forward osmosis fundamentals & hybrids

FO fundamentals & FO-Hybrids

Christos Charisiadis

1. FO fundamentals p.1

1.1 Concentration Polarization in FO membranes p.4

1.1 External concentration polarization (ECP) p.4

1.1.2 Internal concentration polarization (ICP) p.5

1.1.3 Concentrative ICP p.5

1.1.4 Dilutive ICP p.6

1.2 Advances in draw solutions for FO desalination p.6

1.3 Challenges in FO desalination p.8

1.4 FO short notes p.9

1.5 FO hybrid systems p.11

2. Desalination p.12

2.1 Direct desalination p.14

2.2 Indirect desalination p.16

2.3 FO membrane fouling p.17

2.3.1 Micropollutants p.17

2.3.2 Organic fouling in FO membranes p.17

3. Life cycle cost of a hybrid FO & LPRO system for SW desalination and WW recovery

p.18

3.1 Hybrid system fundamentals p.19

3.2 Technologies analyzed p.20

4. Rejection of Trace Organic Contaminants p.23

5. Energy demand in desalination and water treatment processes

p.24

5.1 Major challenges for commercialization p.26

6. Wastewater treatment to Biogas Production p.28

6.1 Biogas production case study p.28

6.2 Conclusions p.30

7. Other FO-RO hybrid configurations p.30

7.1 Pressure retarded osmosis: special FO application for energy recovery in water

industry p.32

7.1.1 Large-scale applications of PRO p.33

7.2 The Concept of Pressure Assisted Osmosis (PAO) p.34

8. Optimization on a new hybrid Forward osmosis-Electrodialysis-Reverse osmosis

seawater desalination process p.36

8.1 Process description p.37

8.2 Results and discussion; Energy consumption of hybrid FO-ED-RO process

p.39

8.2.1 Effect of conductivity p.39

8.2.2 FO-ED-RO energy consumption and effect of membrane resistance

p.39

8.2.3 Effect of concentration and conductivity p.40

8.3 Optimization of the FO-ED-RO process p.40

1st Case Study; FO system for wastewater recovery and seawater desalination

p.43

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FO & RO fundamentals/ Christos Charisiadis

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1. FO fundamentals

During the FO process, the membrane rejects the salts and consequently, the feed

solution is concentrated and the draw solution is diluted with time. Pure water has

then to be recovered from the diluted draw solution by using an energy-efficient

separation technique. Unlike RO, FO utilizes the natural osmotic pressure difference

across the membrane to transport the water molecules and does not require

application of hydraulic pressure to overcome the osmotic pressure of the feed.

In short, the process of FO desalination can be divided into two steps. In the first step,

water molecules are permeated from the feed to the draw solution across the

semipermeable membrane. In the second step of FO desalination, the draw solution

is subsequently recovered by separating pure water from the diluted draw solution

obtained in the first step of the process. A simplified schematic of this two-stage FO

desalination process is shown in Fig. 3.

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In general, any dense, non-porous, and semipermeable material can be employed as

a membrane in the FO process. However, the membrane should exhibit high water

flux (based on lower structural parameter (S) values), low reverse solute flux, low

fouling, high salt rejection, low internal concentration polarization (ICP is one of the

main challenges in FO desalination and will be discussed at a later stage), and high

chemical stability. On the other hand, research on the draw solutions takes into

account that the draw solute used must be capable of generating high osmotic

pressures compared to the feed solution. In addition, the draw solute must be non-

toxic, chemically inert to the membrane, highly soluble in water, and easily

regenerated and separated from the pure product water.

It is important to note that the majority of the FO membranes are asymmetric in

nature and are synthesized with an active dense layer (for salt rejection) supported on

a porous support layer. This results in two distinct membrane orientations. In the first

orientation, the active membrane layer faces the draw solution and is typically the

case in pressure retarded osmosis (PRO). In the second membrane orientation, the

active layer of the membrane is towards the feed solution (such as seawater) while

the porous support layer is towards the draw solution. This is the case in FO

desalination. Generally, the osmotic pressure difference across the active layer of the

FO membrane is significantly less compared to the bulk osmotic pressure difference

and therefore, the actual water flux is significantly lower than the theoretical flux. This

lower water flux in FO desalination is attributed to the inevitable and complicated

membrane-related transport phenomenon known as concentration polarization (CP).

The concentration polarization effects are in fact one of the most challenging

problems in practical realization of the FO desalination process. The overall effect of

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CP is to increase the feed concentration and its osmotic pressure and at the same time

decrease the concentration and the osmotic pressure of the draw solution at the

active layer of the membrane.

1.1 Concentration Polarization in FO membranes

1.1 External concentration polarization (ECP)

ECP in osmotic-driven processes occurs at the membrane active layer and may be

concentrative ECP or dilutive ECP depending on the membrane orientation.

Concentrative ECP occurs when the active layer of the membrane faces the feed

solution (as in case of FO desalination, Fig. 5).

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On the other hand, dilutive ECP occurs when the membrane active layer faces the

draw solution which is the case during PRO (Fig. 6). The overall effect of concentrative

ECP in FO desalination is to increase the concentration of the feed at the active layer

of the membrane due to buildup of solutes on the membrane active layer resulting

from the flow of water from the feed solution. The increase in feed concentration

increases the osmotic pressure of feed at the active layer of the membrane and hence,

reduces the water flux due to decrease in the net osmotic driving force. However, the

effect of ECP on water flux in FO desalination can be relaxed by increasing flow velocity

and turbulence at the membrane surface or by optimizing the water permeation rate.

In addition, FO desalination operates under no or low hydraulic pressure which does

not allow considerable solute buildup on the membrane. The effect of ECP on the

water flux is, therefore, minimum and not a major concern.

1.1.2 Internal concentration polarization (ICP)

However, research has shown that the major factor contributing to the decline in

water permeation rate in FO desalination is the ICP, a phenomenon that takes place

within the asymmetric FO membrane. Like ECP, ICP may be concentrative or dilutive

depending on the membrane orientation.

1.1.3 Concentrative ICP

Concentrative ICP is the result of water and solute permeation through the porous

support layer causing accumulation of solute in the porous support and formation of

a polarized layer on the active layer of the membrane. This happens in cases where

the active layer of the asymmetric membrane faces the draw solution (as in PRO, Fig.

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6). As depicted in Fig. 6, the concentrative ICP is similar to concentrative ECP with the

only difference that concentrative ICP occurs within the porous support of the

membrane and therefore, its effect cannot be reduced by simple manipulation of flow

and turbulence. The overall effect is to reduce the effective osmotic pressure driving

force available for water permeation.

1.1.4 Dilutive ICP

Dilutive ICP is of more importance in the context of this review paper since only

dilutive ICP needs to be accounted for FO desalination processes. The active layer

faces the feed solution in FO desalination and the transport of water from the feed to

the draw solution causes dilution of the draw solute within the porous support layer

of the membrane as shown in Fig. 5. This is referred to as dilutive ICP and represents

one of the most challenging concerns in FO desalination research. In fact, ICP can

result in up to 80% decline in water permeation rate compared to the original flux.

This is also evident if the bulk and effective osmotic pressure differences are compared

in both Figs. 5 and 6.

1.2 Advances in draw solutions for FO desalination

The draw solution, based on the osmotic pressure properties, is the source of the

driving force for the FO desalination process and represents another extensive area of

research besides membrane development. The ideal draw solute must be highly

soluble in water and capable of generating high osmotic pressures compared to the

feed solution. In addition, the draw solute must exhibit low ICP, nontoxic and

chemically inert to the membrane. However, the recovery of pure water from the

diluted draw solution is the most important consideration in successful selection of a

suitable draw solute. Since interest in FO desalination is driven by its energy efficient

nature, the draw solution must require no or minimum energy for regeneration and

for separating pure water from the diluted draw solution.

The draw solution research can be categorized by the methods used to recover the

draw solutions.

1. No draw solute recovery

Glucose draw solution and cellulose acetate membrane to desalinate seawater.

Transport of water molecules from seawater to the glucose draw solution simply

resulted in potable glucose drink. Therefore, recovery of product water was not

required. However, the proposed method can only be used for emergency water

supply.

