Comparison of Nanofiltration with Reverse Osmosis in Reclaiming

membranes

Article Comparison of Nanofiltration with Reverse Osmosis in Reclaiming Tertiary Treated Municipal Wastewater for Irrigation Purposes

MhdAmmar Hafiz 1 , Alaa H. Hawari 1,*, Radwan Alfahel 1 , Mohammad K. Hassan 2 and Ali Altaee 3

1 Department of Civil and Architectural Engineering, Qatar University, Doha P.O. Box 2713, Qatar; [email protected] (M.H.); [email protected] (R.A.) 2 Center for Advanced Materials, Qatar University, Doha P.O. Box 2713, Qatar; [email protected] 3 School of Civil and Environmental Engineering, University of Technology in Sydney, 15 Broadway, Ultimo, NSW 2007, Australia; [email protected] * Correspondence: [email protected]; Tel.: +974-4403-4184

Abstract: This study compares the performance of nanofiltration (NF) and reverse osmosis (RO) for the reclamation of ultrafiltered municipal wastewater for irrigation of food crops. RO and NF technologies were evaluated at different applied pressures; the performance of each technology was evaluated in terms of water flux, recovery rate, specific energy consumption and quality of permeate. It was found that the permeate from the reverse osmosis (RO) process complied with Food and Agriculture Organization (FAO) standards at pressures applied between 10 and 18 bar. At an applied pressure of 20 bar, the permeate quality did not comply with irrigation water standards in terms of chloride, sodium and calcium concentration. It was found that nanofiltration process was not suitable for the reclamation of wastewater as the concentration of chloride, sodium and calcium exceeded the allowable limits at all applied pressures. In the reverse osmosis process, the highest



 recovery rate was 36%, which was achieved at a pressure of 16 bar. The specific energy consumption at this applied pressure was 0.56 kWh/m3. The lowest specific energy of 0.46 kWh/m3 was achieved Citation: Hafiz, M.; Hawari, A.H.; Alfahel, R.; Hassan, M.K.; Altaee, A. at an applied pressure of 12 bar with a water recovery rate of 32.7%. Comparison of Nanofiltration with Reverse Osmosis in Reclaiming Keywords: irrigation water; reverse osmosis; nanofiltration; treated sewage effluent; water reuse Tertiary Treated Municipal Wastewater for Irrigation Purposes. Membranes 2021, 11, 32. https://doi. org/10.3390/membranes11010032 1. Introduction Water scarcity is a major challenge that affects food security worldwide, particularly Received: 21 December 2020 in arid regions. The United Nations (UN) estimates that agriculture accounts for 70% of Accepted: 29 December 2020 water usage around the world [1]. Municipal treated wastewater could be an economical Published: 2 January 2021 solution to be used as irrigation water and a source of nutrients [2]. Treated wastewater can improve soil health and reduce fertilizers consumption. Treated wastewater is rich Publisher’s Note: MDPI stays neu- in pathogens, organics, sodium and chloride; therefore, it could damage soil. The water tral with regard to jurisdictional clai- quality for irrigation water is mainly characterized in terms of total dissolved salts, pH, and ms in published maps and institutio- different concentrations of ions and cations (e.g., Na, Cl, NO , SO , PO , K, Ca, and Mg). nal affiliations. 3 4 4 Enhancing the quality of treated wastewater to meet irrigation standards has become a necessary practice. In order to reach the required quality of treated wastewater, membrane technologies are considered to be a critical element. Copyright: © 2021 by the authors. Li- Shanmuganathan et al. (2015) assessed the performance of nanofiltration (NF) and censee MDPI, Basel, Switzerland. reverse osmosis (RO) for the treatment of microfiltered municipal wastewater to be used This article is an open access article for the irrigation of food crops [2]. The used nanofiltration membranes were NP 010, distributed under the terms and con- NP 030 and NTR 729HF with a molecular weight cut-off (MWCO) 1000, 700 and 400, ditions of the Creative Commons At- respectively. The reverse osmosis membrane was the seawater reverse osmosis membrane tribution (CC BY) license (https:// produced by Woongjin Chemical with an MWCO of 100. The used RO membrane must be creativecommons.org/licenses/by/ operated at high applied pressure (i.e., a minimum of 40 bar). Their study focused on the 4.0/).

