THE FEASIBILITY OF A HYBRID SYSTEM CONSISTING OF A PELLET REACTOR AND ELECTRODIALYSIS FOR REVERSE OSMOSIS CONCENTRATE TREATMENT

Transcription

1 Proceedings of the 13 th International Conference on Environmental Science and Technology Athens, Greece, 57 September 2013 THE FEASIBILITY OF A HYBRID SYSTEM CONSISTING OF A PELLET REACTOR AND ELECTRODIALYSIS FOR REVERSE OSMOSIS CONCENTRATE TREATMENT A.T. K. TRAN 1, Y. ZHANG 1, N. JULLOK 1, B. MEESSCHAERT 2,3, L. PINOY 1,4, and B. VAN DER BRUGGEN 1 1 Department of Chemical Engineering, ProcESS Process Engineering for Sustainable Systems, KU Leuven, W. de Croylaan 46, B3001 Leuven, Belgium 2 Department of Industrial Sciences and Technology, Katholieke Hogeschool Brugge Oostende, Associated to the KU Leuven as Faculty of Industrial Sciences, Zeedijk 101, B8400 Oostende, Belgium 3 Department of Microbial and Molecular Systems, Centre for Surface Chemistry and Catalysis, KU Leuven, Kasteelpark Arenberg 23, B3001 Leuven, Belgium 4 Department of Industrial Engineering, Laboratory for Chemical Process Technology, KAHO St.Lieven, Associated to the KU Leuven as Faculty of Industrial Sciences, Technologie Campus, Gebroeders Desmetstraat 1, B9000 Gent, Belgium Corresponding author: anh.tran@cit.kuleuven.be (A.T.K. Tran) bart.vanderbruggen@cit.kuleuven.be (B. Van der Bruggen) EXTENDED ABSTRACT Reverse osmosis (RO) has become a standard technology for desalination and wastewater treatment for water reuse, but it is limited by the disposal of RO concentrate. The aim of the study described in this paper was to evaluate the feasibility of a hybrid system consisting of a pellet reactor and electrodialysis (ED) to treat RO concentrates in which the pellet reactor was used to remove the scaling potential before ED treatment. The results showed that the average removal efficiency by pellet reactor decreased from 84 over 80 to 73% for calcium removal and from 13 over 11 to 8% for magnesium removal when the superficial velocity (SV) respectively increased from 48 m/h and 61 m/h to 73 m/h. This can be explained by the reduced hydraulic retention time. Furthermore, 2% NaOH was added to the wastewater to have an initial ph value of 11 and When the NaOH dose was 0.3 L/h (initial ph =11), the removal efficiency of calcium was 80%; at a NaOH dose of 0.45 L/h (initial ph =11.5), the efficiency was 96%. Simultaneously, the efficiency to remove magnesium went up from 11 to 25%. It was found that when operating electrodialysis on a RO concentrate first in batch mode for 80 min to get 60% conductivity removal, electrodialysis then continued to run with feed and bleed mode in 44 min; at this time, maximal electrical potential was reached due to scaling on the membrane surface. After pellet reactor treatment with 80% calcium removal efficiency, the ED system could operate in a stable way (after 24h with slight scaling). After pellet reactor and ED treatment, the concentrations of all components were below the characteristics of the influent of reverse osmosis system, which makes the stream suitable for reinserting into RO; Na +, K + and NO 3 concentrations were slightly higher than the average value of RO feed water quality. Keywords: Hybrid system, pellet reactor, electrodialysis, RO concentrate treatment 1. INTRODUCTION In recent years, the disposal of reverse osmosis concentrates emerged as a significant problem. Water treatment or water reuse from wastewater treatment by using RO produces a large volume of RO concentrate, which contains several contaminants threatening the aquatic environment (Mohameda et al., 2005). Since high concentrations

