

exacerbated the river salinity problem.

Since 1988, the Australian Federal Government has

funded and installed a number of salt interception

schemes (SIS) for controlling river water salinity. The

SIS consists of large-scale groundwater pumping

stations and drainage projects that intercept brackish

groundwater flows and dispose them generally

through open pond evaporation (Fig. 3a). This has

significantly reduced the salinity downstream of the

MDB (Goss 2003); however, groundwater is simply

lost through evaporation and therefore does not enable

the sustainable use of groundwater.

As such, a sustainable SIS in the MDB is required,

which not only serves for salt interception but also

allows sustainable use of saline groundwater. One way

of doing this is by making full use of the brackish

groundwater for irrigation as an alternative source to

Fig. 3 Cross sectional view

of the a existing salt

interception scheme (SIS)

installed (18 in total) along

the Murray and Darling

Rivers and b the alternate

SIS scheme which integrates

FDFO desalination for the

sustainable use of brackish

groundwater for irrigation

Rev Environ Sci Biotechnol (2012) 11:147–168 153

123

river water. The use of groundwater will also reduce

the pressure on the river water and make more water

available for environmental flows in the river system.

However, direct irrigation of groundwater is imprac-

tical due to the high salinity content, with the salinity

in some places as high as seawater. The high salinity

content in the water can have a deleterious effect on

the productivity of agricultural crops (Cheeseman

1988). Therefore, the brackish groundwater has to be

desalted first in order to make the brackish water fit for

irrigation purpose.

Since current desalination technologies are energy

intensive, technology such as low energy FDFO

desalination could play a significant role in providing

a sustainable SIS in the MDB (Phuntsho et al. 2011b).

FDFO desalination can be integrated to the current SIS

to make sustainable use of the brackish groundwater

for irrigation as shown in Fig. 3b. Although, FDFO

desalination can be applied to any other areas, there

are few specific reasons that have merit for this

particular case. The agricultural farms in the MDB

have already access to water from the rivers but the

amount of water they can withdraw for irrigation will

be significantly reduced (up to 40%) in the future in

order to make more water available for environmental

flows. Therefore, FDFO can be used to augment the

additional water required for irrigation once the water

restriction is imposed. The existing water can be

combined together with the FDFO product water for

fertigation especially if FDFO product water requires

further dilution to make fertiliser concentration

acceptable for irrigation. Such approaches offer

multiple advantages, including: making water avail-

able for irrigation; sustainable use of groundwater; and

reducing dependence on river water for irrigation,

thereby making adequate water available in the river

for environmental flows while still serving the original

purpose of salt interception.

The above application was specific to MDB in

Australia however; FDFO desalination technology has

potential application in any parts of the world

particularly in the arid and semi-arid countries where

water scarcity is frequent and where saline water exists

in abundance. This technology could be useful as a

means to augment additional irrigation water required

during the drought season or scanty rainfall season.

The limited existing water resources can be used for

providing additional dilution water for FDFO product

water.

4 Choice of fertiliser and the performance

of fertilisers draw solutions

Different draw solutions have been used in the FO

process, including compounds such as ammonium

chloride (Achilli et al. 2010), potassium chloride,

ammonium sulphate and calcium nitrate (Achilli et al.

2010), potassium sulphate (Hancock and Cath 2009),

ammonium bicarbonate (McCutcheon et al. 2006;

Hancock and Cath 2009; Achilli et al. 2010; Yip et al.

2010) etc. Although such chemicals were used for

different reasons, some of these compounds are in fact

used as fertilisers as well. While most literatures have

reported using only one or two draw solutions in their

studies (the most popular being sodium chloride

solution), only the articles by Achilli et al. (2010)

and Phuntsho et al. (2011b) contain a comprehensive

list of different inorganic draw solutions.

The choice of fertiliser DS will be guided by many

factors. The main factors include fertiliser economics

and its performance as draw solute for FDFO process.

Fertiliser economics relates to the availability and the

cost of the fertilisers. The fertiliser must also have its

physicochemical properties suitable for use as draw

solute in FO process such as solubility, pH compatible

to FO membrane used, types of species formed in the

solution, amount of water a unit mass of fertiliser can

extract which depends on the molecular weight and the

osmotic pressure of the fertiliser DS, etc. Finally, the

choice also would be guided by the nutrient require-

ments for the particular target crop. Moody and Kessler

(1976) showed that FDFO can extract up to 80 kg of

water with a kilogram of fertiliser from brackish water

(salinity 3,200 mg/L), and 14 kg of water per kilogram

of fertiliser from seawater. Phuntsho et al. (2011b)

observed that most soluble fertilisers can be used as DS

for FO desalination but the fertiliser solution should

have pH compatible to the FO membrane used. The

existing commercial CTA FO membrane has a limited

pH range between 4.0 and 8 while the recently reported

thin film composite membrane have been reported to

have significantly higher pH range (Wang et al. 2010b;

Yip et al. 2010). Phuntsho et al. (2011b) estimated that

each kilogram of fertiliser can extract between 11 and

29 litres of water from seawater (salinity of 35,000 mg/

L), and 90–215 L of water from brackish water

(salinity of 5,000 mg/L). At lower feed salinity,

fertilisers can extract even more water. The water

extraction capacity also depends on the molecular

154 Rev Environ Sci Biotechnol (2012) 11:147–168

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weight of the fertiliser compound and the osmotic

pressure of the fertiliser. The fertiliser compound with

lower molecular weight and higher osmotic pressure

can extract significantly more water than high molec-

ular weight fertilisers. The osmotic pressures of some

of the commonly used fertilisers in the world for crop

production are shown in Fig. 4.