Membranes 2021, 11, 32. https://doi.org/10.3390/membranes11010032 https://www.mdpi.com/journal/membranes Membranes 2021, 11, 32 2 of 13

permeate water quality without taking into consideration the high energy consumption of SWRO operated at high pressure. It was found that the water produced using NF and RO alone was not suitable for irrigation for food crops due to poor water quality. However, the hybrid NF-RO process was capable of producing permeate which meets the water quality standards for irrigation of food crops. Li et al. (2016) studied the performance of nanofiltration for the treatment of municipal wastewater using various applied pressures and feed solution pH [3]. It was found that the optimum performance was obtained at an applied pressure of 12 bar, a flow rate of 8 LPM and pH = 4. A pilot-scale study conducted by Oron et al. (2006) showed that by using a hybrid ultrafiltration–reverse osmosis (UF- RO) technology, water suitable for irrigation can be produced from secondary treated municipal wastewater [4]. The cost of the process was between 0.16 and 0.24 USD/m3 water. Mrayed et al. (2011) used a hybrid nanofiltration–reverse osmosis (NF-RO) system to produce irrigation water from secondary treated effluent [5]. They used polyacrylic acid (PAA) as a chelating agent. The addition of PAA helped in the formation of covalent bonds among different nutrients in the feed which improved the rejection rate for those nutrients. Egea-Corbacho et al. (2019) tested the performance of a pilot-scale nanofiltration membrane for the treatment of secondary treated wastewater effluent [6]. It was found that the product water quality complies with the Spanish Royal Decree 1620/2007. This was concluded by considering Escherichia coli, total suspended solids and turbidity. Still, the authors did not compare the concentration of various elements in the permeate water with allowable limits (i.e., phosphates, nitrates, total dissolved solids, ammonium, sodium and chloride). A study from Chon et al. (2012) used hybrid technology composed of a membrane bioreactor and nanofiltration to produce irrigation water from municipal wastewater [7]. It was found that the physicochemical properties and molecular weight cut- off were the most critical aspects in the removal of nutrients from the water. Gu et al. (2019) evaluated the performance of the trihybrid anaerobic membrane bioreactor (AnMBR)– reverse osmosis (RO) –ion exchange (IE) process for the transformation of microfiltered municipal wastewater to high-grade clean water [8]. The net energy consumption of the process was 1.16 kWh/m3, and the product water was found to be suitable for industrial and indirect human applications. Hafiz et al. (2019) used FO to produce irrigation water from treated sewage effluent (TSE) [9]. The feed solution and draw solution for the FO were TSE, and an engineered fertilizing solution (0.5 M NaCl and 0.01 M (NH4)2HPO4), respectively. The draw solution was regenerated using RO. The specific power consumption was between 2.18 and 2.58 kWh/m3. Liu et al. (2011) tested NF and RO for the reclamation of textile wastewater in terms of COD rejection and salinity removal [10]. It was found that NF had more severe flux decline compared to RO due to high membrane fouling in the NF process. The total salt rejection of RO was higher than NF. Qi et al. (2020) analyzed the removal efficiency of pollutants in municipal wastewater treatment plants in China along with the operational costs [11]. It was found that biological oxygen demand (BOD5) was the highest removed pollutant, while total nitrogen (TN) was the lowest to be removed. It was recommended that higher TN removal efficiencies should be achieved in order to obtain a better effluent quality. Previous studies assessed the performance of various membrane processes for the treatment of secondary treated wastewater. However, little information is available on the performance of reverse osmosis and nanofiltration to further treat tertiary treated wastewater to generate irrigation water for food crops. This paper is focused on optimizing the performance of the processes to generate a permeate suitable for the irrigation of food crops. The product water quality must comply with the Food and Agriculture Organization (FAO) standards. It is recommended to select a single membrane process that can generate high-quality irrigation water from treated sewage effluent with minimal energy requirements. This study aims to compare the performance of nanofiltration and reverse osmosis for the further treatment of ultrafiltered municipal wastewater for the irrigation of food crops. The performance of each technology was evaluated under different applied pressures in terms of water flux, energy consumption and quality of permeate. Membranes 2021, 11, 32 3 of 13

2. Materials and Methods 2.1. Feedwater The feed solution used in this study was ultrafiltered tertiary treated sewage effluent (TSE). TSE samples were collected from Doha north wastewater treatment plant. The wastewater treatment plant has three treatment stages: (1) the preliminary treatment that contains step screens and vortex degritters; (2) the secondary treatment stage that contains a bioreactor and clarifier; (3) the tertiary treatment stage that contains a chlorine dosing tank, a multimedia filter, an ultrafiltration membrane process and UV disinfection. In total, 20 L of the treated sewage effluent was collected from the treatment plant twice a week and then stored at 2 ◦C to preserve the water quality. The characteristics of the TSE sample are shown in Table1. The maximum limit of the listed parameters was recommended by FAO [11]. The use of this feed water on food crops was unsuitable because of excessive total dissolved solids (TDS) and high ions/cations. The concentration of heavy metals was below the maximum limit recommended by FAO [12]. The conductivity of samples was measured using an OAKTON PCD650 multimeter. Anion concentration was measured by ion chromatography (Metrohm 850 Professional IC) (Metrohm, Herisau, Switzerland), and cation concentration was measured using plasma emission spectroscopy (iCAP 6500-ICP- OES CID) (Thermo Scientific, Waltham, MA, USA). Before measuring the concentration of anions and cations, samples with a conductivity value above 1 mS/cm were diluted using deionized water to a conductivity value below 1 mS/cm. This was carried out to eliminate the interference of high peaks of Na and Cl which may affect the readings of other elements. The turbidity was measured using a turbidity meter (Hach 2100p) (Hach, Dever, CO, USA). Metal concentration was measured using inductively coupled plasma—mass spectrometry (Nexion 300D) (Nexion, Gujarat, India).