2. Thermal draw solute recovery

The research interest FO desalination draw solutions grew rapidly after the use of

ammonium bicarbonate as the draw solution by Elimelech. Being highly soluble and

capable of generating high osmotic pressures, the ammonium bicarbonate draw

solute resulted in high water flux. Pure product water was recovered by moderate

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heating up to 60 °C which resulted in decomposition of ammonium bicarbonate to

form ammonia and carbon dioxide gases. The gases escape the draw solution to give

pure water. However, the energy extensive draw solute recovery and reconstitution

steps of this process remain a big concern for the desalination applications. The

schematic diagram is shown in Fig. 12.

3. Applied magnetic field draw solute recovery

Magnetic nanoparticles to dewater RO concentrate. A magnetic separator was used

to product water from the draw solution. However, the magnetic nanoparticles were

found to be unsuccessful in generating high osmotic pressure due to their large

molecular weight and low solubility.

4. Precipitation draw solute recovery

Precipitation reaction with calcium hydroxide to recover pure water from diluted

aluminum sulfate draw solution. The process does not require energy but involves

toxic products of the precipitation reaction. Two draw solutes, copper sulfate and

magnesium sulfate, were used to desalinate brackish and seawater. Schematic

diagram for the FO desalination method utilizing copper sulfate draw solution is

shown in Fig. 15.

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1.3 Challenges in FO desalination

In summary, the challenges faced in commercial and successful realization of FO

desalination include concentration polarization, membrane fouling, reverse solute

flux, lack of suitable commercial membranes, and unavailability of ideal draw

solutions.

The biggest challenge for the commercialization of the FO process comes from the

economic feasibility. The economic feasibility of a process can be related directly to

its scalability and commercialization.

An economically feasible FO process should satisfy all such factors in order to answer

commonly known issues and drawbacks in the FO process. These issues and

drawbacks can be summarized as:

• Concentration polarization

• Membrane Fouling

• Reverse solute flux

• Suitable commercial membranes

• Availability of ideal draw solutions

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1.4 FO short notes

- Forward osmosis requires less feed pre-treatment than reverse osmosis, due to the

mild process conditions membrane fouling is less pronounced. This, along with the

fact that the process does not require pressure rated vessels to be operated means

that the process can potentially be a feasible alternative to RO desalination when low

equipment weight and volume are among the main requirements.

- In FO a high salinity, i.e. high osmotic pressure ‘draw’ is used to create a chemical

potential difference across a semi-permeable membrane. The membrane is also in

contact with a low osmotic pressure ‘feed’ solution. Consequently water transport

occurs from the low osmotic pressure ‘feed’, to the high osmotic pressure ‘draw’. Both

solutions therefore are either being constantly diluted or concentrated, until the

osmotic pressures of both become equal. To be continuously operated FO therefore

needs constant supply of fresh draw agent, which of course means that in the most

cases FO has to be implemented as a two-stage process.

- As in any other membrane separation significant mass transfer resistances occur in

FO due to concentration polarization (CP) effects. However here, the CP effects occur

not only within the boundary layers on and around the membrane surface, due to

non-ideal hydrodynamics, usually referred to as external concentration polarization

(ECP), but also within the membrane itself. The latter concentration polarization effect

is unique for conventional forward osmosis membranes and is known as internal

concentration polarization or ICP. These effects, being usually the main mass transfer

resistance in membrane separations of course have to be minimized

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- Since ECP is hydrodynamic related it is usually mitigated by suitable membrane

module design and use of spacers. Improved hydrodynamics influence ICP as well, but

the phenomenon is mainly related to the membrane structure (porosity, thickness,

pore tortuosity) and membrane orientation, i.e. whether the porous support side of

an asymmetric membrane is contacting the feed (AL-FS orientation, or FO mode) or

the draw (AL-DS orientation, or PRO mode).

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1.5 FO hybrid systems

FO hybrid membrane systems have been shown to have advantages over traditional

membrane process like high pressure reverse osmosis and nanofiltration for

desalination and wastewater treatment:

(i) chemical storage and feed systems may be reduced for capital, operational and

maintenance cost,

(ii) water quality is improved,

(iii) reduced process piping costs,

(iv) more flexible treatment units, and

(v) higher overall sustainability of the desalination and wastewater treatment process.

Nevertheless, major challenges make FO systems not yet a commercially viable

technology, the most critical being development of a high flux membrane, capable of

maintaining an elevated salt rejection and a reduced internal concentration

polarization effect, and the availability of appropriate draw solutions (cost effective

and non-toxic), which can be recirculated via an efficient recovery process.

There are mainly two clusters of applications concerning FO in the water production

and water treatment industry (Zhao et al. 2012b) (Figure 7.2): (i) desalination and (ii)

water reuse.

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2. Desalination

As forward osmosis only produces diluted draw and concentrated feed solution the

process cannot be used for water desalination on its own, but only when paired with

a secondary process for draw recovery and clean water extraction. FO could however

assist reverse osmosis desalination - the possibilities here are several.

One of the main advantages of FO is the limited amount of external energy required

to extract water from the feed solution, using only a very low amount of energy to

recirculate the draw solution on one side of the membrane, while the feed solution is

passively in contact with the other side of the membrane.

FO could be used to dilute input streams to reverse osmosis desalination plants. As a

result the operating pressure and energy consumed in RO would be lower.

Additionally beneficial is the fact that impaired or waste water, which cannot be

normally mixed with seawater streams (for example due to contaminants, etc), could

also be subjected RO desalination or purification when FO is used together with RO

For FO membranes, water flux decline due to fouling is lower when compared to RO

systems, especially when wastewater with high fouling propensity is used as feed

solution, because the FO process itself does not induce suspended solids and other

organic contaminants into the membrane, reducing as well the need for an extensive

pretreatment.

This application of forward osmosis has been experimentally proven to result in

significant energy savings. It is clear that RO is really approaching the limit. However,

energy costs related to the pre- and post-treatment processes are also an important

aspect of the overall desalination expenses. For example, energy consumption of

some pre-treatment options is higher than 1 kWhm-3. This of course drives the search

for either higher water recovery in the RO, or lower energy demand of the whole

treatment scheme, or a combination of both. The current cost of seawater

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desalination is evaluated in average around 0.76 US $m-3, but typically falls within a

wide range of 0.5–2 US $m-3, depending mainly on local energy cost. As such

desalination remains quite costly, limiting its broader usage. Operational costs

(OPEX—include energy and all other costs associated to maintenance, labour and the

use of chemicals) account for two third of the total desalination costs for full-scale

plants, while the last third of the costs is related to capital cost (CAPEX). In the OPEX,

energy accounts for about half of the cost.

In a report there was an energy consumption of 1.5 kW.h.m-3 for FO-RO, compared to

2.5 - 4 kW.h.m-3 for RO only. Forward osmosis could be used in the same way to dilute

brine exiting RO desalination plants, and in this case wastewater could be employed

usefully. Such a utilization of FO will also reduce the environmental impact of RO, as

diluted brines are considerably less harmful to marine life. Finally, FO could be used

for osmotic cleaning of fouled RO membranes, thus potentially replacing some of the

traditionally used chemical cleaning and membrane backflush. The real benefits of

such a process however are still being discussed.

In depth analysis of RO and combined FO-RO desalination processes reveal that, at

least in terms of power consumption any real advantages are unlikely. It was reported

that the power consumption of FO process is only 2 to 4% of the overall power

consumption of an FO-RO desalination, and that the overall power consumption of a

FO-RO process is actually higher than RO alone.

FO-RO could be competitive when applied for desalination of seawater with high

salinity and therefore selection of draw solution and optimal process conditions is

crucial. The FO process is influenced not only by the osmotic pressure of the draw

solution, but also by the type of draw used. This influence becomes less significant at

higher osmotic pressures.

Research has identified the potential for hybrid forward osmosis/low-pressure reverse

osmosis (FO/LPRO) systems for several applications, including seawater desalination,

and to reduce the cost and fouling propensity of producing fresh water from impaired-

quality water sources, compared to conventional high pressure RO systems.

