CALIFORNIA S DESALINATION AMENDMENT: OPPORTUNITIES FROM THE COLOCATION OF DESAL FACILITIES WITH WASTEWATER TREATMENT PLANTS.

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1 CALIFORNIA S DESALINATION AMENDMENT: OPPORTUNITIES FROM THE COLOCATION OF DESAL FACILITIES WITH WASTEWATER TREATMENT PLANTS Andrea Achilli, PhD, PE, Humboldt State University, 1 Harpst St., Arcata, CA Aa2767@humboldt.edu, Ph: Lori Jones, University of Illinois Urbana Champaign, Urbana, IL Introduction Reverse osmosis (RO) is the main technology employed in seawater desalination; however, RO consumes more energy than alternative technologies used to secure fresh water (at least 2 kwh per m 3 of permeate) and produces a brine, twice as saline as seawater. When the brine is discharged to the ocean, a high salinity layer near the ocean floor is built up due to the increased density of the brine and decreased wave energy at the ocean floor. Studies addressing the sensitivity of marine life to increased salinity are sparse, but suggest that for certain species, small changes in salinity can be devastating. As RO desalination facilities are being implemented in drought-affected areas, California has responded by adding a desalination amendment to the California Ocean Plan in which desalination plants are required to dilute the discharged brine with wastewater effluent before being discharged to the ocean. The colocation of desalination facilities with wastewater treatment plants offers the opportunity to utilize the wastewater effluent in ways that exceed the benefits of simply diluting the concentrated brine before discharge. Dilution of a high saline solution (such as seawater) with a low saline solution (such as treated wastewater effluent) is the basis of osmotically driven processes. When a high salinity solution (draw solution) is separated from a low salinity solution (feed solution) via a semi-permeable membrane, water spontaneously crosses the membrane from the feed solution to the draw solution, following the osmotic pressure gradient. This process is called forward osmosis (FO) (Zhao et al., 2012) (Figure 1a). In a similar process, called pressure retarded osmosis (PRO), the water passes through the membrane in the same direction as FO but the high salinity solution is pressurized. As water crosses the membrane, it is moving from atmospheric pressure to an elevated pressure, effectively turning the chemical potential between the solutions into potential energy (Loeb & Mehta, 1979) and depressurizing the high salinity solution across a turbine generates power from that potential energy (Figure 1b). 1

2 Figure 1. Schematics of forward osmosis (A) and pressure retarded osmosis (B). In FO, water from the low saline solution spontaneously crosses the membrane and dilutes the high saline solution. In PRO, water also moves from the low saline solution to the high saline solution. The osmotic pressure difference between the two solutions of different concentrations is converted to a hydraulic pressure difference when water permeates across the membrane in the PRO process. The higher concentration solution, that is now pressurized, can be depressurized across a turbine, generating power. Treated wastewater is ideal to use for the low salinity solution for these processes because it is readily available (and usually discharged to the ocean or a river) and has a low osmotic pressure. The augmentation of RO with FO and PRO has been shown to offer a promising solution to the current problems with desalination (Blandin et al., 2015; Prante et al., 2014). FO can be used as a pre-treatment step to dilute seawater with water from the treated wastewater source (Figure 2). This reduces the osmotic pressure (and thus the energy requirement) of the RO feed water (Cath et al., 2010). In addition to reducing the energy requirement of seawater desalination via beneficial reuse, an FO-RO system provides two membrane barriers between the wastewater effluent and product water, as well as reducing fouling in the RO membrane due to the dilution and lower required applied pressure (Blandin et al., 2016). FO-RO studies have shown that an energy reduction of 20% could be achieved (Shaffer et al., 2012), approximately 97% of ammonia and nitrates were rejected with the two membrane barriers (Cath et al., 2010), and a prototype, commercial scale FO-RO system operated for 19 months with no chemical cleans, indicating large reductions in operation costs (Thompson & Nicoll, 2011). 2

