w a t e r r e s e a r c h 4 6 ( 2 0 1 2 ) 1 1 4 5 e1 1 5 4 Available online at www.sciencedirect.com journal homepage: www.elsevier.com/locate/watres A comparative life cycle assessment of hybrid osmotic dilution desalination and established seawater desalination and wastewater reclamation processes Nathan T. Hancock, Nathan D. Black, Tzahi Y. Cath* Colorado School of Mines, Division of Environmental Science and Engineering, 1500 Illinois St., Golden, CO 80401, USA article info abstract Article history: The purpose of this study was to determine the comparative environmental impacts of Received 19 August 2011 coupled seawater desalination and water reclamation using a novel hybrid system that Received in revised form consist of an osmotically driven membrane process and established membrane desalina- 4 December 2011 tion technologies. A comparative life cycle assessment methodology was used to differ- Accepted 5 December 2011 entiate between a novel hybrid process consisting of forward osmosis (FO) operated in Available online 13 December 2011 osmotic dilution (ODN) mode and seawater reverse osmosis (SWRO), and two other processes: a stand alone conventional SWRO desalination system, and a combined SWRO Keywords: and dual barrier impaired water purification system consisting of nanofiltration followed Life cycle assessment by reverse osmosis. Each process was evaluated using ten baseline impact categories. It Forward osmosis was demonstrated that from a life cycle perspective two hurdles exist to further devel- Osmotic dilution opment of the ODN-SWRO process: module design of FO membranes and cleaning inten- Reverse osmosis sity of the FO membranes. System optimization analysis revealed that doubling FO Potable water reuse membrane packing density, tripling FO membrane permeability, and optimizing system Wastewater treatment operation, all of which are technically feasible at the time of this publication, could reduce Seawater desalination the environmental impact of the hybrid ODN-SWRO process compared to SWRO by more Energy recovery than 25%; yet, novel hybrid nanofiltration-RO treatment of seawater and wastewater can achieve almost similar levels of environmental impact. ª 2011 Elsevier Ltd. All rights reserved. 1. Introduction Seawater and wastewater effluents are two sources of impaired water that are ubiquitous in coastal areas; however, both sources require substantial treatment to comply with EPA standards for potable and non-potable uses (Asano et al., 2007). The development of modern membrane separation processes such as reverse osmosis (RO) and nanofiltration enabled widespread utilization of these sources. Reclaimed water usually contain much lower concentration of total dissolved solids than seawater, and rarely requires desalination to achieve reuse standards (WHO, 2003). Yet, reclaimed water may still require treatment with RO or nanofiltration to remove dissolved contaminants and enable potable reuse (Asano et al., 2007; Letterman, 1999). While desalination technologies are well developed, they are not without drawbacks. Energy demand associated with removing salts from seawater and dissolved contaminants from reclaimed water is far greater than treatment of fresh water by conventional water treatment processes (Asano et al., 2007). Desalination technologies are also subject to negative environmental impacts and poor public * Corresponding author. Tel.: þ1 303 273 3402; fax: þ1 303 273 3413. E-mail address: email@example.com (T.Y. Cath). 0043-1354/$ e see front matter ª 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2011.12.004 1146 w a t e r r e s e a r c h 4 6 ( 2 0 1 2 ) 1 1 4 5 e1 1 5 4 perception related to marine life impingement and entrainment and the discharge of chemicals and concentrated brine into receiving environments (Lattemann and Höpner, 2008). 1.1. Water desalination and direct potable reuse e energy implication The energy required for desalination by RO and nanofiltration (NF) is a function of the osmotic pressure (p) differential across the membrane. The osmotic pressure of a solution must be overcome with hydraulic pressure to produce water through a semipermeable RO or NF membrane, and water flux (Jw) may be approximated by Jw ¼ Aw(DpDp), where Aw is the membrane water permeability coefficient and Dp is the hydraulic pressure differential across the membrane. Typical seawater RO (SWRO) plants operate at approximately 50% recovery; thus, the osmotic pressure generated by concentrating seawater into brine requires operation at pressures close to 70 MPa (w1000 psig) (Wilf, 2007). At these pressures, SWRO requires a substantial input of energy to power high-pressure pumps. Energy recovery devices are commonly employed in conjunction with SWRO to transfer energy (i.e., pressure) from the brine stream into the incoming feed stream; more than 60% energy saving can be achieved with the assistance of modern energy recovery devices (Stover, 2007). Low salinity wastewater effluent enables pressure driven membrane processes to achieve more than 80% water recovery with substantially lower pressure and less energy demand than desalination of seawater (Wilf, 2007). Overall, osmotic pressure plays an important role in treatment of impaired water. 