Table 1. Characteristics of the feed water (treated municipal sewage effluent).

Max Limit Parameter Value Standard Testing Method (Irrigation Water) APHA-2540 C/total dissolved TDS (ppm) 1461 ± 5 750 solids dried at 180 ◦C Turbidity (NTU) 0.2 ± 0.1 2 APHA-2130B/nephelometric Electrical conductivity (mS/cm) 2.56 ± 0.2 0.7 APHA-2510B/conductivity Dissolved organic carbon (ppm) 6.67 ± 0.05 - Diaphragm electrode method Fluoride (ppm) 0.27 ± 0.2 1.5 Chloride (ppm) 897.5 ± 0.2 106.5 APHA-4110/determination of Bromide (ppm) 0.96 ± 0.2 1 anions by ion chromatography Nitrate (ppm) 25.84 ± 0.2 20 Sulfate (ppm) 320.3 ± 0.2 400 Sodium (ppm) 200.3 ± 0.2 69 Potassium (ppm) 12.4 ± 0.2 10 APHA-3120/determination of metals by plasma emission Calcium (ppm) 87.7 ± 0.2 40 spectroscopy Magnesium (ppm) 21.4 ± 0.2 24 ASTM D1068-15/standard Iron (ppm) 0.59 ± 0.02 5 test methods for iron in water Membranes 2021, 11, 32 4 of 13

Table 1. Cont.

Max Limit Parameter Value Standard Testing Method (Irrigation Water) Boron (ppb) 158.97 ± 0.1 500 Vanadium (ppb) 0.11 ± 0.1 100 Manganese (ppb) 11.54 ± 0.1 200 Cobalt (ppb) 0.17 ± 0.1 50 ± Nickel (ppb) 23.11 0.1 200 EPA method 200.8 Copper (ppb) 13.08 ± 0.1 200 Zinc (ppb) 151.58 ± 0.1 2000 Cadmium (ppb) 0.2 ± 0.1 10 Beryllium (ppb) 2.02 ± 0.1 100

2.2. Experimental Setup A schematic sketch for the bench-scale membrane testing skid is shown in Figure1.A crossflow CF042D cell made of acetal copolymer provided by Sterlitech-USA was used in the nanofiltration and reverse osmosis processes. The cell dimensions are 12.7 × 8.3 × 10 cm with an active length of 9.2 cm, width of 4.6 cm and 0.23 cm slot depth. Two aluminum tanks were used to store the feed and the permeate water. A HYDRACELL pump (M-03S) (230 V, 50 HZ, 3 PH, 6.7 LPM) was used to pressurize the feed water into the system. A water chiller (PolyScience Chiller) was used to maintain the feedwater temperature at room temperature (25 ± 2 ◦C). The pressure was regulated through the system using a back pressure control valve. The flow rate of the feed solution was measured using a flow meter (Read Panel Mount Flow Meter) supplied by (Sterlitech, Washington, WA, USA). The permeate flux was measured using a Mettler Toledo—ICS 241 digital balance that was connected to a computer. A specific quantity (3 L) of TSE was used as a feed solution in both processes. The applied pressure in the RO and the NF processes varied between 10 and 20 bar with an increase of 2 bar for each experiment. The flow rate was 3.5 LPM, and the experimental running time was 4 h. The cross-flow velocity in the experiments was kept at 0.55 m/s. The used RO membrane was BW30LE produced by DOW FILMTEC. The used RO membrane is a polyamide thin-film composite (TFC) membrane with a molecular weight cut-off 100 Da. The used NF membrane was NF90 produced by DOW FILMTEC. The used NF membrane is a polyamide TFC membrane with a molecular weight cut-off 200–400 Da. All experiments were repeated three times. and the average of the results is presented. Characteristics for the nanofiltration and reverse osmosis membrane used in the experiment were summarized in Table2. Three samples (50 mL each) were collected after each experiment for the evaluation of product water quality.

Table 2. Characteristics for the nanofiltration and reverse osmosis membrane used in the experiment.

Properties NF 90 [13] BW30LE [14,15] Contact angle 72.2◦ 63.7◦ Zeta potential (mV) −60 −32 Root mean square roughness (nm) 61 49.7 Molecular weight cut-off (Da) 200–400 100 Thickness (nm) 293 150 MembranesMembranes2021 2021,,11 11,, 32x FOR PEER REVIEW 55 ofof 1313

Pressure regulating valve

Recirculation valve

Feed water tank Pressure relief Pressure valve Flow meter gauge High pressure Cross flow filtration compartment pump Permeate water tank

