Wastewater treatment by membrane distillation

CHAPTER 1

Wastewater treatment by membrane distillation Antonio Comite, Marcello Pagliero and Camilla Costa Department of Chemistry and Industrial Chemistry, University of Genoa, Genoa, Italy

1.1 Introduction Water is considered a resource at risk due to pollution and the world’s increasing population. Efficient policies for effective water distribution and consumption are needed to protect this resource and to minimize wastewater generation.Therefore, to achieve water sustainability it becomes mandatory to consider wastewater as a valuable resource for generating again safe water. Technological processes such as membrane technology are key to advanced water and wastewater treatments. To effectively implement policies for water and wastewater management, membrane processes such as microfiltration (MF) and reverse osmosis (RO) have been widely applied on a commercial scale for water depuration, desalination, and wastewater treatments. Nevertheless, other emerging membrane processes are attracting greater attention and, among these, membrane distillation (MD), which, although still in the initial commercialization phase in the desalination field, is among the membrane processes expected to enable zero-liquid-discharge (ZLD) or to maximize water recycling while minimizing wastewater volumes [1]. The first MD patent was filed by Bodell in 1963 [2] back when suitable inherently hydrophobic membranes were not yet available. When hydrophobic polymer membranes appeared on the market the first studies on MD were devoted to desalination and then to food processing. While researchers have studied the application of MD to wastewater over the last two decades there are challenges that must be addressed before this technology can be applied at a commercial level for wastewater treatment. Fig. 1.1 shows the general trend of MD in scientific publications from the mid-1980s up to 2018. As can be seen, the number of papers addressing wastewater treatment is still limited. In recent years many books and reviews have been published on the subject of MD [3
9]. At the beginning of this chapter, we give an overview of MD and later we discuss the main

Current Trends and Future Developments on (Bio-) Membranes. DOI: https://doi.org/10.1016/B978-0-12-816823-3.00001-0 © 2020 Elsevier Inc. All rights reserved.

3

4

Chapter 1 200 180 Number of publications

160 140

"membrane distillation" "membrane distillation wastewater"

"membrane distillation desalination"

120 100

a

80 60 40

b

20 0 1995

c 2000

2005

2010

2015

2020

Year

Figure 1.1 Publication trend in the last two decades using the keywords (a) “membrane distillation,” (b) “membrane distillation desalination,” and (c) “membrane distillation wastewater” in the title search field of ScienceDirect (Elsevier).

parameters for its application in the wastewater sector. In particular selected examples found in the literature will be reviewed.

1.2 Overview of membrane distillation The distillation process exploits the differences in volatility to separate the components of a liquid solution. The vapor pressure of a pure liquid can be empirically estimated by using the Antoine equation: log10 ð p Þ 5 A 2

B T 1C

(1.1)

where p is the vapor pressure (Pa or bar); T is the temperature (K or  C); and A, B, and C are the constants for each substance. For pure water expressing the vapor pressure in Pa and the temperature in K, the Antoine’s constants are A 5 23.1964, B 5 3816.44, and C 5 46.13 [10,11]. By increasing the temperature the vapor pressure increases. When nonvolatile solutes are diluted in a water solution the Raoult equation can account for the change in the vapor pressure pf: pf 5 xw pow

(1.2)

Wastewater treatment by membrane distillation 5 where xw is the molar fraction of water. For more concentrated solutions in which the solute
water interactions are more important xw needs to be replaced by the water activity: pf 5 aw pow 5 γ w xw pow

(1.3)

where aw is the water activity and γ w is the activity coefficient for water. Therefore when a solute is present usually the vapor pressure is lower than for pure water. The evaporation takes place at the interface between the liquid and the vapor phases and thus the molar flow rate of evaporation depends on the evaporation surface area and on the water flux. When the evaporation interface is mediated by a porous membrane that is not filled by the liquid phase, but only by the vapor phase, the distillation process is deferred to as MD. Considering an aqueous feed, a hydrophobic porous membrane can create a controlled and known (from geometrical considerations) evaporation surface. Then only the vapors of the volatile components, solvent, and/or other volatile species in the feed will diffuse through the porous structure of the membrane to the other side where they can be drained out by vacuum or a sweep gas, or condensed in a liquid phase that may have direct contact with the membrane surface. Which is an isothermal process that exploits a concentrated salt solution, known as draw solution, on the condensing side of the membrane to create a vapor pressure gradient which drives the water mass transfer [4]. MD is essentially a thermally driven separation process in which a hydrophobic porous membrane in contact with a hotter liquid solution (usually an aqueous one) works as an artificial evaporation interface. By simultaneously exploiting a gradient of temperature between the feed phase and the collecting phase on the permeate side and a sufficiently high contact area, high evaporation flow rates are possible even at operating temperatures lower than boiling point of the feed. Since in MD the flow rate can be easily raised by increasing the membrane contact area, the feed does not need to be heated up to the solvent boiling point and low-grade thermal sources, such as solar or geothermal, can be conveniently exploited. Furthermore, since the driving force of the process is a difference of partial pressure of water at the two sides of the membrane, the operating pressures of MD are well below the ones used for pressuredriven membrane processes such as RO, resulting in less severe fouling phenomena and lower operating costs for the same concentration factor. Moreover, unlike RO, the osmotic pressure is not a limiting factor for achieving both very high solute concentrations and water recovery. Since only water vapor can pass through the membrane, theoretically MD can provide complete rejection of all nonvolatile compounds. All these features make MD a feasible competitive process for water treatment. However, MD does have some limiting factors. The thermal conductivity of the membrane as well as the evaporation and condensation processes lead to high heat losses. Moreover, the

6

Chapter 1

permeate flux is lower compared to RO performance and heavily affected by feed fluid dynamics and by temperature and concentration polarization effects. Although in most MD applications the feed temperature is lower compared to traditional distillation processes, in some processes a temperature of the feed close to its boiling point at ambient pressure or even higher for pressurized feeds may be required. Indeed, it is well known that a higher feed temperature results in a higher vapor flux through the membrane and a higher thermal efficiency of the process [12,13].

1.3 Membrane distillation configurations Four main MD configurations have been developed: • • • •

vacuum membrane distillation (VMD); sweeping gas membrane distillation (SGMD); direct contact membrane distillation (DCMD); and air-gap membrane distillation (AGMD).

