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ssio n sNanofiltration for water and wastewater treatment –

a mini review

H. K. Shon1, S. Phuntsho1, D. S. Chaudhary1, S. Vigneswaran1, and J. Cho2

1School of Civil and Environmental Engineering, University of Technology, Sydney (UTS), P.O. Box 129, Broadway, NSW 2007, Australia

2Department of Civil and Environmental Engineering, Yonsei University, Yonsei-ro 50, Seodaemun-gu, Seoul 120-749, Korea

Correspondence to:H. K. Shon ([email protected])

Received: 19 February 2013 – Published in Drink. Water Eng. Sci. Discuss.: 13 March 2013 Revised: 26 April 2013 – Accepted: 8 May 2013 – Published: 5 June 2013

Abstract. The application of membrane technology in water and wastewater treatment is increasing due to stringent water quality standards. Nanofiltration (NF) is one of the widely used membrane processes for water and wastewater treatment in addition to other applications such as desalination. NF has replaced reverse osmo- sis (RO) membranes in many applications due to lower energy consumption and higher flux rates. This paper briefly reviews the application of NF for water and wastewater treatment including fundamentals, mechanisms, fouling challenges and their controls.

1 Introduction

Membrane filtration is a pressure driven process in which membrane acts as selective barriers to restrict the passage of pollutants such as organics, nutrients, turbidity, microor- ganisms, inorganic metal ions and other oxygen depleting pollutants, and allows relatively clear water to pass through (Mulder, 1997). With technological advances and the ever- increasing stringency of water quality criteria, membrane processes are becoming a more attractive solution to the challenge of quality water, and water reuse (Shannon et al., 2008). Several studies have been done on the application of microfiltration/ultrafiltration for wastewater treatment and reuse (Vigneswaran et al., 1991; Seo et al., 1996, 1997; Snoeyink et al., 2000; Visvanathan et al., 2000; Ben Aim and Semmens, 2001; Kim et al., 2001; Matsui et al., 2001a, b).

The membrane process has been classified into four broad categories as depending on their pore sizes as: microfiltra- tion (MF), ultrafiltration (UF), nanofiltration (NF) and re- verse osmosis (RO) membranes. This paper briefly reviews the application of NF for water and wastewater treatment in- cluding for water reuse. This article begins with the brief fundamentals of membrane process followed by discussion on the mechanisms of NF process and its few basic models.

The article also covers the challenges of NF fouling and their control mechanisms adopted to mitigate fouling. Finally the article concludes with a brief summary.

2 Fundamentals of membrane process

There are many types of membrane processes in use. RO membrane is essentially non-porous, and it preferentially passes liquid and retains most of the solutes including ions. The RO is characterized by high operating pressure (20 to 100 bar). NF has pore size 1–5 nm and it can retains ions, and low molecular weight organics. It has significantly higher water permeability than that of RO membrane and operates at lower pressure (typically 7 to 30 bar). Similarly, UF mem- brane has pore size typically 5 to 20 nm and retains fine col- loids, macromolecules, and microorganism. The UF operates with pressure range of 1 to 10 bar. The other membrane pro- cesses that are used in liquid separation process are micro- filtration (MF), electrodialysis (ED), liquid membrane (LM), pervaporation (PV), vapour permeation (VP), and gas per- meation (GP). The types of membrane processes, the parti- cle size typically removed by the membrane, and the driv- ing force of the processes are illustrated in Fig. 1. The major

Published by Copernicus Publications on behalf of the Delft University of Technology.

48 H. K. Shon et al.: Nanofiltration for water and wastewater treatment

Figure 1 Effective range of membrane processes

Applications

D ri

v in

g fo

rc e

C

E

P

T

0.1 1 10 100 1000

1001010.1nm

µm

Organics

Ions

Macromolecules

Colloids

Fines

Dialysis

Liquid membrane

Pervaporation

Electo-dialysis

Reverse osmosis

Nanofiltration

Ultrafiltration

Microfiltration

Conventional filtration

Gas Perm

Membrane distilation









Applications

D ri

v in

g fo

rc e

C

E

P

T

0.1 1 10 100 1000

1001010.1nm

µm

Organics

Ions

Macromolecules

Colloids

Fines

Dialysis

Liquid membrane

Pervaporation

Electo-dialysis

Reverse osmosis

Nanofiltration

Ultrafiltration

Microfiltration

Conventional filtration

Gas Perm

Membrane distilation









Figure 1. Effective range of membrane processes.

difference between these membrane processes is shown in Table 1.