2 of Ca 2+, Mg 2+, bicarbonate HCO 3 were present in RO concentrates from desalination plants (Ji et al., 2010) and wastewater treatment plants (Ahmed et al., 2003), scaling and fouling are the main problems for treating RO concentrate. Many researchers studied methods to reduce the scaling potential of the RO concentrate. Coagulation was used to treat RO brine as pretreatment followed by advanced oxidation to treat the organic matter (Zhou et al., 2011). Removal of bicarbonate was studied by adding HCl into the aerated feed stream of electrodialysis in order to reduce the potential of CaCO 3 scaling when treating RO concentrate (Zhang et al., 2010). Moreover, Rahardianto et al. (2010) studied the method of two steps of seeding precipitation for desupersaturation of RO brine from a desalting plant of agricultural drainage water of high mineral scaling potential, in which the first step was CaCO 3 precipitation and the second step was CaSO 4 precipitation. Furthermore, membrane distillation crystallization was used to study the efficiency of water recovery and NaCl crystallization from RO brine (Ji et al., 2010), which yielded 90% water recovery and 21 kg/m 3 NaCl. Although these methods are efficient, RO concentrate treatment for water recovery is not straightforward. Recently, pellet reactors have been introduced for softening of drinking water (Mahvi et al., 2005), and for removing metals from waste water (Zhou et al., 1999), in which the efficiency of calcium and metals removal could reach 92 to 95% depending on the ph, superficial velocity, and metal concentrations in the influent. During the pellet reactor treatment, three reactions occur for calcium and magnesium: HCO 3 + OH CO H 2O pk a = at 25 o C (1) Ca 2+ + CO 3 2 CaCO 3 K sp = at 25 o C (2) Mg OH Mg(OH) 2 K sp = at 25 o C (3) A pellet reactor can be a good method to reduce the amount of calcium and magnesium in RO concentrate. Furthermore, electrodialysis is proved to be efficient in removing ions from the solution (Zhang et al, 2010). The aim of this study was to evaluate the feasibility of a hybrid system consisting of a pellet reactor and electrodialysis (ED) to treat RO concentrates; the pellet reactor was used to remove the scaling potential before ED treatment. After pellet reactor pretreatment, the wastewater was treated by electrodialysis to remove cations and anions to produce an effluent suitable to reuse in reverse osmosis (RO) system for increasing the water recovery of the RO. 2. MATERIALS AND METHODS 2.1 RO concentrate The synthetic wastewater was made by adding CaCl 2, 6H 2O, MgSO 4.7H 2O, NaNO 3 (ChemLab, Belgium), NaHCO 3 (Fisher Scientific, UK), K 2SO 4 (FlukaGaranyie, Switzerland) into deionized water to have the following characteristics: ph , Ca mg/l, Mg mg/l, HCO mg/l, Cl mg/l, NO mg /L, SO mg/l. 2.2 Pellet reactor In this study, a pellet reactor with a height of 2.2 m and a, 20 mm diameter was used to study the effect of ph and superficial velocity on the precipitation efficiency. The fluidized bed was filled with garnet sand (400 g, µm) from Minelco, Germany. The bed height without flow rate was 0.55 m. Wastewater was pumped from the bottom to the top with a flow rate from 15 to 23 L/h, 2% NaOH was injected at a point 3 cm away from the bottom of the pellet reactor to adjust the ph. The alkalinity dose was varied from 0.3 to 0.45 L/h to investigate the effect of ph to the precipitation.