Chiller

Figure 1. A schematic diagram for the crossflow lab-scale membrane test skid. Figure 1. A schematic diagram for the crossflow lab-scale membrane test skid. 3. Results and Discussion 3. Results and Discussion 3.1. Effect of Feed Pressure on Water Flux and Recovery Rate 3.1. Effect of Feed Pressure on Water Flux and Recovery Rate The water flux (Jw) in the RO process and the NF process was calculated using (퐽 ) EquationThe (1)water [16 ]: flux 푤 in the RO process and the NF process was calculated using   Equation (1) [16]: Vp Jw = (1) Am ×푉푝 t 퐽푤 = ( ) (1) 퐴푚 × 푡 2 where VP is the permeate volume (L), Am is the membrane area (m ), and t is the time 2 ofwhere operation 푉푃 is the (h). permeate Figure2a,b volume present (L), the 퐴푚 change is the membrane in water flux area in (m RO), and and NF t is withthe time time, of respectively.operation (h). It Figure can be 2a seen,b present from Figure the change2a,b that in the water water flux flux in decreasedRO and NF with with time time, at allre- appliedspectively. pressures. It can be The seen decrease from Figure in the 2a,b water that flux the with water time flux is duedecreased to membrane with time fouling at all andapplied the concentrationpressures. The of decrease the feed solution,in the water as the flux reject with solution time is wasdue recycledto membrane back intofouling the system.and the concentration In addition, the of reductionthe feed solution of the water, as the flux reject could solution be due was to recycled the accumulation back into the of organicsystem. matterIn addition, on the the surface reduction of the membraneof the water [17 flux]. Figure could2 abe shows due to that the in accumulation the RO process, of theorganic water matter flux aton an the applied surface pressure of the membrane of 12–20 bar [17] was. Figure within 2a theshows same that range, in the but RO at pro- an appliedcess, thepressure water flux of at 10 an bar, applied the water pressure flux wasof 12 much–20 bar lower. was within In the NFthe process,same range, the waterbut at fluxan applied at an applied pressure pressure of 10 bar of, the 12 barwater was flux higher was much than thelower. water In the flux NF in process the other, the studied water pressureflux at an values. applied pressure of 12 bar was higher than the water flux in the other studied pressure values.

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180 RO-10bar 160 RO-12bar 140 RO-14bar RO-16bar 120 RO-18bar .h) 2 100 RO-20bar

(L/m 80

w (a) J 60

40

20

0 0 40 80 120 160 200 240 Time (Minutes)

140.0 NF-10bar 120.0 NF-12bar NF-14bar 100.0 NF-16bar NF-18bar .h) 2 80.0 NF-20bar

(L/m 60.0

w (b) J 40.0

20.0

0.0 0 40 80 120 160 200 240 Time (Minutes)

Figure 2. Water flux using treated sewage effluent (TSE) as feed water at different applied pressures in( a) reverse osmosis Figure(b) nanofiltration. 2. Water flux using treated sewage effluent (TSE) as feed water at different applied pressures in (a) reverse osmo- sis (b) nanofiltration. Figure3 shows the average water flux for RO and NF at the different studied feed pressures. In RO, the average water flux was 21.3 L/(m2·h) at a pressure of 10 bar. The Figure 3 shows the average water flux for RO and NF at the different studied feed average water flux increased by almost 69% to reach a value of 68.1 L/(m2·h) as the pressure pressures. In RO, the average water flux was 21.3 L/m2.h at a pressure of 10 bar. The av- increased to 12 bar. The average water flux reached a value of 71.5 L/(m2·h) when the erage water flux increased by almost 69% to reach a value of 68.1 L/m2.h as the pressure applied pressure increased to 14 bar. The maximum water flux was 77.7 L/(m2·h), which increased to 12 bar. The average water flux reached a value of 71.5 L/m2.h when the ap- was obtained at an applied pressure of 16 bar. As the applied pressure further increased, plied pressure increased to 14 bar. The maximum water flux was 77.7 L/m2.h, which was the average water flux decreased. The average water flux was 71.6 and 67.5 L/(m2·h) at obtained at an applied pressure of 16 bar. As the applied pressure further increased, the a pressure of 18 and 20 bar, respectively. In the NF process, excluding the experiment average water flux decreased. The average water flux was 71.6 and 67.5 L/m2.h at a pres- with an applied pressure of 10 bar, it was found that the average water flux decreased as sure of 18 and 20 bar, respectively. In the NF process, excluding the experiment with an the pressure increased (Figure3). The maximum average water flux was 44.5 L/(m 2·h), applied pressure of 10 bar , it was found that the average water flux decreased as the which was obtained at an applied pressure of 12 bar. At an applied pressure of 14 bar, pressure increased (Figure 3). The maximum average water flux was 44.5 L/m2.h, which the average water flux decreased to almost 37%, reaching a value of 28.1 L/(m2·h). As wasthe obtained applied pressureat an applied further pressure increased, of 12 thebar. averageAt an applied water pressure flux continued of 14 bar, to the decrease, aver- 2 agereaching water aflux minimum decreased value to almost of 20.6 37%, L/(m reaching2·h) at an a applied value of pressure 28.1 L/m of.h. 20 As bar. the The applied water permeability is expected to increase as the feed pressure increases; however, applying