In Fig. 1.2 the different MD configurations are schematized along with the general temperature and concentration profiles of the nonvolatile and volatile components in the feed solution. The subscripts f and p stand for feed and permeate, respectively. In the VMD configuration the vacuum is applied on the permeate side of the membrane and the vapor is condensed an external cooler. The heat loss is lower than for the other MD configurations but the pore wetting on the feed side and fouling phenomena can be more relevant due to the presence of a pressure gradient [14
16]. In VMD the conductive heat loss is negligible due to the lower thermal conductivity of vapors at low pressure, while the permeate flux is high and the recovery of the most volatile compound occurs. Due to the pressure gradient between the two membrane sides, the risk of pore wetting is higher in VMD in addition to the possibility of fouling the membrane. In VMD an external vacuum pump and an external condenser need to be included in the process layout. In the SGMD configuration a carrier gas flows in the distillate channel of the membrane module and carries the vapor to an external cooler. The permeate flux is lower than in VMD and in addition large condensers are necessary due to the low vapor concentration in the sweep gas. SGMD is less affected by thermal polarization than the DCMD as discussed in the following, and it is can remove volatile compounds and dissolved gases as does VMD. In the DCMD configuration the hot feed and the cold distillate are directly in contact through the membrane. Since it is the simplest configuration and provides a stable flux of distillate, DCMD is already at a demonstration scale for the desalination process. However, DCMD is also characterized by low energy efficiency and high pore wetting because of the

Wastewater treatment by membrane distillation 7 Tf

Tf Cf , solvent

Cf , solvent

Tp

Tp

Cp, solvent

Cf, solute

Cp, solvent

Cf , solute

δ

VMD

SGMD

Feed

Feed

Sweep gas

Tf

Tf

Tp Cp, solvent

C f , solvent

Cold surface

Cf , solvent

Tp

Cf , solute

Cp, solvent

C f , solute

DCMD

Feed

Cold permeate

AGMD

Feed

Air gap

Condensed water

Figure 1.2 Simplified membrane distillation configurations. AGMD, Air-gap membrane distillation; DCMD, direct contact membrane distillation; SGMD, sweep gas membrane distillation; VMD, vacuum membrane distillation.

direct contact between hot and cold liquids. While DCMD is the simplest configuration it has the highest heat loss by conduction and thus cannot recover other volatile compounds or dissolved gases. In order to limit the disadvantages of DCMD, in the AGMD configuration a space filled with stagnant air separates the permeate side of the membrane from a cooled plate where the distillate condenses. This reduces the wetting at the permeate side, and the thermal loss for conduction through the membrane and the flux is also lowered. AGMD has less conductive heat loss, low tendency for fouling, and high flux but there is additional mass transfer resistance and module design is more difficult. Variants of AGMD are the liquid gap membrane distillation (LGMD) or permeate gap membrane distillation (PGMD) configurations where a liquid (usually the permeate) fills the gap between the membrane and the condenser surface [17].

8

Chapter 1

As it can be seen from the temperature and concentration profiles (Fig. 1.2) temperature and concentration polarization phenomena can occur depending on the distillation rate and on the fluid dynamic regimes on the feed and permeate sides. Details on the polarization phenomena and mass transfer in the porous membrane applied to MD can be found in the literature [18,19].

1.4 Membranes Like in other membrane processes the membrane itself is one of the most important factors contributing to the MD performance. The hydrophobic character is essential since it prevents pore wetting but other important requirements include high porosity to extend the effective liquid
gas interface as much as possible and narrow pore size distribution [20]. In fact, the largest pores are flooded more easily and can allow the feed solution to pass through the membrane compromising the separation properties of the whole process [15,21]. Since during the distillation operation the membrane porosity should not be flooded by the liquid phase, one of the most important parameters is the liquid entry pressure (LEP), which is defined as the lowest feed pressure that allows the passage of liquid through the membrane. From this value, the largest pore size can be estimated using the Cantor
Laplace equation: LEP 5

2Bγl cosθ rmax

(1.4)

where B is a geometric factor accounting for the pore shape (0 , B , 1 for noncylindrical shapes; B 5 1 for cylindrical pores); γl is the liquid surface tension; rmax is the largest pore size; θ is the contact angle between the membrane and the liquid feed. The LEP usually decreases by increasing the temperature due to decreasing contact angle and surface tension. LEP is of particular significance in VMD, which works under a pressure gradient. Therefore for VMD the requirement of a very narrow pore size is more important than with the other configurations. In MD polymeric membranes originally designed for MF are still widely used, and therefore optimization of the structure/material and proper membrane functionalization can strongly improve the performance of the MD process [15,20]. It has been suggested that the maximum pore size to prevent wetting is between 0.1 and 0.6 µm [14]. Jacob et al. [22] studied the effect of process variables on the membrane wettability. As seen LEP depends on the liquid surface tension and the contact angle. In wastewater treatment the wettability and consequently the flooding of the porosity of the membrane can

Wastewater treatment by membrane distillation 9 be influenced by the presence of organic molecules, and in particular of oils and surfactants, which act on the surface tension and on the contact angle. An example of how the presence of a surfactant can affect membrane wettability was reported by Eykens et al. [23]. It should be noted that partial membrane flooding may occur even at pressures lower than LEP determined with either pure or saline water. Since pore wetting can take place slowly, assessment of long-term stability at specific operating conditions is needed before application at industrial scale. The main polymers used because of their lower surface energy are polytetrafluoroethylene (PTFE), polyvinilidene fluoride (PVDF), and polypropylene (PP). Commercial membranes made of those polymers are readily available. In Fig. 1.3 the surface morphologies of some commercial PP and PTFE membranes are given. The manufacturing of these membranes involves different techniques depending on the polymer such as either nonsolvent or thermally induced phase separation (NIPS or TIPS), melt extrusion stretching and sintering [16]. PTFE is characterized by excellent thermal and chemical stability as well as low surface energy that allow the preparation of membranes with good hydrophobicity and wetting resistance. The melting point of PTFE is high (327 C) and since it does not dissolve in any solvent at room temperature the preparation of PTFE membranes is complicated. PTFE membranes are prepared by extrusion, rolling, stretching, sintering, or in some instances by melt processing techniques [15,16,20]. PP membranes can be prepared with a stretching method or by the TIPS process. In the latter case a PP is mixed with a suitable diluent and heated up to its melting point, resulting in a homogeneous solution being formed and cast. The phase separation is then obtained by cooling [24]. PVDF is a soluble polymer and therefore the TIPS and NIPS processes are used for manufacturing. In contrast to the thermal process, in NIPS the cast solution is submerged in an liquid that extracts the solvent from the solution. Thus the polymer precipitates forming

Accurel PP 2E HF—0.4 µm

1 µm

Celgard 3500—PP

1 µm

Membrane Solutions PTFE—0,22

1 µm

Membrane Solutions PTFE—1,0

1 µm

Figure 1.3 Examples of surface morphology of the four commercial membranes.