3 CATHODE () 2.3 Electrodialysis A labscale electrodialysis system (Berghof BEL500) was used to perform the experiments. Five pieces of anion exchange membranes (AM) and seven pieces of cation exchange membranes (CM) were used in the stack, each with an effective surface area of 58 cm 2. Cation exchange membranes PCSK and anion exchange membranes PCSA from PCAPolymerchemie Altmeier GMbH, Germany were installed in the stack. PCSK ( SO 3Na) with a thickness of 130 µm has 1 meq/g ion exchange capacity and a surface potential of Ω/cm 2. PCSA (NR 4Cl) with a thickness of µm has 1.5 meq/g ion exchange capacity and a surface potential of Ω/cm 2. The equipment further consisted of three separated circuits for the diluate, the brine and the electrode rinsing solutions, each 3 L in volume and recirculated by a centrifugal pump. The current was kept constant (I = 0.45A) and the maximum electric potential that was applied for the system was 12V (max. 2V /cell pair). The electrodialysis was run first in batch mode and then in feed and bleed mode to test the scaling and efficiency of RO concentrate treatment. The experimental setup for pellet reactor treatment and the electrodialysis system for treatment of RO brine is illustrated in Fig. 1. EFFLUENT DILUATE 200cm CM AM CM AM CM AM CM 150cm 20mm NO3 NO3 NO3 100cm Flow meter ANODE (+) Cl Na+ K+ Cl Na+ K+ Na+ Cl SO42 50cm Valve Bypass flow SO42 SO42 K+ NaOH 10cm 3cm Dosing Pump Pump Tank CONCENTRATE RO SYSTEM RO CONCENTRATE 2.4 Samples analysis Figure 1. Experimental set up for RO concentrate treatment Samples of the pellet reactor effluent were taken at 20 min intervals during the first hour and at every 1 h thereafter. Samples were divided into two; one without filtered and one with filtered by a 0.45 µm membrane filter (Milipore). Samples of ED were taken at 20 min intervals. Ca 2+, Mg 2+, Na + and K + concentrations were determined by ICPMS Inductivelycoupled Plasma Mass Spectroscopy (Thermo Electron Corporation X series ICPMS). SO 4 2, Cl, NO 3 concentrations were determined by ion chromatography (Dionex ICS 2000). Pellets and membranes were analyzed by scanning electron microscopy (SEM) (Philips XL 30 FEG The Netherlands).

4 3. RESULTS AND DISCUSSION 3.1. Scaling potential removal of pellet reactor Experiments with the influent of the RO concentrate. were conducted continuously in the pellet reactor for 6 hours. The pellet reactor was operated at ph 11 with three superficial velocities (SV) of 48 m/h, 61 m/h and 73 m/h respectively corresponding to a flow rate of 15 L/h, 19 L/h and 22 L/h (Run 1, 2, 3) and changing the NaOH dose from 0.3 L/h (initial ph = 11) to 0.45 L/h (initial ph =11.5) (Run 4) while keeping flow rate at 19 L/h. Fig. 2 shows that the calcium removal efficiency of filtered and nonfiltered samples were 84 and 83% (SV 48 m/h); 80 and 78% (SV 61 m/h); 73 and 73% (SV 73 m/h), respectively. In addition, magnesium removal efficiency was 13 and 12%; 11 and 6%; 8 and 7% with increasing SV from 48 to 73 m/h. It can be seen for runs 1 to 3 and for the filtered samples that the average calcium removal efficiency during the experiment decreased from 84% to 73% when SV increased from 48 m/h to 73 m/h. Similarly, the efficiency of magnesium also dropped from 13% to 8%. When the superficial velocity and the flow rate increased, the hydraulic retention time (HRT) lowers. A decreased HRT and an increasing flow rate resulted in a reduced calcium and magnesium removal efficiency. Furthermore, the precipitation of calcium carbonate and magnesium hydroxide from supersaturated solution was accompanied by a ph decrease. After treatment, the ph decreased to the range between 7.5 and 8.2. The ph changed during the process of precipitation because of the hydroxide and carbonate consumption in reaction (2) and (3) to have CaCO 3 and Mg(OH) 2 precipitation. Figure 2. Effect of superficial velocity and NaOH dose on Ca & Mg removal efficiency (%) However, since the difference of removal efficiency of the filtered sample and nonfiltered sample was below 1%, virtually no fines were observed in the effluent of pellet reactor. Most precipitation of calcium and magnesium was attached on the sand. The high supersaturation in the solution could have lead to a quick and massive nucleation and therefore also to fines (Aldaco et al., 2005). The conditions of our experiments apparently