1.2. Osmotic dilution In two previous studies (Cath et al., 2010 and Hancock et al., 2011b) we investigated the technical merits, advantages, and limitations of the hybrid osmotic dilution (ODN) process, which is a forward osmosis (FO) process operated without draw solution reconcentration (Cath et al., 2006) e ODN was integrated with SWRO to perform simultaneous seawater desalination and wastewater reclamation. The hybrid ODNSWRO process is shown schematically in Fig. 1. Water from Fig. 1 e Hybrid OND-SWRO process for simultaneous seawater desalination and wastewater reclamation. A PRO variant of the ODN-SWRO process to achieve energy savings from the mixing of saline seawater and low salinity wastewater effluent sources can be utilized by replacing the ODN processes with PRO. impaired stream diffuses spontaneously through an FO membrane in ODN1 to dilute seawater before SWRO desalination. The diluted seawater has a lower osmotic pressure and enables production of more water at the same applied hydraulic pressure; thus, reducing the energy required to produce a unit volume of product water. Previous studies demonstrated that osmotically driven membrane processes, and specifically ODN and FO, have very high rejection of contaminants common to impaired water and that they experience low irreversible membrane fouling even when treating highly impaired water (Cath et al., 2010). Consequently, FO membranes require less pretreatment of impaired water and less periodic cleaning than pressure driven membrane processes. Additional energy savings may be achieved by implementing the pressure retarded osmosis process (PRO) (Achilli and Childress, 2010; Yip and Elimelech, 2011) in place of the ODN process (Fig. 1). PRO provides a method of converting chemical energy into mechanical energy by depressurizing diluted draw solution through a turbogenerator. Current generation FO membranes used in PRO mode demonstrate power density (i.e., a measure of electric power saved or produced by a unit surface area of membrane) output of up to 5 W per m2 of membrane at the bench-scale and less than 1 W/ m2 at the pilot scale (Patel, 2010). Results in this study will be compared to the current state of PRO performance. 1.3. Investigating environmental impacts of desalination technologies with life cycle assessment methodology Life cycle assessment (LCA) is a methodology to holistically evaluate the environmental impacts associated with the development of products for human consumption or use (ISO, 2006a). In recent years studies were published that explored the use of LCA methodology to evaluate the environmental impact of water treatment technologies (Biswas, 2009; Muñoz and Fernández-Alba, 2008; Raluy et al., 2005a, 2005b; Vlasopoulos et al., 2006). The majority of these studies focused on the construction and operations phases of SWRO plants. Each study developed a life cycle inventory (LCI) to account for various reference flows such as construction materials, chemicals required for operation (primarily antiscaling and disinfection chemicals), and materials used for membrane fabrication. A primary conclusion from these studies is that energy consumption during the operation phase of SWRO plants is the single greatest contributor to its negative environmental impacts, accounting for greater than 85% of the environmental impact (Raluy et al., 2005a). Thus, the main objective of this study was to investigate the comparative environmental impacts of established and novel processes for seawater desalination and water reclamation. A specific focus of this study was to explore if the hybrid ODN-SWRO system provides intrinsic benefits over established SWRO desalination or other hybrid technologies for seawater desalination and wastewater reclamation when environmental impacts are considered. Through system optimization analysis, a secondary objective was to identify required improvements to the hybrid ODN-SWRO process that will make it more favorable from a life cycle perspective. w a t e r r e s e a r c h 4 6 ( 2 0 1 2 ) 1 1 4 5 e1 1 5 4 2. 1147 Material and methods LCA consists of four phases, including goal and scope definition, LCI analysis, life cycle impact assessment (LCIA), and interpretation (ISO, 2006b). The first two phases are described in this section while the LCIA and interpretation are discussed in the Results and Discussion section. 