This hybrid desalination system using forward osmosis, where the feed water is a

primary or a secondary wastewater effluent, and the draw solution is seawater, with

the purpose of recovering fresh water from impaired quality sources with the use of

minimum hydraulic pressure. This hybrid system has two clear advantages:

(i) the diluted seawater resulting from the FO dilution process is further treated in a

LPRO unit to produce fresh water, using less energy than conventional high pressure

SWRO systems;

(ii) the concentrated wastewater effluent produced by FO enables low-cost

processing.

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The results show that FO recovers water from wastewater, rejects nutrients and

micropollutants, and outperforms traditional SWRO systems in terms of fouling

resistance and control, having a high flux recovery when applying physical cleaning

methods.

2.1 Direct desalination

The direct FO desalination concept is similar to other conventional membrane based

desalination processes (e.g. reverse osmosis, nanofiltration) in which fresh water is

directly extracted from a saline water (seawater or brackish water). Direct FO

desalination uses saline water as the feed solution (FS) and an osmotic reagent such

as a non-volatile salt like NaCl, or a volatile salt such as ammonium bicarbonate,

among others, as the draw solution (DS). In this process, an additional step, a draw

solution recovery process, is needed to separate the DS from the solution in the

diluted DS to recover fresh water

One of the most studied processes for direct desalination involves the use of

ammonium bicarbonate (NH4HCO3) as a draw solution, and a thermal process to

recover fresh water and regenerate the osmotic agent.

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Modern Water has deployed FO as a platform technology to produce desalinated

water via direct FO desalination. It is the only company in the world with a commercial

FO plant for direct seawater desalination (located in Al Najdah, Oman). Construction

of the plant was completed in September 2012 and it is currently under operation.

The process has shown to deliver significant operating and capital expenditure

savings, reduced chemical consumption, robust fouling resistant membranes and a

lower carbon footprint than competing technologies such as conventional high

pressure RO membrane systems (Modern Water 2013). These benefits are mainly

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associated to the reduction in RO membrane fouling due to the use of FO as a

pretreatment step.

2.2 Indirect desalination

Indirect desalination uses a high salinity water as the draw solution and an impaired-

quality water source such as wastewater effluent or urban stormwater runoff is used

as feed solution. Seawater and brackish water are potential DS for indirect

desalination. In addition to the free-of-charge draw solution (seawater/brackish

water), the attractiveness of this process is to extract clean water from the feed using

free osmotic energy, leading to partially desalinated water (diluted DS) which can be

further desalinated by a subsequent low-pressure reverse osmosis (LPRO) step as part

of an FO-LPRO hybrid process, and thus reduce the cost of the entire desalination

process.

Indirect desalination experiments showed the ability of FO membranes to reject

nutrients from wastewater, particularly chemical oxygen demand (COD) and

phosphate. Rejection of nitrogen compounds was moderate. Additionally, in a 2013

study, partial desalination of seawater with the use of a submerged membrane

module which makes it possible to adapt the process to a primary clarifier tank. This

work also showed that FO membranes have high rejection of heavy metals present in

the wastewater (˃98%). An analysis of fractional organic carbon composition in the

fouling layer on the FO membrane showed it is mainly formed by biopolymers and

protein-like substances.

FO has been tested for dewatering of nutrient-rich anaerobic digester centrate. A RO

process could be used to recover the fresh water from the clean and diluted DS (i.e.

NaCl solution) while organic compounds were rejected in the FO step. The FO process

was also used to dewater activated sludge. EDTA sodium salt was tested as a possible

DS for dewatering of high nutrient sludge. The nutrients in the sludge were

successfully removed by the FO process. The macromolecular DS (i.e. EDTA based salt)

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can be post-treated by a NF process for fresh water recovery. Alternatively, RO brine

was used as the DS in Zhu’s research. A good thickening efficiency of sludge was

reached. In addition to reduction of sludge volume for lower cost of further

transportation and handling, the concentrated RO brine was also osmotically diluted

and could be safely disposed to limit the negative impact to the environment which is

a problem in the conventional RO process.

2.3 FO membrane fouling

In FO membrane filtration, water flux decline due to fouling is less severe than in RO,

because the FO process itself does not induce suspended solids and other organic

contaminants into the membrane. Both reversible and irreversible membrane fouling

were found to be negligible when using a FO membrane submerged in secondary

wastewater effluent. In a different study with a FO/RO hybrid system, using impaired

water as feed solution, results show the low fouling propensity of the FO process, and

the ability to treat large volumes of water with almost no need for physical or chemical

cleaning. One of the few studies on biofouling in FO membrane filtration suggests that

the effect of the biofilm layer is less severe than for RO processes in the same

hydrodynamic cross-flow channel conditions

2.3.1 Micropollutants

In a study, a coupled forward osmosis – low pressure reverse osmosis (FO-LPRO)

system. When considering only FO with a clean membrane, the rejection of the

hydrophilic neutral compounds was between 48.6% and 84.7%, for the hydrophobic

neutrals the rejection ranged from 40.0% to 87.5%, and for the ionic compounds the

rejections were between 92.9% and 96.5%. With a fouled membrane, the rejections

were between 44.6% and 95.2%, 48.7%–91.5% and 96.9%–98.6%, respectively. These

results suggest that, except for the hydrophilic neutral compounds, the rejection of

the micropollutants is increased by the presence of a fouling layer, possibly due to the

higher hydrophilicity of the FO fouled membrane compared to the clean one, the

increased adsorption capacity of hydrophilic compounds and reduced mass transport

capacity, membrane swelling, and the higher negative charge of the membrane

surface, related to the foulants composition, mainly NOM acids (carboxylic radicals)

and polysaccharides or polysaccharide-like substances. However, when coupled with

RO, the rejections in both cases increased above 96%. The coupled FO-LPRO system

was an effective double barrier against the selected micropollutants.

2.3.2 Organic fouling in FO membranes

Natural organic matter (NOM) showed high reversibility, up to 90% when air scouring

for 15 minutes is used as a cleaning technique. Chemical cleaning with a mixture of

Alconox, an industrial detergent containing phosphates, and sodium EDTA showed to

increase the reversibility (93.6%). In a study, the irreversible fouling was found to be

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8.2% after chemical cleaning. On the support layer of the membrane, TEP formed

clusters, containing a significant amount of particles, reducing the flux of the FO

membrane. Chemical cleaning with 1% NaOCl for 10 minutes proved to compromise

the integrity of the FO membrane, as proved with flux and conductivity

measurements; 94.5% of flux was recovered with Alconox, showing that the

chemically irreversible fouling for the FO membrane is on the order of 5.5%, which

might be associated with the adsorption of biopolymers on the AL and some TEP

residuals on the SL. Physical cleaning (air scouring) of the AL proved to be the most

effective way to control fouling

3. Life cycle cost of a hybrid FO & LPRO system for SW desalination and WW recovery

The economic advantage of FO in comparison to conventional processes for seawater

desalination and municipal wastewater treatment has not been clearly addressed.

This chapter presents a detailed economic analysis on capital and operational

expenses (CAPEX and OPEX) for: i) a hybrid forward osmosis e low-pressure reverse

osmosis (FO-LPRO) process, ii) a conventional seawater reverse osmosis (SWRO)

desalination process, and iii) a membrane bioreactor e reverse osmosis e advanced

oxidation process (MBR-RO-AOP) for wastewater treatment and reuse. The most

important variables affecting economic feasibility are obtained through a sensitivity

analysis of a hybrid FO-LPRO system. The main parameters taken into account for the

life cycle costs are the water quality characteristics (similar feed water and similar

water produced), production capacity of 100,000 m3.d-1 of potable water, energy

consumption, materials, maintenance, operation, RO and FO module costs, and

chemicals. Compared to SWRO, the FO-LPRO systems have a 21% higher CAPEX and a

56% lower OPEX due to savings in energy consumption and fouling control. In terms

of the total water cost per cubic meter of water produced, the hybrid FO-LPRO

desalination system has a 16% cost reduction compared to the benchmark for

desalination, mainly SWRO. Compared to the MBR-RO-AOP, the FO-LPRO systems

have a 7% lower CAPEX and 9% higher OPEX, resulting in no significant cost reduction

per m3 produced by FO-LPRO. Hybrid FO-LPRO membrane systems are shown to have

an economic advantage compared to current available technology for desalination,

and comparable costs with a wastewater treatment and recovery system. Based on

development on FO membrane modules, packing density, and water permeability, the

total water cost could be further reduced.