3 Nanofiltration

NF is the most recently developed pressure-driven membrane process for liquid-phase separations. NF has replaced reverse osmosis (RO) in many applications due to lower energy con- sumption and higher flux rates (Cadotte et al., 1988; Goza- lvez et al., 2002). The properties of NF membranes lie be- tween those of non-porous RO membranes (where transport is governed by a solution-diffusion mechanism) and porous ultrafiltration (UF) membranes (where separation is usually assumed to be due to size exclusion and, in some cases, charge effects). Commercial NF membranes possess a fixed charge developed by dissociation of surface groups such as sulphonated or carboxyl acids. The properties of NF mem- branes, therefore, allow ions to be separated by a combina- tion of the size and electrical effects of UF and the ion inter- action mechanisms of RO (Bowen and Welfoot, 2002).

The NF membrane is the relatively newly introduced tech- nology in wastewater treatment system. The size of pores in NF membranes (nominally∼ 1 nm) is such that even small uncharged solutes are highly rejected while the surface elec- trostatic properties allow monovalent ions to be reasonably well transmitted with multivalent ions mostly retained. These characteristics make NF membranes extremely useful in the fractionation and selective removal of solutes from complex process streams. The development of NF technology as a vi- able process over recent years has led to a remarkable in- crease in its application in a number of industries such as treatment of pulp-bleaching effluents from the textile indus- try, separation of pharmaceuticals from fermentation broths,

demineralization in the dairy industry, and metal recovery from wastewater and virus removal (Bowen et al., 2002).

NF is one of the promising technologies for the treat- ment of natural organic matter and inorganic pollutants in surface water. Since the surface water has low osmotic pres- sure, a low-pressure operation of NF is possible. There is a high rejection of organic substances such as disinfection-by- products precursors by the NF process. In the NF of surface waters, natural organic compounds, which have relatively large molecules compared to membrane pore size, could be removed by sieving mechanism, whereas the inorganic salts by the charge effect of the membranes and ions (Thanut- tamavong et al., 2001, 2002). The past studies on NF are summarized in Table 2 (Ernst et al., 2000; Xu and Lebrun, 1999; Tsuru et al., 2000; Seidel and Elimelech, 2002; Van der Bruggen et al., 2002; Lee et al., 2002; Choi et al., 2002; Trebouet et al., 2001).

4 Separation mechanisms in NF

Since NF membrane exhibits properties between those of ul- trafiltration (UF) and reverse osmosis (RO), both charge and size of particle play important role in NF rejection mecha- nism. Simpson et al. (1987) has described NF as a charged UF system whereas Rohe et al. (1990) has referred it as low pressure RO system. However, NF has advantages of lower operating pressure compared to RO, and higher organic re- jection compared to UF. For the colloids and large molecules, physical sieving would be the dominant rejection mechanism whereas for the ions and lower molecular weight substances, solution diffusion mechanism and charge effect of membrane play the major role in separation process. Macoun (1998) presented the NF rejection mechanisms into following five steps.

– Wetted surface – water associates with the membrane through hydrogen bonding and the molecules which form the hydrogen bonding with the membrane can be transported.

– Preferential sorption/Capillary rejection – membrane is heterogeneous and microporous, and electrostatic repul- sion occurs due to different electrostatic constants of so- lution and membrane.

– Solution diffusion – membrane is homogeneous and non-porous, and solute and solvent dissolve in the active layer of the membrane and the transport of the solvent occurs due to the diffusion through the layer.

– Charged capillary – electric double layer in the pores determines rejection. Ions of same charge as that of membrane are attracted and counter-ions are rejected due to the streaming potential.

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H. K. Shon et al.: Nanofiltration for water and wastewater treatment 49

Table 1. Difference between MF, UF, NF, and RO.

Particular MF UF NF RO

Membrane Porous isotropic Porous asymmetric Finely porous Nonporous asymmetric/composite asymmetric/composite

Pore size 50 nm–1µm 5–20 nm 1–5 nm –

Transfer Sieving and adsorptive Sieving and preferential Sieving/electrostatic Diffusive (solutes migrate mechanism mechanisms (the solutes adsorption hydration/diffusive by diffusion mechanism)

migrate by convection)

Law governing Darcy’s law Darcy’s law Fick’s law Fick’s law transfer

Typical solution Solution with solid Solution with colloids Ions, small molecules Ions, small molecules treatment particles and/or macromolecules

Typical pure water 500–10 000 100–2000 20–200 10–100 flux (L m−2 h)

Pressure 0.5–5 1–10 7–30 20–100 requirement (atoms)

Table 2. Past studies of nanofiltration.

Membrane Pretreatment/Feed solution Remarks

– Four types of flat sheet membranes: DK5 (polysulfone + polypiperazinamide), PES10 (polyethersulfone), C5F (cellulose), and MP 36 (modified PVDFK) with MWCO 200, 1000, 5000, and 1000, respectively – Two types of flat sheet membranes: NF70 (negatively charged), NF45 (hydrophilic, pore size 2–5 nm) – Three inorganic stirred cell membranes with MCO 200, 600, and 2000.