5 did not favour the formation of fines. Observation by SEM (Fig. 3) showed the morphology and crystallizes of CaCO 3 and Mg(OH) 2 on the garnet sand. Similarly, to examine the influence of ph on the calcium and magnesium removal efficiency, experiments were conducted by changing the NaOH dose from 0.3 L/h (Run 2) to 0.45 L/h (Run 4). The relationship between the NaOH dose and the efficiency can be seen in Fig. 2, the precipitation of CaCO 3 rose from 80% to 96%, and precipitation of magnesium hydroxide increased from 11 to 25%, respectively. When increasing NaOH dose, OH concentration in solution also increased which made the reaction (1) shift to the right hand side to get the equilibrium. Thus, CO 3 2 and OH concentrations increased, so that the efficiency of precipitation was higher in run 4 than in run 2. Figure 3. SEM with magnification 51x of pellets before and after pellet reactor treatment 3.2. RO concentrate treatment by electrodialysis The following experiments were conducted in order to investigate the scaling on the cation exchange membranes during treatment with electrodialysis. Electrodialysis was run with two kinds of wastewater, original wastewater (Exp. 1), and wastewater with 80% calcium removal (Exp. 2, 3). The diluate stream flow rate was 1.6 L/h, the flow rate of the concentrate stream was 0.2 L/h. The first experiment was carried out after 80 min of batch mode operation to achieve an efficiency of 60% removal, i.e., the conductivity decreased from 3.97 ms/cm to 1.56 ms/cm and then continued with feed and bleed mode. Figure 4. Conductivity vs time of the diluate during ED treating untreated and pretreated water (80% Ca removal)

6 From fig. 4, it can be seen that after 44 min feed and bleed operation; the conductivity in the diluate vessel was 1.59 ms/cm. The effluent contained 260 mg/l Na +, 8.3 mg/l Mg 2+, 60.2 mg/l K + and 98.8 mg/l Ca 2+. The conductivity of the effluent was stable at a conductivity of 1.59 ms/cm; however, the voltage reached the maximum value with 12 V. Scaling appeared on the membrane surface (Fig. 5A) which made the system s resistance higher and was the reason for an increasing voltage. With the RO concentrate after 80% of calcium removal by pellet reactor treatment as feed stream (Exp. 2), it took 55 min to get the efficiency of 60%, achieving 1.37mS/cm in batch mode. After 80 min running in the feed and bleed mode, the voltage was 12V. However, not likely as in the the first test (membrane scaling), the reason of reaching maximum voltage was the low conductivity of the solution. The conductivity decreased from 1.37 ms/cm to 1.2 ms/cm (after 40 min); 0.92 ms/cm (after 60 min) and to 0.81 ms/cm (after 80 min). Scaling was not observed on the cationic membrane (Fig. 5B). Since the concentration of ions was apparently not high enough to stabilize the operation of the electrodialysis system, the flow rate of the feed needed to be increased so that more ions would be present in the diluate section of the stack and that the maximum voltage would not be reached. The third experiment the flow rate of the diluate vessel was therefore increased from 1.6 to 2.36 L/h; the flow rate of the feed to the concentrate vessel remained at 0.2 L/h (Exp. 3) After 11 h operation in the feed and bleed mode, the diluate effluent had a constant conductivity of ms/cm with the voltage at 9 V ± 0.2 V. After that, between 11 and 18 hours of operation, the conductivity changed slightly to ms/cm while the voltage slightly increased (from 9.2 V to 9.6 V); in the period between 18 and 24 h of operation the conductivity slightly further increased from 1.35 ms/cm to 1.41 ms/cm while the voltage remained constant at 9.8 ± 0.2 V (Fig. 4). The neglectable increase in conductivity and voltage during this last experiment indicates that electrodialysis can work stably in a long period although some slight scaling on the cationic membrane was observed (Fig. 5C). A B C Figure 5. SEM of cation exchange membrane of RO concentrates without pretreatment (Exp. 1) (A) and with pretreatment (Exp. 2) (B) and (Exp. 3) (C) 3.3. Performance of hybrid system (pellet reactor electrodialysis) The results of present work show the feasibility of a hybrid system consisting of a pellet reactor and an electrodialysis system to treat RO concentrate and also that the treated RO concentrate can be reintroduced into the RO system as the influent of RO to increase the water recovery of RO. Indeed, after electrodialysis of pretreated water, the anions and cations concentration in the effluent was lower than the requirement for the RO influent, except for monovalent ions (sodium, potassium, and nitrate) which are slightly higher than the average value of the RO feed water quality (Table 1).