10

Chapter 1

a porous matrix; the morphology of the membrane is controlled by the composition of the solution and by the interaction between the nonsolvent and the solvent. Membranes based on polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) [25,26] or poly(tetrafluoroethylene-co-hexafluoropropylene-co-vinylidene fluoride) prepared by electrospinning are also being developed [27]. An overview of the techniques and approaches used for the preparation of membranes specifically for MD can be found in Eykenes et al. [20] and Wang and Chung [16]. The development of novel MD membranes aimed at creating membrane surfaces less prone to pore wetting by introducing omniphobic character is in progress [28,29]. As reported in Lu et al. [29] one of the approaches to obtain the omniphobic character is improving the roughness of the membrane surface by, for example, adding hydrophobic nanoparticles. Omniphobic membranes are of high interest for MD wastewater applications with low surface tension. Another approach to mitigating wetting and reducing the fouling impact consists of creating layered hydrophobic
hydrophilic membranes. The hydrophilic surface hosts an hydration layer which prevents the oil wetting [30,31]. With polymeric membranes the maximum temperature of the feed stream is limited by the physical properties of the material itself. On the other hand, ceramic membranes are made of metal oxides (e.g., alumina, silica) and have better mechanical properties and higher thermal resistance, but because of the hydroxyl groups on their surface, they have hydrophilic behavior [32
36]. Various techniques to change the hydrophilicity of membranes, such as plasma modification, microwave plasma-enhanced chemical vapor deposition, and reaction with low surface energy compounds, have been investigated [34]. The development of hydrophobic ceramic membranes can extend the possible applications of MD processes to cases where the operating conditions (e.g., temperature, pressure) prevent the use of polymeric membranes [37].

1.5 Membrane modules One of the reasons for the popularity of membrane technology is its scalability. Membranes assembled in adequate modules should exploit their productivity by optimizing the operational variables. Indeed, module design has the main objective of realizing high specific membrane area per volume ratios, allowing high flow rates in a small footprint. Moreover, the fluid dynamics are highly affected by the module geometry and as mentioned above play a key role in membrane performance, controlling temperature and concentration polarization phenomena along the membrane.

Wastewater treatment by membrane distillation 11 In conclusion optimal design approaches provide the highest heat and mass transfer in a MD module and should: • • •

minimize the concentration polarization effects; minimize the temperature polarization effects; and ensure low fouling tendency.

To minimize the polarization phenomena the module should allow high feed velocity in order to be as much as possible in the turbulent regime near the membrane surface. Most of the studies carried out on MD pilot units used modules developed for other membrane processes and therefore not optimized for distillation purposes. Many membrane modules have been proposed to satisfy different process requirements [38]. In general three main module schemas have been suggested (Fig. 1.4): • • •

shell-and-tube modules for hollow fiber or tubular membranes; spiral wound for flat-sheet membranes; and plate and frame modules for flat-sheet membranes.

The shell-and-tube module has been widely studied and used in all the MD configurations (DCMD, AGMD, SGMD, and VMD) because high surface area/module volume ratios can be reached and it is also easy to produce. The tubular or the hollow fiber membranes can operate in two different configurations: if the feed flows outside the fibers, it is referred to as outside-in configuration, while when the feed is sent into the membrane lumen it is called inside-out configuration. The wide availability of membranes with different pore size and channel diameter makes it possible to treat many different feeds, even with suspended solids (if the outside-in configuration is used). However, low flux and weakness of the fibers are the main disadvantages of this scheme [39]. Outside-in

Inside-out

Membranes Concentrate

Feed Capillary membranes Feed

Feed

Permeate

Concentrate Product Spacer

Resin potting

Feed Spacer

Permeate (A)

Permeate Concentrate (B)

(C)

Figure 1.4 Membrane modules: (A) shell-and-tube, (B) plate and frame, and (C) spiral wound. Source: Adapted from D. Winter, J. Koschikowski, F. Gross, D. Maucher, D. Du¨ver, M. Jositz, et al., Comparative analysis of full-scale membrane distillation contactors—methods and modules, J. Membr. Sci. 524 (2017) 758
771. Available from: https://doi.org/10.1016/J.MEMSCI.2016.11.080.

12

Chapter 1 Table 1.1: Some commercial membrane desalination technologies and their characteristics.

Company Memsys Aquastill

Thermosift Petrosep KMX Econity Solar Spring Scarab AB

Technology V-MEMD DCMD AGMD LGMD Joule Tompson effect VMD VMD VMD PGMD AGMD

Module geometry

Productivity 3

Reference 21

Plate and frames Spiral wound

3
1000 m day 4800
24,000 1 m3 day21

[41] [42]

Hollow fiber Hollow fiber Hollow fiber Hollow fiber Spiral wound Plate and frames

1
50 m3 day21 50
1000 m3 day21 10
1000 m3 day21 400 m3 day21 0.6 m3 day21 2 m3 day21

[43] [44] [45,46] [46] [46] [46]

Instead, flat-sheet membranes can be assembled in plate and frame or spiral wound modules. Since this kind of membrane is thinner than hollow fiber, it can provide higher permeability but also requires a porous support that provides mechanical strength. The two module types have different strengths and weaknesses: the plate and frame modules allow higher tangential flow rates, diminishing concentration polarization and fouling effects and the membranes can be easily replaced. However, the plate and frame modules provide low packing density (100
400 m2 m23) and poor energy efficiency [20,40]. The choice of the appropriate module configuration as well as the MD process mode depends on the feed characteristics and the recovery needed. Thus various companies provide plants with different configurations and scales [6]. Some examples are listed in Table 1.1. The development of MD modules requires practical and engineering skills to properly configure an effective exploitation of the membrane area during the distillation process. Modeling is needed to design and foresee module behavior during the distillation process since both mass and heat transfer dynamics in the module itself need to be understood and controlled. Theoretical descriptions of the different MD configurations have been given elsewhere and will not be discussed [47]. Examples of theoretical analysis and simulation of MD modules can be found in Refs. [48,49].

