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Title: Hollow fibre membrane contactors for ammoniarecovery: Current status and future developments

Authors: Mariam Darestani, Victoria Haigh, Sara J.Couperthwaite, Graeme J. Millar, Long D. Nghiem

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Please cite this article as: Mariam Darestani, Victoria Haigh, Sara J.Couperthwaite,Graeme J.Millar, Long D.Nghiem, Hollow fibre membrane contactors for ammoniarecovery: Current status and future developments, Journal of Environmental ChemicalEngineering http://dx.doi.org/10.1016/j.jece.2017.02.016

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Hollow Fibre Membrane Contactors for Ammonia Recovery: Current Status and Future

Developments

Mariam Darestani, Victoria Haigh, Sara J. Couperthwaite and Graeme J. Millar*

Institute for Future Environments and School of Chemistry, Physics and Mechanical

Engineering, Science and Engineering Faculty, Queensland University of Technology (QUT),

Brisbane, Queensland, Australia

Long D. Nghiem

School of Civil Mining and Environmental Engineering, University of Wollongong,

Wollongong, New South Wales, Australia

*Corresponding author:

Graeme J. Millar | Professor

Science and Engineering Faculty | Queensland University of Technology

P Block, 7th Floor, Room 706, Gardens Point Campus, Brisbane, Qld 4000, Australia

ph (+61) 7 3138 2377 | email graeme.millar@qut.edu.au

Abstract

Hydrophobic membrane contactors represent a credible solution to the problem of

recycling ammoniacal nitrogen from waste, water or wastewater resources. This study

critically evaluated existing literature in terms of process principles, membrane types and

functionality, membrane contactor application, technology status, and future research

required. The key operational parameter was the presence of ammonia gas and thus pH

should be above 9. Hollow fibre membranes are usually employed, composed of primarily

polypropylene, polyvinylidene fluoride, or polytetrafluoroethylene. The stripping solution is

normally sulphuric acid which reacts with ammonia to create ammonium sulphate. The acid

is best circulated inside the lumen with any suitable velocity, and kept in excess

concentration. In terms of operational parameters: feed fluid velocity is important in open

loop configurations due to the effect upon ammonia residence time at the membrane

surface; and, ammonia concentration did not notably impact the mass transfer coefficient

which was ca. 1 x 10-5 m/s until in excess of 2,000 mg/L wherein the transport process

diminished. The greatest quantity of ammonia was recovered in the initial membrane

stages where the driving force is greatest. Bench and pilot plant studies concerned

wastewater treatment plants, anaerobic digesters, manure management, industrial

manufacturing, and animal rearing operations. It is recommended to focus upon challenges

such as development of new membrane types customised for ammonia removal, a greater

understanding of the process engineering and economics involved, consideration of the

impact of osmotic distillation, integration of membrane contactors with other water

treatment technologies and development of cleaning in place procedures.

Key Words: ammonia; membrane contactor; water; wastewater; hydrophobic; waste.

1. Introduction

Ammonia is a key component for fertilizer production. Indeed, it is indispensable to

agricultural activities. On the other hand, ammoniacal nitrogen (ammonium and ammonia)

is also a major global pollutant in the aquatic environment as excessive concentrations can

cause algal blooms, fish kill events and toxicity problems [1]. Traditionally, ammoniacal

nitrogen in water and wastewater is often removed by a variety of biological nitrogen

removal (BNR) processes [2-4]. The standard treatment approach involves nitrification and

denitrification stages, albeit in recent years considerable efforts have been directed towards

technologies which may offer benefits in terms of reduced energy consumption. For

example, anaerobic ammonium oxidation (Anammox) has been the subject of considerable

interest [2, 5]. The core function of Anammox bacteria is to anaerobically convert

ammonium to nitrogen gas using nitrite as electron acceptors. The nitrite can be generated

by oxidation of ammonium ions or partial denitrification of nitrate to nitrite ions [2]. As

noted by Licon Bernal et al. [6] biological methods for ammonia control in wastewater are

not currently viewed as economically feasible when very low ammonia concentrations are

required in the effluent (<1 mg/L). With gas emissions various approaches have been

implemented to either control or eliminate ammonia. Catalytic oxidation of ammonia to

nitrogen has been used extensively in the power generation sector [7]. Sorbents such as

clay minerals have also been tested to reduce odours associated with ammonia release from

manure [8]. Thermal methods which involve heating of manure to evolve ammonia gas

coupled with acid absorption have also been described [9].

It has been recognised that ammonia can actually be viewed as a resource and not solely a

problem to be solved [10]. Consequently, there has been a renewed interest in recent years

to recover nutrients from waste streams due to a combination of economic, environmental

and energy considerations [11]. Physicochemical methods have been widely used for the

removal and recovery of ammonium species from wastewater and gas sources in a usable

form to supplement existing ammonia production. Zeolites have been extensively studied in

terms of their ability to remove and recover ammonium species from water and wastewater

via ion exchange [12, 13]. Although zeolites can be effective [14-16] they suffer from

drawbacks such as slow diffusion of ammonium ions in the microporous structure and

consumption of significant volumes of regenerant chemicals [13, 17]. Synthetic resins have

also been evaluated for ammonium capture but they presently lack the required selectivity

to ammonium ions in the presence of common competing cations such as sodium

potassium, calcium and magnesium [18]. The recovery of both ammonium and phosphate

species from aqueous solution by struvite (MgNH4PO4.6H2O) precipitation has also been

intensively researched [19, 20]. Struvite appears to be a viable fertilizer and economics of

recovery depend significantly upon the price of the added magnesium component. Air

stripping has been used for ammonia removal with optimal conditions said to be high

ammonia concentration (1440 mg/L), high pH (10.7) and elevated temperature (ca. 37 oC)

[21]. However, air stripping can have limitations relating to use in for example anaerobic

digesters; wherein ammonia is stripped from a side stream. Use of air introduces oxygen

which can inhibit growth of the microbial species in the anaerobic digester and application

of biogas instead still has the disadvantage of carbon dioxide inhibition of ammonia

stripping [22]. Membrane processes such as reverse osmosis and forward osmosis have

been demonstrated to exhibit some potential for ammonia recovery from wastewater.