2.1. Goal and scope The goal of this study was to evaluate and compare the environmental impacts of the hybrid ODN-SWRO process to those of competitive processes during the desalination of seawater and reclamation of wastewater. A comparative LCA of three functionally equivalent seawater desalination and wastewater reclamation processes was chosen to achieve this goal (Fig. 2). The first scenario (A) is a baseline case of seawater desalination using SWRO and traditional wastewater treatment with discharge to the environment. The second scenario (B) employs a dual pass NF-RO membrane process for wastewater reclamation to augment the water supplied by a SWRO desalination plant. The third scenario (C) uses the hybrid ODN-SWRO process to achieve simultaneous seawater desalination and wastewater reclamation. It is assumed that treating seawater with the SWRO membrane produces sufficient product water quality for use (permeate total dissolved solids concentration of 200 60 mg/L), while purifying the wastewater requires two tight membrane barriers (e.g., nanofiltration-RO or ODN-RO) to achieve a functionally equivalent product water quality. The environmental impacts associated with each scenario are normalized to the production of 1 m3 of water produced, which is defined as the functional unit. The scope of this study includes three gate-to-gate LCIs based on primary chemical inputs for clean-in-place systems, materials consumed during membrane replacement, and energy required for operation and heating of fluids during clean-in-place. This study is not limited to one geographic location, and is therefore unable to include a comprehensive inventory based on a bill of materials for the construction phase. Although the capital construction costs may vary substantially between the different scenarios, the environmental impacts resulting from the construction and decommissioning phase of a membrane based treatment process are typically negligible over a 30 year operation lifetime (as used in this study) when compared to the operation phase (Raluy et al., 2005a, 2005b). Without a specific site to consider, the logistics and impacts of transporting chemicals and materials to the site were not considered for this LCA. LCI boundaries for each scenario are shown in Fig. 2. Each LCI encompasses the core technologies and an auxiliary pretreatment process that includes a 100 mm disk strainer for all feed streams and an additional treatment stage with ultrafiltration membranes for suspended solids removal prior to RO and NF. Waste streams generated during the operation phase (i.e., landfill disposal of membranes at the end of their assumed 5 year service life and discharge of clean-in-place chemicals to the sewer) are included in each LCI. Initial treatment of wastewater feed, post-treatment of the product water, and the distribution Fig. 2 e Gate-to-gate LCI system boundaries for (a) baseline scenario (A) SWRO desalination and wastewater discharge to the environment, (b) alternative scenario (B) NF/RO treatment of wastewater and blending with SWRO permeate, and (c) alternative scenario (C) simultaneous OND/SWRO treatment of seawater and wastewater. Acronym: SW-seawater, WW-wastewater, SDW-solid waste landfill, Pr-T-pretreatment, Po-T-post treatment. Note that only the portion of the wastewater treatment plant that treats wastewater from the three scenarios was considered in the LCA. 1148 w a t e r r e s e a r c h 4 6 ( 2 0 1 2 ) 1 1 4 5 e1 1 5 4 system of each scenario is assumed to be equivalent between all scenarios and is therefore irrelevant for a process-based comparative LCA (Hancock, 2011). Energy and material requirements for each scenario were estimated based on systems designed to produce 3785 m3/day (1 million gallons per day (MGD)) of drinking water. The relevant flow rates for each process are summarized in Table 1. Water recovery of the disk filter and ultrafiltration membrane process were assumed to be 99% and 90%, respectively. The SWRO system was designed to achieve 50% water recovery in all cases. A high water recovery design was assumed for the dual pass NF-RO system for wastewater treatment in scenario B (80% for the first pass (NF) and 85% for the second pass (RO)) based on suggested values from the literature (Wilf, 2007). The ODN process in scenario C was initially assumed to achieve 50% recovery of the wastewater stream. The energy consumption and system design of unit processes for each scenario was either calculated from known principles of hydraulic flow or derived from manufacturer data (see Table S1 in Supplementary Content). Design of the ODN process was facilitated with a Microsoft Excel based software program developed by the authors and described in our previous publication (Cath et al., 2010). Energy for wastewater pumping to the system in each scenario was not considered