1.6 Wastewater treatment by membrane distillation Traditionally MD has been used for desalination. However, as the literature shows MD in wastewater treatment is becoming more common [6,16,50
52]. In MD total retention of nonvolatile solutes and the possibility of achieving high concentration factors are expected. Moreover, the MD process can be configured to use waste heat or heat from natural sources. These features characterize MD compared to other conventional membrane-based separation processes. MD is expected to allow the development of more flexible processes able to deliver high product quality and a high degree of separation, with overall low

Wastewater treatment by membrane distillation 13 energy consumption when integrated with a suitable energy source. The possibility of using solar collectors as well as photovoltaic cells makes the application of MD feasible even in remote areas. All these features make MD very attractive for wastewater treatments inspired to a ZLD approach. The MD performance indicators are: • • • •

permeate flux, separation selectivity, stability over the time, and energy efficiency.

The permeate flux is related to both the thermal and vapor pressure gradients, which in turn depend on the fluid dynamics of the concentrate and permeate side streams and on the thermal conductivities of the membrane and of the vapor phase. Moreover, the flux depends on the MD configuration (i.e., DCMD, AGMD, VMD, SGMD), on the membrane and module characteristics, and on the operating parameters specific for each configuration. The efficiency of the separation is evaluated as in pressure-driven membrane processes by the rejection factor:   Cf 2 Cp 100 (1.5) αð%Þ 5 Cf where α is the rejection factor, Cf and Cp are the overall concentration of the dissolved/ suspended compounds in the feed and in the permeate, respectively. The various MD modules exhibit different tendencies for fouling as found in other membranebased separation processes. For example, tubular configurations are less prone to fouling while side spiral wound and capillary configurations can exhibit higher fouling tendencies. In addition to the process layout adopted, the module choice must also consider a feed pretreatment aimed at lowering the fouling potential of wastewater. While the Silt Density Index or the Modified Fouling Index (MFI) tests [53] are commonly used to assess the fouling potential for nanofiltration and RO processes, a combined fouling index, which takes into account also the effects of membrane hydrophobicity, should be developed for MD.

1.6.1 Parameters affecting membrane distillation performance In desalination MD performance is generally affected by the process, membrane, and membrane module-related parameters used [7]. When MD is used for the treatment of wastewater, its characteristics have a strong impact on performance. Fig. 1.5 summarizes the main parameters influencing the use of MD for wastewater treatment. The characteristics of the wastewater are of paramount importance since its

14

Chapter 1 Membrane • Type • Material •LEP • Structure

•Thickness •Pore size •Porosity •Tortuosity •Thermal conductivity

Module •External or submerged •Configuration Hollow fiber Tubular Plate and frame Spiral wound

Wastewater • Inorganic dissolved solids • Organic dissolved compounds Volatile compounds • Oil, fat, wax • Organic colloids • Microorganisms •Low Kps solutes •Pollutants concentration

MD configuration Performance •Rejection •Flux •Stability •Recovery •Wetting pressure •Membrane flooding •Fouling •Scaling •Energy efficiency

•DCMD •VMD •AGMD •SGMD

Process •Feed temperature •Permeate temperature •Fluid dynamics •Waste or renewable energy

Figure 1.5 Parameters affecting MD application to wastewater.

components can strongly affect the rejection and flux as well as their stability over time. The composition of wastewater depends on its source, which affects the feasibility of MD, and configuration and performance need to be investigated in each case. When studying the application of MD to wastewater the following should be considered: • • •

wetting and flooding of membrane porosity; scaling and fouling; and low rejection of other volatile compounds.

In the following sections examples of MD application to specific types of wastewater treatment are given. Finally some examples of integration of MD with other treatment processes will be reviewed concluding with remarks on fouling phenomena.

1.6.2 Desalination and brine treatment While a review of MD for desalination is not the aim of this chapter and there is plenty of literature available on the topic, we would like to note that one application of MD in desalination is the further concentration of concentrated brines from RO. RO, with a

Wastewater treatment by membrane distillation 15 market share over 60%, is the most commonly applied treatment process for desalination in the Middle East and Africa, comprising more than the share of thermal processes since it requires less electrical and thermal energy. One of the main problems associated with desalination is related to the production of concentrated brine that has to be disposed of in an environmentally friendly manner. Concentrated RO brines are characterized not only by high sodium chloride concentrations but also by the presence of other elements under stringent regulatory restrictions. Since MD can use solar energy it can be applied in poorer regions and can achieve high water recovery and reduce the volume of brine that needs to be disposed [54]. Basically, if a MD crystallization process is applied, salts or other specific elements (e.g., lithium seems to be of great interest) can be recovered for either the commercialization or other more environmental-friendly disposal approaches.

1.6.3 Industrial wastewater treatment Industrial wastewater is usually treated to meet the discharge limits imposed by environmental authorities. When toxic or hazardous species are present in wastewater, additional separation or conversion processes are necessary. In such cases MD can retain almost all the nonvolatile toxic solutes and at the same time concentrate them in relatively smaller volumes compared to RO, for example. In the following sections the application of MD to some wastewater will be reviewed on the basis of some literature examples. 1.6.3.1 Radioactive wastewater The treatment of radioactive wastewater has received increasing attention especially after the Fukushima disaster (Japan, March 2011) where, even after 8 years, the contaminated water is still a huge problem. Radioactive contaminated water and wastewater sources are nuclear power reactors, factories for the production of radioisotopes and nuclear weapons, uranium enrichment plants, research and medical centers. Radioactive wastes are classified by the International Atomic Energy Agency (IAEA) into six classes depending on both their level of activity and half-life [55]. Usually radioactive water and wastewater fall into the low level waste (LLW) or intermediate level waste (ILW) classes. Since radioactive properties cannot be changed all the proposed methods for the treatment of radioactive water and wastewater aim at concentrating the radioactive components into a smaller volume for controlled disposal or safe storage [56]. MD has been investigated for the treatment of this type of wastewater and several studies can be found in the literature. Besides the rejection, which is calculated using the concentration of the radioactive elements in the wastewater, the decontamination factor D

16

Chapter 1

is very often considered as the index of performance of a successful MD operation on a radioactive wastewater [57,58]: D5