Reverse osmosis for example has some ability to separate ammoniacal nitrogen species

from landfill leachate albeit the efficiency can be relatively low [23]. Forward osmosis has

recently received interest for ammonia removal and recovery from solution, especially in

relation to use of magnesium based draw solutions which can promote struvite precipitation

reactions [24]. However, fundamental issues concerning the achievement of high solute

selectivity while sacrificing water permeability with forward osmosis membranes needs

further research to improve performance.

Membrane contactors have been the subject of interest in terms of industrial application for

control of ammonia [25], aromatics [26], carbon dioxide [27] and deoxygenation of fluids

[28]. Membrane contactors appear prospective for removal and recovery of ammonia from

wastewater as they are relatively simple in concept and able to produce a saleable fertilizer

product such as ammonium sulphate [29]. Membrane distillation has been studied for its

ability to recover ammonia from wastewater solutions. For example, Zhang et al. [30] used

a polyvinylidene fluoride (PVDF) hollow fibre membrane with an applied vacuum on the

strip side (Vacuum Membrane Distillation), to treat wastewater comprising of high

concentration of ammonia (497 mmol/L) and salt (100 g/L). The degree of ammonia

removal was promoted by raising solution temperature, increasing vacuum and greater salt

content which was ascribed to reduction in ammonia solubility. Wu et al. [31] confirmed

the latter results and also added that faster feed velocities could further improve ammonia

removal due to greater mass transfer coefficients. Tun et al. [32] used Direct Contact

Membrane Distillation wherein permeate at 20 oC was used on the strip side to capture the

volatile ammonia from a urine feed typically heated to 60 or 70 oC. Addition of low

concentration sulphuric acid to the permeate can increase ammonia recovery rates to 99 %

[24]. The presence of volatile organic compounds can present challenges to the use of

membrane distillation as they can potentially cross the hydrophobic membrane boundary

[24].

Consequently, the use of hydrophobic membrane contactors at ambient temperature may

be attractive, with the driving force being the chemical reaction of acid with ammonia gas

[33, 34]. This methodology offers the prospect of being: selective to ammonia gas removal;

able to operate without the need for significant energy input as in the case of membrane

distillation and air stripping; relatively cost effective as it does not require the use of

regenerant chemicals inherent to employment of ion exchange media; suitable for removal

of ammoniacal nitrogen to very low levels. The latter method has considerable promise for

ammoniacal nitrogen capture, but until now there has not been a comprehensive, critical

review completed of the literature. Therefore the aim of this study was to evaluate the

process fundamentals, determine the applicability of membrane contactors, examine the

technology status, and outline the challenges which remain to be solved for ammonia

removal and recovery.

2. Principles of Membrane Contactor Operation

The basis of membrane contactor technology is the use of hydrophobic membranes,

typically in a hollow fibre configuration across which gaseous species such as ammonia gas

can transfer [35]. Hollow fibre membrane contactors present very high specific contact

areas (m2/m3), <1 mm internal diameter, with a wall thickness of <1 mm and pore size <0.3

microns [36, 37]. Normally, a tube in shell configuration is used as it has the advantages of

being physically robust, requires limited space for use, and protects the delicate hollow fibre

membranes. The main disadvantages are that irreversible fouling may occur and that fibres

may break under high strain situations. Tubular membrane contactors have also been

reported, primarily by Mukhtar and co-workers [38-42] as this type of membrane

configuration is amenable to treat systems wherein the propensity for fouling is high and

the need for rigorous cleaning methods such as mechanical or harsh chemicals is required.

Samani Majd et al. [42] reported the use of PTFE tubular hydrophobic membranes of 6.72

mm internal diameter with a wall thickness of 0.66 mm and pore size 2.46 microns, for the

treatment of ammonia from liquid manure, wherein sulphuric acid solutions between pH 2

and 5.4 were flowed inside the tubes. It was reported that up to 50 % of the ammonia could

be captured from the manure over a 20 day period [38].

With either membrane type, typically an acid such as sulphuric acid is flowed counter-

currently on the lumen side of the membrane in order to react with ammonia gas to create

ammonium sulphate [Figure 1 and Equation 1].

Equation 1: 𝟐 𝐍𝐇𝟑 + 𝐇𝟐𝐒𝐎𝟒 → (𝐍𝐇𝟒)𝟐𝐒𝐎𝟒

This latter process ensures that the difference in ammonia partial pressure on both sides of

the membranes remains significant and thus provides the chemical potential which drives

the separation process [43]. The methodology of using a membrane to remove gaseous

species from solution into an acid wherein the gas can undergo a chemical reaction is

generically termed “TransMembraneChemiSorption” (TMCS) [43] or Perstraction with

Chemical Reaction (PCR) [44].

A critical aspect of the operational mode for membrane contactors is the presence of

ammonia gas which can traverse the pores in the hydrophobic membrane. Figure 2

illustrates the ammonia/ammonium equilibrium as a function of pH, salinity, and

temperature [45] as predicted using the unionized ammonia calculator provided by Florida

Department of Environmental Protection [46].

Once ammonia gas transfers across the hydrophobic membrane the ammonium-ammonia

equilibrium on the shell side will be perturbed and as such ammonium species would be

expected to convert to ammonia gas and concomitantly create protons (hydronium ions)

which would reduce the solution pH [Equation 2] [47].

Equation 2: 𝐍𝐇𝟒+ → 𝐍𝐇𝟑 (𝐠) + 𝐇+ (𝐚𝐪)

Wäeger-Baumann and Fuchs [47] observed a notable decrease in solution pH from 10 to 9.2

with time on stream when treating a digester effluent. Zhu et al. [48] also recorded a

significant decrease in the ammonia flux rates with polypropylene membranes with time

which corresponded to a reduction in feed pH due to the use of a closed loop system. Licon

Bernal et al. [49] used a “closed loop” ammonia removal process and recorded a decrease in

solution pH in the feed from 9.3 to 7.2 due to the process illustrated in Equation 2.