– Thin film composite, flat sheet membranes

– Flat sheet membrane NF70

– Polyamide TFC flat sheet membrane with MWCO 250 Dalton

– Cellulose acetate hollow fibre submerged membrane, aerated. – MPT-20 and MPT-31 made of polyster with polyacrilonitrile support and polypropy- lene with polysulphone support, respectively, MWC 450 Da and negatively charged – Polyamide, negatively charged, 200 Da MWCO

PAC adsorption, Tertiary wastewater efflu- ent for ground water recharge

No pretreatment, Electrolytic solutions (NaCl, Na2SO4, and polyethylene glycol)

No pretreatment, different electrolytic solutions

No pretreatment, NOM solutions

No pretreatment, 14 chemical solutions

No pretreatment, different NOM solutions

No pretreatment, synthetic wastewater

Ferric chloride coagulation – filtration (10µm polypropylene mesh), Landfill leachate

Millipore filtered (10µm and 1µm) feed, Secondary sewage effluent

Cut-off characteristics of membrane play impor- tant role to remove smaller molecular weight substances

The transport of solute through NF depends on sieving mechanism and surface force interaction.

The rejection of solutes decreased but the perme- ate volume increased with an increase in temper- ature. NOM fouling of NF membranes is governed by the combined effects of initial permeate flux or applied pressure, crossflow velocity and divalent (calcium) ion concentration. Flux decline is caused by the molecules that fill the pores of the membrane and adsorption of molecules on the membrane surfaces which is enhanced by the hydrophobicity of the solutes. Mass transport is more affected by difference in NOM structure than solution chemistry, and is dominated by diffusion. Low pressure NF bioreactor can be used for long- term without fatal fouling and cleaning. Presence of Fe3+ ions may change the surface charges, ionic force of solutions and the struc- ture of membrane surface, and thus may reduce the organic retention capacity of the membrane Fouling due to colloids (such polysaccharides or proteins) are more severe than the hybropho- bic and transphilic fractions of organics in the sewage effluent.

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50 H. K. Shon et al.: Nanofiltration for water and wastewater treatment

– Finely porous – membrane is a dense material punc- tured by pores. Transport is determined by partitioning between bulk and pore fluid.

The characteristics of NF membranes lies between the non-pores reverse osmosis membranes (where the rejection is due to solution-diffusion mechanism) and porous UF mem- branes (where the rejection is by size exclusion and elec- trostatic charge effects). Thus, the rejection of uncharged molecules is dominated by size exclusion, while that of ionic species is influenced by both size exclusion and electrostatic interactions. Electrostatic characteristics of NF membranes have been known as playing an important role in rejection anions, namely, negative zeta potential on the membrane sur- face varies with different pH and concentration of an elec- trolyte solution (Choi et al., 2002) .

5 Mathematical modelling of nanofiltration process

NF is a complex phenomenon. The NF membranes exhibit properties between those of RO membranes and UF mem- branes, and hence the solution-diffusion mechanism, the size exclusion, and charge effects need to be considered in mod- eling the governing phenomenon of NF process. The basic equation to describe the transport of ions/solutes through the membranes is given by the extended Nernst-Planck equation (Eq. 1).

J = Dp dc dx −

zcDp RT

F dψ dx +KccV (1)

where,J= Ion flux based on membrane area (mol m−2 s−1), Dp =Hindered diffusivity (m

2 s−1), c= Ion concentra- tion in the membrane (mol m−3), x=distance from the membrane (m), z=Valence of ion, R=Gas constant (J mol−1 K−1), T =Absolute temperature (K),F =Faraday constant (C mol−1), Kc =Hindrance factor for conversion, ψ=Potential difference, andV =Solvent velocity (m s−1).

The terms on the right hand side of the equation rep- resent transport of solutes due to diffusion, electric gradi- ent, and convention respectively. Thus the equation can pre- dict solute rejection as a function of feed concentration, ion charge, convection across the membrane, and solute diffu- sion (Braghetta, 1995). It can be used to calculate the effec- tive pore size (which does not necessarily mean that pores exist), and to determine the thickness and effective charge of the membrane (Bowen and Mukhtar, 1996).