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3.1 Hybrid system fundamentals

The energy consumption for desalination using conventional seawater reverse

osmosis (SWRO) systems lies between 2.5 and 4 kWh.m-3 depending on many

parameters (i.e. intake type, pretreatment, seawater salinity, etc.). Typical costs of

water desalination by SWRO is in the range of 0.5-1 USD.m-3, which has been achieved

by advances in energy recovery devices and membranes with improved performance;

however, a decrease in costs due to technological developments is not foreseen as

equipment and energy costs will increase. At the same time, brine discharge

regulations are getting more stringent, raising the costs for new projects.

Water production costs from wastewater recovery and reuse typically lie in the range

between 0.40 and 1.26 USD.m-3, depending on which level the treatment is initiated

(i.e. primary or secondary wastewater), and the treatment level required for its reuse

(i.e. direct/indirect potable or non-potable reuse, industrial water, irrigation).

A hybrid system uses wastewater on one side of the FO membrane and seawater on

the other side of the membrane, thus recovering water from the wastewater stream.

By eliminating a draw solution and energy intensive water recovery from the draw

solution, osmotic dilution becomes a low energy FO process. This FO process achieves

two objectives: i) volume-reduction treatment of wastewater, and ii) reduction of

osmotic pressure of seawater prior to RO desalination. Benefits of reducing the

volume of wastewater are reduced energy consumption for treatment, lower volume

transported, lower chemical use, and the possibility of harvesting energy (e.g. biogas)

and nutrients (e.g. phosphates) from the concentrated wastewater more efficiently.

The big opportunity relies in the use of a low-value wastewater effluent, i.e. primary

effluent, which at the same time is high in organics for further concentration. In

contrast, secondary effluent is a higher-value water with lower organics for biogas

production.

Osmotic dilution can also be adapted in a conventional seawater desalination facility

as a forward osmosis e low pressure reverse osmosis unit (FO-LPRO), offering the

potential for energy and cost savings in a SWRO facility by lowering the operating

hydraulic pressure, enabling the use of brackish water RO membranes (BWRO) instead

of SWRO membranes, and increasing the water recovery ratio of the whole system

(higher flux). Environmental impacts may be diminished by reducing electricity

requirements, and also by discharging brines with lower salinity and lower volumes to

the aquatic ecosystem. Moreover, reducing the volume of the impaired water offers

additional benefits, previously described.

The driving factor for considering implementing a FO-LPRO system versus a reverse

osmosis (RO) system (for desalination purposes) or versus an

ultrafiltration/nanofiltration (UF/NF) - advance oxidation process (AOP) (for

secondary wastewater recovery) or a membrane bioreactor-reverse osmosis-

advanced oxidation process (MBR-RO-AOP) hybrid system (for primary wastewater

recovery) should be the energy savings compared to the capital expenses. FO has been

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depicted as a near horizon low energy desalination technology considering that the

recovery rate of actual desalination/treatment processes is changed.

Energy savings associated with the integrated FO-LPRO system compared to a

conventional SWRO system are mainly related to the reduction in the osmotic

pressure of the partially desalinated water and the hydraulic operational pressure

required by the recovery process (i.e. low pressure RO system) to produce fresh water.

Lower energy consumption is needed as the dilution rate increases; however, this

requires a higher capital cost for the membrane area. For a hybrid FO-LPRO seawater

desalination system, the specific energy consumption (SEC) associated to the FOLPRO

process, after an energy consumption analysis based on a conservative estimate,

ranged between 1.3 and 1.5 kWh.m-3 using a secondary wastewater effluent as feed

and seawater as draw solution (total production capacity of 2,400 m3.d-1), which is

lower than the energy consumption of conventional SWRO.

It is important to compare similar processes in terms of influent and effluent water

quality. A previous study compared RO for both seawater desalination and tertiary

wastewater treatment, which cannot produce water with the same quality. The study

reports that capital costs for a plant producing water from seawater are about twice

the costs of a plant reusing secondary effluent (not considering the costs of the

primary/secondary wastewater treatment facility). Similarly, the operation and

maintenance (O&M) costs for producing RO water from seawater are 2 times higher

than the cost of reusing secondary sewage. The total life cycle costs for producing RO

water from secondary effluent and seawater are 0.28 and 0.62 USD m-3, respectively.

The final cost of water can differ by a factor of 2 due to inaccuracies (i.e. not

considering the cost of treating raw wastewater effluent) in the calculation method.

Several studies have shown that an MBR-RO-AOP system is a multi-barrier approach

that could be/has been implemented in water reuse projects

A critical aspect in the cost of FO-RO hybrid systems was identified as the FO

membrane. If FO membrane modules can be commercially produced at a reasonable

price (i.e. comparable to production costs of RO modules with the same packing

density), it is anticipated that use of FO-RO may be viable in the near future.

3.2 Technologies analyzed

a) Hybrid forward osmosis e low pressure reverse osmosis (FOLPRO) process for

seawater desalination and wastewater recovery. The feed water for the FO process is

considered to be a primary municipal wastewater effluent with an osmotic potential

of approximately 0.50 bar, both based on the osmotic pressure generated by small

organic molecules and ions present in a wastewater sample. The draw solution is

considered to be seawater with total dissolved solids (TDS) of 40,625 mg.L-1 and an

osmotic potential of 27.6 bars. The diluted seawater (50% dilution) is then fed to the

LPRO unit with a TDS of 20,313 mg.L-1 and an osmotic potential of 13.8 bars

21


b) Seawater reverse osmosis (SWRO) desalination process. The feed water is

considered to be seawater with a TDS of 40,625 mg.L-1 (value reported for the Red

Seawater) and an osmotic potential of 27.6 bars. A 50% total recovery was considered

for the SWRO system.

c) Membrane bioreactor - reverse osmosis - advance oxidation (MBR-RO-AOP) process

for wastewater treatment and reuse. The AOP was composed of a UV irradiation

system with the addition of hydrogen peroxide (H2O2). AOP has been selected because

of its ability to remove organic pollutants from wastewater streams not treatable by

conventional techniques. More information on AOP can be found elsewhere.

Reclaimed water has an average TDS of 210 mg.L-1.

The FO membrane water flux shows an asymptotic behavior when related to the total

water costs (Fig. 4). As the water flux increases, the reduction in the total water cost

is lower, even when the flux doubles. For an average cost of USD 1,500 for the FO

module, and considering a flux of 5 L.m-2.h-1, the cost of each m3 of water produced

is USD 0.692.Whenthe flux doubles to 10 L.m-2.h-1, the water total cost decreases 9%

(USD 0.637). As the flux increases, the effect on the cost is reduced: when the flux

doubles from 10 to 20 L.m-2.h-1, the total water cost decreases only by 4%.

22


For a higher FO membrane water flux, the cost of the module starts to be less

significant for the final water cost of the whole FO-LPRO system. If the FO membrane

flux is low (≈5 L.m-2.h-1), the cost of the FO module has a higher impact on the total

water cost.

Fig. 5 shows the relation between the total cost of a FO-LPRO plant and the cost of the

FO membrane module. The module cost represents an important parameter in the

calculation of the CAPEX. When a 10 L.m-2.h-1 water flux was considered for the FO

membrane, there was a considerable increase in the total cost of the plant when the

module cost doubled. For example, when the FO module cost was USD 1,500, the total

cost of the FO-LPRO plant was around USD 146 million; if the module cost doubles to

USD 3,000 without changing any other parameter in the calculation, the total cost of

the FO-LPRO plant rises to USD 170 million, an increase of 16%.

23


A scenario can be seen where the total water costs may be further reduced thereby

making this technology a viable option to treat and recover wastewater when

compared to a MBR-RO-AOP system.

4. Rejection of Trace Organic Contaminants

To ensure water safety in FO-RO hybrids, TrOCs is of course of concern. TrOCs include

endocrine-disrupting chemicals, pharmaceutically active compounds, pesticides, and

disinfection by-products. They are present in impaired water in ng/L to g/L levels, and

could represent a human and environmental threat, even at low concentrations.