because it was assumed to already be flowing under pressure to a pre-existing discharge point. Energy demand of seawater intake pumping was calculated assuming that seawater with a density of 1024 kg/m3 is pumped 1.6 km (1 mile) and requires 30 m (100 ft) of lift using a pump with a standard motor and pump efficiencies of 94% and 86%, respectively (Wilf, 2007). Environmental impacts associated with the energy consumed in each treatment scenario were estimated from the Ecoinvent 2000 Union for the Co-ordination of Transmission Energy (UCTE) energy mix (Swiss Center for Life Cycle Inventories, 2011). The UCTE energy mix is composed of approximately 40% fossil fuels (i.e., hard coal and natural gas), 30% nuclear, 10% hydroelectric, and the remaining 20% is Table 1 e Flow rates into specific unit processes and RO product waters in each treatment scenario. Flow m3/min (MGD) Unit process Scenarios A Seawater Disk filter UF SWRO SWRO Permeate Wastewater Disk filter UF WW NF WW RO WW RO Permeate ODN feed ODN Permeate Total product a After dilution. 5.8 5.8 5.3 2.6 B (2.2) (2.2) (2.0) (1.0) 2.9 2.9 2.6 1.3 e e e e e e e 2.6 (1.0) 2.1 2.1 1.8 1.6 1.3 (1.1) (1.1) (1.0) (0.5) (0.8) (0.8) (0.7) (0.6) (0.5) e e 2.6 (1.0) C1 2.9 (1.1) 2.9 (1.1) 5.3 (2.0)a 2.6 (1.0) 5.3 (2.0) e e e e 5.3 (2.0) 2.6 (1.0) 2.6 (1.0) derived from several other sources including biomass, wind, and photovoltaic. Both seawater and wastewater are assumed to have a temperature of 20 C. Systems were designed with an assumed total dissolved solids concentrations of 35,000 mg/L in the seawater and 850 mg/L in the wastewater (Tchobanoglous et al., 2003; Wilf, 2007). Concentrations of chemical species are provided in Table S2 in the Supplementary Content. To further reduce mineral scaling, a scale inhibitor is frequently added to SWRO feed and is essential for operation of high water recovery membrane applications (Asano et al., 2007; Sauvet-Goichon, 2007). The ultrafiltration process requires pre-coagulation to achieve enhanced removal of membrane foulants (Voutchkov, 2010). Each unit process also requires periodic clean-in-place at an elevated fluid temperature with one or more chemicals such as HCl and H2SO4 (acid), NaOH (base), NaOCl (oxidizing agent), sodium dodecylbenzenesulfonate (SDBS) (surfactant), and tetrasodium ethylenediaminetetraacetic acid (Na4EDTA) (chelating agent) added to maintain long-term process performance. Materials used for the fabrication of membrane modules are another important component for the comparative LCA of the different water treatment scenarios. The types of materials used in ultrafiltration, nanofiltration, and RO membrane modules are generally well known (Baker, 2004; Beerya et al., 2011; Mulder, 1997); yet, membranes and modules for FO and ODN are not standardized. More detailed description is provided in the Supplementary Content. The inventory analysis was performed based on the scope of the study presented above. The LCI for three scenarios is summarized in Table S3 in the Supplementary Content. 2.2. Life cycle impact assessment methodology SimaPro LCA software (Pré Consultants, 2011) was employed to aggregate the chemical and material requirements for each scenario and translate that data into values associated with ten environmental impact categories. Characterizing chemical and material use into impact categories was performed with the Leiden University Institute of Environmental Sciences (CML) 2001 method (CML, 2011). The ten baseline impact categories provided by the CML method include abiotic depletion potential (ADP), acidification potential (AP), eutrophication potential (EP), fresh water aquatic ecotoxicity potential (FAETP), global warming potential (GWP) (100 years), human toxicity potential (HTP), marine aquatic toxicity potential (MAETP), ozone layer depletion potential (ODP), photochemical oxidation potential (POCP), and terrestrial ecotoxicity potential (TETP). The categories and their descriptions are summarized in Table S4 in the Supplementary Content. SimaPro was also used to perform interpretation of the LCIA data. Based on findings from the interpretation phase, the software was also used to perform a perturbation analysis of system design parameters that were found to have the most significant impact on the comparative performance of the hybrid ODN-SWRO process relative to the other two scenarios. The hybrid ODN-SWRO process was exclusively used for the perturbation analysis because the design and material requirements of the unit processes in the other two scenarios followed industry standards or were designed and optimized w a t e r r e s e a r c h 4 6 ( 2 0 1 2 ) 1 1 4 5 e1 1 5 4 with system design software from major vendors (i.e., DOW Filmtec ROSA (Dow Filmtec, 2010) and ERI Power Model (ERI, 2011)). 