A New Mode of Reverse Electrodialysis Operation to Reduce Seawater RO Energy Demand

Abstract

Introduction Water and energy are two of the main challenges facing our modern world today. With water shortages and pollution reaching alarming levels, research is pushed to look for alternative water sources, such as (secondary treated) waste water and seawater. In the case of seawater, the main issue is the large amount of energy needed for its desalination, around 2-3 kWh/m³ at a recovery of 50% (35 g/l total dissolved solids) in the case of state-of-the-art reverse osmosis (RO). By coupling RO with reverse electrodialysis (RED), the energy demand can be decreased in two ways: (1) energy can be produced in RED by the salinity gradient between the seawater and for example impaired water and (2) the concentration of the seawater decreases in RED, entailing a lower energy demand in the RO step. This type of hybrid can theoretically decrease the energy demand of seawater desalination to the point of energy neutrality and can even be energy producing [1,2]. However, the viability of the process is limited by the slow desalination kinetics in RED, resulting in a high required membrane area and consequent high capital costs. The desalination rate in RED is mainly limited in the first stages of desalination, where the low salinity compartment causes a high resistance of both the solution itself and of the membrane. Indeed it was shown that the membrane resistance mainly depends on the low salinity solution it is in contact with [3,4]. Figure 1. Envisioned hybrid (A)RED-RO process To overcome this initially high resistance, a new mode of RED operation was developed: assisted RED or ARED. Here, instead of producing energy, a small potential difference is applied in the same direction as the natural salt transport to increase the desalination rate. ARED can be incorporated into the hybrid system as shown in Figure 1. This study is one of the first to explore the possibilities of ARED on lab-scale. Materials and methods A 5-cell pair RED set-up was used for the ARED tests. Fujifilm type I and type II membranes were used, with an active membrane area of 7.8x11.2 cm². Spacers with a thickness of 485 μm were used to create the low and high salinity compartments. A constant current density (0-136 A/m²) was applied and the resulting voltage was recorded to create current-voltage curves. Experiments were executed in once-through mode to keep the influent concentrations constant. Results To study the influence of the low salinity compartment (impaired water compartment), the seawater compartment concentration was kept constant, at 0.5M NaCl, while the impaired water compartment concentration was varied (0.01, 0.05, 0.1 and 0.25M NaCl). All obtained current-voltage curves at the lower concentrations (0.01 and 0.05M NaCl) show a clear downward declination, indicating a significant decrease in resistance at increasing currents. At lower flow rates, this decrease in resistance becomes more apparent, as the residence time in the system increases. At higher concentrations (0.1 and 0.25M NaCl), the relation is linear, as theoretically expected. The observed decrease in system resistance is caused by an increase in concentration of the low salinity compartment due to the salt transport in the system. Galama et al. (2014) and Geise et al. (2014) showed that the low salinity compartment concentration determines the membrane resistance [3,4] and that an increase in the concentration of this compartment thus leads to a decrease in the membrane resistance as well. The rapid decrease in resistance observed in ARED is its main advantage. By incorporating it into the RED-RO hybrid, it increases the economic viability of the system. Further characterisation and modelling of the system is under way to further assess its value.