Recently, extended research was performed to evaluate FO as a barrier against TrOC,

especially in association with RO. A recent review summarized recent studies

dedicated to the fate of TrOC in the FO process. Among the studies cited, it was

observed that the FO process may provide a robust barrier for most TrOCs, but for

some TrOCs, only limited rejection was found. In addition, most of the FO studies on

TrOC were carried out using the commercial HTI CTA membrane, which demonstrates

relative low permeation fluxes (which could impact the low TrOC rejection).

FO-RO hybrid processes offer a promising solution not only to lower desalination

energy needs, but also to increase water reuse efficiency by combining seawater

desalination and water reuse. Interestingly, due to the lower fouling propensity

compared to pressure driven membrane system, FO has the potential to treat feed

water of various qualities (potentially even including raw sewage), allowing to lower

wastewater treatment costs. FO-RO schemes do require further validation but also

radical shift in current consideration of water supply. Societal (public perception of

24


water reuse) and water management (proximity of wastewater and desalination

plants) challenges clearly need to be overcome.

This review clearly emphasized the need for flux increase to allow for more favorable

FO economics and discussed the required technical development (i.e., novel

membranes, PAO mode). However, flux improvement is of course also associated with

drawbacks, such as increased fouling, lower rejection of TrOCs in PAO operation, and

the limits of membrane mechanical resistance.

At this stage, it is, therefore, questionable if the FO/PAO-RO hybrid process will allow

sustainable and long-term operation at high flux. Additional studies are required to

support successful implementation of FO-RO hybrids in the industry:

Up-scaling: most of the studies in literature have been conducted using small

flat-sheet coupons. More pilot scale and full scale tests are needed to assess

up-scaling challenges in term of mass transfer limitations on module scale, the

effects of spacer design on pressure drop, effects of fouling and the feasibility

of cleaning strategies.

Improved economic assessment: The economic models used for FO should be

updated by incorporation of fouling models that are better able to simulate

practical implementation of FO/PAO-RO hybrids. In addition, a better

integration of cost savings from the water recycling scheme may be considered

as any treatment step avoided in the water recycling scheme as a result of

combination with desalination will help to support FO/PAO-RO hybrids

economic credentials

5. Energy demand in desalination and water treatment processes

One of the main advantages of FO is the limited amount of external energy required

to extract water from the feed, only using a very low hydraulic pressure to recirculate

the DS on one side of the membrane, while the feed is passively in contact with

membrane on the other side. However, it should be noted that special attention must

be taken into the water quality, because the dilution might contaminate the water

and affect the downstream RO process. This might turn into an energy intensive

solution, having to add an advanced pretreatment process, which turns the research

on contaminant removal with FO membranes critical.

The driving factor for considering implementing a FO system versus a RO system (for

desalination purposes) or versus a ultrafiltration/nanofiltration (UF/NF) - advance

oxidation process (AOP) (for secondary wastewater recovery) or a MBR - RO - AOP

hybrid process (for primary wastewater recovery) should be the energy savings

related to the capital expenses. FO has been depicted as a near horizon low-energy

desalination technology (see Figure 7.5)

25


The energy consumption for desalinating water with RO membranes lies between 2.5

and 4 kWh·m-3, as a result of the development of new efficient membranes and the

use of energy recovery devices over the last decade.

According to Table 7.4, the specific energy consumption (SEC) of a direct desalination

FO system coupled with a RO process is lower when compared to a two-pass RO

process for a total system recovery higher than 25% (total production capacity of

100,000 m3·d-1).

Energy savings associated with the integrated FO-RO system compared to a two-pass

RO system are mainly related to the reduction in the osmotic pressure of the partially

desalinated water and the hydraulic operational pressure required by the recovery

process (i.e. low pressure RO system) to produce fresh water, compared to a

traditional RO system (Table 7.5). It is clear that lower energy consumption is possible

as the dilution rate increases. This, however, represents a higher capital cost for the

membrane area required.

26


With current commercial technology, hybrid FO systems for both desalination and

water recovery applications have proven to have higher capital cost compared to

conventional technologies. Nevertheless, due to the demonstrated lower operational

costs of hybrid FO systems, the payback time for the construction and the unit cost for

each m3 of fresh water produced with the FO system is shorter than conventional

desalination/water recovery technologies (i.e. ultrafiltration/RO systems). Minimum

water flux considerations are made for these evaluations, where the average FO water

flux should be 10.5 L·m-2·h-1 when competing against water reuse technologies (UF-

LPRO), and a water flux of 5.5 L·m-2·h-1 is needed to recover and desalinate water at a

lower cost compared to RO systems.

5.1 Major challenges for commercialization

FO is not yet a mature technology, but it is expected to help reduce the cost of

producing fresh water from impaired-quality sources in the next decade. According to

the 25th Desal Data IDA Worldwide Desalting Plant Inventory, the installed SWRO

capacity of desalination plants around the world as of June 30, 2012 was 74.8 million

m3·d-1. The cumulative contracted capacity, which includes plants that are contracted

or under construction, reached 80.5 million m3·d-1. It is expected that the installed

capacity will reach 120 million m3·d-1 by 2020. Clearly there is a need for fresh water;

therefore, new generation FO systems will play a significant role in the desalination

and water treatment industries. The main advantages, disadvantages and challenges

of FO technology compared to present RO standards are depicted in Table 7.6.

27


The

major challenges of FO to be a commercially viable technology are:

(i) developing a high flux membrane, capable of maintaining an elevated salt rejection

and a reduced internal concentration polarization (ICP) effect,

(ii) for PRO applications, the membrane has to additionally give adequate mechanical

support to withstand hydraulic pressure,

(iii) in some applications, the availability of appropriate draw solutions, which can be

recirculated via an efficient recovery process,

(iv) better understanding of fouling and biofouling occurrence in osmotically driven

processes,

(v) assuring the high quality of the water produced,

(vi) hybridization with other technologies that can increase the benefits of FO use (i.e.

water recovery, energy production, etc.).

It was suggested the use of RO concentrate instead of seawater as the draw solute for

water reuse to lower the operational cost and bring FO closer to commercialization. It

was found that the integration of FO and RO may result in a high flux of fresh water

and a stable performance under some operating conditions; however, the internal

concentration polarization in FO is an important factor affecting the overall

performance, since it reduces the effective osmotic pressure that drives the filtration

process.

28


6. Wastewater treatment to Biogas Production

Wastewater has been considered as an energy source to produce biogas by an

anaerobic process. The crucial factor for successful anaerobic treatment of

wastewater with biogas production is the concentration of organic matter compounds

including polysaccharides, lipids, protein, simple aromatics. The soluble organic

fraction in wastewater cannot be concentrated easily with a low cost process,

consequently becoming a barrier to the direct anaerobic treatment of wastewater. In

the treatment of wastewater, the idea of osmotic concentration of raw wastewaters

using FO membranes has not been studied to its full extent as an alternate method to

provide high organic concentration for an anaerobic process, and avoid primary and

aerobic treatment of wastewater.

The concentrated wastewater can then be utilized for anaerobic treatment and biogas

recovery, and the diluted seawater can be further reused in industrial/agricultural

activities or desalinated with significantly reduced energy cost by LPRO to produce

potable water.

Cath et al. studied the removal of micropollutants by a FO-RO system using a

membrane biological reactor (MBR) and the effluent from an activated sludge tank as

feed waters for the spiral wound FO membrane, and synthetic seawater as draw

solution (DS). They measured the concentration of the micropollutants present in the

MBR and the effluent (no micropollutants were spiked in the water), varying from 2

to 400 ng·L-1 (in some cases the compounds were below the limit of quantification).

The rejections were mostly high with the FO membrane, except for Bisphenol A. When

the hybrid process was considered, very high percentages of rejection (>99%) were

achieved.