Af Ap

(1.6)

where Af and Ap are the specific activities of the feed and the permeate, respectively. Most of the studies on radioactive wastewater have been carried out using the DCMD configuration and some examples based on VMD and AGMD can be found. For example, Zakrzewska-Trznadel et al. [58] studied DCMD with a PTFE spiral wound membrane for the treatment of radioactive wastewater and found good decontamination for several nuclides. Khayet in collaboration with Zakrzewska-Trznadel studied the application of DCMD to radioactive wastewater on several membranes. Membrane characterization is described in Ref. [59], while application to both real and simulated radioactive wastewater containing 60Co and 137Cs is reported in Ref. [60]. The authors used both commercial and lab-made membranes made of PP, PVDF, and PTFE with pore size between 0.2 and 1.0 µm and some of them were in shell-and-tube, plate and frame, or spiral wound modules. Khayet et al. [57] investigated the application of DCMD on radioactive wastewater with surface functionalized membranes. In this recent report, the authors compared a commercial PTFE membrane (TF200, Gelman, which is supported on PP) with others based on polysulfone and polyethersulfone and with a surface modified by fluorinated macromolecules. The tests were performed with both a model solution of Co, Ce, and Sr up to a conductivity of about 1 mS cm21 and a contaminated water with 60Co, 137Cs, and 85Sr up to a feed activity between 35 and 42 Cps. In all the experiments the activity of the permeate was around 4.3
4.7 Cps and the permeate conductivity was 9
16 µS cm21. With all the tested membranes high decontamination factors were found for 60Co (D from 400 to 1000), 137Cs (D from 900 to 1400), and 85Sr (D from 400 to 800). It was interestingly found that, after the tests, the specific activity of the modified membranes was lower than that of the commercial TF200 membrane. This is probably attributed to the higher LEP of those membranes (around 4 bar) compared to the commercial ones (2.76 bar). Jia et al. [61
64] studied the application of VMD to water containing elements with radioactive isotopes. Even if radioactive isotopes were not used that study showed the ability of MD with hollow fibers in vacuum configuration to retain such elements. They showed that the vacuum pressure to be applied should exceed the vapor pressure of the feed in order to get a flux through the membrane and the upper limit to the flux is represented by the pressure-build effect on the permeate side due to water condensation on the wall of the module. Liu et al. reported that the dusty gas model is suitable for estimating the mass transfer during the MD of low level radioactive wastewater containing Cs1, Sr21, and Co21 [65].

Wastewater treatment by membrane distillation 17 MD is a promising technology for the treatment of radioactive wastewater since, in addition to the other well-known advantages (e.g., the possibility to use low-grade thermal energy), the probability of pore contamination due to the adsorption of radioactive elements is lower and a high concentration level can be achieved in a single stage. Considering the specific case of the treatment of wastewater in a nuclear power plant where heat should be easily available, the MD process is clearly an attractive option. However, further investigation is needed to prove the resistance of membrane materials to ionizing radiation. 1.6.3.2 Textile wastewater The textile industry has high water consumption and its wastewater contains harmful substances such as azo dyes, heavy metals, surfactants, and several salts and can be either alkaline or acidic. MD has been proposed for its treatment since it offers the possibility to simplify the process of treating wastewater with variable composition. DCMD is one of the most commonly studied configurations for the treatment of textile wastewater and most of the tests are carried out using simplified wastewater (i.e., only the dye without other solutes and suspended matter). However, textile wastewater usually contains surfactants whose presence plays a major role in exalting the pore wetting phenomenon with a detrimental impact on rejection. Leaper et al. [66] studied AGMD with a simulated wastewater containing sodium chloride, sodium dodecyl sulfate, and two dyes, sunset yellow and rose Bengal. Commercial hydrophobic flat-sheet PTFE membranes (Sterlitech, United States) laminated onto a PP nonwoven backing layer and characterized by a mean pore size of 0.20 µm were used. The membrane module had an air-gap width of 3 mm. The feed was preheated at 70 C and on the permeate side, tap water at 20 C was circulated on a stainless steel condenser plate. Garcı´a et al. [67] tested a real textile wastewater in a textile mill with a DCMD pilot plant with a nominal capacity of about 480 L day21 to produce high-quality water recovery for reuse exploiting waste heat as a source of energy. In particular, they compared a hydrophobic PTFE membrane with a hydrophilic-coated hydrophobic PTFE membrane in order to reduce the wetting effect of surfactants. The coated membrane showed less tendency for wetting than the conventional PTFE but unfortunately wetting occurred in any case during chemical cleaning with NaOH. To reduce the wetting effects of various types of surfactants, Lin et al. added an agarose hydrogel layer to the surface of a PTFE membrane, which resulted in a more stable flux and high retention over time but a strong reduction of the permeate flux of about 71% with respect to the pristine membrane was observed [68].

18

Chapter 1

Banat et al. [69] and Criscuoli et al. [70] investigated the behavior of VMD with a PP membrane and an exponential decrease of the permeate flux over the time and a dependence on the operating conditions adopted on the feed flow (e.g., velocity) were observed. They reported complete retention of the dye into the concentrate. By using a PTFE membrane Baghel et al. [71] reported that the percentage removal of Sudan III dye decreased by increasing the feed circulation rate, permeate pressure, and feed dye concentration. 1.6.3.3 Olive mill and olive wastewater Several MD studies have been devoted to the treatment of wastewater generated during the processing of olives for either olive oil or table olive production. Olive mill wastewater (OMW) contains solids and colloids, dissolved inorganic solids, and an interesting soluble organic fraction composed of carbohydrates and polyphenols [72]. Table olive wastewater (TOW) contains a lower concentration of polyphenols but a higher dissolved salt concentration (mainly NaCl) [73]. Both types of wastewater are of interest due to the amount and kind of polyphenols that if recovered could have applications in the nutriceutical, cosmetic, and pharmaceutical industries. OMW with a chemical oxygen demand (COD) of 156 g L21 and total polyphenols around 4 g L21 was treated by DCMD using PTFE and PVDF membrane and again a permeate flux decline over the time was observed [74]. The authors [75] tested MF and coagulation/flocculation as pretreatment before the DCMD at 40 C with a commercial 0.2 µm PTFE membrane. While both were found to be beneficial MF resulted in the highest flux (6
7 L m22 h21) in MD. In every case, a flux decline over time was observed even after the MF pretreatment. However, MD was less fouled than nanofiltration or RO. Recently Carnevale et al. [76] studied the application of DCMD e VMD with capillary PP membranes to OMW. Prior the membrane process the OMW was filtered on a net to remove most of the suspended matter. Then OMW was sent into the fiber lumen while the distillate was collected outside the capillary in both the DCMD e VMD configurations. During the production of table olives, water with high NaCl and NaOH concentration is used for olive debittering and fermentation. TOW in addition to the salts contains polyphenols (less than in OMW) and lactic acid. The organic load can be very high, which could limit the feasibility of a MD process aimed at recovering the saline solution for reuse in table olive productions. Kiai et al. [77] studied the application of DCMD to TOW by commercial PTFE membranes supported on PP with different nominal pore size from 0.2 to 1 µm. Starting from feed with conductivity in the range 105
130 mS cm21 after 4 h of operation a permeate with

