Abstract
Although seawater reverse osmosis (RO) is nearing its thermodynamic minimum energy limit, it is still an energy-intensive process, requiring 2–3 kWh/m³ at a recovery of 50%. Pre-desalination of the seawater by reverse electrodialysis (RED), using an impaired water source, can further decrease this energy demand by producing energy and reducing the seawater concentration. However, RED is hampered by the initial high resistance of the fresh water source, resulting in a high required membrane area (i.e., high investment costs). In this paper, a new process is presented that can overcome this initial resistance and decrease the RED investment cost without the need for additional infrastructure: assisted RED (ARED). In ARED, a small potential difference is applied in the direction of the natural salinity gradient, increasing the ionic transport rate and rapidly decreasing the initial diluate resistance. This decreasing resistance is shown to outweigh any negative effects caused by, for example, concentration polarization, resulting in a process that is more efficient than theoretically expected. As this effect is mainly important at low diluate concentrations (up to 0.1 M), ARED is proposed as a first step in an economic and energy efficient (A)RED-RO hybrid process.
Highlights
Reverse osmosis (RO) is currently the benchmark for seawater desalination, accounting for ±60% of the worldwide desalination capacity.[1]
reverse electrodialysis (RED) has been compared to other osmotic dilution processes, pressure-retarded osmosis (PRO), forward osmosis (FO), and pressure-assisted osmosis (PAO), as a pre-desalination step before RO.[13,14,15]
Whereas in ED the potential difference increases with increasing current densities relative to the ideal curve, the opposite is observed for assisted RED (ARED)
Summary
Reverse osmosis (RO) is currently the benchmark for seawater desalination, accounting for ±60% of the worldwide desalination capacity.[1]. It can be assumed that the increased low salinity compartment concentration in ARED results in a lower resistance of the compartment itself, and of the membranes.
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