6.1 Biogas production case study

The hybrid FO-LPRO system proposed in this chapter considers a concentration ratio

of 2:1 (i.e. 50% volume reduction) for the wastewater effluent. Considering that the

initial volume is 100,000 m3.d-1, the volume to be treated after the FO process would

be 50,000 m3.d-1. Therefore, a calculation on the amount of biogas that could be

produced from a

50,000 m3.d-1 capacity

AnMBR was made

(≈266,000 P.E.). This is

an attempt to

quantify one of the

added benefits of

using a hybrid system

that reduces the

volume of

wastewater to be

29


treated, increasing the concentration of carbon (mixed liquor suspended solids) in the

feed, and recovering the energy within the wastewater. The addition of an FO

concentration step before the AnMBR was a promising way for net energy recovery

from typical municipal wastewater in temperate areas. Table 3 describes the

parameters taken into consideration for the calculation of the total energy production

(kWh.y-1) from an ideal AnMBR.

The MBR-RO-AOP system could potentially be used as well to produce biogas if the

sludge produced is process in anaerobic reactor. Table 4 shows the calculation of the

total energy production (kWh.y-1) for a conventional 100,000 m3.d-1 capacity MBR,

considering an average sludge yield of 0.5 kg of volatile solids (VS) per kg of COD and

a conversion rate of 0.4 from VS to biogas. The MBR-RO-AOP system has the potential

to produce nearly 2.9 million cubic meters of methane per year, 17% less than the

amount calculated for the FO-LPRO system coupled with an AnMBR (Table 3). The FO-

LPRO system has the potential to extract more of the energetic value of the

wastewater compared to the energy produced from treating the sludge in the MBR-

RO-AOP system.

Based on the integration of a heat pump and FO concentration technology into an

AnMBR for municipal wastewater treatment, one can achieve a net energy recovery

for the system. Table 5 compares the net energy recovery (kWh.m-3) from municipal

wastewater (concentration factor of 1) to the FO-concentrated municipal wastewater

(concentration factor of 5 and 10). At 3 different temperatures (10 oC, 20 oC and 30

30


oC), an increased concentration of 5 times in volume can achieve positive net energy

recoveries. A smaller volume of concentrated wastewater can produce energy and it

needs less energy to transport it.

6.2 Conclusions

FO-LPRO has been depicted as a near horizon low-energy desalination technology

considering the use of a hybrid system for desalination and wastewater recovery,

using the principle of osmotic concentration/dilution. Based on the economic analysis

of water treatment systems producing 100,000 m3.d-1 of water presented in this

study, it can be concluded that:

_ A hybrid FO-LPRO system has lower costs for producing water compared to

conventional seawater desalination by SWRO.

_ Compared to SWRO, FO-LPRO systems result in a higher CAPEX, but present a

significant reduction in OPEX (56%). As a result of the reduction in OPEX, the total cost

per unit of water (USD.m-3) for the proposed hybrid FO-LPRO system is lower than

benchmark conventional desalination technologies (SWRO).

_ The sensitivity analysis showed that the most critical aspect in terms of economic

feasibility for these hybrid FO-LPRO systems is the FO module cost.

_ The proposed hybrid FO-LPRO system has a comparable cost to wastewater

treatment and recovery system (MBR-RO-AOP).

_ Additional advantages of hybrid FO-LPRO systems include the reduction in

wastewater volume to be post-treated, recoverable biogas production based on

anaerobic post-treatment of concentrated wastewater effluent, and the reduction of

greenhouse gas emissions compared to conventional high-energy desalination

technologies.

7. Other FO-RO hybrid configurations

Bamaga et al. proposed to combine the FO-RO hybrid described in Figure 3 with a

second FO stage implemented as an RO post-treatment. In this configuration, the

additional FO stage is used to dilute the RO brine with the concentrated impaired

water from the first FO step to

(1) further concentrate the wastewater stream and so facilitate its post-treatment (for

example via digestion) and

31


(2) dilute the RO brine before disposal to limit its environmental impact.

Although the additional FO presents some potential environmental benefits, the

economic and technical feasibility is questionable due to the low permeation fluxes

observed, especially in the second FO. Ultimately, recommendations to focus on the

first FO stage and optimization of module design were given.

FO has been demonstrated to be a robust and simple process allowing to treat difficult

streams such as anaerobic digester concentrate or sludge, and as such could also be

well adapted to treat difficult wastewaters, mainly due to its low fouling propensity.

Thus, instead of using secondary treated wastewater (Figure 3), new concepts have

emerged to consider the implementation of FO upstream in the wastewater

treatment scheme, i.e., on primary treated wastewater or even the direct

implementation on raw sewage. It is expected that thanks to the avoidance of some

purification steps, significant cost reduction could be obtained.

One example is the concept of osmotic membrane bioreactor (OMBR) where FO is

implemented within the secondary (biological) treatment. The OMBR has been

developed by analogy with membrane bioreactors (MBRs), where biological

degradation and clarification were operated in a single step. However, instead of using

a porous ultrafiltration or microfiltration membrane as for MBR, a dense FO

membrane is submerged in the bioreactor of the OMBR. As such, higher rejections of

contaminants were observed than for MBRs, yet at lower fouling propensity and thus

OMBRs can produce the high water quality which is crucial in the context of potable

water reuse. One major limitation in OMBR operation remains the salt accumulation

in the OMBR tank, resulting from the high rejection of dissolved solids by the FO

membrane and the reverse solute diffusion occurring in the FO process. This salinity

build up can only be mitigated by the development of more selective membranes, or

by decreasing the sludge retention time. Another proposed solution was the addition

of ultrafiltration or microfiltration system to OMBR to create salt bleeding, but this

32


process is more complex to operate since two sets of well-balanced membrane

systems are needed.

As the first experiences with the OMBR operating in a secondary biological treatment

are positive, with little fouling observed, it is of course interesting to envision FO

treatment further upstream, for example on the raw wastewater (or sewage) after

primary treatment, as stated above (and shown in Figure 4). The interest of the water

treatment community in the scheme in Figure 4 is high, as FO offers a double

advantage here: not only can high quality water be recovered, in addition the

concentrated sewage stream can be more easily converted to energy via digestion

(due to the higher COD concentration). Initial experiments using FO on primary

treated (screened) wastewater demonstrated that the accumulated fouling layer was

loose and easily reversible, and thus fouling can indeed be controlled. Further

validations are of course required, especially with regards to clogging issues in the

feed channels, and also in terms of long term behavior—but implementing FO directly

after primary treatment in the future could allow for significant savings in wastewater

(and moreover water reuse) treatment costs.

7.1 Pressure retarded osmosis: special FO application for energy recovery in water

industry

Pressure retarded osmosis (PRO) is a special type of FO application, where the primary

and end product is energy instead of water. Potential energy of a natural or

engineered salinity gradient system can be utilized by a PRO process in the form of

electricity or hydraulic pressure. The main difference between a modern PRO and an

FO process is the applied pressure on the high salinity draw solution. The pressure of

the high salinity draw solution can be kept relatively constant during the PRO process,

even though the volumetric flow rate is to be increased. Therefore, the draw side of

the PRO process can be assumed to be isobaric, in most cases. The near-isobaric

behavior of the draw side of the PRO process can be explained by the harvested Gibbs

free energy of mixing and volumetric expansion. Thus, PRO can be used to increase

33


the internal energy of the draw solution with respect to the ratio of the permeated

water flux.

7.1.1 Large-scale applications of PRO

Until recently, the Norwegian electric company, Statkraft, had been the pioneer of

large scale PRO applications by constructing and operating the first prototype PRO

facility in the world. Statkraft adopted a system where seawater was used as the draw

solution in their system (see Figure 7.7). Spiral wound membrane modules, which

were supplied by Hydranautics, were used with a projected power density of 5 W·m-2

at around a 12.5 bar hydraulic pressure difference across the membrane. However,

low extractable work values of the selected system configuration and parameters

(seawater as the draw solution), inefficiencies in the hydraulic power generation

systems and high capital investment requirements, forced the company to cease their

osmotic power operations in December 2013.