In many early studies with the FO process, the

actual flux observed was very low due to concentration

polarisation (CP) effects (described more in separate

section). The degree of CP effects varies depending on

the properties of DS used, membrane properties, and

the operational hydrodynamic conditions of the FO

process (Gray et al. 2006). In order to account for the

CP effects, the performance of DS is assessed in terms

of performance ratio, defined as the ratio of actual flux

to theoretical flux calculated as a percentage. This

ratio indicates the percentage of the effective bulk

osmotic pressure difference that is effectively gener-

ating water flux across the FO membrane (McCutch-

eon et al. 2006; Phuntsho et al. 2011b). The

performance of each fertiliser DS varies considerably.

Table 2 provides the data for a few selected fertiliser

DS. Amongst the nine listed fertilisers, potassium

chloride has been reported to have the highest pure

water flux, with more than 22 LMH (L/m2/h) at 2 M.

Potassium chloride also showed the highest

performance ratio. Higher water flux has been

observed at higher fertiliser concentrations. The

performance ratio of the selected fertiliser solutions

ranges from 9 to 16%. Diammonium phosphate (DAP)

and monoammonium phosphate (MAP) have compar-

atively much lower water flux amongst all the selected

fertiliser draw solutions.

The mixture of ammonium bicarbonate and ammo-

nium hydroxide in specific proportions produces a

mixture of ammonium bicarbonate, ammonium car-

bonate and ammonium carbamate solution (McCutch-

eon et al. 2005; Cath et al. 2006). These DS have been

widely studied because it can be easily recovered and

regenerated after FO desalination by heating at low

temperature (McCutcheon et al. 2005; Cath et al. 2006).

The compounds formed in these mixtures are fertiliser

compounds and, therefore their performance as DS is

worth discussing here. Ammonium carbamate is highly

soluble and therefore has the ability to generate high

osmotic pressure. This particular DS has been found to

have performance ratio of up to 20% (McCutcheon et al.

2006) and the water flux was observed to be comparable

to some of the fertiliser draw solutions indicating that

this DS can also be suitably used for FDFO process.

The performance of fertiliser DS is also assessed in

terms of the maximum recovery rate at which it can be

operated in the FO desalination process, using both

0

50

100

150

200

250

300

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Osm

o�c

pres

sure

(atm

)

Fer�lizer concentra�on (M)

Ca(NO3)2

(NH4)2SO4

NH4Cl

KCl

(NH4)2HPO4

NH4H2PO4

NaNO3

NH4NO3

KNO3

CO(NH2)2

KH2PO4

Fig. 4 Osmotic pressure of

some of the selected

fertilisers commonly used

around the world for

agriculture. Osmotic

pressure was predicted using

OLI Stream Analyser 3.2

(OLI Inc. US)

Rev Environ Sci Biotechnol (2012) 11:147–168 155

123

brackish water (5,000 mg/L or 0.086 M sodium chlo-

ride solution) and seawater. The calculations were

made based on the maximum osmotic pressure that a

particular fertiliser can generate in solution, based on

its maximum solubility. The osmotic pressure was

predicted using OLI Stream Analyzer 3.1 software.

Table 2 also shows the theoretical maximum recovery

rate at which the FO can be operated for particular

types of feed water. The movement of water across the

membrane towards the DS will occur until the osmotic

equilibrium is reached between the DS and the feed

solution (FS) (Phuntsho et al. 2011b) irrespective of the

rate of water transport across the membrane. Theoret-

ically, 100% recovery is possible if the fertiliser draw

solution can generate osmotic pressure higher than the

maximum solubility of the sodium chloride solution

(6.15 M) with an osmotic pressure of 404 atm. For

example, ammonium nitrate (being highly soluble in

water) can easily generate osmotic pressure in excess

of 404 atm. Similarly, calcium nitrate and sodium

nitrate can also generate osmotic pressure in excess of

404 atm; therefore, the use of these fertilisers as DS for

FO desalination can theoretically achieve a 100%

recovery rate. Other fertilisers, such as ammonium

sulphate, can achieve about 94 and 87% recovery rates

with brackish water and seawater, respectively. Most

fertiliser DS listed in Table 2 can in fact achieve more

than 90% recovery rate with brackish water except for

potassium nitrate, with only about 85% due to its low

solubility (4.03 M). The recovery rates with seawater

for all fertilisers are higher than 80%, except with

potassium nitrate.

However, it must be noted that 100% recovery is not

practical since at higher concentration, the feed solution

could start to precipitate and cause scaling on the feed

side, further impacting the water flux. Precipitation may

occur earlier because in practice, the saline water from

the natural sources can include many other dissolved

elements such as Ca, Mg, etc. which have different

solubility rates. The energy required to keep the fluid

flowing will also rise because of the increase in the

viscosity of the feed water at higher concentration.

Reverse movement of draw solutes (specific reverse

solute flux or SRSF) also occurs by natural diffusion

during the FO process, which is not actually desirable

(implications of reverse solute are described more in

the other section). The performance of fertiliser draw

solutions, in terms of SRSF, varied widely depending

Table 2 Performance of the selected fertilisers as FO draw solution

Name of

fertilisers

pH at

2 MaMax.

solubility

(M)

p at 2 M

(atm)

Jw

(LMH)

PR

(%)

SRSF

(m moles/L)

With BW (0.086 M) With SW

Vol.

(L)aMax.

recovery

rate (%)a

Vol.