3. Results and Discussion 3.1. Baseline comparative LCIA of water treatment scenarios The LCIA of the inventories (Table S3) was performed with the CML 2001 method. The ten impact categories (Table S4) were used to investigate the relative environmental impact of the different treatment scenarios. The normalized impacts of the three scenarios across the ten categories are shown in Fig. 3. Each scenario is normalized by the maximum value observed between the three scenarios, such that the dominant scenario records 100% of the relative impact compared to the other processes. Results in Fig. 3 indicate that the SWRO system (scenario A) and the ODN-SWRO system (scenario C1 with membrane Aw0 ¼ 0.36 L/m2 h bar) produce a similar degree of environment impact, while the dual barrier NF-RO and SWRO system (scenario B) achieves the design objective with substantially less environmental impact. Scenario C1 has the greatest impact potential in the acidification, eutrophication, ozone layer depletion, human toxicity, terrestrial ecotoxicity, and photochemical oxidation categories. For these categories, the dominant product flows associated with the greatest contribution to the impact were the Na4EDTA and scale Fig. 3 e Relative impacts of scenarios A, B, and C1 evaluated using ten impact categories. Scenario C1 is for the ODN membrane with the baseline membrane Aw0 value (0.36 L/m2$h$bar). The maximum values observed for each impact category are: ADP: 9.9$10L3 kg Sb, AP: 6.9$10L3 kg SO2, EP: 5.5$10L3 kg PO4, GWP: 1.4 kg CO2, ODP: 8.1$10L8 kg CFC-11, HTP: 0.72 kg 1,4-DB, FAETP: 0.74 kg 1,4-DB, MAETP: 1600 kg 1,4-DB, TETP: 5.3$10L3 kg 1,4-DB, POCP: 2.9$10L4 kg C2H4. 1149 inhibitor required for the monthly clean-in-place of the ODN membranes. These impacts are the result of the manufacturing processes required to produce these chemicals. The treatment of the clean-in-place waste at the wastewater treatment plant also contributed substantially to the eutrophication, human toxicity, and terrestrial ecotoxicity categories of scenario C1. Thus, the baseline ODN-SWRO scenario does not appear favorable for reducing the environmental impacts associated with seawater desalination and water reclamation; however, relatively simple improvement to the process that are readily achievable at the time of this publication can decrease chemical consumption of scenario C and reduce the environmental impacts of the process. Normalized scoring of the impact categories in Fig. 3 are provided in Figure S1 in the Supplemental Content document. Normalization scoring divides the results in each impact category by a reference value to provide a measure of their relative importance. The figure shows the relative importance of fresh and marine aquatic ecotoxicity potentials for the three scenarios, but especially for scenario A and C1. Note that specific impact categories such as ozone layer depletion, human toxicity, terrestrial ecotoxicity, fresh and marine ecotoxicity, and photochemical oxidation potentials are difficult to quantify because their local impacts make them difficult to aggregate with the traditional global impact categories used (Guinée, 2002). 3.2. FO membrane optimization Unique aspects of the hybrid ODN-SWRO process were identified while collecting data for the LCI of scenario C. One aspect is that membrane modules for ODN are not yet optimized with respect to their materials and membrane packaging compared to RO membranes. The water permeability coefficient of the FO membrane used to construct the ODN design model (Cath et al., 2010), and used in the baseline LCIA (Hydration Technology Innovations (HTI) membrane cast in 2005 (Hancock and Cath, 2009)), was relatively low (Aw0 ¼ 0.36 L/m2$h$bar) and the packing density of an 8-inch membrane module was 9.5 m2. These parameters, in addition to the inherently low flux through FO membranes when seawater is the draw solution, resulted in a packing density ratio of FO membrane modules to SWRO membrane module of 14:1 to achieve the same water recovery described in scenario C1. Newer HTI membranes that were cast in 2008 and 2010 have permeability coefficients 1.4 and 2 times higher (Aw1 ¼ 0.50 and Aw2 ¼ 0.71 L/m2$h$bar, respectively) (Hancock et al., 2011a), and a corresponding membrane packaging ratio of 4.8:1 and 3.4:1, respectively. In the near future, HTI membranes with three times higher permeability will be available (Aw3 ¼ 1.08 L/m2$h$bar) which will further reduce the ratio to approximately 2:1. New innovative thin-film composite FO membranes might further improve this ratio (Yip et al., 2010; Yip et al., 2011). Significantly higher packing density of FO modules, approaching that of 8-inch RO membranes, is unlikely because of the need to use feed spacers that provide wider flow channels and allow flushing out of solids from the feed channels. Yet, 8-inch diameter FO