FULL-SCALE FEASIBILITY OF THE FO-MBR PROCESS FOR WASTEWATER RECLAMATION

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1 FULL-SCALE FEASIBILITY OF THE FO-MBR PROCESS FOR WASTEWATER RECLAMATION Marina Arnaldos 1, Teresa de la Torre 1, Carlos Rodríguez 1, Jorge Malfeito 1 1. Acciona Agua, SA R&D Department, Barcelona, CAT, Spain ABSTRACT In this study, two optimal draw solutions (DSs) for wastewater reclamation using forward osmosis (FO) were evaluated after a systematic selection combining the effect of fundamental DS properties on system performance and experimental results: potassium formiate and potassium pyrophosphate. Both salts were shown to present higher water flux values than the NaCl reference. Additionally, the selected salts presented a sustained process behaviour with no observable fouling for the entire operation time. Calculation of the replacement cost of each selected DS showed that potassium formiate was a 34 % more costly from an operational perspective as compared to potassium pyrophosphate. INTRODUCTION Forward osmosis (FO) has emerged as a promising alternative to conventional pressure-driven membrane processes in the recent years (Yap et al., 2012). FO processes are driven by an osmotic pressure difference across a membrane; water flows from the solution of low osmotic pressure (feed) to a high-osmotic-pressure solution (DS). Recently, a novel type of FO-based membrane bioreactor (FO-MBR) for wastewater treatment has been reported (Achilli et al., 2009). Several studies have pointed out the potential of FO-MBRs to substitute current MBRs based on the anticipated lower energy costs of the novel system; Nevertheless, FO systems are in need of further research to determine their actual applicability. Firstly, the operational behaviour and feasibility of FO-MBR processes has not been fully characterized; some studies have used deionized water (DI) as feed (Qin et al., 2009; Xiao et al., 2011), while others have employed synthetic feeds (Achilli et al., 2009; Mi and Elimelech 2010; Lay et al., 2011). Those studies using more representative feeds have not operated the process in a continuous mode or evaluated operational behaviour during long-term operation (Holloway et al., 2007). Secondly, a thorough study of highperformance DSs for FO-MBR applications has not been carried out. Most FO studies targeting wastewater treatment have used NaCl solutions as DS due to the high solubility and low molecular weight of NaCl (causing high osmotic pressures and low internal concentration polarization (ICP) effects, respectively) and the fact that it can be easily recovered by RO (Achilli et al., 2009; Zhang et al., 2012; Xiao et al., 2011). Cornelissen et al. (2008) compared the effects on permeate flux of different DS, namely NaCl, NaNO 3, MgSO 4 and ZnSO 4 when treating activated sludge from an MBR facility. They found that DSs with monovalent electrolytes resulted in approximately 100% higher water fluxes than bivalent electrolyte solutions. This conclusion was also reached by Qin et al. (2009) when treating DI, and the cause has been attributed to a higher diffusion coefficient of monovalent electrolytes as compared to bivalent electrolytes (Cornelissen et al., 2008). This causes an increased ICP for divalent electrolytes when the feed is facing the active side of the membrane, and thus a lower water flux. The same conclusion was reached by Bowden et al. (2012) when evaluating the performance of organic ionic salts as draw solutions; the lower intrinsic diffusion coefficients of organic molecules as compared to most inorganic salts was shown to bring about lower water fluxes. Nevertheless, some studies have shown bivalent salts (CaCl 2, KHCO 3, MgCl 2, and NaHCO 3 ), to outperform NaCl as draw solutions (Achilli et al., 2010; Zou et al., 2011). This is perhaps due to fact that bivalent salts have higher van t Hoff factors that cause higher osmotic pressures at equal molar concentrations. It is probable that the performance of the FO-MBR process depends on two basic properties of the DS, the diffusion coefficient and the van t Hoff factor; clear understanding of the relative contributions of these properties to final