(L)

Max.

recovery

rate (%)a

Ammonium nitrate 4.87 101.9 64.9 15.037 14.95 189.34 129.87 100 15.236 100.0

Ammonium sulphate 5.46 5.80 92.1 19.408 13.60 1.03 103.95 98.1 12.659 86.9

Ammonium chloride 4.76 7.35 87.7 19.253 14.16 62.27 215.34 98.5 29.069 89.3

Calcium nitrate 4.68 22.04 108.5 18.079 10.75 1.80 90.55 100 11.498 100.0

Sodium nitrate 5.98 10.95 81.1 20.54 16.34 48.63 134.62 100 17.691 100.0

Potassium chloride 6.80 4.82 89.3 22.812 16.48 35.03 154.89 97.8 21.011 84.8

MAP 3.93 4.56 86.3 15.66 11.71 15.95 100.39 97.4 13.672 81.8

DAP 8.12 7.13 95.0 14.01 9.52 2.50 115.78 98.2 14.392 87.6

Potassium nitrate 5.99 4.03 64.9 15.94 15.85 109.65 111.89 95.7 13.700 70.3

Osmotic potential assumed for seawater (SW) is 28 atm (0.6 M sodium chloride solution) and brackish water (BW) 3.93 atm

(0.086 M sodium chloride solution or 5,000 mg/L of sodium chloride solution). PR: performance ratio. p: osmotic pressure. SRSF

(Js/Jw): specific reverse solute flux

MAP mono-ammonium phosphate, DAP diammonium hydrogen phosphatea pH values as measured at 2.0 M. Solubility and speciation data were adapted from OLI Stream Analyzer speciation results. Data

adopted from (Phuntsho et al. 2011b) and rest calculated by the authors

156 Rev Environ Sci Biotechnol (2012) 11:147–168

123

on the type of fertilisers used (as shown in Table 2)

(Phuntsho et al. 2011b). Ammonium sulphate, calcium

nitrate and MAP were observed to show comparatively

much lower SRSF than any other fertilisers selected for

this study. However, ammonium nitrate and potassium

nitrate had the highest SRSF. In general, the ammo-

nium compounds of sulphate and phosphate, and

calcium nitrate containing monovalent elements, per-

formed well in terms of SRSF since lower RSF is

preferred for any FO process. NH4HCO3 certainly has

reported higher RSF (Achilli et al. 2010). The extent of

reverse solute diffusion could depend on many factors,

such as types of species formed in the solution and their

properties (e.g. charge, valency, hydrated size, etc.)

including other factors such as solution pH and

membrane properties (Phuntsho et al. 2011b). There-

fore it is important to consider all these factors while

selecting a suitable fertiliser as DS for FDFO desali-

nation process.

5 Limitations and options for fertiliser drawn

desalination

Ideally, the FO process offers several novelties for

desalination in comparison to conventional membrane

processes such as RO. However, FO still faces several

technological barriers because of which it has not been

able to compete on a commercial scale with other

desalination technologies. While some of the short-

comings elaborated below are applicable to the FO

process in general, some are specific to FDFO

desalination processes.

5.1 FO membranes

One of the major technical barriers to the commercial

application of the FO process has been the lack of

suitable FO membrane that could produce high water

flux comparable to the RO process based on the

theoretical bulk osmotic pressure gradient (Wang et al.

2010b; Yip et al. 2010; Tiraferri et al. 2011). The

existing membranes are dense semi-permeable mem-

branes that were originally designed for pressure-

driven membrane processes such as RO (Cath et al.

2006). These pressure-based membranes have an

asymmetric structure with a thin selective active layer

supported on thick layers of porous polymer and fabric

(Cath et al. 2006). Initially, the performance of the FO

process using these asymmetric membranes was

observed to be very low (Kravath and Davis 1975;

Lee et al. 1981). This lower than expected water flux

has been later explained to be due to the phenomenon

known as internal and external concentration polari-

sation (ICP and ECP) during the mass transport

process, which significantly reduces the effective

osmotic driving force between the two solutions

(Lee et al. 1981; Gray et al. 2006; McCutcheon and

Elimelech 2007). The asymmetric structure of the

membrane makes the CP effect even worse due to the

presence of ICP on the porous support layer side of the

membrane. Since the ICP phenomenon occurs inside

the membrane support layer, it cannot be easily

mitigated simply by providing hydrodynamic shear

forces like in pressure-driven membrane processes.

Until now, commercial FO membranes, made up of

cellulose triacetate supported on embedded polyester

screen mesh manufactured by Hydration Technologies

Inc. (HTI, USA) have been widely used in FO studies.

The FO membrane is also manufactured by Catalyx

Inc. (Anaheim, California), although no literature is

currently available about the performance of this

particular membrane (Wang et al. 2010b).

The ideal FO membrane should have high water

permeability and salt rejection, should be thin without

a porous support layer to remove the ICP effects, and

should also have good mechanical strength. However,

providing a thin membrane without support layers is a

big challenge since it does not provide adequate

mechanical strength to carry the water flow inside the

membrane module. However, several breakthroughs

have been reported on membrane synthesis recently.

The thin film composite (TFC) FO membranes are

reported to have much higher water flux and salt

rejection than the existing CTA FO membrane (Wang

et al. 2010b; Yip et al. 2010; Wei et al. 2011). TFC

membranes have been long used for RO desalination

because of its excellent properties such as high salt

rejection, high chemical resistance and high mechan-

ical strength (Yip et al. 2010). However, the thick and

dense support layer used for TFC-RO is not suitable for

FO process as it causes severe ICP. The innovative

claim for this TFC has been the modification of the

support layer which is significantly thinner and also

highly porous making it more suitable for FO process.

In particular, the hollow fibre thin film composite FO

membrane (Wang et al. 2010b) is a significant

breakthrough since flat sheet membranes are more

Rev Environ Sci Biotechnol (2012) 11:147–168 157

123

complicated for the design of spiral-wound modules

accommodating two different and independent flows in

the module separately. Recently, two companies have

already announced the commercialisation of TFC FO

membranes and the membranes are expected to be

available in the market very soon (GWI 2011a, b). With

the commercialisation of TFC FO membranes emi-

nent, the future prospects of FO process and its

applications are certainly high.