The mass transport through a membrane could have as many as five steps such as (i) diffusion from the water phase to the surface of the membrane, (ii) selective portioning into the membrane phase, (iii) selective transport (diffusion) through the membrane, (iv) desorption from the permeate side of the membrane, and (iv) diffusion away from the mem- brane and into the bulk fluid of the extracting phase. Among these transfer operations, steps (i), (iii), and (v) may control

the rate of mass transfer as the slowest step. Mass trans- port through NF can be described as diffusion-controlled processes. The different mechanisms and models used to describe the transport of solutes through a semi-permeable NF membrane are Donnan equilibrium, extended Nernst- Planck, hindered transport, and irreversible non-equilibrium thermodynamics model. The use of the extended Nernst- Planck model in conjunction with the Donnan equilibrium condition suggests the possibility of characterizing the ef- fective membrane pore size and effective charge density to predict the separation of mixtures of electrolytes at the membrane/solution interface. Secondly, the non-equilibrium thermodynamic model provides a real description of ion transport through membranes even though the membrane is treated as a black box and the model gives little insight into the physico-chemical processes involved in solute and solvent transport across a membrane. Thus, the thermody- namic model can be accepted especially when the param- eters used in the model are experimentally measured. The steric-hinderance pore (SHP) model is the modification of pore model where the gradient of mechanical pressure across a membrane is taken into account. Summary of the previ- ous study on NF modelling is given in Table 3 (Lee and Lee, 2000; Yoon et al., 2002; Thanuttamavong et al., 2001; Ratanatamskul et al., 1998).

6 Membrane fouling in the NF process

Like any other membrane processes, NF is also suscepti- ble to membrane fouling. Membrane fouling is one of the significant challenges in any membrane process and there- fore understanding the fouling mechanism and identifying a suitable control option is one of the essential components of the membrane applications (Hong and Elimelech, 1997; Mulder, 1997; Lee et al., 2010; Phuntsho et al., 2011b). The solutions to fouling issues require a multipronged approach involving the membrane properties, operational conditions, feed characteristics, etc. (Chapman et al., 2002; Shon et al., 2005, 2009; Phuntsho et al., 2011a). The NF membrane foul- ing could be due to inorganic precipitation or scaling, col- loidal fouling, organic adsorption and/or biofouling. While biofouling is important in long term, most likely, biofouling occurs only after organic or inorganic or colloidal fouling. Since interactions between solutes and the membranes are poorly understood, it is possible that effects like charge in- teractions, bridging, and hydrophobic interactions may play an important role in NF fouling. Normally, membranes with larger pores exhibit a greater flux decline as filtration pro- ceeds because of internal clogging. However, flux decline is not necessarily due to fouling. Other phenomena such as concentration polarization or osmotic pressure or membrane compaction can appear as fouling during the NF process.

Reiss and Taylor (1994) compared three parameters, silt density index (SDI), modified fouling index (MFI), and the

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H. K. Shon et al.: Nanofiltration for water and wastewater treatment 51

Table 3. Summary of the mathematical modeling work on Nanofiltration

Modeling Types Remarks

dm·s dt =−

d( Jb Jw

)

dt Cross flow dm·s

dt : the degree of concentration polarization

dm·c dt =

d( Jb Jt

)

dt dm·c

dt : deposition probability and the bulk

crystallization rate

R= 1− [1/(t2−t1)

∫ t2 t1

Ls(Cm−Cp)dt]

[ ( f /(t2−t1)

∫ t2 t1

Lv∆Peffdt ] Stirred cell The measured rejection matches the model

results quite well

Js =ω∆π+ (1−σ)CavgJv (non-equilibrium thermodynamic equation)

Cross flow Solute flux is governed by convection for a relatively large pore size membrane

P= HDSD DS( Ak ∆x

) HD = 1 SD = (1−η)

2

η= r s/r p (steric hindrance pore model)

Cross flow Ak ∆x

: the ratio of porosity to membrane thick- ness (value: 4.4×103 ∼ 7.8×104 for NF) P: solute permeability

Ji =−ui RT dci dx −Zi ci ui F

dΨ dx + ci Jv

(the extended Nernt-Planck equation) Cross flow The difference in rejection between chloride

and nitrate was well explained by introduc- ing a new parameter∑

i ZiCi = 0 outside membrane∑

i ZiCi +øX = 0 inside membrane

( ci Ci

)1/Zi = exp(−F∆ΨD RT

) (Donnan equilibrium)

linear correlation of the mass transfer coefficient (MTC) to investigate the NF fouling. However, no correlation between these parameters was obtained indicating that the simple fil- tration laws might not be valid for NF process. DiGiano et al. (1994) found that the organic compounds with molecular weight higher than 30 kDa was responsible for NF fouling. They further noticed the change in fouling mechanism after 20 h operation of NF, possibly due to the interactions of the hydrophobic and hydrophilic fractions of organics. Thorsen et al. (1999) recommended the use of highly hydrophilic NF membranes with pore size of 1–2 nm and low operating pres- sure to reduce fouling. They found that hydrophilic mem- branes were more fouling resistant irrespective of the pore size of the membranes. Membrane fouling would be very se- vere in positively charged membranes which can attract the negatively charged organics easily (Nystrom et al., 1995). In- organic ions such as calcium, phosphorus, aluminium and iron etc. were found to enhance the membrane fouling dur- ing water treatment process (Baker et al., 1995). Hong and Elimelech (1997) showed that membrane fouling by NOM was increased in the presence of calcium ions, at lower pH, and higher ionic strength. They further noted that permeation drag and electrostatic double layer repulsion controlled the membrane fouling. Chellam et al. (1997) found that colloidal materials could cause more fouling than organic in NF. The NF membrane fouling can occur due to the following rea-

