An optimal design approach of forward osmosis and reverse osmosis hybrid process for seawater desalination

The forward osmosis (FO) and reverse osmosis (RO) hybrid process uses seawater and wastewater treatment plant effluent as the FO draw solution and feed water, respectively, and the diluted seawater by FO is used as the RO feed water resulting in the less energy consumption than the conventional seawater reverse osmosis applications. This work developed an optimal design approach of the hybrid process by finding the optimal RO recovery and FO permeate flow rate. The optimized RO recovery (e.g. 38.5–66.7% according to the FO permeate flow rate) determined by solving an optimization problem based on the mass balance in the FO-RO hybrid process, minimizes the RO energy consumption (1.86–3.49 kWh/m³ at 25°C and 2.41–3.86 kWh/m³ at 5°C). The RO energy consumption decreases as the RO recovery increases until it reaches an optimal value. The optimal FO permeate flow rate can be defined with three different perspectives: (1) to minimize the RO energy consumption, (2) to minimize the RO feed flow rate, and (3) to minimize the environmental impacts of the concentrate discharge. Thus, the optimal FO permeate flow rate should be determined based on the weights of the three perspectives. The energy saving achieved by the optimal design approach in this work ranges from 37.6 to 46.7% according to the temperature.