Other works on FO membranes include, the synthesis

of nanofiltration-like FO membranes using layer-by-

layer (LbL) assembly (Saren et al. 2011), double

skinned FO membrane (Wang et al. 2010a), polyamide

hollow fibre FO membrane (Setiawan et al. 2011) etc.

most of which reported a substantially better perfor-

mance than the existing commercial CTA FO mem-

brane in terms of water flux and salt rejection. Carbon

nanotube is another promising candidate for FO mem-

brane, which has been found to have a water flux higher

than the theoretical water flux (Jia et al. 2010).

Since the FO process has comparatively lower

water flux than the RO process using the current

commercially available FO membranes, it should be

acknowledged that the membrane surface area

required will be significantly higher than RO plants

depending on the capacity of the plant. Therefore, the

capital cost of the FO desalination plant is likely to be

comparatively higher than the RO plant, on the basis

of current membrane performance and modular

design. However, with further research, the perfor-

mances of FO membranes and their modular designs

are expected to be improved in the future.

5.2 Low water flux due to concentration

polarisation and dilutive effects

External concentration polarisation (ECP) in a pressure-

based membrane process is a phenomenon where the

concentration of the solution at the membrane surface

increases from that of the bulk solution. This phenom-

enon reduces the water flux and is well-understood and

modelled (Elimelech and Bhattacharjee 1998). The CP

effect particularly ECP is largely mitigated by providing

horizontal crossflow shear and turbulence on the mem-

brane surface instead of the dead end filtration process.

However, the FO process is unique since two

independent solutions flow in contact with the mem-

brane on each side of the membrane surface. The

existing membranes are asymmetric in structure, with a

thin active rejecting layer on top of a thick porous

support layer. Besides ECP, ICP, a closely-related

phenomenon to ECP, occurs within the porous support

layer of the membrane in the FO process. In ICP, the

solute concentrations at the transverse boundaries

within the porous support layer reduce the osmotic

pressure gradient across the active layer of the mem-

brane, therefore resulting in a corresponding reduction

in water flux (Gray et al. 2006). ICP is intrinsic to the FO

process and is found to be critical because it occurs

within the membrane support layer; therefore, it cannot

be mitigated simply by altering hydrodynamic condi-

tions (as in the case of ECP). In fact, it has been found

that ICP, particularly dilutive ICP, is the key factor

responsible for reducing the performance of water flux

in the FO process (Gray et al. 2006).

Another phenomenon that is also intrinsic to the FO

process is the dilutive CP (dilutive ECP or ICP,

depending on the orientation of asymmetric FO

membrane). The concentration of the draw solution

at the membrane surface is lower than the bulk

concentration because of the dilution of the DS when

the water diffuses across the membrane towards the

DS from the feed solution. This dilution phenomenon

reduces the osmotic pressure of the DS on the

membrane surface and therefore, reduces the net

driving force across the membrane, significantly

lowering the water flux. The models for concentrative

ECP or ICP and dilutive ECP or ICP in the FO process

have been comprehensively discussed elsewhere

(McCutcheon and Elimelech 2007; Tan and Ng

2008). However, with the improvement in the mem-

brane design and by identifying more suitable draw

solutions through more research, it may be possible to

mitigate the CP effect to a certain extent. The ideal

membrane should have a symmetric structure or a very

thin support layer to minimise the ICP effects. Double

skinned FO membrane has also been investigated to

remove the ICP effects during the FO process,

although this has been found to slightly increase the

water resistance (Wang et al. 2010a).

Another aspect of the FO process is that the DS will

be diluted as it travels along the length of the

membrane module, which further reduces the net

osmotic pressure along the length of the membrane

module, and therefore decreases the water flux. With

low water flux, the osmotic equilibrium of DS and FS

may not be able to be reached in a single stage FO

process; therefore, they may have to process through

158 Rev Environ Sci Biotechnol (2012) 11:147–168

123

multiple FO stages. This indicates that the total

membrane area required may increase, which may

result in the initial capital cost remaining slightly

higher.

5.3 Issues with salt rejection and reverse

movement of draw solutes

In the FO process, solutes are present on both sides of

the membrane surface. Therefore, movement of sol-

utes occurs in both directions. Since none of the

synthetic membranes are ideal membranes, the solute

rejection is therefore not 100% (Phillip et al. 2010).

Solute rejection in fertiliser driven FO processes is

even more important because solute movement can

occur in both directions: forward movement of feed

salt measured in terms of salt rejection rate, and

reverse movement of draw solutes by diffusion.

Reverse solute movement is particularly important

because fertiliser DS contains nitrogen (N) and phos-

phorus (P) elements, which can be detrimental for

further management of concentrated brine. N and P

elements can cause eutrophication and algal blooms of

water bodies if they are released to the environment

indiscriminately (Hails 2002). The presence of NaCl

salt in the FDFO product water will also mean the

potential to cause sodium toxicity to the plants.

The extent of feed salt rejection and reverse

diffusion of draw solutes will depend on membrane

properties as well as the draw solute properties. The

existing CTA FO membrane has low working pH

range, low pure water permeability and low salt

rejection which are a cause of concern for FO process

(Yip et al. 2010). However, these issues are not going

to be significant once new generation of FO mem-

branes is available in the market. Depending on the

solute properties (such as diffusivity, ionic charge,

hydrated diameter, etc.), the reverse solute flux varies

considerably for each fertiliser. However, DS with

divalent ions or ions with larger hydrated size, showed

much lower reverse solute flux than the DS containing

monovalent ions or ions with smaller hydrated size

(Hancock and Cath 2009; Achilli et al. 2010; Phuntsho

et al. 2011b). The DS containing larger hydrated ionic

sizes or divalent ions (such as SO42?, PO4

3?, Ca2?)

showed comparatively lower reverse solute flux

(Achilli et al. 2010; Phuntsho et al. 2011b). Therefore,

fertilisers such as MAP, DAP, and calcium nitrate,

showed much lower reverse solute flux than the other

fertilisers containing monovalent ions. As such, the

selection of fertiliser as DS requires careful consider-

ation of the reverse solute flux of the fertiliser in order

to minimise the adverse environmental impact.