Wastewater treatment by membrane distillation 19 conductivity in the range 100
355 µS cm21 was obtained with a salt rejection .99.7%, but the permeate flux was decreasing over the time. The 0.2 µm membrane was less affected by the irreversible fouling but in general the 0.45 µm membrane performed better. It was found that feed temperature should not exceed 50 C since the fouling phenomena strongly affect performance. Within the development of suitable membranes, the effectiveness of pretreatment solutions should be studied. 1.6.3.4 Coking wastewater Coking wastewater is generated by the purification processes of the coke gas and contains a variety of pollutants such as ammonia, phenol, polycyclic aromatic hydrocarbons, nitrogen heterocyclic compounds, etc. A biological treatment is usually applied but because of the presence of inhibitory compounds, full oxidation of the organic compounds is not achieved. Li et al. tested DCMD with real coking wastewater after an anaerobic
anoxic
oxic biological treatment using commercial PTFE membranes with a nominal pore size of 0.22 µm. In particular, the authors compared the membrane performance of DCMD on the raw water with the one after chemical coagulation with poly-aluminum chloride and nonionic polyacrylamide and found that this pretreatment was useful to enhance membrane performance in terms of permeate flux stability since scaling phenomena on the membrane surface were no longer observed [78].

1.6.4 Membrane distillation hybrid systems MD showed high flexibility for integration with other physical and chemical processes, in particular with other membrane processes. These so-called hybrid processes offer the opportunity to achieve a ZLD target. An example of this type of integration was the concentration of RO brine discussed above. One of the opportunities offered by MD is clearly when a waste heat is available and then this technique becomes competitive against other technological concentration options. In the following, we discuss some examples of MD coupled with other membrane separation processes and with reactive systems. Among this last class the coupling of biological reactors with MD will be discussed. 1.6.4.1 Nanofiltration or reverse osmosis and membrane distillation An interesting example of integration was given by Tun and Groth [79] where two integrated configurations were examined using MD as crystallizer to realize a ZLD process. In the first configuration an industrial effluent discharged from a SO2 scrubber was treated by integrating MF
nanofiltration with a MD crystallizer to recover sodium sulfate and water to be reused in an industrial process. MF provided a suitable pretreatment to reduce the fouling on the nanofiltration membrane and the role of the nanofiltration was to reduce

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the hydraulic loading to the MD crystallizer. The MD crystallizer is a MD unit where the concentration of the feed proceeds until a condition of supersaturation of the solute; the feed is then sent to another dedicated unit where the solute is crystallized. The second configuration concerns the use of MD to reach a supersaturation condition on the RO brine of a desalination unit. The authors claimed that up to 95% overall water recovery could be achieved. Based on these examples, it appears that MD may be effective in specific industrial wastewater treatments and for desalination when compounds that can affect membrane wettability and fouling are absent. 1.6.4.2 Forward osmosis and membrane distillation MD couples well with forward osmosis (FO). FO has the ability to treat several types of wastewater and is less influenced by fouling phenomena than RO. As described later in this book, FO exploits a high salinity solution to draw water from a less concentrated solution (e.g., wastewater) through a water permeable membrane. In that way without application of an external pressure, the wastewater concentration can be obtained. During FO the high salinity solution (draw solution) is therefore diluted and then MD is an effective option to again concentrate the draw solution. Since in this integration usually MD deals only with the draw solution, in principle should not be affected by any fouling problem. Moreover, FO can occur spontaneously and is also less affected by fouling; therefore, this process integration is one of the most promising for wastewater treatment. Application of FO
MD integrated processes is emerging but is limited by the availability on the market of membranes and modules for both the FO and the MD. As follows some examples of integrated FO
MD processes found in the literature will be mentioned. The integration of FO and MD was studied for the treatment of several types of wastewater including high nutrient sludge [80], dye wastewater [81], and oily wastewater [82]. Here we would like to mention the following wastewater types: •

•

Real hazardous waste landfill leachates with different salinity. The operating conditions of the integrated process were optimized by the Response Surface Methodology (RMS) approach and both total organic carbon (TOC) and total nitrogen (TN) rejections were higher than 98% while hazardous metals like Hg, As, and Sb were completely removed. DCMD was used to concentrate the draw solution and a commercial PTFE
PVDF membrane supported by polyethylene terephthalate (PET) was used [83]. Secondary effluent of a civil wastewater treatment plant. Husnain et al. [84] in their paper described the integration of FO and MD in the same testing cell where the FO membrane and the MD membrane were facing the same channel for the draw solution. The integrated FO
MD system showed an inherent flux balancing mechanism that enabled stable operation over about 20 h. For MD a 0.22 µm commercial PP membrane was used.

Wastewater treatment by membrane distillation 21 •

Flue gas desulfuration wastewater from coal-fired power plant. Lee et al. [85] showed that the application of FO and MD enabled water recovery higher than by using RO. A pretreatment of the feed by MF was necessary to reduce the problem of fouling and antiscalants were added in both the feed and draw solution to reduce scaling problems. The draw solution was concentrated by DCMD with a commercial 0.2 µm PTFE membrane.

1.6.4.3 Electrodialysis and membrane distillation The integration of MD with electrodialysis (ED) for wastewater treatment is still not well explored, but some examples can be found for desalination. Ren et al. [86] studied the treatment of acidic rare earth element wastewater by integrating ED and VMD. By using ED the acid was separated and recovered and VMD allowed the concentration of cerium and the recovery of reusable water. The test was carried out on a synthetic wastewater and the PVDF hollow fiber membranes tested in VMD were either commercial or lab-made. 1.6.4.4 Chemical conversion and membrane distillation MD can be integrated with a chemical conversion process with the aim of transforming the pollutants into inert or less problematic compounds. The chemical conversion process can be catalytic and has the role of destroying a contaminant while MD can concentrate the feed to recover a purified water stream and concentrate the feed with a beneficial effect on the degradation kinetics. Two examples of coupling MD with chemical conversion systems are photocatalysis MD and ozonation MD. The combination of photocatalysis and MD is an interesting integration that has been investigated in several papers [87
91], and in some cases a beneficial effect of the photocatalyst on the fouling tendency was observed. Since this integration is the focus of another chapter in this book, we will not cover it here. The integration of ozonation and MD is also very interesting. Zang et al. [92] combined DCMD with a homogenous catalytic ozonation reactor. In the reactor the organic compound were catalytically oxidized while the MD unit was used to extract the purified water and to retain the catalyst together with all the nonvolatile dissolved compounds. A model saline wastewater with potassium hydrogen phthalate was used as feed and a laboratory-made PVDF hollow fiber membrane was obtained by dryjet wet spinning. After 60 h membrane changes were observed mainly related to a decrease of the contact angle, swelling, and adsorption of the organic molecules on the membrane surface while mechanical properties remained unaltered. The permeate flux of the hybrid homogeneous catalytic ozonation MD reactor was higher than the one found by using only MD due to minor fouling deposition on the membrane during the ozonation process.