2 performance is thus of great interest when selecting a DS. In this study, the FO-MBR process has been investigated with real activated sludge. Two DSs have been selected taking into account their fundamental properties (the diffusion coefficient and the van t Hoff factor) and their effect on process performance in terms of water fluxes and reverse salt fluxes using a mathematical modelling approach. The process has been run continuously during 1 month for each DS and the operational behaviour has been characterized in order to evaluate the potential for full-scale application. METHODOLOGY/ PROCESS Selection of Promising DS through Mathematical Modelling The FO model equations have been implemented in Matlab 2009b (The Mathworks, US). The different salt fluxes contributing to ICP have been shown in Figure 1. When performing a salt balance around the membrane support, the following Equation 1 can be obtained. J w C + J s = D eff dc dx Equation 1. Salt balance around the support layer of a FO membrane. Where: Jw is the water flux; C is the salt concentration at a certain point of the support layer; Js represents the salt flux; Deff is the effective diffusion coefficient and x is the distance in the support layer. When solving Equation 1 to get Jw, the equations shown in Table 1 can be obtained (Loeb et al., 1996). These equations describe the behaviour of the system when there exists ICP. The identification of the DS properties driving FO performance has been carried out by examination of the FO model developed by Loeb et al. (1996) (Table 1, positions1 and 2). The fundamental DS properties contained in the model are the van t Hoff factor ( ) and the effective diffusion coefficient (Deff); these are embedded in the DS osmotic pressure and the mass transfer coefficient, respectively (Table 1, positions3 and 4). The effect of both these variables on Jw and Js has been evaluated carrying out combined sensitivity analyses; van t Hoff values have been increased from 0.5 to 4 while diffusion coefficients have been varied from to m 2 /s. These values cover most of the existent organic and inorganic salts. The rest of the parameters have been kept constant with the characteristic values shown in Table 2. Finally, the solute permeability of the membrane for each DS has been approximated from their diffusion coefficients using the following Equation 2, which takes into account that permeability coefficients for charged inorganic and organic solutes have been proven to be dominated by the diffusion component (Ghiu et al., 2004; Bellona et al., 2004). B = B NaCl D D NaCl Equation 2. Calculation of solute membrane Experimental Setup permeability. The setup of the FO-MBR system with a membrane area of cm 2 used in this study has been shown in Figure 2. The membrane employed was a FO cellulose triacetate (CTA) membrane (flat sheet, woven) from Hydration Technology Innovation (HTI, US). Activated sludge was continuously fed at 4 L/min to the process from the aerobic tank of a 6 m 3 /h MBR wastewater treatment plant. DS concentration was controlled at around 1.5 M by automatically dosing concentrated DS solution when conductivity in the DS tank decreased below the setpoint. The DS was supplied to the membrane at 1 L/min. The pressure of the feed and DS were 1 bar and 0.5 bar, respectively. Water flux was calculated using the increase in level measured in the DS tank, while the salt flux was calculated carrying out a mass balance of the salt loss in the DS tank. RESULTS/ OUTCOMES Figure 1. Salt fluxes (red) and water fluxes (grey) in a FO membrane with ICP. Selection of Promising DS through Mathematical Modelling The effect of Deff and on Jw and Js has been evaluated carrying out combined sensitivity