Consequently, during operation of a membrane contactor for ammonia removal it may be

necessary to regularly adjust the solution pH by addition of an appropriate alkali in order to

maintain the driving force for the membrane separation process [49]. Notably, the majority

of wastewater and water streams to be treated for ammonia removal are in the pH range 6

to 8, and thus alkali addition is expected to be a major cost for membrane contactor

technology implementation.

Two options exist in terms of the configuration the membrane is used for ammonia

removal: (1) feed solution in the lumen side and acid in the shell side; or, (2) feed solution in

the shell side and acid in the lumen. Agrahari et al. [45] preferred to flow the feed in the

lumen side as did Mandowara and Bhattachrya [33] whereas Hasanoğlu et al. [50] circulated

the ammonia feed in the shell side of the membrane contactor module. Evidence has been

presented which shows that flowing the feed in the shell side gives superior ammonia

removal performance compared to the situation where the feed was in the lumen side [50].

As the greatest opportunity for fouling of the membranes would be expected to occur from

the feed solution, it would also be logical from a practical point of view to flow the feed on

the shell side where the potential for blocking of the flow path was reduced (due to the

larger space available relative to the inside of the much smaller membrane fibres).

3. Membrane Properties

The most important phenomenon relating to membrane contactors [51] is the mass transfer

driven by vapour pressure or concentration difference across the membrane. When one of

the fluids is water or an aqueous liquid, it is critical that the membrane is not wetted due to

surface tension. Wettability is controlled by chemical composition of the membrane and

geometry of the pores [52]. Commonly used membranes for ammonia removal are made of

hydrophobic polymers such as polyethylene (PE) [53], polypropylene (PP) [53, 54],

polyvinylidene fluoride (PVDF) [55, 56] or polytetrafluoroethylene (PTFE) [54, 56, 57].

Recently, super-hydrophobic membranes [58-63] have been manufactured using

electrospinning technology. Super-hydrophobic surfaces have water contact angles of over

150°. Low wettability and self-cleaning properties of super hydrophobic surfaces are

desirable for many applications such as antifouling surfaces and have received interest in

academia and industry [64]. Super-hydrophobic membranes fabricated by careful control of

the surface topography of hydrophobic material have been examined for membrane

distillation [63], biofuel recovery [60] but to the best of our knowledge are yet to be used

for ammonia removal. Large scale manufacturing of these membranes may be beneficial for

the membrane contactor industry.

The most commonly studied hollow fibre membrane modules are those supplied by

Membrana under the trade name Liqui-Cel® which comprise of PP membrane [Figure 3] [45,

65]. Alternate hydrophobic membranes can be supplied by a range of vendors, albeit not all

in the hollow fibre configuration [66]. Key performance criteria include the ability to

withstand a wide pH range, to be chemically and thermally resistant, mechanically stable,

and hydrophobic.

Tan et al. [35] applied a PVDF membrane for ammonia removal which was synthesised by

spinning a dope comprising of K-760 PVDF polymer and N,N-dimethylacetamide in the

presence of water and lithium chloride. PVDF offers advantages such as being able to form

asymmetric membranes which potentially can reduce membrane resistance by means of

possessing a pore size distribution instead of uniform pore sizes especially when the pore

sizes become smaller and Knudsen diffusion dominates [67-69]. Tan et al. [35] discovered

that post treatment of the PVDF membrane with ethanol enhanced not only the

hydrophobicity of the material but also the effective surface porosity. PTFE membranes

have also been evaluated for use in hollow fibre membrane systems. Jia et al. [70]

described means to control the physical properties of PTFE membranes to allow them to be

customized to different applications. For example, maximum pore size was in the range 1.1

to 3.0 microns, contact angle from 107 to 122 o and porosity up to 76.5 % depending upon

the stretching ratios and temperatures used. Tang et al. [71] were of the opinion that PTFE

membranes were superior with regards to their ability to not only maintain hydrophobicity

over extended operation periods but also to maintain a comparatively stable flux rate for

carbon dioxide removal from a gas stream. Literature regarding the use of PTFE membranes

for ammonia recovery is relatively scarce. Wang et al. [72] reported the integration of a

PTFE membrane contactor with a membrane distillation unit to produce concentrated

ammonia solutions from wastewater. It was claimed that up to 97.5 % ammonia removal

was obtained and that overall energy consumption was remarkably low (20 % of the energy

used in a conventional steam stripping process).

Pores sizes in hydrophobic membranes have been demonstrated to influence performance.

For example, Xu et al. [73] varied the draw ratio used in the preparation of polypropylene

membranes to discover that gas permeability and the mass transfer coefficient both

exhibited a maximum at 260 % elongation which corresponded to an average pore size of

ca. 74 nm.

Outlined above are the three most common hydrophobic membrane types (PP, PVDF &

PTFE). However, there are many other potential materials for construction of hydrophobic

membranes. Eykens et al. [66] studied in excess of twenty different hydrophobic membrane

materials used for membrane distillation. Resistance to wetting was a primary criteria

studied by investigating the contact angles which relate to hydrophobicity of the materials.

The surface energy, pore size, and roughness were critical parameters. The optimal pore

size is recommended to be ca. 0.3 µm. No one membrane was discovered to be the optimal

choice and membrane selection was dependent upon process conditions used.

4. Process Fundamentals

The transfer of gaseous species such as ammonia from aqueous solution across a

hydrophobic membrane is assumed to occur in 5 distinct stages [74]: (1) transfer of

ammonia from the bulk solution to the boundary layer at a membrane pore; (2) equilibrium

of the ammonia solution with gas (air) present in the membrane pores; (3) transfer of the

ammonia gas across the air filled pore; (4) reaction of the ammonia gas with the receiving

component (usually acid) in the strip solution; (5) transfer of the ammonium salt through

the boundary layer into the bulk strip solution. Notably, water is repelled by the

hydrophobic membrane surface unless a pressure exceeding the breakthrough pressure is

applied [74].

