Akihiko Tanioka

Reverse osmosis (RO) membranes have been widely applied to seawater desalination and wastewater reclamation, and many large RO plants (>100,000 m³/d) have been constructed since 2000. Energy efficiency is indispensable for large plants, especially for future mega-ton scale seawater reverse osmosis (SWRO) plant (1,000,000 m³/d). In order to reduce energy demand in RO operation, high-efficient low-pressure RO membrane, energy recovery device (ERD), and pressure-retarded osmosis (PRO) which would recover power from salinity gradient between freshwater and concentrated brine were studied in the “Mega-ton Water System” project. A PRO hollow fiber membrane module was newly developed, and a practical continuous operation has been examined at a prototype plant for one year. The maximum 13.5 W/m² of membrane power density using 10-inch module was established at our prototype PRO plant. As the results of the plant cost estimation and the financial impacts on the energy saving operation at the mega-ton scale SWRO plants, not only 20% energy reduction by the high-efficient low-pressure RO membrane and the ERD but also further 10% energy saving was possible by using the PRO system.

Salinity power generation using hollow fiber modules was examined using the pressure-retarded osmosis (PRO) system between pure water and concentrated brine. Pure water and concentrated brine were supplied from a regional sewage treatment facility and sea water desalination (sea water reverse osmosis [SWRO]) plant. To minimize the effect of the concentration polarization near the membrane surface on the pure water side, the number of open ports in the module was increased from 3 to 4 and that modification was found to be effective because non-permeating pure water, which left the module through fourth port, flushed leaked salt from the brine side through the membrane. Our prototype PRO plant got the maximum output power density, 7.7 W/m² at a 2.5 MPa hydraulic pressure difference and a 38% permeation of pure water into the brine. To remove the organic foulant in the pure water, a low pressure Reverse Osmosis (RO) membrane and coagulation–sedimentation method with ozonation showed good results. However, the pressure drop across the RO membrane itself and cost concerns have not yet been solved. Based on the pure water’s flow simulation, the hollow fiber element was found not to effectively work if the module and element for the SWRO were used without modification because the flow pattern of pure water and brine inside the module and element during the PRO operation was different from that during the RO operation.

In the present paper, we report a novel ion-exchange carbon nanomaterial, surface sulfonated carbon nanofiber (S-CNF) fabrics, with a high surface area and high ion-exchange and adsorption capacities. S-CNF fabrics bearing SO₃H groups were prepared by electrospinning, successive carbonization and sulfonation. We observed a nanosize effect, in which the surface area, and ion-exchange and adsorption capacities significantly increase with a decrease in the fiber diameter. The prepared fabrics with a diameter of 80 nm had the maximum values for the total pore surface area of 32.2 m² g⁻¹, the ion-exchange capacity of 2.94 mmol g⁻¹, and an adsorption capacity of 943.1 mg g⁻¹ for methylene blue (MB). The MB adsorption capacity is two times higher than that of commercial activated carbons. These results highlight the fact that the surface functionalized CNF fabrics with ionic groups are promising materials for high-capacity ion-exchangers.

This letter reports the enhancing effects of a nanofiber network structure on stimuli-responsive wettability switching. Thermoresponsive coatings composed of nanofibers were prepared by electrospinning from thermoresponsive polymer poly(N-isopropylacrylamide) (PNIPAAm). The nanofiber coatings showed a large amplitude of thermoresponsive change in the wettability from hydrophilic to hydrophobic states compared to a smooth cast film. In particular, the combination of the surface chemistry and unique topology of the electrospun nanofiber coatings enables a transition from the Wenzel state to the metastable Cassie–Baxter state with an increase in temperature and consequently an enhanced amplitude of change in the water contact angles: the apparent contact angle differences between 25 and 50 °C are Δθ*_25–50 °C = 108 and 10° for the nanofiber coatings with a diameter of 830 nm and a smooth cast film, respectively. The fabrication of the 3D nanofiber network structure by electrospinning from stimuli-responsive materials is a promising option for highly responsive surfaces in wettability.