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Salinity gradient energy is a prominent alternative and maintainable energy source, which has considerable potential. Reverse electrodialysis (RED) is one of the most widely studied methods to extract this energy. Despite the considerable progress in research, optimization of RED process is still ongoing. In this study, effects of the number of membrane pairs, ratio of salinity gradient and feed velocity on power generation via the reverse electrodialysis (RED) system were investigated by using Fujifilm cation exchange membrane (CEM Type 2) and FujiFilm anion exchange membrane (AEM Type 2) ion exchange membranes. In the literature, there is no previous study based on a RED system equipped with Fujifilm AEM Type II and CEM Type II membranes that have homogeneous bulk structure. Using 400 µm of intermembrane distance, maximum obtainable power density by 5 pairs of Fujifilm membranes at 1:45 salinity ratio and with a linear flow rate of 0.833 cm/s was 0.426 W/m2.

BACKGROUND Increasing demand on energy sources has motivated research studies on renewable energy originating from biomass. Water lettuce (Pistia stratiotes L.) as an invader lignocellulosic biomass with high crop yield can be utilized in anaerobic digestion to produce biomethane. Substrate and inoculum concentrations, and temperature are important parameters for kinetic analysis of methane production that was performed with water lettuce and waste sludge for the first time. Kinetic analysis can be used in reactor design, scale up and identification of optimum conditions. RESULTS In the present study, anaerobic digestion of water lettuce with waste sludge as inoculum was performed in a batch system. Substrate concentration [30, 40 and 50 g total solid (TS) L−1], waste sludge concentration (3.4 and 6.8 g TS L−1) were investigated at different digestion temperatures (35, 45, 55 and 65 °C). In the studied range, the highest biogas yield [321 mL g–1 volatile solid (VS) with 72.5% of methane content) was reached at substrate concentration of 30 g TS L−1 and waste sludge concentration of 6.8 g TS L−1 at 35 °C. The Cone model fitted well to the actual methane production compared to the modified Gompertz model. The effect of digestion temperature on methane production potential was described by the Ratkowsky model (R2 = 0.91) and optimum digestion temperature was found as 45 °C. Experimental methane yields were 81.5–232.7 mL methane g–1 VS which were 27.7–79.0% of theoretical maximum methane yield. Energy conversion efficiency was reached up to 78.4%. CONCLUSION The results revealed that anaerobic digestion of water lettuce with waste sludge for high methane production rate was accomplished at certain substrate and inoculum concentrations and temperature. © 2019 Society of Chemical Industry