The synergy between osmotically assisted reverse osmosis (OARO) and the use of thermo-responsive draw solutions for energy efficient, zero-liquid discharge desalination

Abstract

Conventional zero-liquid discharge (ZLD) processes alleviate environmental concerns associated with brine discharge, but are not widely applied, due to their high cost and significant energy consumption. However, new and promising ZLD technologies, such as those integrated with osmotically assisted reverse osmosis (OARO), may be technically and economically more favourable than existing technologies.In this study, a numerical comparison between an FO and an OARO integrated ZLD process is performed, in which a thermo-responsive draw solution is utilised for both. The findings from the parametric and techno-economic analysis indicate that OARO mitigates several shortcomings of FO, making it more energy efficient (energy savings of ≈10.8%), less susceptible to internal concentration polarisation and more economical (cost savings of ≈15.5%). Furthermore, a key advantage of OARO is that it operates with lower draw solute concentrations allowing for the draw solute to be regenerated at temperatures 12 °C lower than with FO operation. This means that low-grade waste heat (LGWH), which is essentially ‘free energy’, is more easily utilised as an energy source.In comparison to other hydraulic pressure-driven ZLD technologies, such as low-salt-rejection RO (LSRRO), the electrical energy consumption of OARO can be significantly offset when powered by LGWH and using a thermo-responsive draw solute. The specific electrical energy consumption can be as low as 3.47 kWh/m3 when concentrating a saline stream from 0.6 M to 4 M using 4 membrane stages. At the same time, a 4-stage LSRRO system would consume approximately 48% more electricity to achieve the same concentration factor. Furthermore, OARO processes operating with a thermo-responsive draw solution can outperform those using non-responsive draw solutes (i.e. NaCl). Brine concentrations of up to 245 g/kg can be achieved using technically feasible operating pressures and draw solute concentrations, while the number of required membrane stages is almost halved from 6 to 4.