3 analyses. The results of this analysis showed that simultaneous high diffusion and van t Hoff coefficients caused the highest water fluxes and lower salt fluxes (Figure 3). The combined effect of high water and low salt fluxes has been expressed using a combined ponderated index shown in Figure 3. In order to validate the model, data from salts evaluated in the literature (Achilli et al., 2010; Bowden et al., 2012; Phuntsho et al., 2011; Qin et al., 2009) were classified from higher to lower water fluxes. Then, the van t Hoff coefficients of those salts were estimated and taken as integers and the diffusivities were calculated following Cussler (1997). The results in terms of water flux for each of the DSs reported (or proposed to be used) in the literature have been shown on Figure 4. The predicted order of water fluxes predicted by the model matched that found in the literature. Thus, the model was validated for its ultimate purpose, which is to discern between better and worse performing draw solutions. Additionally, the simulations shown in Figure 3 were carried out for different DS concentrations and the average of Jw, Js and the ponderated index was calculated for each scenario. The result has been shown in Figure 5. As can be seen, the previously selected DS concentration of 1.5 M corresponds with the concentration at which the Jw reaches an almost maximum value while keeping Js values relatively low. Thus, this optimal concentration was chosen as reference to be used in the selection and testing of DS. The information drawn from the previous simulations was used in order to select a set of promising DSs to test experimentally. Firstly, an extensive list of organic and inorganic salts presenting solubility values higher than 1.5 M was obtained. From this initial list, highly toxic, carcinogenic and unstable salts were dismissed. Additionally, salts containing ions regulated for agricultural reuse purposes were dismissed. Of the remaining salts, those having higher diffusivities than the reference DS for each van t Hoff coefficient (DS already tested in the literature, namely NaCl, CaCl 2 ) were preliminarly selected. Of these, a shortlist was created based on the results of the ponderated performance index (Figure 5). As can be seen, no salt better than CaCl 2 was found for the van t Hoff coefficient of 3; K 2 HPO 4 was identified as a possible alternative, but its performance was similar to that of CaCl 2. For the van t Hoff coefficient of 2, two draw solutions have been detected as promising, HCOOK and MgSO 4 ; the latter was finally dismissed due to its very low water fluxes. For the van t Hoff coefficient of 4, no DS has been experimentally tested in previous studies (as far as the knowledge of the authors go) and thus no reference existed. Using this information, a final set of two DSs to be tested was selected. Specifically, potassium formiate (HCOOK) was selected on the basis of its improved Jw as compared to the NaCl reference and its low cost (Table 3), while potassium formiate (K 4 P 2 O 7 ) was selected on the basis of its simultaneous high Jw and low Js (Figure 6). Experimental Process Evaluation The performance of the previously selected DS was evaluated in the FO-MBR pilot plant. The average results of continuous operation for HCOOK and K 4 P 2 O 7 has been shown in the following Table 3. Sustained long-term operation with minimal fouling and consistent water fluxes for both salts have been achieved without need for chemical membrane cleanings. Additionally, and as predicted by the model, both salts presented greater water fluxes (normalized at 20ºC) as compared to sodium chloride (average water flux of approximately 4.5 LMH, results not shown). Finally, K 4 P 2 O 7 presented similar water fluxes as HCOOK but lower reverse salt fluxes; this makes it potentially the most economical solution. Table 4. Performance comparison between Draw Solution different DSs. Water flux at 20ºC (LMH) Salt (grams/m 2 /h) K 4 P 2 O HCOOK Flux Assuming an operational water flux of 6 LMH (theoretically corresponding to 1.5 M of HCOOK and 1.25 M of K 4 P 2 O 7 ) and taking into account the reverse salt fluxes of each selected DS, as well as their cost (Table 3), the specific cost of replacing lost DS in the FO process has been shown in Table 5 for each selected salt ( Specific Cost of FO Replacement ). Assuming that the DSs have to be recovered in a reverse osmosis (RO) system (with an assumed solute rejection of 99.6%), the operational cost of each DS will be the sum of the cost of FO and RO replacement. The result for each DS has been shown in Table 5 ( Specific Operational Cost ). As can be seen, potassium formiate is approximately 44 % more economical from an operational perspective as compared to potassium pyrophosphate. Table 5. Cost comparison between different DSs. Draw Solution Specific Cost of FO Replacement ($/m 3 ) Specific Operational Cost ($/m 3 ) K 4 P 2 O HCOOK