sons: (i) biological fouling which is the growth of biologi- cal species on the membrane surface, (ii) colloidal fouling which results in a loss of permeate flux through the mem- brane, (iii) organic fouling due to the deposition of organic substances, and (iv) scaling which is defined as the forma- tion of mineral deposits precipitating from the feed stream to the membrane surface (Duranceau, 2001).

6.1 Membrane fouling control

Membrane fouling is normally controlled either by oper- ating the system within the critical flux range or adding chemicals (especially to prevent inorganic scaling and foul- ing), and/or by pretreatment. Pretreatment is emerging as the most promising solution to control the fouling as it is simple and easy to implement. Gusses et al. (1997) and Glucina et al. (1997) found that conventionally used filter media was not sufficient to reduce the fouling of NF, and suggested a combi- nation of coagulation, ozonation and biofiltration as a better alternative to reduce the NF fouling. Normally for coagula- tion as a pretreatment, iron or aluminium sulphate are com- monly used. However, Nystrom et al. (1995) observed that when humic acid was filtered alone, it was retained to 100 %, but when filtered together with FeCl3, humic acid retention decreased. Thus, it requires to be wise on the use of physico- chemical pretreatments before a NF module. Levenstein et

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52 H. K. Shon et al.: Nanofiltration for water and wastewater treatment

al. (1996) found that addition of a polyelectrolyte enhanced the ion rejection in NF. Activated carbon adsorption is a very effective pretreatment process. Many researchers have used activated carbon adsorption as a pretreatment to mem- brane processes (Kim et al., 2001; Matsui et al., 2001a, b; Vigneswaran et al., 2003). Since the initial decrease in the permeate flux is mainly due to rapid, irreversible adsorption of organic substances on the membrane surface (Ben Aim et al., 1993), providing a pre-treatment such as adsorption or flocculation of organics before passing the feed solution through the membrane is very effective solution to the mem- brane fouling problem (Chapman et al., 2002). Backwashing, backflushing or chemical cleaning are some other options to reduce the NF fouling.

6.2 Importance of pre-treatments prior to NF process

Pretreatment of the feed to NF is one of the important con- siderations to protect the membrane and to improve the per- formance of NF. Protection refers usually to the prevention of fouling, but also includes the protection against mechani- cal and chemical damage. A high solids load can damage the membrane surface mechanically and restrict the flow in the filtration system. Meanwhile, oxidation agent, e.g. chlorine and ozone, are harmful to many membrane materials.

NF can be used in the tertiary wastewater especially to re- move persisting organic pollutants. In order to improve the filtration flux of NF and extend the operation of NF without extensive organic fouling, effective pretreatment is necessary. In recent years, high rate flocculation and magnetic ion ex- change resin have been tried to remove hydrophobic and hy- drophilic organics respectively. This can greatly reduce the organic fouling on the NF membranes.

7 Summary

NF that is the widely used membrane process for water and wastewater treatment in addition to other applications such as desalination where its application is increasing plays an important role to partially replace RO, which reduces en- ergy and operational costs. The fundamentals of membrane process in general and the mechanisms of the NF process in particular with some of its basic models were discussed and the issues and challenges of the membrane fouling with NF applications have also been identified including the pre- treatment options to mitigate the membrane fouling with the NF process. For the future, NF on behalf of RO will be pref- erentially considered if it meets water quality requirements.

Acknowledgements. This research was funded by Australian Research Council (ARC). This work was supported in part by the National Research Foundation of Korea (NRF) grand funded by the Korea government (MEST) (No. 2012047029, TOC).

Edited by: I. S. Kim

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Reiss, C. R. and Taylor, J. S.: Prediction of nanofilter productivity decline using fouling indices, Proceedings AWWA Membrane Technology Conference, Nevada, USA, 1994.

Rohe, D. L., Blanton, T. C., and Marinas, B. J.: Drinking water treat- ment by nanofiltration, National conference on Environmental Engineering, 1990.

Seidel, A. and Elimelech, M.: Coupling between chemical and physical interactions in natural organic matter (NOM) fouling of nanofiltration membranes: implications for fouling control, J. Membr. Sci., 203, 245–255, 2002.

Seo, G. T., Suzuki, Y., and Ohgaki, S.: Bilogical powdered activated carbon (BPAC) microfiltration for wastewater reclamation and reuse, Desalination, 106, 39–45, 1996.