membrane modules containing 20 m2 of membrane area are already commercialized (Hutchings et al., 2010). A second observation is that the power density of the ODN may be calculated by multiplying the specific energy savings 1150 w a t e r r e s e a r c h 4 6 ( 2 0 1 2 ) 1 1 4 5 e1 1 5 4 (kWh/m3) of the SWRO process due to seawater feed dilution by the nominal water flux through the FO membrane used in the ODN process. The energy demands and power densities for the SWRO process in scenarios A and C are summarized in Table 2. Results from the energy balance in scenario C1 show that the power density of the ODN-SWRO process (with Aw0 ¼ 0.36 L/m2$h$bar) is 3.6 W/m2 when an energy recovery device is used in the SWRO process. However, the values reported in Table 2 are not the maximum achievable energy density by the hybrid ODN-SWRO process under the design specifications listed in Table 1. A plot of the potential power density as a function of the fraction of water recovered from the wastewater achievable by the ODN is shown in Fig. 4 for FO membranes with different permeability coefficients. With an energy recovery device operating in conjunction with the SWRO the maximum potential power density of the ODN process is 3.7, 5.2, 7.4, and 11.1 W/m2 for Aw values of 0.36, 0.50, 0.71, and 1.08 L/m2$h$bar, respectively. Without the use of energy recovery devices (unlikely in most SWRO desalination cases) the maximum power density achieved by the ODN could be 7.5, 10.5, 15.0, and 22.4 W/m2, respectively. For the design conditions listed in Table 1 the maximum power density occurs at approximately 61% water recovery from the 7570 m3/day (2 MGD) wastewater stream (Fig. 4). The potential power density of the ODN-SWRO process is significantly greater than any values measured for PRO membranes to date. Using a membrane with an Aw value of 0.67 L/m2 h bar, Achilli and Childress (2010) measured a power density of 5.1 W/m2 with a 60 g/L NaCl draw solution. A similar membrane used in the ODN-SWRO process would achieve power density in excess of 7.4 W/m2 with an energy recovery device or 15.0 W/m2 without an energy recovery device when using 35 g/L sea salt draw solution. 3.3. New comparative LCIAs of water treatment scenarios Before attempting to optimize the ODN-SWRO process to reduce its environmental impact, it is important to revisit the operating conditions to achieve peak power density. Operating the hybrid system at peak power density represents a conservative approach to chemical demand. This is because more Table 2 e Energy demands and power densities for the SWRO process in scenarios A and C1. The nominal water flux determined by our design program (Cath et al., 2010) at 50% water recovery from a 7570 m3/day (2 MGD) wastewater feed is 3.8 L/m2$h. Description SWRO energy consumption in scenario A, kWh/m3 SWRO energy consumption after ERD in scenario A, kWh/m3 SWRO Energy consumption after dilution in scenario C1, kWh/m3 Energy recovered with ERD in scenario C1, kWh/m3 Power density including ERD, W/m2 Power density excluding ERD, W/m2 Energy value 4.0 2.4 2.1 0.6 3.6 7.2 Fig. 4 e Power density of the FO membrane in the hybrid ODN-SWRO process as a function of the fraction of water recovered from wastewater. A baseline water permeance (Aw0) of 0.36 L/m2$h$bar was assumed for model development, and sensitivity analysis was performed with 1.43, 23, and 33 the membrane permeance. The model assumed utilization of ERD in the SWRO process (Table 2). membrane area is required, and therefore creates more system volume to clean, to achieve 11% more water recovery from the wastewater stream. To decrease chemical demand of the ODNSWRO system, the membrane packing density and membrane permeability coefficient were varied. Doubling the membrane packing density reduced by approximately 50% the number of pressure vessels that would otherwise need to be filled with heated cleaning solution during clean-in-place. Increasing Aw of the FO membrane will reduce the total surface area of membrane required to achieve the desired water recovery. The relative impact of various improvements to the ODN membrane (i.e., permeance coefficient) and module (i.e., packing density) in scenario C are summarized with respect to the ten impact categories in Fig. 5. The greatest environmental impact is observed when the ODN water recovery is 61% and membrane Aw0 is utilized (scenario C2). Increasing the water recovery required an additional 14,612 m2 of FO membrane area (35% increase) due to the reduction in the average water flux when the seawater draw solution is diluted. This increase results in significantly more membrane area to clean. The amount of membrane area required to perform the dilution is reduced by a maximum of 33% when Aw increased by a factor of three (Aw3) (scenario C6). The substantial