Agrahari et al. [45] published a comprehensive mathematical model which adequately

simulated hollow fibre membrane contactors for removal of ammonia from aqueous

solution using a sulphuric acid strip. Key aspects of the model included incorporation of not

only radial and axial diffusion through the lumen side of the membranes but also Knudsen

diffusion of ammonia through the membrane pores and the gas adsorption and desorption

behaviour at the pore walls.

Ashrafizadeh and Khorasani [75] performed a laboratory study of a polypropylene hollow-

fibre membrane module for the treatment of solutions comprising ammonium hydroxide

and purified water. The strip solution was prepared by dilution of sulphuric acid to make

appropriate concentrations of 3 to 7 % for testing. The major factor influencing the mass

transfer coefficient was solution pH, and in particular increasing the pH from 8 to 10 had the

greatest impact. Kartohardjono et al. [76] reported that increasing feed pH (10 to 12)

accelerated the degree of ammonia removal from an ammonium sulphate solution, with

ammonia mass transfer coefficients changing from 5 x 10-7 to 1.5 x 10-6 m/s, respectively.

This latter result was surprising in terms of the fact that at pH 10 essentially all the

ammoniacal nitrogen is in the form of ammonia [Figure 2]. PVDF hollow fibre membranes

were prepared and tested by Tan et al. [35] for ammonia removal from an ammonium

chloride solution in the pH range 8 to 12, a temperature of 25 oC and a 0.05 M sulphuric acid

stripping solution. The mass transfer coefficient rapidly increased as expected when the pH

rose from 8 to 10 (0.1 to 1.39 x 10-5 m/s) and only slightly improved to 1.52 x 10-5 m/s upon

further raising the pH to 11 as expected based on the degree of ammonia present [Figure 2].

The observation by Kartohardjono et al. [76] of increasing ammonia removal at pH values

above 10 cannot be explained in terms of the low acid concentration employed (0.1 M) in

the strip solution, as the study by Tan et al. [35] used only 0.05 M sulphuric acid. Without

further information it is not pertinent to speculate further on the aforementioned anomaly.

A lesser promotion of the mass transfer coefficient was observed when the feed velocity

was increased and this behaviour was ascribed to a reduction in the liquid film resistance

[75, 76]. Tan et al. [35] elaborated on this latter observation by noting that once the feed

velocity was greater than 0.59 m/s or a Reynolds number > 0.32, membrane diffusion now

became the rate controlling step thus increasing feed velocity had negligible impact at that

point.

The concentration of ammoniacal nitrogen in the feed was not found to significantly change

the mass transfer coefficient which was reported as 1.47 x 10-5 m/s and the main constraint

with acid concentration was that it should be sufficient strip the ammonia from the feed

[75]. In contrast, it has been reported that for concentrations of ammonia less than 800

mg/L as the ammonia concentration increased in the feed, the ammonia mass transfer

coefficient actually decreased from 4 to 1.5 x 10-6 m/s [76]. Zhu et al. [48] have also noted

that the mass transfer coefficient can significantly decrease with increasing ammonia

concentration in the feed. It was noted that below concentrations of ca. 2000 mg/L

ammonia that the mass transfer coefficient only varied by 5 %, whereas with concentrations

ranging up to 10000 mg/L the mass transfer coefficient measurably decreased by up to 25

%. Increasing viscosity of the high concentration ammonia solutions was invoked as being

partially responsible for the observed decrease in mass transfer coefficient. However, other

factors may need to be considered to explain the relationship between mass transfer

coefficient and ammonia concentration. The presence of salts in the feed solution did not

inhibit or accelerate the rate of ammonia removal by the membrane contactor [75].

The effect of feed temperature and stripping solution temperature should also be important

with respect to ammonia transfer across the hydrophobic membrane. Higher feed

temperature not only promotes the formation of ammonia gas [Figure 2] but also can in

theory accelerate diffusion rates for ammonia [36].

Acid stripping velocity was found not to notably impact the ammonia removal rate [75],

which was proposed to be indicative of the fact that the interface between the membrane

outermost surface and the acid stripping solution was where reaction occurred [35]. The

concentration of phosphoric acid used to strip ammonia from aqueous solution was shown

to be important by Lai et al. [77] who found that increasing the acid molarity from 0.1 to 0.3

M significantly reduced the effluent ammonia concentration. Use of 0.4 M phosphoric acid

subsequently did not notably improve the ammonia removal rate compared to 0.3 M

solutions. Further, these authors also found that sulphuric acid was more preferable than

phosphoric acid, albeit no rationale for this behaviour was provided. It was interesting that

these reported studies regarding the impact of acid concentration focussed on the removal

efficiency of ammonia, but made no mention of the possibility of interference from osmotic

distillation [78]. This latter process could promote the transfer of water vapour from one

side of the membrane to the other, the direction of which would be related to the relative

concentrations of acid on the strip side and dissolved species on the feed side.

Various modelling approaches have been undertaken in relation to predicting the

performance of hydrophobic hollow fibre membrane contactors. Tan et al. [35] examined

the material balance in the membrane contactor system for ammonia removal from water

using assumptions such as plug flow, ideal mixing and stable flow rates. Mandowara and

Bhattacharya [36] considered that assumption of plug flow behaviour was not conducive to

simulation of larger membrane contactors. Nosratinia et al. [79] focussed on understanding

the distribution of ammonia species in the contactor and also the effluent concentration

using Computational Fluid Dynamics (CFD). These authors assumed that Navier-Stokes

equations represented the velocity distribution and continuity equations were sufficient to

calculate the ammonia concentrations. Finite element methods revealed that the ammonia

concentration was predicted to be at a maximum in the centre of the fibre, to gradually

decrease as it neared the wall and to rapidly decrease in concentration at the wall. In the

axial direction, ammonia removal majorly was achieved in the initial 0.5 m of fibre length

and decreased in rate after this point due to a reduction in concentration gradient.