Seo, G. T., Ohgaki, S., and Suzuki, Y.: Sorption characteristics of bi- logical powdered activated carbon in BPAC-MF (biological ac- tivated carbon-microfiltration) system for refractory organic re- moval, Water Sci. Technol., 35, 163–170, 1997.

Shannon, M. A., Bohn, P. W., Elimelech, M., Georgiadis, J. G., Marinas, B. J., and Mayes, A. M.: Science and technology for water purification in the coming decades, Nature, 452, 301–310, 2008.

Shon, H. K., Vigneswaran, S., Aim, R. B., Ngo, H. H., Kim, I. S., and Cho, J.: Influence of Flocculation and Adsorption as Pretreatment on the Fouling of Ultrafiltration and Nanofiltration Membranes: Application with Biologically Treated Sewage Ef- fluent, Environ. Sci. Technol., 39, 3864–3871, 2005.

Shon, H. K., Vigneswaran, S., Zareie, M. H., Aim, R. B., Lee, E., Lee, J., Cho, J., and Kim, I. S.: Physico-chemical pretreatment to seawater reverse osmosis (SWRO): organic characterization and membrane autopsy, Desalination, 236, 282–290, 2009.

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www.drink-water-eng-sci.net/6/47/2013/ Drink. Water Eng. Sci., 6, 47–53, 2013

Responses to the reviewers’ comments

Reviewer #1

The reviewer’s comments are general and therefore there are no specific responses required.

Reviewer #2

1. Some mistakes including typographical errors are also founded in the text. Author’s response: The authors have corrected the typo errors in the manuscript revised. The revised manuscript will be submitted after the DWES preparation and submission procedures.

2. Abstract Page 60, line 8 fouling challenges and their control mechanisms adopted. –> Please revise the above sentence.

Author’s response: The authors have revised the sentence.

3. Introduction With technological advances and the ever-increasing stringency of water quality criteria, membrane processes are becoming a more attractive solution to the challenge of quality water, and water reuse (Shannon et al., 2008). The use of membrane technology in wastewater reclamation is increasing as the requirement of stringent water quality is being felt by the pollution control authorities around the world. Please revise the above sentences. In my view, those sentences have the same meaning in a sense.

Author’s response: The authors have corrected the sentences in the manuscript revised.

4. Page 61, line 11 Similarly, Ultrafiltration (UF)_ –> Please check the style.

Author’s response: The authors have corrected the style.

5. Page 64, line18 x=distance normal to –> Please check the style.

Author’s response: The authors have corrected the typo errors in the manuscript revised. The revised manuscript will be submitted after the DWES preparation and submission procedures.

6. Referenes Page 72 Shon, H. K., Vigneswaran, S., Aim, R. B., Ngo, H. H., Kim, I. S., and Cho, J.: Influence of Floc-culation and Adsorption as Pretreatment on the Fouling of

Ultrafiltration and Nanofiltration Membranes: Application with Biologically Treated Sewage Effluent, Environ. Sci. Technol.,39, 3864–3871, 2005.

Author’s response: The authors have corrected the typo errors in the manuscript revised. The revised manuscript will be submitted after the DWES preparation and submission procedures.

7. Page 73 Vigneswaran, S., Chaudhary, D. S., Ngo, H. H., Shim, W. G., and Moon, H.: Application of a PAC-Membrane Hybrid System for Removal of Organics from Secondary Sewage Effluent: Experiments and Modelling, Separ. Sci. Technol., 38, 2183–2199, 2003. Please carefully check once again the reference style. Interactive comment on Drink. Water Eng. Sci. Discuss., 6, 59, 2013.

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Review Process

The process of peer-review and publication in the interactive scientific journal Drinking Water Engineering and Science (DWES) differs from traditional scientific journals. It is a two-stage process involving the scientific discussion forum Drinking Water Engineering and Science Discussions (DWESD), and it has been designed to use the full potential of the internet to foster scientific discussion and enable rapid publication of scientific papers.

Initial access review by competent editors assures the basic scientific and technical quality for papers published in DWESD. Subsequent interactive discussion and public commenting by the referees, authors and other members of the scientific community is expected to enhance quality control for papers published in DWES beyond the limits of the traditional closed peer-review. Also in cases where no additional comments from the scientific community are received, a full peer- review process in the traditional sense, but in a more transparent way, is assured before publication of a paper in DWES.

The individual steps of the DWES process of peer-review, publication and interactive discussion are described below, and complementary illustrations are given in the following Flow Chart .

1. Submission of Original Manuscript and Editor Assignment

An original manuscript is submitted electronically and assigned to a Topical Editor covering the relevant subject areas (for details see Editor Assignment).