reduction in membrane area achieves significant reductions across all environmental impact categories. Terrestrial ecotoxicity potential is most sensitive to the reduction of chemicals required for system operation and maintenance. The two primary factors that contribute to terrestrial ecotoxicity potential are energy production and treatment of clean-inplace chemicals by a wastewater treatment plant. w a t e r r e s e a r c h 4 6 ( 2 0 1 2 ) 1 1 4 5 e1 1 5 4 Fig. 5 e Relative impact of various improvements to the ODN membrane and module in scenario C evaluated using the ten impact categories. %R represent percent ODN water recovery, PD1 represent packing density of 9.5 m2/module, PD2 represent 20 m2/module, and Aw represent the different FO membrane permeance coefficients. The maximum values observed for each impact category are: ADP: 0.01 kg Sb, AP: 7.3$10L3 kg SO2, EP: 5.9$10L3 kg PO4, GWP: 1.4 kg CO2, ODP: 9$10L8 kg CFC-11, HTP: 0.78 kg 1,4-DB, FAETP: 0.76 kg 1,4-DB, MAETP: 1400 kg 1,4-DB, TETP: 6.1$10L3 kg 1,4-DB, POCP: 3.1$10L4 kg C2H4. Decreasing the volume of the FO membrane modules (scenarios C3 through C6) decreases the volume of cleaning solution that requires heating and reduces the mass of chemicals required to prepare the clean-in-place solution. Further reduction in the clean-in-place chemicals used in scenario C may be achieved by reducing the frequency of clean-in-place. It is unrealistic to expect that high flux FO membranes would be able to continually treat impaired water (e.g., secondary effluent) without frequent cleaning. Adding ultrafiltration pretreatment to the wastewater stream (similar to scenario B) is a simple method that may reduce the expected frequency of clean-in-place to twice per year. Normalized scoring of the impact categories in Fig. 5 are provided in Figure S2 in the Supplemental Content document. Figure S2 also shows the relative importance of fresh and marine aquatic ecotoxicity potentials for all the scenarios. The effect of adding an ultrafiltration pretreatment to the wastewater stream prior to the ODN process is shown in Fig. 6 and normalized scoring of the impact categories are provided in Figure S3 in the Supplemental Content document. Adding ultrafiltration pretreatment before the ODN process with FO membrane Aw2 (scenarios C5) further reduces the environmental impacts by achieving a reduction in the total amount of chemicals required during clean-in-place and reducing the volume of clean-in-place wastewater requiring treatment 1151 Fig. 6 e Relative impact of UF pretreatment of the ODN feed stream prior to the FO membranes across the ten impact categories of scenarios C5 and C6. In all cases the ODN water recovery is 61% and the membrane packing density is 20 m2/module. The maximum values for each impact category are: ADP: 7.5$10L3 kg Sb, AP: 5.2$10L3 kg SO2, EP: 3.8$10L3 kg PO4, GWP: 1.0 kg CO2, ODP: 5.3$10L8 kg CFC-11, HTP: 0.52 kg 1,4-DB, FAETP: 0.56 kg 1,4-DB, MAETP: 1100 kg 1,4-DB, TETP: 3.2$10L3 kg 1,4-DB, POCP: 2.1$10L4 kg C2H4. (both by a factor of 6 by performing biannual rather than monthly clean-in-place events). Conversely, adding ultrafiltration pretreatment to the ODN process when membrane Aw3 is used (scenarios C6) slightly increases the environmental impact of this scenario relative to a scenario with more frequent (monthly) clean-in-place cleaning intervals. The increase in nine out of the ten categories may be primarily attributed to an increase in the energy required by the process to operate the additional ultrafiltration system (an increase in 0.07 kWh/m3 over the scenario with more frequent clean-inplace events). These results are significant because they show that the high power density of the FO membrane (with Aw3) provides enough energy savings to over compensate for the increase in negative impacts associated with the greater chemical demand of this scenario. Yet, none of the scenario C variants in Fig. 6 achieved substantially less environmental impact than the other variants. This relative indifference in environmental impacts raises the issue of tradeoffs in optimization of the process and will be discussed later in this section. The comparative analysis of the three scenarios presented in Fig. 3 is revisited in light of the FO membrane module design modifications, performance improvements, and operation adjustments discussed above. A comparative LCA for scenarios A and B with two of the modifications to scenario C are shown in Fig. 7 and normalized scoring of the impact categories are provided in Figure S4 in the Supplemental Content document. 