Abstract
•Energy optimization in asymmetric temperature FO systems is explored.•Colder stream temperature maintained and not maintained scenarios are compared.•Unmaintained conditions caused increased flux and RSF and reduced energy demand.•Energy savings of 67%−75% are achievable with TFC and CTA membranes.•If RSF is not a priority, unmaintained colder stream temperature is advantageous.
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Water and energy are key components to assess the feasibility of any water treatment technique. For industrial applications of forward osmosis (FO), energy optimization needs to be explored further. Asymmetric temperature conditions, having different feed solution (FS) and draw solution (DS) temperatures, are likely to occur in industrial FO applications and are least explored. Some standard practices are followed without any scientific evidence of their impact on the system performance and energy utilization, like maintaining the colder stream temperature instead of letting it reach a steady state with the hotter stream. This study compares water flux, reverse solute flux (RSF), and energy utilization when the colder stream temperature is maintained and not maintained. Thin film composite-poly amide (TFC-PA) and cellulose triacetate (CTA) membranes were used at laboratory-scale in 24 h batch experiments with 0.75 M NaCl as DS and DI water as FS. All possible configurations of membrane orientation (AL-DS and AL-FS) and streams heating and cooling strategies were studied. The module material was made up of plexi-glass with 4 cm thick plates and flexible tubing of PVC was used. Results revealed that under not maintained conditions, on average, the water flux increased by 9.1–17%, RSF increased by 13.8–27.3%, and energy consumption reduced by 75.8–67% with CTA and TFC membranes, respectively. Therefore, if the RSF is not the primary concern of the application, it is advantageous not to maintain the colder stream temperature. However, it is recommended that scientists verify and compare the efficiencies between maintained and not maintained temperatures under their specific experimental conditions.