3 Figure 2. A schematic of the FO-RO hybrid system. FO is used as a pre-treatment step to dilute seawater with water from an impaired water source (such as treated wastewater). This reduces the osmotic pressure (and thus the energy requirement) of the RO feed water. After the diluted seawater is passed through the RO system, the produced brine is approximately the concentration of seawater, and can be discharged to the ocean after passing through a pressure exchanger to further reduce the energy consumption of the system. If PRO is implemented after RO, the concentrated brine from RO is diluted to recover energy, which is then used to pre-pressurize the RO feed water (Figure 3) (Prante et al., 2014). A model based scenario of the RO-PRO configuration was shown to offer a 31% decrease in the power consumption of RO desalination (Altaee et al., 2014). In this paper, we analyze and compare the specific energy consumption and amount of dilution of the FO-RO and RO-PRO hybrid treatment schemes. 3

4 Figure 3. A schematic of the RO-PRO system. In the RO-PRO system, seawater is fed directly to the RO system. The concentrated brine produced from RO is diluted via PRO using treated wastewater to recover energy and reduce the concentration of the brine. The diluted brine, now approximately the concentration of seawater, is then used to pre-pressurize the RO feed water via a pressure exchanger. Materials and Methods In both FO-RO and RO-PRO hybrid system simulations, the RO subsystem consists of four Dow SW modules modeled by the Dow Water & System Processes system design software package ROSA (Dow, 2016). The RO subsystem is maintained at a 40% RO recovery for the varying FO or PRO dilutions. In the FO-RO simulations, the FO subsystem is first simulated using an FO model based on the PRO model previously developed by Prante et al. (Prante et al., 2014) and the outputs of the FO simulation (flow rates, pressures, and concentrations) are used as inputs for the RO subsystem modeled in ROSA. In the RO-PRO simulations, the RO subsystem is first modeled in ROSA, and the outputs of the ROSA simulation are used as the inputs to the PRO model previously developed by Prante et al. (Prante et al., 2014). In the FO-RO system, the reduction in specific energy is calculated by adding zero to four FO elements before the RO subsystem. The specific energy of the system, normalized to the RO permeate flow, is determined by: SSSS nnnnnn,ffff RRRR = SSSS RRRR + SSSS FFFF SSSS rrrrrrrrrrrrrrrr Equation 1 Where SSSS nnnnnn,ffff RRRR is the net specific energy of the FO-RO system (kwh/m 3 ), SSSS RRRR is the specific energy of the RO subsystem (kwh/m 3 ), SSSS FFFF is the specific energy of the FO subsystem (kwh/m 3 ), and SSSS rrrrrrrrrrrrrrrr is the energy recovered via an energy recovery device (kwh/m 3 ). The 4

5 specific energy of the RO subsystem is determined by the ROSA software. The specific energy of the FO subsystem is the energy input into the pumps, normalized to the RO permeate flow rate, evaluated as: SSSS FFFF = QQ dd,iiii PP dd + QQ ff,iiii PP ff η QQ p pp,rrrr Equation 2 Where QQ dd,iiii is the incoming flowrate of the draw solution (m 3 /hr), PP dd is the pressure drop along the draw channel (kwh/m 3 ), QQ ff,iiii is the incoming flowrate of the feed solution (m 3 /hr), PP ff is the pressure drop along the feed channel (kwh/m 3 ), QQ pp,rrrr is the flowrate of the RO permeate (m 3 /hr), and η p is the pump efficiency. The specific energy recovered by using an isobaric energy recovery device is calculated by: SE recovery = P c,roq c,ro η Q e p,ro Equation 3 Where P c,ro is the pressure of the exiting concentrate stream (kwh/m 3 ), Q c,ro is the flow rate of the exiting concentrate stream (m 3 /hr), and η e is the efficiency of the pressure exchanger. The efficiency of the FO-RO system pumps was assumed to be 80% and the efficiency of the pressure exchanger was assumed to be 98%. The entering flow rate into the FO subsystem was held constant at 4 gpm, and the incoming concentration of the draw solution was 35 g/l (the concentration of seawater). In the RO-PRO system, the reduction in specific energy is also calculated by adding zero to four PRO elements after the RO subsystem. The specific energy of the system, normalized to the RO permeate flow rate, is given by: SSSS nnnnnn,rrrr PPPPPP = SSSS RRRR + SSSS pppppppp SSSS PPPPPP SSSS PPPP Equation 4 Where SSSS nnnnnn,rrrr PPPPPP is the net specific energy of the RO-PRO system (kwh/m 3 ), SSSS pppppppp is the specific energy expenditure of the system pumps (kwh/m 3 ), SSSS PPPPPP is the specific energy generation of the PRO subsystem (kwh/m 3 ), and SSSS PPPP is the specific energy recovered via system pressure exchangers (kwh/m 3 ). The specific energy of the PRO subsystem, normalized to the RO permeate flow rate, is given by (Achilli et al., 2014): SSSS PPPPPP = PP aaaaaaaaaaaaaa PP dd + PP ff QQ pp,rrrr Equation 5 Where PP aaaaaaaaaaaaaa is the applied hydraulic pressure difference (kwh/m 3 ). The specific energy expenditure of the pumps, SSSS pppppppp, is determined via Equation 2. In the RO-PRO system, there are two pressure exchangers which recover energy; the first one, given by Equation 6, depressurizes the brine from the RO subsystem to the pressure for the PRO subsystem: 5

