Abstract
The transition toward circular economy principles in water treatment requires advanced process systems engineering tools to evaluate the trade-offs between environmental sustainability and economic viability, particularly for energy-intensive Zero Liquid Discharge (ZLD) systems. While classic ZLD systems treat concentrated brine as waste, circular ZLD (CZLD) systems incorporate salt recovery technologies that generate marketable salt product. This study presents a comprehensive technoeconomic assessment framework for CZLD systems integrated with renewable energy. The framework is developed to evaluate different CZLD configurations that generate saleable sodium chloride. The assessment methodology integrates solar photovoltaic systems with increasing capacities (100-1400 kW) to analyze renewable energy penetration and energy storage requirements. The renewable energy integration model incorporates hierarchical energy dispatch algorithms prioritizing direct solar utilization, battery storage, and grid backup systems. Solar PV integration demonstrates renewable fractions ranging from 15-100%, with corresponding emissions reductions of up to 72% compared to grid-powered baselines. The technoeconomic analysis reveals a critical trade-off: while renewable integration substantially reduces carbon emissions, it significantly increases operational costs due to higher solar costs compared to subsidized grid electricity. However, configurations with substantial salt revenue generation maintain economic viability even under complete renewable operation. Beyond configuration screening, this work makes three specific contributions to process systems engineering: (i) a hierarchical energy-dispatch model for intermittent solar integration in energy-intensive separations; (ii) a revenue-function formulation that embeds salt valorization into ZLD process economics, shifting the design objective from cost minimization to net revenue maximization; and (iii) systematic identification of the thermodynamic basis for membrane-thermal hybrid superiority through analysis of osmotic pressure versus latent heat energy barriers across 26 configurations. These contributions establish a reproducible framework applicable to brine management in resource-constrained industrial settings.