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Membranes 2021, 11, 32 pressure further increased, the average water flux continued to decrease, reaching a7 min- of 13 imum value of 20.6 L/m2.h at an applied pressure of 20 bar. The water permeability is expected to increase as the feed pressure increases; however, applying excessive pressure may result in excessive accumulation of foulants on the surface of the membrane, which excessivemay result pressure in a lower may average result inwater excessive flux [18]. accumulation The lowest of average foulants water on the flux surface for RO of and the membrane,NF was obtained which at may an applied result in pressure a lower of average 10 bar. waterThis is flux due [ 18to]. the The fact lowest that the average water waterdiffusion flux through for RO andthe membrane NF was obtained starts to at o anccur applied when pressurethe applied of 10 pressure bar. This exceeds is due the to thenatural fact thatosmotic the waterpressure diffusion of the throughfeed solution. the membrane It was found starts that to occurthe osmotic when thepressure applied of pressurethe feed exceedssolution thewas natural almostosmotic 9 bar; conseque pressurently, of the a low feed feed solution. pressure It was of found10 bar thatwas thenot osmoticenough pressureto overcome of the the feed osmotic solution pressure was almostof the feed 9 bar; solution. consequently, It can be a low observed feed pressure that the ofwater 10 bar flux was obtained not enough using to RO overcome was higher the osmoticthan NF; pressure this can of be the attributed feed solution. to the Itfact can that be observedtreated wastewater that the water is rich flux in obtained organic foulan using ROts. Therefore, was higher the than membrane NF; this canfouling be attributed could be toa dominant the fact that factor treated in this wastewater experiment. is richAlthough in organic the larger foulants. pore Therefore, size of the theNF membranemembrane foulingcan result could in higher be a dominant water flux, factor the foulants in this experiment.could penetrate Although through the the larger membrane pore size pores, of theunlike NF membranethe RO membrane, can result causing in higher a more water severe flux, thefouling foulants effect could [19,20]. penetrate The contact through angle the membrane pores, unlike the RO membrane, causing a more severe fouling effect [19,20]. of the NF 90 membrane was 72.2°, while that of BW30LE◦ was 63.7°; therefore, the RO◦ Themembrane contact was angle found of the to NFbe more 90 membrane hydrophilic was compared 72.2 , while to the that NF of membrane. BW30LE wasEnhanced 63.7 ; therefore,hydrophilicity the ROof the membrane RO membrane was found means to more be morestrongly hydrophilic bounded water compared layerto at the sur- NF membrane. Enhanced hydrophilicity of the RO membrane means more strongly bounded face of the membrane, which may act as a barrier for foulants and hence reduce fouling water layer at the surface of the membrane, which may act as a barrier for foulants and [13,21]. The zeta potential of NF 90 was −60 mv, and that of BW30LE was −32 mv; the NF hence reduce fouling [13,21]. The zeta potential of NF 90 was −60 mv, and that of BW30LE membrane has a higher negative charge when compared to the RO membrane. This makes was −32 mv; the NF membrane has a higher negative charge when compared to the RO the NF membrane more prone to fouling due to the high attraction force between the membrane. This makes the NF membrane more prone to fouling due to the high attraction membrane and positively charged foulants [22,23]. The RMS roughness of the NF90 mem- force between the membrane and positively charged foulants [22,23]. The RMS roughness brane was 61 nm, and that of BW30LE was 49.7 nm; the membrane with the rougher sur- of the NF90 membrane was 61 nm, and that of BW30LE was 49.7 nm; the membrane face is more prone to fouling [24]. Similar results were obtained in previous studies—the with the rougher surface is more prone to fouling [24]. Similar results were obtained in NF90 membrane displayed severe fouling when used for wastewater treatment, which previous studies—the NF90 membrane displayed severe fouling when used for wastewater resulted in a lower level of water flux than RO [21,25]. A more detailed discussion is pro- treatment, which resulted in a lower level of water flux than RO [21,25]. A more detailed discussionvided on membrane is provided fouling on membrane to support fouling the findings to support regarding the findings water regarding flux. water flux.

90.0

80.0 77.7 71.5 71.6 68.1 70.0 67.5 .h)

2 60.0

50.0 44.5 RO

40.0 NF 28.1 30.0 23.5 Avarage Flux (L/m Avarage Flux 21.3 21.1 20.6 20.0 17.1

10.0

0.0 10 12 14 16 18 20 Pressure (bar)