4 CONCLUSION In the presented study, the full-scale potential of the FO-MBR technology has been evaluated. Firstly, two best-performing DSs were selected through a mathematical modelling approach on the basis of their improved performance in terms of Jw and the Jw to Js ratio as compared to the NaCl reference (commonly employed in FO applications). The selected DSs were HCOOK and K 4 P 2 O 7 ; their long term performance to treat real activated sludge was assessed through a one month period operation in a FO-MBR pilot plant. Both salts were shown to present higher Jw values than the NaCl reference. Additionally, the selected salts presented a sustained process behaviour with no observable fouling for the entire operation time. Calculation of the replacement cost of each selected DS showed that HCOOK was a 28 % more costly from an operational perspective as compared to K 4 P 2 O 7. ACKNOWLEDGMENT The research leading to these results has received funding from the LIFE+ Programme of the European Commission (LIFE12/ENV/ES/ LIFE OFREA) as well as from the People Programme (Marie Curie Actions) of the European Union s Seventh Framework Programme FP7/ under REA agreement (SANITAS Project). This publication reflects only the author's views and the European Union is not liable for any use that may be made of the information contained therein. REFERENCES Achilli, A., Cath, T. Y., Marchand, E. A., & Childress, A. E The forward osmosis membrane bioreactor: A low fouling alternative to MBR processes. Desalination, 239(1-3), Achilli, A., Cath, T.Y. and Childress, A.E Selection of inorganic-based draw solutions for forward osmosis applications. Journal of Membrane Science, 364, Bellona, C., Drewes, J. E., Xu, P., & Amy, G Factors affecting the rejection of organic solutes during NF/RO treatment a literature review. Water Research, 38(12), Bowden, K. S., Achilli, A., & Childress, A. E Organic ionic salt draw solutions for osmotic membrane bioreactors. Bioresource Technology, 122(0), Cornelissen, E. R., Harmsen, D., de Korte, K. F., Ruiken, C. J., Qin, J.-J., Oo, H., & Wessels, L. P Membrane fouling and process performance of forward osmosis membranes on activated sludge. Journal of Membrane Science, 319(1 2), Cussler, E.L Diffusion: mass transfer in fluid systems. Cambridge University Press, 525 pp. Ghiu, S. M. S., Carnahan, R. P., & Barger, M Permeability of electrolytes through a flat RO membrane in a direct osmosis study. Desalination, 144(1 3), Holloway, R. W., Childress, A. E., Dennett, K. E., & Cath, T. Y Forward osmosis for concentration of anaerobic digester centrate. Water Research, 41(17), Lay, W. C. L., Zhang, Q., Zhang, J., McDougald, D., Tang, C., Wang, R., & Fane, A. G Study of integration of forward osmosis and biological process: Membrane performance under elevated salt environment. Desalination, 283, Loeb, S., Titelman, L., Korngold, E., & Freiman, J Effect of porous support fabric on osmosis through a Loeb-Soujourian type assymetric membrane. Journal of Membrane Science, 197, Mi, B., & Elimelech, M Organic fouling of forward osmosis membranes: Fouling reversibility and cleaning without chemical reagents. Journal of Membrane Science, 348(1 2), Phuntsho, S., Shon, H. K., Hong, S., Lee, S., Vigneswaran, S A novel low energy fertilizer driven forward osmosis desalination for direct fertigation: Evaluating the performance of fertilizer draw solutions. Journal of Membrane Science 375, Qin, J.-J., Oo, M. H., Tao, G., Cornelissen, E. R., Ruiken, C. J., Korte, K. F., Wessels, L. P., & Kekre, K. A Optimization of operating conditions in forward osmosis for osmotic membrane bioreactor. The Open Chemical Engineering Journal, 3, Xiao, D., Li, W., Chou, S., Wang, R., & Tang, C. Y A modeling investigation on optimizing the design of forward osmosis hollow fiber modules. Journal of Membrane Science, , Yap, W. J., Zhang, J., Lay, W. C. L., Cao, B., Fane, A. G., & Liu, Y State of the art of osmotic membrane bioreactors for water reclamation. Bioresource Technology, 122, Zhang, H., Ma, Y., Jiang, T., Zhang, G., & Yang, F Influence of activated sludge properties on flux behavior in osmosis membrane bioreactor (OMBR). Journal of Membrane Science, , Zou, S., Gu, Y., Xiao, D., & Tang, C. Y The role of physical and chemical parameters on forward osmosis membrane fouling during algae separation. Journal of Membrane Science, 366(1 2),

5 Figure 2. Operational setup of FO-MBR pilot plant. AS=activated sludge; CIT=conductivity sensors; LIT=level sensors; TIT=temperature sensors; FI=rotameters. Figure 3: Effect of DS properties on water fluxes

6 Figure 4: Effect of DS properties on water fluxes Figure 5: Effect of draw solution concentration on salt and water fluxes.

7 Figure 6: Effect of DS properties on FO performance

8 Table 1. Forward Osmosis model equations taking into account ICP effects. Table 2. Parameters from the ICP equations assumed to have fixed values. Table 3. Cost comparison between different DSs in units of $/(g LMH)x10 2.