5.4 Challenges in meeting irrigation water quality

standards and their options

As long as the fertiliser is soluble and generates

osmotic pressure higher than the saline feed water, any

draw solutions can extract water from saline water

(Phuntsho et al. 2011b). The recovery rate of such FO

desalination processes would vary depending on the

water salinity, fertiliser solubility and the osmotic

potential of the DS. Although water flux and recovery

rate can be increased by increasing the DS concentra-

tion in the FO process, there is an ultimate limit to

which the osmotic process can continue occurring. In

such a case, each DS can extract water only up to the

concentration where its osmotic potential equals that

of the feed water (i.e. osmotic equilibrium), irrespec-

tive of their performance in terms of water flux or

reverse solute flux. Beyond this point, the DS cannot

be further diluted since the osmotic pressure of the

diluted DS is equivalent to that of the saline water. At

this osmotic equilibrium, depending on the feed

salinity, the fertiliser concentration may be still too

high for direct fertigation. Fertilisers are also salts and

can increase the concentration of ionic species or

enhance the conductivity to the irrigation water.

Moreover, the fertiliser nutrient concentration may

also exceed the required limit and therefore could

prove to be a problem for some of the more sensitive

crops.

Table 3 shows the assessment of the final nutrient

concentrations of the nine selected standard fertilisers

when operated with feed water of different salinities.

The osmotic pressures were calculated using OLI

Stream Analyzer software (OLI Inc.) It is clear from

the table that there is a limit to which a particular type

of DS could achieve its final concentration and this

will depend on the TDS or the osmotic pressure of the

feed water. The lower the salinity or TDS of the feed

water, the lower will be the final DS concentration of

the fertiliser DS.

For example, when ammonium nitrate is used as the

DS with seawater as the feed solution, the final

ammonium nitrate concentration after FO desalination

would be 0.802 M, which contains an actual N

Rev Environ Sci Biotechnol (2012) 11:147–168 159

123

concentration of 22.5 g/L. Likewise, ammonium sul-

phate, calcium nitrate and diammonium phosphate

(DAP) could contain 16.4, 15.1 and 14.5 g/L of N

nutrient, respectively. The lowest N concentration was

observed for monoammonium phosphate (MAP) and

ammonium chloride, with only little more than 8 g/L

with seawater feed. The P concentration also remained

high, with 19.3 and 16.0 g/L for MAP and DAP,

respectively. The potassium (K) nutrient was by far the

highest amongst the major nitrogen, phosphorus,

potassium (NPK) nutrients, with 24.4 and 27.6 g/L

for potassium chloride and potassium nitrate, respec-

tively. Although the required nutrient concentration

for fertigation would vary depending on many factors

such as types of crops to be irrigated, cropping

seasons, soil nutrient conditions, etc. (Oliver and

Barber 1966), this concentration is too high for direct

fertigation. For example, the required nutrient con-

centration varies from 50 to 200 mg/L for N,

12–60 mg/L for P, and 15–250 mg/L for K, depending

on the types of crops and growing seasons (Phocaides

2007). Therefore, the data in Table 3 with seawater as

feed, indicate that a significant amount of water is

required to further dilute the final fertiliser DS before

fertigation.

However, the assessment with brackish water

(5,000 mg/L sodium chloride solution) as the feed

solution could result in significantly lower nutrient

concentration in the final DS, as also shown in

Table 3. The nutrient concentration in the final

fertiliser DS also depends on the type of fertiliser

used as the DS and the salinity of the feed water. Based

on the results, FDFO desalination is more suitable for

brackish water.

If the nutrient concentration does not meet the

fertigation standard, the DS must be further diluted to

make the desalted water fit for fertigation. Dilution

could be easily done if the site has access to a limited

source of potable water for irrigation, such as in the

Murray-Darling Basin once the government applies

further caps on river water use. However, this is a

challenge for sites without access to any sources of

potable water for irrigation. Since maintaining the

required nutrient concentration is essential for ferti-

gation, an additional process could be integrated with

the FO unit. Although such a unit requires energy, it is

nevertheless essential for maintaining a low final

nutrient concentration and acceptability for direct

fertigation. Therefore, such unit must have low energy

consumption so that the total energy cost of the

irrigation water remains comparatively lower than the

conventional desalination processes making the desa-

lination feasible for irrigation purpose. There are

several possible options which can be used as addi-

tional process either as pre-treatment or post-treatment

system and they are all described below.

Table 3 Final concentration of fertiliser draw solution at osmotic equilibrium with feed water after desalination

Type of fertiliser DS Type of feed solution

Seawater feed or 0.60 M sodium chloride solution

feed (p = 27.4 atm)

BW feed or 0.086 M sodium chloride solution feed

(p = 3.93 atm)

M at 27.4 atm g/L of N g/L of P g/L of K M at 6.8 atm g/L of N g/L of P g/L of K

Ammonium nitrate 0.802 22.456 0.0962 2.69

Ammonium sulphate 0.585 16.380 0.0728 2.04

Ammonium chloride 0.629 8.806 0.0868 1.22

Calcium nitrate 0.539 15.092 0.0673 1.88

Sodium nitrate 0.651 9.114 0.0874 1.22

Potassium chloride 0.624 24.397 0.0866 3.39

MAP 0.623 8.722 19.297 0.0866 1.21 2.68

DAP 0.515 14.420 15.952 0.0654 1.83 2.03

Potassium nitrate 0.706 9.884 27.603 0.0884 1.24 3.46

The fertiliser concentration is also expressed in terms of the actual nutrient concentrations of NPK