Increasing the feed velocity inhibited the extent of ammonia removal due to a diminished

contact time. Rezakazemi et al. [80] similarly applied CFD principles to investigate ammonia

behaviour in a membrane contactor wherein the feed solution was in the lumen and the

acid strip solution in the shell side. At the beginning of the ammonia removal process the

driving force was highest, thus the mass transfer rate was greatest at this point. Radially,

ammonia was calculated to be at the highest concentration in the centre of the lumen, with

a dramatic decrease in concentration predicted in close vicinity to the lumen wall. CFD

simulations also revealed that the fluid velocity was slower at the lumen walls, and varied

axially with maximum velocity obtained about the middle of the lumen fibre. It was

concluded that the velocity at the inlet to the lumen was not fully developed and that a

satisfactory simulation model required consideration of entrance effects to ensure accuracy.

The number of contactor modules required to be used in series is a key aspect to be

considered [34]. Thus, the question arises as to the optimal ammonium concentration

range to be treated by membrane contactors as the amount of ammonia removed per

contactor in series will reduce as the ammonia concentration in the feed diminishes.

Economic considerations may restrict the use of membrane contactors to certain water

types as increasing the number of modules required can significantly increase capital costs

due to the requirement for associated instrumentation, pumps, pipework, and pH

adjustment capacity.

5. Case Studies

As outlined in Section 1, agricultural emissions of ammonia are of particular concern in

relation to development of a suitable control methodology. As such, Garcia-Gonzáles and

co-workers [81-83] studied the recovery of ammonia from manure by means of a

hydrophobic membrane contactor. The basic principle was to raise the pH of the manure by

addition of sodium hydroxide to generate ammonia gas which was transferred into PTFE

membranes submerged in the solution. The ammonia gas was subsequently reacted with a

circulated acid solution in the lumen. In accord with Equation 2 the authors found it

necessary to intermittently add sodium hydroxide whenever the solution pH dropped to 7.7

to obtain the working pH of ca. 8.5 to 9.0 [81]. The ammonium concentration in the acid

solution was found to be 3.5 times the corresponding amount in the manure solution [81].

Aeration of the manure sample was discovered to be advantageous in that elevation of the

system pH was achieved by means of reaction with bicarbonate alkalinity which produced

carbon dioxide and hydroxyl species [83]. Consequently, operational costs which

presumably considered the additional cost of the aeration stage, were estimated to be

overall reduced by >50 % while ammonia removal efficiency was maintained at 98 %.

Further studies which included a nitrification inhibitor suggested that aeration of the

manure could reduce costs by up to 70 % and economic analysis indicated that a net cost of

only US$3924 per annum was incurred for a 4000 head swine utility [82]. If water quality

credits were factored in the calculation then it was possible to gain a financial benefit of

US$77,427.

The presence of ammoniacal nitrogen in water and wastewater is particularly prevalent in

industry and municipal water situations [12]. Licon et al. [34] evaluated the ability of

membrane contactors to remove relatively low concentrations of ammonia (5 to 25 mg/L)

from water which was used for electrolysis to create hydrogen. A notable increase in

ammonia removal rate was evidenced once the pH surpassed the pKa value of 9.3 albeit

once pH was attained the ammonia removal rate was relatively constant. With an “open

loop” process configuration it was noted that increasing the feed velocity past a certain

value severely diminished the degree of ammonia removal. Once the Reynolds number for

the fluid in the lumen (the feed) exceeded 6 the ammonia mass transfer coefficient was

stable. A “closed loop” system was also tested by Licon Bernal et al. [49] which employed

buffered solutions in an effort to keep the feed pH at values sufficient to ensure ammonia

was the main nitrogen containing species present. Bicarbonate/carbonate buffer systems

were not recommended due to limited buffer capacity, instead a boric acid/borate buffer

was preferred. Using the aforementioned system ammonia removal of >95 % was observed.

In the case of a “closed loop” configuration increasing the feed velocity actually increased

the ammonia removal as the number of times the feed contacted the membrane surface

was enhanced. More recently, Licon Bernal et al. [6] examined the ability of hydrophobic

membrane contactors to concentrate ammonia from a regenerant solution. The latter

solution arose from a zeolite ion exchange process which had been used to recover

ammonium species from a tertiary treatment facility at a wastewater treatment plant.

Ammonium nitrate and diammonium phosphate could both be recovered and a predictive

model was developed which could accurately simulate the experimental data under a range

of operational conditions. Integration of the membrane contactor with the zeolite ion

exchange process allowed recovery of highly pure salts.

Liu and Wang [84] described the recovery of ammonia from a radioactive wastestream

comprising of 238U, 25,000 mg/L ammonia, tetrahydrofurfuryl alcohol (THFA) and nitrate

species. It was found that the ammonia mass transfer coefficient was independent of the

presence of either urea or THFA and was a value of 8.9 x 10-6 m/s. As the feed velocity was

increased from 0.0048 to 0.248 m/s a significant promotion in the degree of ammonia

removal was observed (from 75.8 to 90.4 %, respectively). It was postulated that the

elevated flow rates enhanced turbulence and decreased mass transfer resistances. In

addition, it was shown that the optimal velocity was 0.049 m/s as above this value minimal

increase in ammonia removal was evident but a significant increase in energy cost was

apparent. In accord with Ashrafizadeh and Khorasani [75] large changes in ammonia

concentration (ca. 2211 to 23899 mg/L) did not impact ammonia removal rates.