7 Table 1. Performance of a hybrid system (pellet reactor electrodialysis) Concentration, ppm Ca 2+ Mg 2+ K + Na + 2 SO 4 NO 3 Cl RO concentrate After pellet reactor After eletrodialysis RO feed water* * RO feed water quality (Juby, 2008) 4. CONCLUSIONS The feasibility of RO concentrate treatment by a hybrid system consisting of a pellet reactor and electrodialysis was successful demonstrated. Superficial velocity and sodium hydroxide dosing played an important role in calcium and magnesium precipitation; high calcium removal efficiency in the pellet reactor could be achieved from 70% to 95% with initial ph and a superficial velocity from 48 m/h to 73 m/h. After pretreatment with 80% efficiency, the electrodialysis worked in a stable way with less scaling compared with to the electrodialysis directly carried out on the untreated RO concentrate. A pellet reactor is a good method to reduce the scaling potential of RO concentrate during eletrodialysis. The diluate of eletrodialysis can be reinserted to the RO system as the influent of RO to obtain a higher water recovery. REFERENCES 1. Ahmed, M., Arakel, A., Hoey, D., Thumarukudy, M.R., Goosen, M.F.A., AlHaddabi, M., Al Belushi, A., Feasibility of salt production from inland RO desalination plant reject brine: a case study. Desalination 158, Aldaco, R., Irabien, A., Luis, P., Fluidized bed reactor for fluoride removal. Chem. Eng. J. 107, Ji, X., Curcio, E., Al Obaidani, S., Di Profio, G., Fontananova, E., Drioli, E., Membrane distillationcrystallization of seawater reverse osmosis brines. Sep. Purif. Techno. 71, Juby, G., Reverse Osmosis Recovery Maximization. U.S. Department of the Interior, Bureau of Reclamation, DWPR Report No Mahvi, A.H., Shafiee, F., Naddfi, K., Feasibility study of crystallization process for water softening in a pellet reactor. Int. J. Environ. Sci. Technol. 1, Mohameda, A.M.O., Maraqaa, M., AlHandhaly, J., Impact of land disposal of reject brine from desalination plants on soil and groundwater. Desalination 182, Rahardianto, A., McCool, B.C., Cohen, Y., Accelerated desupersaturation of reverse osmosis concentrate by chemicallyenhanced seeded precipitation. Desalination 264, Zhang, Y., Ghyselbrecht, K., Meesschaert, B., Pinoy, L., Van der Bruggen, B., Electrodialysis on RO concentrate to improve water recovery in wastewater reclamation. J. Membr. Sci Zhou, P., Huang, J., Alfred, W.F.L.I., Wei, S., Heavy metal removal from wastewater in fluidized bed reactor. Water Res. 33, Zhou, T., Lim, T., Chin, S., Fane, A.G., Treatment of organics in reverse osmosis concentrate from a municipal wastewater reclamation plant: feasibility test of advanced oxidation processes with / without pretreatment. Chem.Eng.J. 166,