MAP mono-ammonium phosphate, DAP diammonium hydrogen phosphate

160 Rev Environ Sci Biotechnol (2012) 11:147–168

123

5.4.1 Pre-treatment of feed water to achieve lower

nutrient concentration in the final FDFO

product water

One of the options for additional process is the

integration of the nanofiltration (NF) process, as a pre-

treatment unit mainly for reducing the TDS of the feed

water. Besides other uses, NF has been used as pre-

treatment for seawater RO desalination since NF has

several advantages such as low operating pressure and

energy cost, high flux, high retention of multivalent

anion salts and relatively low investment and low

operation and maintenance costs (Lu et al. 2002;

Hassan et al. 1998). NF is operated at optimum

pressure of 8–10 bars (Van der Bruggen and Vandec-

asteele 2002) which is significantly lower than the RO

process. NF could reject more than 50–80% of

monovalent ions and 60–99% of divalent ions (Bartels

2007). Brackish groundwater could contain significant

concentrations of divalent ions such as Ca2?, Mg2?,

SO42?, etc., in addition to monovalent ions such as Na?

and Cl-. Therefore, NF can considerably reduce the

TDS and the osmotic pressure of the feed water.

Moreover, the reduction in the multivalent ions would

also eliminate the scaling potential of the FO feed

water and therefore FDFO can operate at much higher

recovery rates. This in turn can achieve much lower

nutrient concentration that is acceptable for direct

fertigation in the FDFO desalination process. The

process layout for this option is presented Fig. 5.

5.4.2 Post-treatment options to achieve lower

nutrient concentrations in the final FDFO

product water

Alternatively, NF can also be used for post treatment

of the diluted DS instead of pre-treatment as shown in

Fig. 6. Recovery and recycling of draw solutes by NF

process have been evaluated by Tan and Ng (2010)

and more recently by Zhao et al. (2011). NF can

significantly remove multivalent ions from the final

diluted fertiliser DS and therefore it may be suitable

for fertilisers containing multivalent ions such as

calcium nitrate, MAP, DAP, ammonium bicarbonate,

Fig. 5 FDFO desalination

process integrated with

nanofiltration (NF) pre-

treatment process for

reducing the total dissolved

solids (TDS) of the feed

water

Rev Environ Sci Biotechnol (2012) 11:147–168 161

123

sulfate of ammonia, etc. Tan and Ng (2010) observed

that, two stages NF post treatment can recover draw

solutes (MgSO4 and Na2SO4) that meet WHO drink-

ing water quality standards and similar results were

reported by Zhao et al. (2011). Therefore, NF perme-

ate will contain significantly reduced concentration of

fertiliser nutrient which can then be either used

directly for fertigation. Since, NF post treatment could

reject high percentage of multivalent ions, only a

certain percentage of FDFO product water has to be

post-treated with NF and the rest can be diluted using

the NF permeate to achieve the required nutrient

concentration. The NF concentrate can be recycled

back as DS for FDFO desalination process to extract

more water from the feed water. The other advantage

of the post-treatment is that, NF can operate more

efficiently since the final diluted DS contains only

dissolved fertiliser and any foulants present in the feed

water could have removed during FDFO process.

However, a detail comparative study of these two

options could provide a more useful insight on the

advantages of the options.

Another potential post treatment alternative is by

using fertilisers with thermolyte properties such as

ammonium bicarbonate, ammonium carbonate and

ammonium carbamate. In such case, certain amount of

thermolyte draw solutes from the final FDFO product

water can be recovered and recycled for reuse of the

DS. The DS can be prepared containing thermolyte

solutions such as mixture of ammonium bicarbonate

or ammonium carbonate or ammonium carbamate

(McGinnis 2002; McCutcheon et al. 2005, 2006;

McGinnis and Elimelech 2007) which can be further

mixed with other essential fertilisers in specific

proportions to meet specific crop requirements. These

ammonium compounds would provide N nutrient

while other macro nutrients such as P or K can be

provided by mixing together other fertilisers. The final

diluted DS containing a mixture of thermolyte solu-

tions and other fertilisers after FDFO desalination can

then be heated up to 60�C to recover certain percent-

age of ammonium compounds in the form of ammo-

nia-carbon dioxide which can be regenerated and

recycled back as concentrated DS to further extract

water. This recovery concept was first developed by

McGinnis (2002) and later reported in many other

studies. Since the final product water still needs to

contain certain amount of ammonia N nutrients, only

certain fraction of the final diluted DS may be required

to undergo ammonia-carbon dioxide recovery and

regeneration process. The final product water after

recovering and recycling ammonia-carbon dioxide

will contain significantly lower nutrient concentration

which can be directly used for fertigation. Since only

Fig. 6 FDFO desalination

process integrated with

nanofiltration (NF) post-

treatment process. The NF

permeate containing

reduced concentration of

fertiliser nutrient can be

used directly for irrigation

while the concentrate can be

recycled back as DS to

extract more water by FDFO

desalination process

162 Rev Environ Sci Biotechnol (2012) 11:147–168

123

certain percentage of ammonium compound is

required to be removed from the diluted DS for

recovery and recycling, the diluted DS may not be

required for heating up to 60�C since ammonium

bicarbonate decomposition starts at about 35�C (Go-

kel 2004) and therefore the amount of energy required

is expected to be much lower than recovering 100% as

required for drinking water. Figure 7 shows the

concept of the FDFO desalination process with

ammonia-carbondioxide recovery and recycling

process.

5.4.3 Blending of fertilisers containing different

nutrients to provide improved nutrient

distribution

One of the other ways of achieving lower final nutrient

concentration could be by using DS containing

multiple elements or ionic species. It is evident from

Table 3 that the presence of more numbers of different

elemental species in the DS can provide adequate

osmotic pressure at lower fertiliser concentration. At

lower fertiliser concentration, the actual nutrient

concentration will also be correspondingly lower.