Figure 3. Average water flux of reverse osmosis (RO) and nanofiltration (NF) at different feed pressures. Figure 3. Average water flux of reverse osmosis (RO) and nanofiltration (NF) at different feed pressures. SEM images of unused and used RO and NF membranes at different applied pressures are shownSEM images in Figure of 4unusedb–d. The and used used RO RO membranesand NF membranes at an applied at different pressure applied of 12, pres- 16 andsures 20 are bar shown can be in observed.Figure 4b–d. It The can used be seen RO frommembranes the SEM at an images applied that pressure as the appliedof 12, 16 pressure increased, more accumulation of foulant materials occurred on the surface of the membrane. A similar observation was detected on the nanofiltration membrane, where the amount of the accumulated foulants increased on the surface of the membrane as the feed pressure increased (Figure4f–h). TSE is rich in organic matter that could be attracted Membranes 2021, 11, x FOR PEER REVIEW 8 of 13

and 20 bar can be observed. It can be seen from the SEM images that as the applied pres- Membranes 2021, 11, 32 sure increased, more accumulation of foulant materials occurred on the surface of8 ofthe 13 membrane. A similar observation was detected on the nanofiltration membrane, where the amount of the accumulated foulants increased on the surface of the membrane as the feedto the pressure membrane increased surface (Figure by electrostatic 4f–h). TSE forces. is rich Moreover,in organic matter studies that showed could that be attracted divalent toions the such membrane as calcium, surface which by electrostatic is attracted toforces. the surfaceMoreover, of the studies membrane showed by that electrostatic divalent ionsforces, such could as calcium, bridge organic which mattersis attracted to the to membrane the surface surface, of the causingmembrane further by electrostatic fouling [26]. forces,From thecould EDX bridge analysis organic shown matters in Table to the3, membrane it can be seen surface, that aftercausing the further use of thefouling RO and[26]. FromNF membranes, the EDX analysis new elements shown werein Table detected 3, it ca onn thebe surfaceseen that of after the membranes, the use of the such RO as and Na, NFMg, membranes, Si, P, S, Cl, K, new Ca, elements and Fe; this were indicates detected the on accumulation the surface of of the these membranes, ions on the such surface as Na,of the Mg, membrane. Si, P, S, Cl, Table K, Ca,3 shows and Fe; that this the indi amountcates ofthe accumulated accumulation ions of onthese the ions surface on the of surfacethe RO of membrane the membrane. was higher Table than 3 shows that that for thethe NFamount membrane. of accumulated This indicates ions on the the higher sur- facerejection of the rate RO of membrane these ions was by RO higher when than compared that for to the NF. NF membrane. This indicates the higher rejection rate of these ions by RO when compared to NF.

(a) (e)

(b) (f)

(c) (g)

Figure 4. Cont.

MembranesMembranes 20212021, ,1111,, x 32 FOR PEER REVIEW 99 of of 13 13

(d) (h)

FigureFigure 4. 4. SEMSEM images images of of (a ()a a) clean a clean RO RO membrane, membrane, (b) ( ba) tested a tested RO RO membrane membrane at a at feed a feed pressure pressure of 12of bar, 12 bar,(c) a ( ctested) a tested RO membrane RO membrane at a feed at a pressure feed pressure of 16 bar, of 16 (d bar,) a tested (d) a testedRO membrane RO membrane at a feed at a pressurefeed pressure of 20 bar, of 20 (e bar,) a clean (e) a NF clean membrane, NF membrane, (f) a tested (f) a NF tested membrane NF membrane at a feed at pressure a feed pressure of 12 of bar,12 bar, (g) (ag tested) a tested NF NF membrane membrane at a at feed a feed pressure pressure of of16 16 bar, bar, and and (h ()h a) atested tested NF NF membrane membrane at at a a feed feed pressurepressure of of 20 20 bar. bar.

Table 3. Analysis of elements wt% on the surface of a clean and tested RO and NF membranes. Table 3. Analysis of elements wt% on the surface of a clean and tested RO and NF membranes. Weight % Membrane Weight % Membrane (C) (O) (Na) (Mg) (Si) (P) (S) (Cl) (K) (Ca) (Fe) (C) (O) (Na) (Mg) (Si) (P) (S) (Cl) (K) (Ca) (Fe) RO—clean 87.39 9.53 0 0 0 0 2.98 0 0 0 0 RO—cleanRO—tested 87.39 70.64 9.53 19.45 0 0 0.5 0 0.08 00.22 0.66 2.98 3.32 0 0.3 0 0.05 0 0.75 1.79 0 RO—tested 70.64 19.45 0.5 0.08 0.22 0.66 3.32 0.3 0.05 0.75 1.79 NF—cleanNF—clean 89.78 89.78 6.66 6.66 0 00 0 0 0 0 3.560 3.56 0 0 00 00 00 NF—testedNF—tested 75.2 75.2 17.28 17.28 0.26 0.05 0.26 0.08 0.05 0.53 0.08 0.53 3.94 3.94 0.1 0.1 0 0 0.34 0.34 1.75 1.75