The Japanese Mega-ton Water System project, however, adopted a PRO system which

uses seawater reverse osmosis (SWRO) brine as the draw solution (see Figure 7.8), in

which the highest theoretical extractable work was calculated as 2.75 kWh per m3 of

draw solution used as opposed to 0.75 kWh per m3 in seawater systems. The Megaton

project selected hollow fiber PRO membrane modules, which were supplied by the

Toyobo company, with a predicted power density of more than 12 W·m-2 at around a

30 bar hydraulic pressure difference across the membrane. The achievement of 13.3

W·m-2 for a 10 inch diameter module was announced during the “Mega-ton Water

System” International Symposium (2013). The Mega-ton project, as Statkraft did, used

hydraulic turbines to convert the salinity gradient potential of SWRO brine to

electricity in its 460 m3·day-1 brine flow rate capacity pilot plant, in Fukuoka, Japan.

34


So far, there have not been many PRO applications where the salinity gradient is

harvested and converted to the hydraulic pressure in order to be used in different

pressure dependent systems. A recently initiated Korean national research project,

Global MVP (Membrane Distillation, Valuable Source Recovery, PRO), is one of the

first multimillion dollar projects, where PRO is used to utilize hydraulic pressure

without converting it to electricity. Global MVP adopts an SWRO-PRO hybrid system,

where diluted draw solution from PRO is used to pre-pressurize the seawater prior to

the SWRO process. Seawater pre-pressurizing is established by deploying isobaric

pressure exchangers, which can reach up to 97% efficiency in terms of energy

recovery. Woongjin Chemical (CSM membranes) is the membrane producer partner

in the project and supplies spiral wound PRO membrane modules with a predicted

power density of 7.5 W·m-2. Energy Recovery Inc. (USA) is another key partner in the

project in order to optimize and supply high efficiency isobaric pressure exchangers

which are customized for the specified purpose.

7.2 The Concept of Pressure Assisted Osmosis (PAO)

As an alternative to FO, the concept of pressure assisted osmosis (PAO) has arisen

recently, and appears promising to overcome the current flux limitations of FO.

Opportunities and challenges of novel membranes, novel modules and the use of PAO

operation, to improve permeation flux in osmotic processes, are critically and

systematically discussed in the following sections.

The concept of pressure assisted osmosis (PAO), relies on the application of moderate

pressure on the feed side of a FO system to enhance water permeation through the

membrane (Figure 7). As such, by a synergistic effect of hydraulic and osmotic

pressure, PAO can improve FO fluxes and thus FO process economics due to lower

membrane surface requirements.

35


Comparative investigations of PAO in continuous and discontinuous mode also

confirmed that water flux increases with hydraulic pressure. Interestingly, and as a

result of more intense ICP in PAO operation, RSD decreased, tackling a second

limitation of current FO operation, and thus rekindling the interest in PAO.

More recent work compared the performance of CTA membranes in PAO mode to

that of commercial TFC membranes. In addition to allow for higher flux in FO process,

the TFC membranes were also more responsive to hydraulic pressure applied in the

PAO process, and thus showed clear flux enhancement (up to 25 L.m-2.h-1) at moderate

hydraulic pressure. In addition to providing extra driving force for permeation flux,

hydraulic pressure was also observed to limit RSD and increase the water permeability

due to membrane deformation when TFC membranes were used. Therefore, PAO

constitutes a promising alternative to tackle the permeability-selectivity trade-off of

FO.

A recent publication therefore proposed an optimised sequence for FO/PAO cleaning,

which consists of osmotic backwashing to detach the foulant cake from the membrane

surface and high CFV operation to flush the feed channel with fresh water to remove

the foulants that were dislodged from the surface. This method proved to be efficient,

even at high FO permeation flux and in PAO operation. A more detailed insight in

fouling and cleaning mechanisms is starting to emerge (Figure 8), which shows that

even high flux membranes operated in PAO mode can be cleaned without the need

for chemicals.

36


8. Optimization on a new hybrid Forward osmosis-Electrodialysis-Reverse osmosis

seawater desalination process

Standalone membrane desalination processes are largely influenced by

thermodynamic irreversibility-driven energy loss. It is one of the reasons that many

studies focus on integrated membrane systems, in addition to creating process

flexibility. A new Forward osmosis-Electrodialysis-Reverse osmosis (FO-ED-RO) hybrid

system employs FO element upstream to ED-RO system for an access to draw

solutions with higher electrical conductivity, aiming at reducing energy consumption

and inheriting various advantages of ED system. Various draw solutes candidates,

including sodium chloride, are selected primarily based on conductivity, and are

further analyzed for best draw solute selection. Then, the optimum values for energy

consumption, unit process sizes, and total unit production cost are determined by a

simulation based on various FO membranes, modules and FO recovery ratio. As a

result, in terms of total unit water production cost, the best draw solute is ammonium

chloride with 0.514 USD/m3. The results prove that the new hybrid process is

competitive in seawater desalination with respect to the established RO as well as

other hybrid systems. Meanwhile, the study also recommends pursuing a research on

cheap, yet high electric conductive draw solutes and low cost-low resistance ED

membranes, to consolidate the applicability of the process.

Integrating various seawater desalination units has become a practice to discover

energy minimizing systems, enhanced flux options, waste recovery alternatives,

stringent water quality achieving mechanisms, and means for utilizing available

resources such as renewable energy. Previously, most of the studies have focused on

two-unit integrated systems, mainly analyzing FO-RO hybrid desalination units and

their networks pertinent to specific problems.

37


Unique properties of ED, such as operating at small differential limit and process

flexibility, also led studies to focus on various electrodialysis hybrid processes with a

goal of improving energy consumption, and capital and operating costs. Although ED

units can remove more than 70% salt in the diluate effluent, the standalone ED

element can rarely be used as a final treatment operation for pure water production.

Besides, dilute solutions impose high electrical resistance which results in low current

efficiency, high current density and split of water at the surface of the ED membrane.

Therefore, prior to the use of water for potable purpose, a need in pre or post

treatment brings out the idea of using a downstream RO process since it is effective

for removing microorganisms.

Furthermore, studies have integrated ED membrane systems aiming at reducing

energy loss due to thermodynamic irreversibility in seawater desalination processes.

8.1 Process description

In the FO-ED-RO hybrid system, only forward osmosis has a direct encounter with

seawater. It utilizes highly concentrated and conductive draw solutes on the opposite

side of the membrane section, for production of freshwater from seawater. The

concentration of the draw solutes must be greater than that of the seawater feed to

ensure higher osmotic pressure. This can be continuously achieved by integrating with

electrodialysis section as a concentrating unit. The draw solute output from the

forward osmosis section is divided and fed as a concentrated and a diluate stream to

the ED unit, which contains a cationic and anionic exchange membrane. In the ED unit,

positive ions of the draw solute in the diluate side migrate through cationic exchange

membrane to the cathode and negative ions through anionic exchange membrane to

the anode.

At the same time, those ions are repulsed by the similarly charged membranes and

retained on the concentrate side. The concentrated stream is recycled back to the FO

unit to provide draw solution as a continuous recycle system. This occurs in an

electrodialysis membrane stack. The diluate effluent with low concentration flows to

the reverse osmosis system for freshwater production. The output of the reverse

osmosis unit contains two parts: a permeate which is the final product water and a

retentate with a concentrated draw solute which is recycled to the forward osmosis

together with the concentrated stream of ED system.

38


In developing this system the following assumptions are considered: seawater is

assumed to be a simulated solution with 3.5% of sodium chloride, concentration

polarization is neglected in the electrodialysis for calculation simplicity, Fouling and

scaling are only considered in the cost analysis part, Loss of draw solutes in the RO

unit is neglected for simplicity of calculation, pumping energy of FO and ED units is not

considered in the design, fouling and scaling is only considered in the cost analysis

part, loss of draw solutes in the RO unit is neglected, circulation pumping energy of

the FO and RO units is not considered in the design (Fig. 1).

Draw solutes used in the hybrid FO-ED-RO system should possess higher electric

conductivity than sodium chloride. Then, the candidate solutes are filtered according

to their hazardous and safety information as far as the produced water is for drinking

and the chance of exposure during the process should be eliminated. This filtration is

carried out based on NFPA 704 code and standard. The standard addresses the health,

flammability, instability, and related hazards that are presented by short-term acute

exposure to a material under a condition of fire, spill, or similar emergencies. The

system indicates the degree of severity by a numerical rating that ranges from four,

indicating a severe hazard, to zero indicating a minimal hazard. In addition, the cost of

solutes is one of the parameters that affect the selection of draw solutes. Therefore,

it will be comprehensively analyzed later with economic feasibility. The osmotic

pressure can supposedly be gained by increasing the concentration of draw solutes at

the expense of cost if the electric conductivity of a solute has an enormous effect on

the energy consumption.

