Membrane Distillation and Reverse Electrodialysis for Near-Zero Liquid Discharge and low energy seawater desalination

Abstract

With a total capacity of 70 million cubic meters per day, seawater desalination industry represents the most affordable source of drinking water for many people living in arid areas of the world. Seawater Reverse Osmosis (SWRO) technology, driven by the impressive development in membrane materials, modules and process design, currently shows an overall energy consumption of 3–4kWh per m3 of desalted water, substantially lower than thermal systems; however, the theoretical energy demand to produce 1m3 of potable water from 2m3 of seawater (50% recovery factor) is 1.1kWh. In order to move towards this goal, the possibility to recover the energy content of discharged concentrates assumes a strategic relevance. In this work, an innovative approach combining Direct Contact Membrane Distillation (DCMD) and Reverse Electrodialysis (RE) is tested for simultaneous water and energy production from SWRO brine, thus implementing the concept of low energy and Near-Zero Liquid Discharge in seawater desalination. DCMD operated on 1M NaCl RO retentate fed at 40–50°C resulted in a Volume Reduction Factor (VRF) up to 83.6% with transmembrane flux in the range of 1.2–2.4kg/m2h. The performance of RE stack fed with DCMD brine (4–5.4M) and seawater (0.5M) was investigated at different temperatures (10–45°C) and flow velocities (0.7–1.1cm/s). Experimental data show the possibility to obtain an Open Circuit Voltage (OCV) in the range of 1.5–2.3V and a gross power density of 0.9–2.4W/mMP2 (membrane pair). In general, optimization is required to find best operating conditions for the proposed system.