3.2. Energy Consumption

3.2. EnergySpecific Consumption energy (𝐸) for RO and NF was calculated using Equation (2) [27]:

Specific energy (Es) for RO and NF was calculated𝑃 using Equation (2) [27]: 𝐸 = (2) 𝑛 × %𝑅  P  E = (2) where P is pressure (bar), n is pump sefficiency,n × and%R %R is recovery rate. From Equation (2), it can be seen that the specific energy depends on both the applied pressure and re- coverywhere Prate,is pressure where the (bar), lowestn is specific pump efficiency, energy wi andll be % obtainedR is recovery at a rate.high Fromrecovery Equation rate and (2), lowit can pressure. be seen Figure that the 5 specificshows the energy specific depends energy on consumption both the applied of the pressure RO process and recoveryand the NFrate, process where at the different lowest feed specific pressures. energy In will the NF be obtainedprocess, the at aspecific high recovery energy consumption rate and low increasedpressure. from Figure 0.685 shows to 2.35 the kWh/m specific3 at energy a feed consumptionpressure of 12 of and the 20 RO bar, process respectively. and the NFAs shownprocess in at Equation different (2), feed the pressures. specific energy In the isNF a function process, of the the specific applied energy pressure consumption and recov- 3 eryincreased rate. At from a low 0.68 feed to 2.35 pressure kWh/m of 10at aba feedr, the pressure specific of energy 12 and consumption 20 bar, respectively. was 1.33 As kWh/mshown3 in. The Equation high specific (2), the specificenergy energyconsumption is a function at such of a the low applied feed pressure pressure is and due recovery to the 3 lowrate. recovery At a low rate feed obtained pressure at of a 10feed bar, pressure the specific of 10 energy bar (Figure consumption 4). The maximum was 1.33 kWh/m energy . consumptionThe high specific was energy2.35 kWh/m consumption3, whichat was such obtained a low feed at apressure feed pressure is due of to 20 the bar. low The recovery high specificrate obtained energy at consumption a feed pressure at such of 10 a bar high (Figure applied4). Thepressure maximum is due energyto the low consumption recovery. 3 Inwas the 2.35 RO kWh/mprocess, the, which same wastrend obtained was obse atrved, a feed where pressure the specific of 20 bar. energy The consumption high specific increasedenergy consumption from 0.46 to at 0.73 such kWh/m a high3 appliedat a feed pressure pressure is of due 12 toand the 20 low bar, recovery. respectively. In the At RO a lowprocess, feed thepressure same of trend 10 bar, was the observed, specific whereenergy the consumption specific energy was consumption1.22 kWh/m3, increasedwhich is 3 duefrom to 0.46 the tolow 0.73 recovery kWh/m rateat obtained a feed pressure at such of a 12low and applied 20 bar, pressure. respectively. It was At found a low that feed 3 thepressure NF process of 10 bar,at an the applied specific pressure energy of consumption 12 bar gave the was highest 1.22 kWh/m water recovery, which israte due and to thethe lowest low recovery energy rateconsumption. obtained atFor such the aRO low process, applied the pressure. lowest energy It was foundconsumption that the was NF foundprocess atat an an applied applied pressure pressure of of 12 barbar, gave while the the highest highest water water recovery recovery rate rate and was thelowest at an energy consumption. For the RO process, the lowest energy consumption was found at an applied pressure of 16 bar. The difference in the water recovery rate at an applied pressure applied pressure of 12 bar, while the highest water recovery rate was at an applied pressure between the 12 and 16 bar in the RO process was only 3.3%, while the energy consumption of 16 bar. The difference in the water recovery rate at an applied pressure between the was 18% higher at an applied pressure of 16 bar when compared to an applied pressure

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12 and 16 bar in the RO process was only 3.3%, while the energy consumption was 18% higher at an applied pressure of 16 bar when compared to an applied pressure of 12 bar. ofThe 12 quality bar. The of thequality produced of the permeateproduced should permeate be analyzed should be to analyzed investigate to whichinvestigate process which and processwhich running and which conditions running should conditions be utilized. should be utilized.

3.00

2.50 2.35 2.16

2.00 )

3 1.70 RO 1.50 1.33 NF (kWh/m 1.22 1.17 s E 1.00 0.73 0.68 0.67 0.56 0.46 0.50 0.50

0.00 10 12 14 16 18 20 Pressure (bar)

Figure 5. Specific energy consumption of RO and NF at different feed pressures. Figure 5. Specific energy consumption of RO and NF at different feed pressures.