39


8.2 Results and discussion; Energy consumption of hybrid FO-ED-RO process

8.2.1 Effect of conductivity

Higher conductivity solutes are potassium bromide, potassium iodide, potassium

chloride, and ammonium chloride in the order of decreasing conductivity. There is a

clear disparity between the lower part of the graph with blue color indicating the

energy consumption of high conductive solutes and a red color for low conductive

solutes, sodium chloride and sodium iodide. In low resistance membranes, more than

15% reduction in energy consumption is achieved when the draw solutes with higher

equivalent electric conductivity are used than sodium greater than that of 1.511

kWh/m3 for a current density 50 A/m2 with similar draw solute. However, the total

membrane area required for the former is 223.5 m2 which is half the membrane area

required for the latter, which is 474 m2. One of the reasons in obtaining low energy

consumption in this study is the modulation of the area of ED membrane along with

the number of cells with respect to available market values in order to keep the

current density constant. In this work, the process design is carried out based on 50

A/m2

8.2.2 FO-ED-RO energy consumption and effect of membrane resistance

Results relating to the size of FO membrane which influences the concentration of the

input draw solute to ED. As a result, the minimum and the maximum desalination

energy values for the hybrid process obtained are 1.511 kWh/m3 and 8.217 kWh/m3

respectively. The lowest energy consumption is a result of very large membrane area,

close to the maximum existing area of FO unit that produces the lowest concentration

feed to the ED, and the lowest-resistance ED membrane. The highest energy

consumption shows the invariable effect of membrane resistance on energy

consumption along with high concentration fed to ED due to very small upstream FO

membrane area. Nevertheless, a tradeoff between membrane cost and energy

consumption cost is to be seen later for all membrane arrangements and types, in the

process optimization, to determine the comprehensive optimized design results.

40


Out of the total energy consumed by the hybrid process, the energy make up by the

ED is greater than that of RO in all the results. This depicts, the corresponding

intermediate concentration between ED and RO at minimum energy consumption

point shifts to a brackish water concentration. At high ED outlet/RO inlet

concentrations, the energy consumption of RO is significantly greater than that of ED

which shows the irreversibility of RO process. The above result supports the

background suggestion that stated, lower energy consumption is obtained for ED-

BWRO hybrid system.

Results also show, membrane resistance has a direct effect on the energy

consumption of the ED unit and its energy percentage make up, which increases

significantly when high resistance membranes are used. It gets higher than that of RO

for the entire intermediate concentration. ED membrane resistance is the main cause

of voltage drop along with stream resistance. Therefore, lower resistance membranes

give better results and high resistance membranes impose high voltage drop

8.2.3 Effect of concentration and conductivity

Conductivity has not only a direct effect on ED energy consumption, but also on hybrid

energy consumption due to their intermediate concentration influence on RO unit.

Draw solutions with higher equivalent conductivity give lower intermediate

concentration between ED and RO and the vice versa. It may be because, for high

conductive draw solutes, results show the smallest hybrid energy consumption when

most of its salt concentration is removed by ED. For lower conductive draw solutes

the concentration shift towards the RO feed to minimize the hybrid energy

consumption because of the low conductivity burden on ED. As a result, working on

higher conductivity draw solutes brings two benefits; it reduces the energy

consumption of ED unit and also reduces the overall energy consumption by shifting

the intermediate concentration to the less irreversible side.

8.3 Optimization of the FO-ED-RO process

Due to the difference in conductivity, and perhaps physicochemical properties, there

is a difference in the cost of desalination energy among draw solutes. Apart from

potassium chloride, the rest show a slight difference in the cost of desalination energy

with respect to that of sodium chloride. However, it is cancelled out by the rise in

amortized capital cost except for low-cost draw solutes. For example, ammonium

chloride shows a 15% reduction in desalination energy cost and a 2% rise in amortized

capital cost demonstrating its distinctive advantage as a draw solute.

The optimum total unit product cost is estimated for all the draw solutes used in this

hybrid process. The costs visibly vary among the draw solutes with a minimum value

of 0.5136 USD/m3 for potassium chloride and a maximum value of 1.1747 USD/m3

for potassium iodide. The lowest value shows its competitiveness with the existing

established RO and other hybrid processes developed.

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In this study, the results show a difference essentially based on the cost of draw

solutes. As a result, high-cost draw solutes tend to result in maximum cost at minimum

FO stack cost and vice versa. On the other side, for lower cost draw solutes, it shows

monotonically increasing graph especially for the maximum recovery ratio value

showing a more dominant effect of FO stack than the draw solute through the entire

membrane. The total unit production cost significantly varies with increasing FO

recovery ratio for all the draw solutes, due to its direct reducing effect on the seawater

feed flow rates and a rising effect on the draw solute concentration upon reaching the

fixed production rate. This in turn raises the cost of draw solute despite a reduction in

the cost of FO stack. Contrary to the initial premise, even though the number of

modules has an influence on the change in the number of membranes, it doesn't seem

to have significant impact on the total unit production cost. In future studies, total

membrane area instead of individual membrane modules should be considered since

it is better optimized in computational applications.

Meanwhile, there is a difference in the investment cost among different selections of

draw solute processes of the hybrid system. The smallest investment cost is

1,256,848.48 USD by potassium chloride and the largest is 4,379,909.09 USD by

potassium iodide. The capital cost is significantly affected by the FO and ED membrane

stack and their replacement costs. The cost of potassium iodide and sodium iodide

draw solutes have similar significance to the stack cost since their cost is

overwhelmingly greater than that of other draw solutes. Except for the draw solutes

with higher chemical costs, the percentage make up cost of membrane stacks and

their replacement is over 50%. The interdependence of draw solutes and membrane

stack is clearly observed for potassium chloride, which has got the least FO membrane

area with 300 membranes and 10 modules. Therefore, the change in FO membrane

area will affect the total cost of unit product water in two ways, primarily, in its direct

cost of FO membrane stack and, secondarily, in its inverse proportional effect on the

concentration of draw solute and desalination energy

The ED membrane cost and the unit cost of desalination energy are interrelated

throughout membrane area and current density. The difference in unit cost of

desalination energy is caused by the difference in draw solute concentration and the

conductivity. In some cases, low conductivity solutes give higher ED membrane stack

costs due to higher diluate concentration out of the ED. The percentage make up-cost

of desalination energy out of annualized cost varies from 15% to 46%. The lowest

values are obtained for highly conductive draw solutes which may also be due to their

good physiochemical properties.

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The cost and energy consumption of the process is compared to some of the

previously developed and studied processes shown Table 7.

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1st Case Study; FO system for wastewater recovery and seawater desalination

The studies comprised in this work were based on the layout of a hybrid forward

osmosis/low pressure reverse osmosis (FO/LPRO) system shown in Figure 1.6.

Wastewater was used as feed water for the FO process; once concentrated, the

wastewater is ready for a post-treatment step to recover energy in form of biogas.

The draw solution for the FO process is seawater; once diluted, the seawater is fed

into a LPRO unit to remove the remaining dissolved salts and produce fresh water.

To better understand the process, an example of FO filtration is given: using a

municipal primary wastewater effluent (conductivity ≈ 2000 μS·cm-1) as feed solution

and Red seawater (conductivity ≈ 52000 μS·cm-1) as draw solution, a 60% seawater

dilution can be achieved, resulting in partially desalinated seawater (conductivity ≈

21000 μS·cm-1), which can further be treated with a LPRO system (feed pressure ≈ 20

bar), to produce fresh water (conductivity ≈ 250 μS·cm-1) at a lower energy

consumption than a traditional RO system (feed pressure ≈ 60 bar). The concentrated

wastewater effluent can be further treated in an anaerobic bioreactor to recover

biogas.

forward osmosis fundamentals & hybrids

Engineering