Wäeger-Baumann and Fuchs [47] were interested in the removal of ammonia from

anaerobic digester effluent. However, they noted that with this type of solution there were

inherent issues associated with the presence of particulate matter which could block

apparatus. Consequently, these authors proposed that hollow fibre membranes should be

directly placed in the digester effluent without any shell casing present. In this case, the

acid stripping solution was passed through the lumen side. A crucial aspect of this study

was the realization that the specific membrane area was required to be relatively high (32

m2/m3) as with lesser values the degree of ammonia removal was unsatisfactory. A solution

pH of 10 was employed which promoted the extent of ammonia removal from solution but

represented a significant cost as digester effluent is buffered by two systems,

ammonia/ammonium and bicarbonate/carbonate. Lauterböck et al. [85] extended the

studies of Wäeger-Baumann and Fuchs [47] by testing submerged hollow fibres for longer

periods (351 days) with anaerobic digester fermentation broth. The outlined system

performed well and an average of 70 % ammonia removal was recorded. It was correlated

that increasing free ammonia decreased the mass transfer coefficient, which was in contrast

to the view of Ashrafizadeh and Khorasani [75] who found minimal influence of ammonia

concentration in solution. Lauterböck et al. [85] proposed that the high viscosity of the

digester solution may be a limiting factor in their case. A slight contamination of the strip

acid solution was noted by the presence of sodium and potassium ions from the digester

solution. More recently, Lauterböck et al. [86] evaluated the benefits of inclusion of a

membrane contactor submerged in anaerobic digestate upon the performance of the

bacteria present. A substantial increase in methane production was noted when the organic

load was 4.2 kg COD/m3/day and hydrogen sulphide concentrations were <3.3 mg/L.

The influence of the strip solution composition was investigated by Kartohardjono et al. [87]

who compared sulphuric acid to Natural Hot Spring Water (NHSW). For the same pH value,

NHSW exhibited slightly higher ammonia removal efficiencies which was attributed to the

presence of various acids in NHSW including hydrochloric and nitric acid. Mandowara and

Bhattacharya [33] dispensed with the acid stripping solution and instead passed the

ammonia containing aqueous solution through the lumen side of a membrane contactor

module and applied a vacuum on the shell side to generate the required concentration

gradient. These authors found that the exit concentration of ammonia was highly

dependent upon the velocity of the aqueous stream in the lumen and removal rates >70 %

were obtained when the velocity was lowered to 0.02 m/s or less. Hossain and Chaalal [65]

proposed an alternative strip solution which comprised of di(2-ethylhexyl) phosphate

(D2EHPA) in sunflower oil solvent. The use of sunflower oil was viewed as being more

sustainable than alternate methods and safer for operators to handle.

6. Pilot/Commercial Plant Trials

In terms of operation of membrane contactors for treatment of significant amounts of

ammonia from aqueous or gas states, it is necessary to consider the configurations of the

membranes used. A generic process flow diagram for pilot testing of a membrane contactor

for ammonia removal from aqueous solution is shown in Figure 4. The illustrated version is

an “open loop” version as the feed water is simply passed through the contactor and

collected in the treated water tank. If the feed solution was recycled to the feed tank after

exiting the membrane contactor this would be a “closed loop” configuration which used

multiple passes of the solution through the membrane to achieve required discharge limits

for ammonia. In practice, several membrane modules could be used in series. The treated

water may be used for beneficial reuse if ammoniacal nitrogen mitigation of the solution

makes it compliant with environmental regulations.

Boehler et al. [88] reported the outcomes of a pilot plant membrane contactor unit for the

removal of ammoniacal nitrogen in the form of ammonium sulphate from wastewater

treatment plant effluent. Three hollow fibre membrane contactors in series were employed

with a total surface area of 120 m2, with flow rates ranging from 5 to 12 L/m2 h and

sulphuric acid solution was passed through the lumen side. The wastewater was pH

adjusted to ca. 9.5 in order to almost entirely convert the ammonium species to ammonia

gas and as such it was found that removal values of 95 % or greater could be achieved in

practice when the ammonium-N content was 700 to 3400 mg/L. Overall, the prospect for

commercial application of the contactor technology was deemed to be good with the main

advantage of the process being that inclusion of a CO2 stripper section reduced the cost

associated with addition of sodium hydroxide to elevate solution pH. Norddahl et al. [29]

also conducted a pilot plant study wherein water from either an anaerobic digester which

was subsequently been filtered by an ultrafiltration unit or a centrifuged sample from a

sludge generated by a municipal solid waste treatment plant was passed through a

polypropylene hollow fibre membrane system. The strip solution consisted of 1 wt/wt%

sulphuric acid. In accord with previous studies [34], pH values of 10 or greater resulted in

substantial acceleration of the removal of ammoniacal nitrogen due to the almost total

formation of free ammonia. Ulbricht et al. [43] summarized the results from a commercially

operating membrane contactor system located in Wuppertal, Germany which was

employed by Membrana GmbH to remove ammonia from their manufacturing facility

effluent. Two membrane contactors were used in series which was sufficient to treat

between 5 and 10 m3/h of water which contained 500 to 2000 mg/L ammonia at a

temperature of 40 to 50 oC and minimal levels of particulates. The feedwater was pH

adjusted to 9 or greater which facilitated ammonia removal values of up to 95 %. A

common theme from the aforementioned pilot plant studies was the need to raise the feed

solution pH to at least 9 and preferably 10 in order to convert the majority of ammonium

ions to ammonia gas. Notably, Boehler et al. [88] found that precipitates correspondingly

formed in the stripper sections of their ammonia removal plant which could lead to

equipment fouling and clogging. In terms of the ammonia membrane contactors calcite

(CaCO3) was the principal precipitate identified. A technical solution was devised which

involved cleaning of the ammonia stripper with hydrochloric acid solution to dissolve the

carbonate species.

The impact of sulphuric acid concentration and the relation to stability of the ammonia

treatment system has also been evaluated. PP membrane contactors were used by Li et al.

