Advancing desalination technology: A novel hybrid reverse osmosis-membrane distillation system for greater efficiency, eco-friendlier, and enhanced freshwater recovery

This paper proposes a novel hybrid configuration (aligned with the sixth United Nations Sustainable Development Goal) of a six-element reverse osmosis (RO) module with thirteen double-stage and parallel membrane distillation (MD) modules (two series stages) for industrial and large-scale plants (not laboratory scale). This education-oriented research presents heat/mass transfer simulation details to perform energy, exergy, economic, and environmental analyses for the proposed RO-MD hybrid configuration for the first time. The specific energy consumption, recovery rate, specific exergy consumption, cost of water (COW), and cost of carbon (COC), have been assessed for the hybrid and stand-alone systems. The findings indicate that the RO-MD hybrid achieves a 45.32% recovery ratio and 61.63% exergy efficiency, outperforming stand-alone RO (35.95%, 51.60%) and stand-alone MD (7.37%, 48.24%), while cutting specific electrical energy consumption by 13.3% compared with RO and specific thermal energy consumption by 79.42% compared with MD. Meanwhile under the studied operating condition (25 °C inlet temperature and 35,000 ppm feed TDS), the hybrid system reduces permeate total dissolved solids (TDS) from 267 ppm (stand-alone RO) to 223 ppm. Economic analysis shows that employing a geothermal heat source for a 30,000 m3/day plant reduces the cost of water from 2.117 to 1.573 $/m3, with a further decrease to 0.872 $/m3 achievable using recycled waste heat. Additionally, the hybrid configuration results in 11.16% and 13.34% reductions in cost of carbon and Carbon dioxide (CO2) emissions compared with stand-alone MD and RO systems, respectively.