Concentration and Temperature Effects on Water and Salt Permeabilities in Osmosis and Implications in Pressure-Retarded Osmosis

On the potential of pristine Cocos nucifera L. tissues for green desalination

Coconut palm tree (Cocos nucifera L.) tissues were used as a readily available, low-cost and green adsorbent to desalinate seawater. The tree bark (CB), husk (CH), leaves (CL) and roots (CR) were examined in their fresh (F) and dry (D) forms. The salinity removal (adsorption) efficiency followed the trend: F_CB ≈ F_CR > F_CL > D_CR > F_CL > D_CR. The sorbents from the coastal region desalinated more efficiently than those from a non-coastal region. Also, the fresh tissues were more effective and efficient than the dry parts. The salinity retention ability (desalination : desorption) follows the trend: F_CR (22.2) > F_CB (19.0) ≫ D_CR (12.3) > D_CB (11.0) > D_CL (6.14) ≈ F_CL (6.10) > F_CH (4.3) > D_CH (2.1). Moreover, the desalination fitted the pseudo-second-order kinetics than the pseudo-first-order, suggesting the predominance of chemisorption over physical removal. Overall, water pH, conductivity, total dissolved solids and dissolved oxygen (DO) correlated positively and strongly with desalination. By contrast, the density and redox potential correlated negatively, whereas temperature and DO showed no definite influence. Conclusively, F_CR and F_CB are the most suitable coconut palm tree tissues for desalination. Future studies should include chemical characterization of the tissues and system optimization for upscaling. This article is part of the theme issue 'Developing resilient energy systems'.

Feasibility of Pressure-Retarded Osmosis for Electricity Generation at Low Temperatures

A membrane-based technique for production of pressure-retarded osmosis (PRO) is salinity gradient energy. This sustainable energy is formed by combining salt and fresh waters. The membrane of the PRO process has a significant effect on controlling the salinity gradient energy or osmotic energy generation. Membrane fouling and operating conditions such as temperature have an extreme influence on the efficiency of the PRO processes because of their roles in salt and water transportation through the PRO membranes. In this study, the temperature impact on the power density and the fouling of two industrial semi-permeable membranes in the PRO system was investigated using river and synthetic sea water. Based on the findings, the power densities were 17.1 and 14.2 W/m2 at 5 °C for flat sheet and hollow fiber membranes, respectively. This is the first time that research indicates that power density at low temperature is feasible for generating electricity using PRO processes. These results can be promising for regions with high PRO potential that experience low temperatures most of the year.

Factors Affecting the Performance of Membrane Osmotic Processes for Bioenergy Development

Forward osmosis (FO) and pressure-retarded osmosis (PRO) have gained attention recently as potential processes to solve water and energy scarcity problems with advantages over pressure-driven membrane processes. These processes can be designed to produce bioenergy and clean water at the same time (i.e., wastewater treatment with power generation). Despite having significant technological advancement, these bioenergy processes are yet to be implemented in full scale and commercialized due to its relatively low performance. Hence, massive and extensive research has been carried out to evaluate the variables in FO and PRO processes such as osmotic membrane, feed solutions, draw solutions, and operating conditions in order to maximize the outcomes, which include water flux and power density. However, these research findings have not been summarized and properly reviewed. The key parts of this review are to discuss the factors influencing the performance of FO and PRO with respective resulting effects and to determine the research gaps in their optimization with the aim of further improving these bioenergy processes and commercializing them in various industrial applications.

Temperature Effects and Entropy Generation of Pressure Retarded Osmosis Process

Pressure retarded osmosis (PRO) is considered as one of the promising and new techniques to generate power. In this work, a numerical model was used to study the effect of the flow streams temperature on the performance of the PRO process and entropy generation. The variation of the feed solution and draw solution temperatures, pressure difference, concentration difference, and flow rates on the power density and entropy generation were discussed. The model results were validated with experimental measurements obtained from literature and showed a good agreement with the model predictions. It was found that the power density increases by about 130% when both feed solution and draw solution temperatures increase from 20 °C to 50 °C. The feed solution temperature has more impact on the power density than that of the draw solution. This is due to the direct effect of the feed solution temperature on the water permeability and diffusion coefficient. The effect of the feed solution temperature becomes significant at higher concentration differences. Whereas, at low concentrations, the power density slightly increases with the feed temperature. Furthermore, it is found that there is an optimum volumetric flow in the channels that maximizes the power density and minimizes the entropy generation when fixing other operating conditions.