[89] to remove ammonia from landfill leachate. Two membranes were deployed in series

and operated with a flow rate of 100 L/h, ammonia levels in the range 1000 to 3000 mg/L,

and a 6 to 10 % sulphuric acid stripping solution which was recirculated at 200 L/h. Notably,

the landfill leachate was pre-treated to reduce the chemical oxygen demand (COD). Under

the outlined conditions almost complete removal of ammonia was reported and it was

stated that the pilot plant was operationally stable for a period of 2 months. The mass

transfer coefficient was approximately 6 x 10-6 m/s for the test duration. It was

demonstrated that acid concentration of 6 % or higher increased both the ammonia mass

transfer coefficient and extent of ammonia removal from the leachate. A distinct promotion

of the ammonia mass transfer coefficient was also found when increasing the leachate

solution temperature from 10 to 40 oC (ca. 4 to 8.75 x 10-6 m/s, respectively) in accord with

the ammonia estimates as a function of temperature in Figure 2. Consequently, use of high

concentrations of acid stripping agent appeared reasonable. Indeed, Boehler et al. [88]

recommended use of 60 % sulfuric acid solutions in order to aid formation of high

concentration ammonium sulphate solutions. However, Shao et al. [90] installed a

membrane contactor pilot unit at a water treatment plant which was using ammonia as part

of a disinfection process involving chloramines and showed that there was an upper limit to

acid concentration which could be employed. The primary objective was to demonstrate

the applicability of the tests unit for remediation of ammonia leaks. Four polypropylene

membrane contactors, each with 42 m2 of membrane surface area were employed. As part

of their studies, an examination of the polypropylene membrane was conducted to

understand the resistance to contact with sulphuric acid solutions. Degradation was not

observed by electron microscopy until 40 % acid solutions were involved and oxidation of

the material was not evident until the acid concentration was increased to 50 %.

Experiments with different concentrations of acid during ammonia removal tests revealed

that 10 % sulphuric acid was optimal. When 20 % sulphuric acid was used the mass transfer

coefficient decreased and this was attributed to greater viscosity of the acid solution.

Increasing the velocity of the acid solution distinctly promoted the mass transfer coefficient

for ammonia in line with previous studies [75, 76]. Almost complete removal of the

ammonia gas (99.9 %) was observed in this study.

The quality of the ammonium sulphate product has also been elucidated in pilot plant trials.

Klaassen and Jansen [91] described a membrane contactor pilot plant for ammonia gas

removal from a dye intermediate facility in the Czech Republic. It was claimed that the unit

which used polypropylene membranes with 0.2 micron pores was able to remove 50 kg/h

ammonia, reduce ammonia emissions by up to 99.9 % and recover a 27 wt% aqueous

ammonia product. Boehler et al. [88] analysed various ammonium sulphate samples

produced using membrane contactor technology and discovered that only low levels of

micro-pollutants were present and that the fertilizer quality should be acceptable for

agricultural use.

A critical aspect of pilot plant tests is to ascertain details regarding process economics.

Rothrock et al. [92] applied a hydrophobic membrane to capture ammonia gas released in a

poultry farm at pilot plant scale. PTFE membrane with a total surface area of 0.3716 m2 was

used along with 1 N sulphuric acid strip solution which was recirculated at a rate of 27

L/day. Liming of the poultry litter was performed in order to volatilize the ammonia and it

was found that 73 to 96 % of ammonia gas was removed by the membrane under the

outlined conditions. Maximum recovery of ammonia was 17.78 g N/m2/day over a period of

4 days. Simple economic calculations based upon estimated annual capital and sulphuric

acid costs with the value of the produced ammonium sulphate suggested a slight loss in

income (ca. US$2689 per annum) for a shed with 20,000 broilers. However, when taking

into consideration reduced energy costs associated with a reduction in ventilation

requirements, better air quality and reduced death rates a net benefit was predicted.

A summary of the studies provided above in relation to application of membrane contactors

for ammonia removal in practical situations is provided in Table 1.

7. Future Directions

As with all membrane based technologies, the issue of fouling of the membrane surface

during operation is an issue that must be addressed. Efforts to improve the resistance of

hydrophobic membranes to fouling events included that by Wang et al. [93] who described

the coating of PVDF membranes with chitosan, sodium alginate and polyfunctional lysine

and demonstrated significant improvement in anti-fouling capability. Preparation of

composite membranes is another strategy which has been pursued, such as the

incorporation of zinc oxide into electro-spun PVDF fibres which controlled fibre roughness

and diameter [94]. Incorporation of silver into the membrane may also aid anti-bacterial

properties [95]. Another strategy for minimization of membrane fouling has been revealed

by Lauterböck et al. [85] who have shown that long term operation of membrane contactors

is possible with tests up to 351 days completed. Notably, in this latter study the membrane

fibres were directly placed in anaerobic digester effluent without the outer casing present in

order to minimize fouling events. The cleaning in place (CIP) methodology is also one which

must be addressed when using membrane contactors for ammonia removal. Woo et al. [96]

examined the impact of a range of cleaning approaches such as backwashing, chemical

enhanced backwashing, air scrubbing and aeration upon hydrophobic PVDF hollow fibre

ultrafiltration modules. It was found that membrane exposure to chemicals could be

detrimental and thus it was important to develop precise CIP procedures which optimize

control of both hydrophilic and hydrophobic contaminants and minimise membrane surface

damage. However, the long term trials of membrane contactors by Ulbricht et al. [43] did

not mention issues with membrane stability, although it is noted that the feedstream

treated was relatively clean. Similarly, Lauterböck et al. [85] noted no decrease in

membrane performance after almost a year of operation, albeit they did not perform any

cleaning cycles. Hence, the importance of the impact of CIP procedures may relate to the

quality of the water to be treated.

Osmotic distillation (OD) can occur at ambient temperature and involves differences in

vapour pressure either side of a hydrophobic membrane. Thus the possibility arises that

transfer of water vapour may occur during the ammonia removal process which may lead to

either a concentration or dilution of the acid stripping solution dependent upon the relative

water activities for the feed and strip solutions. Practically, this could lead to problems with

flooding or emptying of the strip solution vessel and this effect has been mentioned by

Boehler et al. [88] as diluting the concentrated acid solution. Nevertheless, the impact of

osmotic distillation has scarcely been mentioned in existing literature in relation to

ammonia removal from solution using membrane contactors and should be investigated to

determine the impact and how to control this phenomenon.