3.3. Product Product Water Quality Characteristics of of the the produced produced permeate permeate were were measured measured for forthe theRO ROand andthe NF the pro- NF processes. The quality of the produced permeate was compared with the Food and Agri- cesses. The quality of the produced permeate was compared with the Food and Agricul- culture Organization (FAO) standards [12]. As shown in Figure6a, the concentration of ture Organization (FAO) standards [12]. As shown in Figure 6a, the concentration of the the different measured elements in the produced permeate from the RO process at applied different measured elements in the produced permeate from the RO process at applied pressures between 10 and 18 bar complied with the FAO standards. At an applied pressure pressures between 10 and 18 bar complied with the FAO standards. At an applied pres- of 20 bar, multiple parameters (such as TDS; conductivity; and chloride, sodium, and sure of 20 bar, multiple parameters (such as TDS; conductivity; and chloride, sodium, and calcium concentration) exceeded the allowable limits. In the NF process (Figure6b), under calcium concentration) exceeded the allowable limits. In the NF process (Figure 6b), under all applied pressures, the permeate quality did not comply with FAO standards. It was all applied pressures, the permeate quality did not comply with FAO standards. It was observed that the elements that did not comply with the standards were chloride, sodium observed that the elements that did not comply with the standards were chloride, sodium and calcium. The high concentration of these elements in return affected the TDS concen- and calcium. The high concentration of these elements in return affected the TDS concen- tration. For example, at an applied pressure of 12 bar where the highest water recovery rate tration. For example, at an applied pressure of 12 bar where the highest water recovery was attained, the TDS concentration was almost 21% higher than the allowable limit, and ratethe chloride,was attained, sodium the andTDS calcium concentration concentrations was almost were 21% 84%, higher 61% andthan 13%the allowable higher than limit, the andallowable the chloride, limit, respectively. sodium and Itcalcium was observed concentr thatations the were NF membrane84%, 61% and had 13% a low higher rejection than therate allowable for monovalent limit, respectively. ions, where theIt was rejection observed rate that for chloridethe NF membrane and sodium had was a low only rejec- 27% tionand rate 12%, for respectively. monovalent The ions, membrane where the rejection rejection raterate dependsfor chloride mainly and onsodium the molecular was only 27%weight and cut-off 12%, respectively. (MWCO); the The MWCO membrane of NF90 rejection is 200–400 rate Da,depends and the mainly MWCO on the of BW30LE molecular is weight100 Da. cut-off As the (MWCO); MWCO increases, the MWCO the rejectionof NF90 is rate 200–400 decreases Da, [and28– 30the]. MWCO of BW30LE is 100 Da. As the MWCO increases, the rejection rate decreases [28–30].

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(a)

(b)

FigureFigure 6. Characteristics 6. Characteristics of the of thepermeate permeate produced produced from from treated treated sewage sewage effluent effluent (TSE) (TSE) using using (a ()a )reverse reverse osmosis osmosis (b (b) )nan- nanofil- ofiltrationtration at different applied pressures.

It canIt can be inferred be inferred that that using using a single-stage a single-stage NF NFis not is notpossible possible due due to the to thelow low water water quality;quality; thus, thus, it is it recommended is recommended to use to useRO ROat pressure at pressure applied applied between between 10 and 10 and18 bar. 18 bar. SelectingSelecting the themost most suitable suitable applied applied pressure pressure to operate to operate the theRO ROprocess process depends depends on onthe the waterwater flux, flux, the therecovery recovery rate, rate, energy energy consumption consumption and and product product water water quality. quality. The The lowest lowest energyenergy consumption consumption in RO in RO was was obtained obtained at atpressures pressures applied applied between between 12 12 and and 14 14 bar. bar. Af- After ter considering the water quality, itit isis recommendedrecommended toto useuse an an applied applied pressure pressure of of 14 14 bar bar due dueto to the the higher higher water water quality. quality.

4. Conclusions In this paper, a comparative study was conducted on the reclamation of tertiary treated sewage effluent (TSE) by nanofiltration and reverse osmosis for water reuse in

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4. Conclusions In this paper, a comparative study was conducted on the reclamation of tertiary treated sewage effluent (TSE) by nanofiltration and reverse osmosis for water reuse in irrigation. It was found that reverse osmosis (RO) is suitable for the reclamation of tertiary treated sewage effluent (TSE) to be used as irrigation water for food crops. However, NF is not suitable for the reclamation of wastewater due to its low rejection rate for monovalent ions. In NF, the concentration of Na and Cl ions exceeded the maximum allowable limits recommended by FAO. In RO, the highest recovery rate was 36%, which was achieved at an applied pressure of 16 bar. The specific energy consumption at this applied pressure was 0.56 kWh/m3. At an applied pressure of 14 bar, the recovery rate was only 2% lower than that at an applied pressure of 16 bar, while the specific energy consumption was almost 11% lower. At an applied pressure of 12 bar, the specific energy consumption was 8% higher than the specific energy at an applied pressure of 14 bar, while the recovery rate was 7% higher. It is recommended to use the RO process at an applied pressure of 14 bar for the reclamation of TSE. This is due to the high recovery rate, low energy consumption and high water quality. In the future, the capital and operational costs of both processes must be compared at an industrial or pilot scale to ensure the economic feasibility of the process.

Author Contributions: M.H.: conceptualization, data curation, formal analysis, writing—original draft preparation. A.H.H.: conceptualization, methodology, writing—review and editing. R.A.: data curation, formal analysis. M.K.H.: conceptualization, writing—review and editing. A.A.: conceptualization, methodology, writing—review and editing. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Qatar National Research Fund (QNRF) graduate sponsorship research award (GSRA6-1-0509-19021). Data Availability Statement: All data generated or analyzed during this study are included in this published article. Acknowledgments: This research was made possible by an Award (GSRA6-1-0509-19021) from Qatar National Research Fund (a member of Qatar Foundation). We would also like to thank Central Laboratories Unit (CLU) at Qatar University for generating SEM images and the ion chromatography tests. The contents herein are solely the responsibility of the authors. Conflicts of Interest: The authors declare no conflict of interest.

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