6 SSSS PPPP,1 = QQ ff,oooooo PP cc η QQ e pp,rrrr Equation 6 Where QQ ff,oooooo is the exiting brine flow rate from the RO subsystem (m 3 /hr) and PP cc is the pressure difference between the exiting brine from the RO subsystem and the entering draw pressure of the PRO subsystem (kwh/m 3 ). The second pressure exchanger depressurizes the exiting draw solution of the PRO subsystem, given by: SSSS PPPP,2 = QQ dd,oooooopp dd,oooooo η QQ e pp,rrrr Equation 7 Where QQ dd,oooooo is the exiting draw flowrate from the PRO subsystem (m 3 /hr) and PP dd,oooooo is the exiting pressure of the draw from the PRO subsystem (kwh/m 3 ). The efficiency of the RO-PRO system pumps was assumed to be 80% and the efficiency of the pressure exchangers was assumed to be 98%. The entering flow rate into the RO subsystem was held constant at 4 gpm, and the incoming concentration of the RO feed solution was 35 g/l (the concentration of seawater). Results and Discussion FO-RO calculations of net specific energy have been made for 40% RO recovery. The seawater and wastewater streams (Qsw,in and Qww,in) entering the FO subsystem were held constant at 4 gpm (1.09 m 3 /hr). The concentration of the influent seawater was assumed to be 35 g/l. The number of FO elements, FO dilution, system flow rates, exiting brine concentration, and net specific energy for the FO-RO system are shown in Table 1. As expected, as the number of FO elements in the FO subsystem increases, the FO dilution increases. Increasing FO dilution, or increasing FO permeate flow rate causes all system flow rates to increase. The exiting concentration of the brine waste stream from the FO-RO system decreases with increased FO dilution; using four FO elements, the brine concentration is only 1.8 g/l higher than the influent seawater concentration (36.8 g/l vs 35 g/l), thus dramatically reducing potential environmental concerns of brine discharge to the ocean. With four FO elements, the net specific energy of the system decreases 20.1%, while the RO permeate flow rate (the production of fresh water) increases 56.3%. Table 1. Number of FO elements, FO dilution, system flow rates, brine concentration, and net specific energy at 40% RO recovery. Number of FO elements Dilution (%) Qp,FO (m 3 /hr) Qsw,out = Qf,RO (m 3 /hr) Qc,RO (m 3 /hr) Qp,RO (m 3 /hr) Cc,RO (g/l) SEnet (kwh/m 3 )