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1.6.4.5 Membrane distillation bioreactors Several papers on the integration of MD with biological reactors can be found in the literature. One of the main advantages of coupling a membrane process with a biological reactor is related to the ability of this integrated system to control the sludge retention time independently of the hydraulic retention time allowing for the exploitation of high biomass concentration in the bioreactor and resulting in higher removal efficiency. Usually a MD biological reactor (MDBR) combines a biological process based on thermophilic microorganisms with a MD, making MDBR are a category of high retention membrane bioreactors [93,94]. Goh et al. reviewed MDBRs and compared their characteristics with the ones of membrane bioreactor
RO for industrial wastewater reclamation on the basis of greenhouse gas (GHG) emissions, wastewater reclamation performance, and sludge production [95]. The authors concluded that MDBR represents a high retention system capable of producing high-quality water with lower GHG emissions than MBR
RO when waste heat and even nonrenewable energy is used. The MDBR permeate quality is usually comparable to the quality of MBR
RO permeate beside the higher ammonia concentration, which can be a limit for permeate reuse. In any case, the integration of MD and biological reactors is still in an early stage of development and further studies are necessary to develop suitable membranes. In a MDBR the effects of the biofilm on the membrane surface on both the thermal profile and concentration profile must be taken into account. A fouling layer will confer additional resistance, which is dependent on characteristics such as its porosity and thickness [96]. In another study a petrochemical wastewater normally treated with a biological process was treated with a MDBR process [97]. Submerged DCMD was used and a very stable rejection of TOC and salts was observed for more than a month on a real wastewater. The main type of fouling was inorganic and the membrane performance was recovered by a chemical cleaning. Anaerobic MDBRs have been recently studied by Jacob et al. [98] and Song et al. [99]. They found that by using this approach several trace organic contaminants could be removed while phosphorus was accumulated in the system, thereby producing an opportunity for phosphorus recovery. More sophisticated integrated configurations involving both FO and MD have also been studied. In this type of configuration MD is used to regenerate a draw solution and highquality effluent can be obtained. For example, Morrow et al. [100] recently proposed an osmotic membrane bioreactor-MD (OMBR-MD) system where a FO stage is integrated within the biological reactor and the draw solution is continuously regenerated by the DCMD producing water of potable grade. The integrated OMBR-MD system achieved 90.2% NH41
N removal and 98.4% COD removal.

Wastewater treatment by membrane distillation 23 In such a configuration the MD performance is much more stable since its role is only to restore the draw solution concentration [101].

1.6.5 Fouling in membrane distillation Fouling is related to the accumulation of materials on the membrane surface or in the membrane porosity resulting in a decrease of the membrane performance (e.g., in terms of flux and rejection over time). Fouling phenomena are one of the main obstacles for MD technology in some applications. Tijing et al. [102] recently reviewed fouling and its control in MD. Fouling control is usually based on the feed pretreatment and on membrane cleaning. In the specific case of MD we can assume that fouling is (1) porous or (2) almost dense. The porous fouling effect is mainly due to the changes in the overall thermal resistance. The main effect of the almost dense fouling is observable in the flux decline due to increased mass transfer resistance. The factors affecting fouling on membranes are related to: • • • •

Wastewater characteristics: pH, ionic strength, and presence of organic/inorganic matter and compounds. Foulant characteristics: concentration, molecular size, solubility, diffusivity, hydrophobicity or hydrophilicity, charge, etc. Membrane properties: hydrophobicity, surface roughness, pore size and pore size distribution, surface charge, and surface functional groups. Operating conditions: flux, pressure gradients, temperature, and flow velocity on both membrane sides.

Gryta [103] studied fouling phenomena in a DCMD laboratory module by feeding PP capillary membranes on the inside while the distillate stream flowed from the outside cocurrently. Bilge wastewater (an oily wastewater), a saline wastewater from meat processing (a wastewater with proteins and polysaccharides), and a tap water with electrical conductivity of about 600 µS cm21 (for the study on inorganic scaling) were tested in order to understand the different types of fouling. In Fig. 1.6 the protocol used by the authors to evaluate the fouling on the membrane flux is shown. After stabilization with clean water (A), the authors observed a flux decrease over time during the wastewater treatment experiment (B), then the membrane was fed again with clean water (C) and subsequently with a cleaning solution (D). Following the proposed protocol it is possible to estimate the irreversible fouling as a fraction of flux that cannot be recovered. The additional drying step (E) to restore the membrane hydrophobicity should be noted. In fact, due to both fouling deposition and membrane cleaning the membrane porosity might be completely or partially filled by water that has to be removed before starting a new distillation cycle. The drying

24

Chapter 1

Figure 1.6 Typical experimental protocol for investigating fouling in MD [103].

step after a cleaning procedure usually is not present for other membrane processes and must be taken into account during the process design. It has been observed that proteins have high tendency to be adsorbed onto the membrane surface and that a pretreatment to induce their separation from water (e.g., boiling and precipitation) is highly recommended to reduce the effect of fouling on the membrane performance. Also, the oil resulted in an irreversible fouling without full recovery of the reference clean water flux. The investigation on the scaling phenomena using tap water showed that after acid cleaning the water flux is not fully restored probably due to the fact that scaling occurs inside the porosity and might not be fully accessible to the cleaning solutions. Fouling control strategies can be summarized as follows [102]: • • • • • • • •

pretreatment; membrane flushing; temperature and flow reversal; gas bubbling; effect of magnetic field, microwave irradiation, or sonication; use of antiscalant; chemical cleaning; and surface modification for antifouling membranes.