2. Access Review

The Topical Editor is asked to evaluate whether the manuscript is within the scope of the journal and whether it meets a basic scientific quality. The Topical Editor can suggest technical corrections (typing errors, clarification of figures, etc.) before publication in DWESD. Further requests for revision of the scientific contents are not allowed at this stage of the review process but shall be expressed in the interactive discussion following publication in DWESD.

3. Technical Corrections

The authors have the opportunity to perform technical corrections, which may be reviewed by the Topical Editor to verify requested corrections and prevent further revisions, which are not permitted at this stage.

4. Publication of Discussion Paper in DWESD

After final acceptance the manuscript is typeset by the Copernicus Publications Production Office, proofread by the authors, and published as a discussion paper on the DWESD Website. Public accessibility, archiving and citability are guaranteed from this moment on (usually about 2-8 weeks after submission).

5. Open Discussion (6 weeks)

Upon internet publication the paper is opened for public review and discussion, during which interactive comments can be published by designated referees (anonymous or named) and all interested members of the scientific community (named). Normally, every discussion paper shall receive at least two Referee Comments; for more information see Interactive Public Discussion.

6. Final Response

After the open discussion the authors are expected to publish a response to all comments (within 4 weeks, extendable upon request). The editor can also publish additional comments or recommendations. Normally, however, formal editorial recommendations and decisions shall be made only after the authors have had an opportunity to respond to all comments, or if they request editorial advice before responding.

7. Submission of Revised Manuscript

Submission of a revised manuscript is expected only if the authors have satisfactorily addressed all comments, and if the revised manuscript meets the high quality standards of DWES (MS Evaluation Criteria). In case of doubt, the authors shall consult the editor if she/he recommends preparation and submission of a revised manuscript or not. Normally the revised manuscript should be submitted no later than 4 to 8 weeks after the end of the open discussion. If more time is required for manuscript revision, the authors can request an extension.

8. Peer-Review Completion

In view of the access peer-review and Interactive Public Discussion, the Topical Editor either directly accepts/rejects the revised manuscript for publication in DWES or consults referees in the same way as during the completion of a traditional peer-review process. If necessary, additional revisions may be requested during peer-review completion until a final decision about acceptance/rejection for DWES is reached.

9. Publication of Final Revised Paper in DWES

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Announcements

DWES publishes the best papers of the "Crossing the currents, water and social media" conference in a special issue. More information.

After a successful cooperation in 2009 and 2011, we are proud to publish the best papers of "CCWI 2013" in a special issue.

DWES publishes the best papers of the "5th International Desalination Workshop 2012 South Korea" in a special issue.

Overview of Scheduled Special Issues

Recent Papers

02 | DWES, 05 Jun 2013: Nanofiltration for water and wastewater treatment – a mini review

03 | DWESD, 03 Jun 2013: Present challenges for future water sustainable cities: a case study from Italy

04 | DWES, 03 Jun 2013: Predicting the residual aluminum level in water treatment process

05 | DWESD, 27 May 2013: Effects of network pressure on water meter under-registration:

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DWES - Review Process & Interactive Public Discussion

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In case of acceptance the final revised paper is typeset and proofread. Then it is published on the DWES Website with a direct link to the preceding original paper and interactive discussion in DWESD. All publications (original paper, interactive comments, final revised paper) are permanently archived and remain accessible to the open public via internet and the final revised paper is also accessible as print copy.

The timing indicated above is a guideline which may have to be modified according to the availability and response times of editors, referees, and authors.

The submission of comments and replies which continue the discussion of scientific papers beyond the limits of immediate interactive discussion is encouraged. Such peer-reviewed comments undergo the same process of peer-review and publication as described above, i.e. after publication and discussion in DWESD they may also be published in DWES if sufficiently substantial.

If a manuscript that has been published as a discussion paper in DWESD is not accepted for publication as a final paper in DWES, the authors have several options to proceed as outlined under Frequently Asked Questions, Point 7.

Interactive Public Discussion

In the Interactive Public Discussion following the publication of a paper in Drinking Water Engineering and Science Discussions (DWESD), the following types of interactive comments can be submitted for immediate non-peer-reviewed publication alongside the discussion paper:

Short Comments (SC) can be posted by any registered member of the scientific community (free online registration). Such comments are attributed, i.e. published under the name of the commentator.

Referee Comments (RC) can only be posted by the referees involved in the peer-review of the discussion paper. They can be anonymous or attributed (according to the referee's preference).

Editor Comments (EC) can only be posted by the Topical Editor of the discussion paper.

Author Comments (AC) can only be posted by the contact author of the discussion paper on behalf of all co-authors.

All interactive comments are fully citable, paginated, and archived as a supplement to DWESD.

Figures and Supplements

Comments can be composed by using either plain text or LaTeX formatting. Complex content without LaTeX commands can be uploaded as a *.pdf file and will be displayed as a supplement to the comment. In any case, figures can directly be included in the comment.