Therefore, blending together two or more fertilisers in

solution to provide multiple nutrient elements, or

mixed with other soluble agricultural chemicals such

as pesticides/insecticides, fungicides, etc., could

reduce the final nutrient concentration in the DS.

The presence of other ionic or non-ionic species could

increase the osmotic potential of the DS and hence

lower the concentration of essential nutrient elements

in the final DS. However, such mixtures should be

compatible with the membranes, as well as meet the

fertigation requirements. Agricultural chemicals are

often applied through an irrigation system, and as

such, terms such as chemigation, fungigation, etc. are

popular with the fertigation system.

5.4.4 Hybrid FO system to achieve lower nutrient

concentrations in FDFO product water

The other alternative is the use of impaired wastewater

effluent from the wastewater treatment plant (if the

farms have access to such a source). Wastewater

effluent can be used for dilution of fertiliser solution,

either directly if the effluent meets the irrigation water

quality standards, or after further treatment by the FO

process. Figure 8 shows the conceptual process dia-

gram of FDFO desalination using impaired water as

the source of water for further dilution. The concept

here is to use a two-stage FO process as a multiple

barrier for simultaneous wastewater treatment and

desalination of seawater through osmotic dilution

(Cath et al. 2010). The first stage FO process is to

desalinate the brackish water using fertiliser as the DS

and the diluted fertiliser DS goes to the second stage

FO unit to extract water from the impaired water

source. The second stage FO unit could offer dual

advantages of treating wastewater effluent to the

Fig. 7 FDFO desalination

process using DS containing

mixtures of ammonium

bicarbonate or ammonium

carbamate with other

essential fertilisers. Certain

percentage of ammonium

bicarbonate/ammonium

carbamate compound can be

recovered and recycled by

heating up 60�C to extract

more water. The final FDFO

product water will contain

significantly lower fertiliser

nutrient concentration

acceptable for direct

fertigation

Rev Environ Sci Biotechnol (2012) 11:147–168 163

123

required irrigation standard, while at the same time,

providing further dilution to the fertiliser solution so

that it can be applied directly for fertigation. Alterna-

tively, saline water can be used as the DS in the first

stage of the FO unit to extract water from the impaired

water. The diluted saline water can then be used as the

feed water in the second stage FO unit, with concen-

trated fertiliser as the DS. Either way, the recovery rate

could increase and achieve a low final fertiliser

concentration in the DS. In the second option, the

amount of fertiliser lost due to reverse solute flux

would be less than the first option since the fertiliser is

used as the DS only once, whereas in the first option,

the fertiliser is used as the DS twice.

Similarly, FDFO has the potential to be integrated

with the membrane bioreactor (MBR). The application

of osmotic MBR (OMBR) has been recently studied

(Achilli et al. 2009; Xiao et al. 2011). Other than the

salinity increase in the mix liquor due to salt rejection

and reverse draw solute diffusion, the performance of

OMBR was excellent with quite stable flux without

significant fouling and scaling issues (Xiao et al.

2011). Using CTA FO membrane, OMBR was found

to reject more than 99% of organic carbon and 98% of

ammonium-nitrogen, suggesting a better compatibil-

ity than conventional membrane bioreactors (Achilli

et al. 2009). Because the water quality for irrigation is

much lower than the drinking water, the potential

application for OMBR with FDFO is quite significant.

There are two possible combinations for the Osmotic

MBR or OMBR with FDFO as in the case above with

wastewater effluent. As a pre-treatment, the saline

water can be used as the DS to extract from the

wastewater by OMBR in the process diluting the

saline water. This diluted saline water can be further

used as feed water for FDFO process in the process

achieving much lower nutrient concentration in the

final diluted fertiliser DS. Alternatively, OMBR can

be used as post treatment in which case, OMBR

process can use the diluted fertiliser DS after FDFO

desalination as the DS to extract water from the

wastewater.

5.4.5 Other issues and challenges

Fouling is one of the significant issues in any

membrane process and therefore FO performance

can also be influenced by fouling problems. Most

available literature on FO fouling shows that, the rate

of water flux decline due to membrane fouling is

significant and quite similar to RO process. Moreover,

in FO process, the reverse diffusion of draw solutes is

most likely to enhance membrane fouling depending

on the types of draw solutes used and the complexity it

forms with the feed foulant (Lay et al. 2010; Lee et al.

2010). However, in the absence of high pressure

unlike RO process, membrane fouling in FO process

has been mostly characterised by reversible fouling

Fig. 8 FDFO desalination

process using 2 stage FO

process with additional

dilution water from a

secondary wastewater

effluent and limited river

water source

164 Rev Environ Sci Biotechnol (2012) 11:147–168

123

(Lee et al. 2010; Mi and Elimelech 2010). Fouling

studies suggest that, fouling in FO could be controlled

through careful design and optimisation of operating

parameters (Lee et al. 2010). However, there is still a

lack of long term data on FO fouling potential and

mechanism in FO process and therefore, quite a

significant research efforts are required in this area.

Biofouling could be another significant issue which

needs consideration in FO process and currently there

is no literature available on FO biofouling. Membrane

processes are constantly in contact with the water

medium and therefore micro-organisms growth and

biofilm formation could be inevitable as they are not

influenced by the hydrodynamic conditions or the

operating pressure. In FDFO desalination process in

particular, fertiliser DS contains nutrients containing

N and P which could likely enhance biofouling as

nutrients are considered precursors to biofouling

(Melo and Bott 1997). Unlike organic fouling which

is noticed within a short duration of membrane

operation, biofouling usually occurs after a long-term

membrane operation. Biofouling is caused by the

microbial action however, still very little is known

about the fundamental nature of the biofouling process

(Ivnitsky et al. 2010). Organic fouling in NF or RO

processes is usually prevented through pre-treatment

of feed water however, biofouling occurs even with

very low organic concentration (Flemming 2002). FO

process is expected to be operated at high recovery

rates and therefore scaling could also become an issue

to FO desalination process.