Although MD is able to retain colloids and suspended solids, their prior removal can be beneficial for achieving stable operation processes since they can lead to pore clogging and fouling phenomena. The pretreatment can be afforded by integrating in the process more traditional separation processes (e.g., coagulation, flotation, cartridge, sand filtration) or membrane processes such as MF or ultrafiltration (UF). Gryta et al. [104,105] studied the

Wastewater treatment by membrane distillation 25 treatment both of a solution containing NaCl and proteins and of an ion exchange regeneration wastewater by DCMD with PP membranes and observed the formation of several types of fouling on the membrane surface affecting in turn the membrane performance. They showed that after a proper pretreatment the MD performance was higher and more stable. A well-designed feed pretreatment can improve MD performance in terms of both flux and quality of the permeate. Moreover, in some cases the pretreatment stage can reduce the presence of other pollutants associated with the removed matter (e.g., colloids) which might be responsible for other undesired effects (e.g., wetting). Regular membrane flushing with clean water has been studied for DCMD to reduce scaling [104]. This approach seemed to be more effective when applied during the induction phase before the effective scaling formation. As in other membrane processes flux reversal might be a good strategy to control fouling and in MD to reduce the effect of scaling. Flux reversal can be obtained by reversing the feed and permeate streams or by reversing the temperature. The main effect of this strategy is related to the inhibition of homogeneous precipitation and crystal nucleation. Gas bubbling is often used in MF and in membrane biological reactors (MBRs) to control fouling by increasing the shear rate at the membrane surface. Similarly it can be done in MD by setting a module with spaced membranes (e.g., hollow fiber) that can homogeneously distribute fine bubbles. Microwaves and sonication can induce uniform heating and reduce polarization effects but their effectiveness on fouling needs to be further investigated. However, the magnetic field effect has been studied for its ability to mitigate scaling. Antiscalants are frequently used in separation processes such as RO and thermal separation. Antiscalants usually hinder precipitation by interfering with crystal growth at their surface. In the case of MD the use of antiscalants can be beneficial if the antiscalant does not increase membrane wettability. The chemical cleaning approach is applied after the fouling effect is seen on membrane performance. Several cleaning agents are commercially available and should be selected based on the type of fouling. The aim of chemical cleaning is to restore membrane performance and properties (e.g., flux, retention, hydrophobicity, etc.). Unfortunately chemical cleaning is not always able to clean the membrane when pore blocking has occurred and in some cases membrane wettability can be increased by cleaning agents. The development of antifouling membranes is one of the most active research fields in membrane science today. The main aim of researchers is to impart or enhance surface functionalization in order to increase its hydrophobicity.

26

Chapter 1

Another study showed that fouled PTFE membranes from AGMD of synthetic and RO brines can be successfully used for the MF of humic acid aqueous solutions [106].

1.6.6 Economics of membrane distillation The major cost parameters for MD are the thermal energy required for heating, the electrical energy required for pumping, and the membrane (area) material. Although DCMD requires two pumps for operation (one for the feed and one for permeate) the required pressures are lower than those needed for RO operation. The following are some of the main advantages of MD: • • • • • • •

Separation is obtained without the application of a pressure. High rejection of nonvolatile compounds (e.g., up to about 100%). Possibility to remove compounds with higher volatility than water. Low operating temperature compared to other distillation processes (e.g., especially VMD). Possibility of using polymeric materials for most of the membrane distillation plant components (e.g., pumps, pipes, modules, etc.) with fewer corrosion problems. Performance is usually not strongly affected by the feed salt concentration, which can reach very high concentrations (e.g., up to 300 g L21 of TDS if scaling does not occur). Possibility of using waste heat sources or renewable energy sources (e.g., solar and geothermal energy).

The disadvantages of MD include: • • •

Low commercial availability of membranes and modules designed for MD Fouling and wetting issues management, which in some cases requires a feed pretreatment Demonstration-scale plants have been realized especially for desalination while wastewater treatment is still at lab and small pilot scale

For example, Topaloglu et al. [107] showed that, when waste heat is available, the integration of FO and MD to produce drinking water can be both economically and environmentally sustainable since the ZLD objective becomes feasible. One of the main costs is energy consumption, which can be on the order of 40 kWh m23. However, this cost can be considerably reduced if a source of waste heat is available to raise the wastewater temperature to the operating temperature [108,109].

1.7 Conclusion and future trends MD is an emerging membrane process that has promising potential in the wastewater treatment field. Although the application of MD to desalination is ready for commercialization, in the case of wastewater treatment fewer examples are available a

Wastewater treatment by membrane distillation 27 demonstrative scale. For specific application to wastewater treatment validation tests are usually required to assess the medium- and long-term stable application of any MD-based process. One of the main challenges of application of MD to wastewater is wetting of the membrane due to the organic components present in the wastewater composition. Another important issue is related to fouling formation on the membrane surface and in some cases the porosity. Various studies have investigated the application of MD to different types of wastewater and ways to improve membranes and process have been reported. Future trends in MD for application to wastewater treatment will be focused on: • • • •

membrane development to improve the membrane hydrophobicity and its antifouling properties; module development to improve mass transfer and control heat transfer in order to both minimizing heat losses to withstand feed characteristics; integrating MD with more sustainable energy supply systems; and integrating MD with pretreatments and other chemical or membrane processes.

List of acronyms AGMD COD DCMD ED FO GHG IAEA ILW LEP LGMD LLW MBR MD MDBR MF MFI NIPS OMBR OMW PET PGMD PP PTFE PVDF PVDF-HFP RMS RO SGMD TDS

air-gap membrane distillation chemical oxygen demand direct contact membrane distillation electrodialysis forward osmosis greenhouses gases International Atomic Energy Agency intermediate level (radioactive) waste liquid entry pressure liquid gap membrane distillation low level (radioactive) waste membrane biological reactor membrane distillation membrane distillation biological reactor microfiltration Modified Fouling Index nonsolvent-induced phase separation osmotic membrane bioreactor olive mill wastewater polyethylene terephthalate permeate gap membrane distillation polypropylene polytetrafluoroethylene polyvinilidene fluoride poly(vinylidene fluoride-hexafluoropropylene) response surface methodology reverse osmosis sweeping gas membrane distillation total dissolved solids

28

Chapter 1

TIPS TN TOC TOW UF VMD V-MEMD ZLD

thermally induced phase separation total nitrogen total organic carbon table olive wastewater ultrafiltration vacuum membrane distillation vacuum-multieffect membrane distillation zero liquid discharge

List of symbols α aw γl γw θ A, B, and C Af Ap B Cf Cp D p rmax T xw

rejection factor water activity liquid surface tension activity coefficient for water contact angle between the membrane and the liquid feed Antoine’s equation constants specific for each substance specific activity of the feed specific activity of the permeate geometric factor accounting for the pore shape (0 , B , 1 for noncylindrical shapes; B 5 1 for cylindrical pores) overall concentration of the dissolved/suspended compounds in the feed overall concentration of the dissolved/suspended compounds in the permeate decontamination factor vapor pressure largest pore size temperature molar fraction of water

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