The Interactive Public Discussion comprises two Phases

Phase 1: Open Discussion (6 weeks)

The referees are asked to publish one or more Referee Comments and every registered member of the scientific community may publish Short Comments as defined above. The authors of the discussion paper have the option (but no obligation) to reply by publishing their own Short Comments individually, or by posting Author Comments collectively on behalf of all co-authors. The authors of a discussion paper are automatically informed by e-mail about the publication of comments in the Interactive Public Discussion of their paper. Publication alert services will also be available to other members of the scientific community. The publication of interactive comments is supervised by the Topical Editors, who have the option of censoring comments that are not of a substantial nature and of direct relevance to the issues raised in the discussion paper or which contain personal insults. Authors are advised to follow the discussion of their paper and to notify the Topical Editor in case of abusive commenting. The DWES editorial board reserves the right to exclude abusive commentators.

To participate in the interactive discussion of a paper recently published in DWESD, please locate this paper on the DWESD Website and follow the appropriate links there.

Phase 2: Final Response

After the open discussion no more Short Comments and Referee Comments are accepted, but the contact author and the Topical Editor of the discussion paper have the opportunity to publish final Author Comments and Editor Comments, respectively. The final response phase is generally limited to 4 weeks (expandable to 8 weeks) and automatically terminated upon submission of a revised manuscript. Before submitting a revised version of their manuscript for publication in DWES (second stage of publication), the authors are supposed to have answered the Referee Comments and relevant Short Comments cumulatively or individually in one or more Author Comments.

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Drinking Water Engineering and Science (DWES) Editor-in-Chief: Gertjan Medema Executive Editors: Jasper Verberk, Arne Verliefde & Ignaz Worm

Open Access – Public Peer-Review & Interactive Public Discussion – Personalized Copyright under a Creative Commons License – Moderate Article Processing Charges

Indexed in Scopus . Included in the Directory of Open Access Journals (DOAJ) as well as in the Bodleian Library (UK) , Deutsche Digitale Bibliothek (D) and Library of Congress (USA) . Long-term e-archived in Portico and CLOCKSS .

Aims and Scope

Drinking Water Engineering and Science aims to be the leading scientific open access journal for the publication of original research in drinking water treatment. The focus is on fundamental and applied research in water sources, substances, drinking water treatment processes, distribution systems and residual management.

Drinking Water Engineering and Science serves scientists from universities and research institutes and engineers from water supply companies and engineering consulting firms.

Every issue will contain at least one paper on drinking water in developing countries. A fund will be established to give authors from developing countries the opportunity to publish their work.

Journal Topics

Subject Area Topic

Sources - Protection - Pollution - Catchment and reservoirs - Aquifer Management

Treatment - Aeration - Microfiltration, ultrafiltration - Nanofiltration, reverse osmosis - Granular filtration - Adsorption - Advanced oxidation - Disinfection - Coagulation, sedimentation, flotation, flocculation - Conditioning - Ion exchange - Biological treatment - Water treatment in developing countries - Natural Treatment

Substances - Emerging chemical contaminants (endocrine disrupting compounds) - Particles - NOM - Taste and odor

Distribution - Treatment - distribution interaction - Network design - Demand prediction

Search DWES

Announcements

DWES publishes the best papers of the "Crossing the currents, water and social media" conference in a special issue. More information.

After a successful cooperation in 2009 and 2011, we are proud to publish the best papers of "CCWI 2013" in a special issue.

DWES publishes the best papers of the "5th International Desalination Workshop 2012 South Korea" in a special issue.

Overview of Scheduled Special Issues

Recent Papers 01 | DWES, 21 Jun 2013: Development of water use scenarios as a tool for adaptation to climate change

02 | DWES, 13 Jun 2013: Technical Note: Wet validation of optical tomography for drinking water discolouration studies

03 | DWES, 05 Jun 2013: Nanofiltration for water and wastewater treatment – a mini review

04 | DWESD, 03 Jun 2013: Present challenges for future water sustainable cities: a case

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Tools - Process control and automation - Modeling and simulation - Sensoring and monitoring - Computational fluid dynamics - Microbial and chemical risk assessment

Applications - Potable vs. non-potable - Industry - Water reuse and recycling - Swimming pools - Residuals Management

Issuing Body

Drinking Water Engineering and Science (DWES) and Drinking Water Engineering and Science Discussions (DWESD) are published by the Copernicus GmbH (Copernicus Publications) on behalf of the Delft University of Technology (TU Delft).

DWES ISSN 1996-9457 eISSN 1996-9465 http://www.drink-water-eng-sci.net

DWESD eISSN 1996-9481 http://www.drink-water-eng-sci-discuss.net

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