Despite the publication of several studies relating to longer term evaluation of hydrophobic

membranes for ammonia removal and recovery [43, 89], there is minimal examination of

the process economics, and to which problems it is best applied [82]. Jilvero et al. [97]

evaluated the techno-economics of controlling ammonia slip from post combustion

processes. Absorption, acid washing, and steam stripping methods were investigated but

hydrophobic membrane contactors were not. One of the main conclusions was that

approaches which destroyed the ammonia were not cost effective.

The question as to what ammonia concentration range is optimal for implementation of

membrane contactors remains to be answered. Literally hundreds of mg/L ammonia can be

removed in a single pass through one module when the concentrations of ammonia are

relatively high. However, if low emission standards (i.e. < 1 mg/L) need to be met then a

large number of modules in series may be required. As such capital costs would increase

substantially due not only to the increasing number of membrane contactors but also the

associated pumps, monitoring equipment, fittings and piping. An analogous situation has

been identified in relation to the application of nanofiltration or reverse osmosis membrane

technology to the separation of ammonia from manure [98] where it was found that higher

ammonia concentrations favoured the implementation of membrane technologies. If

membrane contactors are best employed for reducing ammonia concentrations to for

example 50 mg/L (thus minimizing number of modules required), then integration with

another ammonia removal technology such as a biological process may need to be

considered. This latter example needs further analysis to allow decisions to be made

regarding overall process design.

A related question concerns the daily flow rates which are amenable to treatment using

membrane contactors. Consider wastewater streams which can range from a few thousand

litres per day to tens or hundreds of mega-litres. To be a viable proposition for large scale

de-ammonification of wastewater the physical footprint would typically need to be

relatively small and the technology simple. Membrane contactors may satisfy these latter

conditions but these criteria need to be verified as yet.

The quality of the ammonium product produced and the market demand should also be

evaluated. As noted by Garcia-Gonzáles and Vanotti [81] hydrophobic membranes tend to

repel undesirable species from entering the fertilizer product, but consideration must also

be taken as to the fertilizer concentration range which is useful and achievable especially in

light of the aforementioned influence of osmotic distillation. The use of phosphoric acid

should be investigated more since as indicated by Licon Bernal et al. [6] ammonium

phosphates are commonly used as fertilizers and the ammonia removal behaviour is

different when using phosphoric acid as the ammonia stripping agent compared with

sulphuric acid solutions. In relation to the value proposition for the fertilizer product,

factors such as risks associated with volatility of fertilizer, sodium hydroxide and transport

costs should also be incorporated in economic models.

8. Conclusions

Hydrophobic membrane contactors appear to exhibit considerable promise for the

sustainable capture of ammonia emissions from waste, water, and wastewater sources.

This technology has been proven to at least pilot plant scale and the underlying scientific

principles are understood. This review has identified that a range of hydrophobic

membrane materials may be used depending upon availability from commercial vendors.

Due to the inherent requirement for the presence of ammonia gas there is universal

agreement that high pH conditions are required for successful operation. It appears that

flowing the ammonia solution in the shell side is preferable to the opposite situation where

the strip acid solution is in the shell side. Feed velocity was determined to be relatively

unimportant in a closed loop configuration system but critical in an open loop design where

contact time with the membrane surface was reduced and consequently the ammonia

concentration in the effluent was raised. The influence of ammonia concentration upon

membrane performance is more complicated with increasing amounts present reducing the

mass transfer coefficient. Sulphuric acid has most often been used as the stripping agent as

it forms a valuable product, ammonium sulphate. The velocity that the acid circulates

through the membrane lumen does not significantly impact ammonia removal. On the other

hand, increasing acid concentrations aid ammonia removal, but the impact of osmotic

distillation needs to be assessed to prevent the strip solution tank either overflowing or

drying out. Pilot plant tests of the membrane contactor technology have all been reported

as being successful and a commercial plant has been operating in Europe for several years.

Based upon our review, there expansion of the use of hydrophobic membrane contactors is

highly prospective especially if the following research questions are addressed. Future

research should be directed towards extending the practical application of the technology

and consideration of issues such as: (1) customization of membrane materials to different

water compositions by modification of hydrophobicity and use of hydrophilic treatments to

minimise fouling propensity; (2) integration of membrane contactors with other treatment

technologies; (3) process design and identification of most suitable ammonia concentrations

to treat; (4) techno-economic evaluation including risk analysis; (5) determination of

cleaning in place conditions and operational means to minimize membrane fouling.

9. Acknowledgements

The support of the Science and Engineering Faculty at QUT is appreciated. Mariam

Darestani is also grateful for the award of an Advance Queensland Fellowship.

10. References

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Figure 1: Illustration of the operating principles of membrane contactors for ammonia

recovery from wastewater

(a) Impact of salinity

(b) Effect of Solution Temperature

Figure 2: Estimated equilibrium between ammonium and ammonia as a function of pH,

temperature, and salinity: 1214 mg/L ammonia used in simulation.

Figure 3: Illustration of a typical membrane contactor (Reproduced with permission from

www.liquicel.com)

Figure 4: Process Flow Diagram for “open loop” ammonia removal from wastewater using

membrane contactor

Table 1: Summary of the studies reported for bench-top, pilot, and commercial trials of membrane contactors for ammonia removal and recovery

Description Reference

Ammonia emissions from manure Garcia-Gonzáles and co-workers [81-83]

Ammonia present in an electrolysis solution Licon et al. [34]; Licon Bernal et al. [49]

Ammonia in a radioactive wastewater

comprising uranium

Liu and Wang [84]

Anaerobic digester effluent Wäeger-Baumann and Fuchs [47];

Lauterböck et al. [85]

Ammonia control in a wastewater treatment

plant

Boehler et al. [88]

Anaerobic digester effluent Norddahl et al. [29]

Manufacturing effluent treatment Ulbricht et al. [43]

Remediation of ammonia from landfill

leachate

Li et al. [89]

Control of ammonia leaks from a

wastewater treatment facility

Shao et al. [90]

Ammonia gas from a chemical (dye) factory Klaassen and Jansen [91]

Ammonia gas emissions from a poultry farm Rothrock et al. [92]