7 The reduction in net specific energy and increase in RO permeate flow rate is also shown in Figure 4. Each data point represents the addition of one more FO element. As expected, when dilution increases, the specific energy decreases, due to the decrease in osmotic pressure. Also shown in Figure 6 is the net specific energy of the FO-RO system at constant 40% system recovery (RO permeate flow rate 1.6 gpm). At a constant system recovery, and therefore a constant production of fresh water, the reduction in net specific energy of the system with four FO elements increases to 24.7%. Figure 4. Net specific energy consumption and RO permeate flow rate of the FO-RO system at a 40% RO recovery and at a 40% system recovery as function of percent FO dilution. Table 2 lists the number of PRO elements, PRO dilution, PRO permeate flow rate, the exiting concentration of the draw solution from the PRO subsystem, and the net specific energy of the RO-PRO system. Four RO elements were used in the RO subsystem at a 40% recovery, and zero to four PRO elements were added in series after the RO subsystem. The entering flow rate of seawater into the RO subsystem was set at 4 gpm, and at a 40% recovery the exiting brine solution was found to be 0.65 m 3 /hr. This was used as the entering draw solution flow rate for the PRO subsystem, and the entering wastewater stream into the PRO subsystem was set at 4 gpm. With increasing the number of PRO elements, the PRO dilution increases. As the PRO dilution increases, the PRO permeate flow rate increases, which decreases the concentration of the brine. With four PRO elements, the concentration of the brine is only 3.1 g/l higher than the influent seawater concentration. With four PRO elements, the net specific energy of the system decreases 32.8%. 7

8 Table 2. The number of PRO elements, PRO dilution, PRO permeate flow rate, brine concentration, and net specific energy of the RO-PRO system at a 40% RO recovery. Number of SEnet (kwh/m 3 ) PRO Elements Dilution (%) Qp,PRO (m 3 /hr) Cds,ex;PRO (g/l) A comparison of net specific energy and exiting brine concentration of the FO-RO and RO-PRO systems as function of increased membrane area is shown in Figure 5. Increased membrane area represents the addition of FO or PRO membrane modules to the hybrid system. As expected, for both systems, as the membrane area increases the net specific energy consumption and brine concentration decreases. While both systems achieve similar reduction in brine concentration, the RO-PRO system appears to require substantial less specific energy to desalinate seawater than the FO-RO system. However, the FO-RO system in addition to reduce energy consumption of desalination, also increases the volume of treated water through direct potable reuse of the FO feed water. Overall, both systems show promise in the integration of water reuse and desalination systems for a more sustainable water supply management Figure 5. Net specific energy consumption and brine concentration of the FO-RO and RO-PRO systems as function of increased membrane area due to the addition of FO or PRO membrane modules to the hybrid system. 8

9 References Achilli, A., Prante, J., Hancock, N., Maxwell, E., Childress, A Experimental Results from RO-PRO: A Next Generation System for Low-Energy Desalination. Environmental Science & Technology, 48(11), Altaee, A., Zaragoza, G., Sharif, A Pressure retarded osmosis for power generation and seawater desalination: Performance analysis. Desalination, 344, Blandin, G., Verliefde, A.R.D., Comas, J., Rodriguez-Roda, I., Le-Clech, P Efficiently Combining Water Reuse and Desalination through Forward Osmosis - Reverse Osmosis (FO-RO) Hybrids: A Critical Review. Membranes, 6(37). Blandin, G., Verliefde, A.R.D., Tang, C.Y., Le-Clech, P Opportunities to reach economic sustainability in forward osmosis reverse osmosis hybrids for seawater desalination. Desalination, 363, Cath, T.Y., Hancock, N.T., Lundin, C.D., Hoppe-Jones, C., Drewes, J.E A multi-barrier osmotic dilution process for simultaneous desalination and purification of impaired water. journal of Membrane Science, 362(1-2), Dow ROSA System Design Software, (Ed.) D.W.P. Solutions. Loeb, S., Mehta, G A two-coefficient water transport equation for pressure-retarded osmosis. Journal of Membrane Science, 4, Prante, J., Ruskowitz, J., Childress, A., Achilli, A RO-PRO desalination: an integrated low-energy approach to seawater desalination. Applied Energy, 120, Shaffer, D.L., Yip, N.Y., Gilron, J., Elimelech, M Seawater desalination for agriculture by integrated forward and reverse osmosis: Improved product water quality for potentially less energy. Journal of Membrane Science, , 1-8. Thompson, N.A., Nicoll, P.G Forward Osmosis Desalination: A Commercial Reality. in: IDA World Congress - Perth Convention and Exhibition Centre (PCEC). Perth, Western Australia. Zhao, S., Zou, L., Tang, C.Y., Mulcahy, D Recent developments in forward osmosis: Opportunities and challenges. Journal of Membrane Science, 396,