Integrated RO-VA-AGMD system for sustainable brine management – a techno-economic analysis

• Validated thermo-economic assessment of a coupled multistage RO–spiral-wound VA-AGMD system for high-recovery desalination. • 37.7% higher freshwater recovery compared to conventional standalone RO systems is achieved. • System performance is predominantly governed by the number of RO stages. • Competitive water cost of 0.76 $/m³ at optimal configuration. This study presents a techno-economic framework for a coupled multistage reverse osmosis (RO) spiral-wound vacuum-assisted air gap membrane distillation (VA-AGMD) system for sustainable brine management and enhanced water recovery. The coupled system is designed to overcome the inherent recovery limitations of standalone RO while minimizing brine discharge and overall energy consumption. A comprehensive mathematical model is developed for both subsystems, incorporating mass and heat transfer, osmotic effects, temperature polarization, and economic considerations. The integrated model is implemented and validated against published experimental as well as theoretical data, exhibiting deviations below 5%. Parametric analyses are conducted to investigate the influence of design and operational variables including feed salinity, flow rate, brine temperature, vacuum pressure, and the number of RO and MD stages on productivity, specific energy consumption, and levelized cost of water (LCOW). The coupled configuration increases overall freshwater recovery by up to 37.7% relative to conventional RO systems while maintaining a competitive LCOW of 0.76 $/m³. A five to six RO stages coupled with four to five MD stages, delivering a balanced trade-off between recovery enhancement, capital and operational expenditures. Increasing feed flow and brine temperature substantially improved productivity, whereas reducing vacuum pressure to 15 kPa (abs) lowers specific thermal energy consumption by up to 40.7%.