1152 w a t e r r e s e a r c h 4 6 ( 2 0 1 2 ) 1 1 4 5 e1 1 5 4 These improvements include doubling the FO membrane packing density from 9.5 to 20 m2 per 8-inch membrane module and either doubling Aw and adding ultrafiltration pretreatment of the wastewater stream prior to the ODN or tripling Aw and maintaining a monthly clean-in-place frequency (C5 with pretreatment of ODN feed and C6). With these two modified cases of scenario C, the ODN-SWRO process is shown to have lower environmental impact than scenario A for all impact categories and all but two categories (ozone depletion and terrestrial ecotoxicity potentials) when compared to scenario B. The contribution of Na4EDTA manufacturing increased the contribution of scenario C variants relative to the scenario B ODP impact category. The contribution of NaOH production and treatment of spent clean-in-place chemicals account for the slightly greater terrestrial ecotoxicity potential of the scenario C variant with monthly chemical cleaning. 3.4. LCIA contribution analysis for hybrid ODN variants An analysis was performed to determine the contribution of individual unit processes to the overall impact category values for two scenario C5 variants presented in Fig. 6 e scenario C5 with or without ultrafiltration pretreatment of the ODN feed. Contribution plots for the two variants are presented in Fig. 8a and b for the variant with and without ultrafiltration pretreatment, respectively. The contribution of the UCTE electricity mix is shown to be the dominant factor for both scenarios and across all impact categories. For the scenario C variant without ultrafiltration pretreatment, chemicals required for the monthly clean-in-place are the second most significant contribution and the scale inhibitor for the SWRO membranes is primarily responsible for it being the third most significant. When ultrafiltration pretreatment of the Fig. 8 e Contribution analysis of various unit processes to the ten impact categories for scenario C5 without UF pretreatment of ODN feed (a) and with UF pretreatment of ODN feed (b). DF indicates the 100 mm disk filter. In the legend, C [ Chemicals, M [ Materials, and EM [ Energy mix. wastewater stream is included and the CIP frequency reduced to twice per year, the consumables (i.e., FeCl3 and scale inhibitor) of the wastewater ultrafiltration and SWRO membrane processes alternate between the second and third greatest contributions among different impact categories. 3.5. Contribution of energy source to environmental impacts Fig. 7 e Comparative LCIA for scenarios A, B, C5 with UF pretreatment of ODN feed, and C6. Legend descriptors are provided in the captions of Figs. 5 and 6. Maximum impact category values are the same as in Fig. 3. The UCTE electricity mix chosen to study the comparative life cycle impacts of the various water treatment scenarios contains a balanced mix of energy derived from fossil fuel sources, renewable sources, and nuclear power. If the electricity mix were composed of less polluting energy sources, the contribution of each unit process to an individual impact w a t e r r e s e a r c h 4 6 ( 2 0 1 2 ) 1 1 4 5 e1 1 5 4 1153 (scenario B) produces the least environmental impact compared to the baseline ODN-SWRO scenario. Currently achievable improvements to the water permeability of FO membranes and membrane packing density for an ODN process can substantially reduce the environmental impacts of the hybrid ODN-SWRO process across all impact categories; yet, at the current state of the technology, its environmental impact is only marginally lower than that of established technologies such as hybrid nanofiltration-RO for water reclamation. Enhancement of water pretreatment to protect the FO membranes and reduce the environmental impact of ODN due to high cleaning frequency may produce additional environmental impacts associated with operation of advanced pretreatment processes, making the environmental impact difference between the two options insignificant. Analysis of the effective power density of the ODN membrane in the hybrid process showcases the remarkable ability of the process to improve the utilization of energy resources. The ODN-SWRO hybrid process can achieve power densities that are higher than levels currently demonstrated by the pressure retarded osmosis process. Acknowledgements We acknowledge the support of California Department of Water Resources (Grant 46-7446-R-08) and the AWWA Abel Wolman Fellowship. Special thanks to Prof. David Muñoz at Colorado School of Mines, Dr. Jean Debroux at Kennedy/Jenks Consultants, and Hydration Technologies Innovations for providing supporting information. Appendix. Supplementary material Fig. 9 e Contribution analysis similar to Fig. 8, but using renewable electricity mix instead of the UCTE electricity mix. The ten impact categories are assessed for scenario C5 without UF pretreatment of ODN feed (a) and with UF pretreatment of ODN feed (b). In the legend, C [ Chemicals, M [ Materials, and EM [ Energy mix. 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