6 Concluding remarks

Membrane-based desalination technology can play a

vital role in solving the water scarcity issues. The

current generation of membrane technologies, partic-

ularly reverse osmosis (RO) desalination, have signif-

icantly improved however, RO desalination still

remains energy intensive in nature and any efforts

towards improving the energy efficiency further

increases the total cost of the water. Due to energy

issues, desalination for large-scale purposes (such as

irrigation) using the current technologies is still not

seen as a viable option. Therefore, new technologies

that address energy issue could extend the scope of

desalination and to large-scale water use applications

such as irrigation, where water consumption accounts

up to 80% of the total fresh water consumption in the

world.

Forward osmosis (FO) process is a promising and

an emerging low energy desalination technology. FO

desalination for potable water however still suffers

from lack of an ideal draw solutes that can be easily

recovered and regenerated without significant energy

consumption. Although, few promising draw solutes

such as thermolyte solutions of ammonia-carbondi-

oxide and magnetic nanoparticles have been investi-

gated recently, their performances are yet to be proven

on commercial scale and more research works are

needed. However, the novelty offered by FO process

can be advantageously used in desalination for non-

potable purpose such as irrigation. In such case, the

diluted draw solution (DS) can be used directly

without the need for the recovery and regeneration

of draw solutes from the diluted DS if it meets the

irrigation water quality standards. When fertilisers are

used as the draw solutes in the fertiliser drawn FO

(FDFO) desalination, the diluted fertiliser solution

after desalination can be directly applied for fertiga-

tion, thereby avoiding the need for separation and

recovery of the draw solution. The energy required for

fertiliser driven FDFO desalination is expected to be

comparatively lower than the current state of the art

RO desalination technologies. Since FDFO is a low

energy process, this particular technology can also be

easily powered by the renewable energy, such as solar

and wind energy, which exits abundant in many arid

countries including Australia.

FDFO technology is suitable for locations in the

inland and remote areas where agricultural farms are

usually located such as in Australia where drought and

water scarcity are frequently experienced and where

the sources of saline water are abundant. Most soluble

fertilisers can be used as draw solutions, and therefore

extract water from saline water with very high

recoveries. Since fertilisers are extensively used for

agricultural production, FDFO desalination does not

create additional environmental issues related to

fertiliser usage. In fact, the FDFO desalination could

add more value to irrigation water; therefore, provid-

ing more opportunities for improving the efficiencies

of water and fertiliser uses. Such technology could

provide irrigation water through alternative sources

such as saline groundwater and wastewater

effluent. Significantly, it could potentially solve

the over-allocation issues of the Murray-Darling Basin

Rev Environ Sci Biotechnol (2012) 11:147–168 165

123

by providing nutrient-rich water from saline

groundwater.

However, FO technology still requires further

research and progress if it is to compete with existing

technologies such as RO desalination technology. But

significant progress has been already made in mem-

brane synthesis recently, especially with thin film

composites and also the promising performances

offered by nanotechnology. These new generations

of FO membranes are expected to solve most of the

issues that have plagued the performance of the FO

process using the current commercial FO membranes,

and therefore the future prospects for FO desalination

are quite promising. Further research is also underway

to identify more suitable draw solutions that can be

applied efficiently for potable water desalination.

However, one of the contentious issues with the

FDFO is the challenge in meeting the irrigation water

quality standards in terms of nutrient concentrations

limiting the direct use of FDFO product water for

fertigation. Estimations indicate that, the final nutrient

concentration would exceed the required limit when

the feed water has high salinity content such as

seawater indicating that FDFO could be more suitable

for low salinity water such as brackish water. Fortu-

nately, several options are available that could be

integrated with the FDFO desalination process in order

to keep the final nutrient concentration low. This

includes options such as integrating nanofiltration

process as a pre-treatment unit to reduce the feed water

salinity since NF can reject up to 50% of monovalent

ions and almost complete multivalent ions while

achieving high water flux at low hydraulic pressure.

NF can also be used as a post-treatment unit to

concentrate and recycle certain percentage of the

diluted draw solution while the permeate containing

significantly lower nutrient concentration used direct

for fertigation and the concentrate can be recycled as

DS to extract more water. Another potential option is

to use FDFO process with wastewater effluent treat-

ment through multiple FO stages, while achieving

effluent treatment and nutrient dilution simulta-

neously. FDFO can also be similarly combined with

osmotic MBR to provide additional water for dilution.

Another promising option is to use a mixture of

thermolyte fertiliser solutions mixed with other fertil-

isers as DS in FDFO process. Certain percentage of the

final diluted DS can be heated to temperature not

higher than 60�C to recover and recycle ammonia-

carbondioxide solutions ultimately reducing the final

N concentration in the product water. Lower nutrient

concentration in the final DS could also be achieved by

using a DS containing multiple elemental or ionic

species through methods such as blending of fertilisers

with two or more fertilisers, and mixing with other

agrochemicals such as pesticides, fungicides, etc. that

could enhance the osmotic pressure of the DS and keep

the final nutrient concentration low.

Although, FDFO offers a very promising concept of

making water available from saline sources, more

researches and long term performances data are

required to make this technology viable and attractive.

In the meantime, RO desalination technology will

continue to dominate the desalination process for

some time to some especially for potable water as this

technology is already well established in the market.

To make FO process competitive with RO technology,

more research needs to focus particularly on minimis-

ing concentration polarisation effects that are the main

limitations of FO process.

Acknowledgments This study was funded by the National

Centre for Excellence in Desalination Australia (NCEDA).

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