Techno-Economic and Life Cycle Assessment of Engineered Systems for Inland Desalination and Concentrate Management

Abstract

The long-term outlook for managing concentrate streams in inland applications remains uncertain due to environmental impacts and high disposal costs. Membrane distillation (MD) has emerged as a promising desalination and concentrate management solution across industries, with the potential to maximize resource recovery, support reuse, and enhance water supply security. MD is a thermally driven membrane process capable of producing high-quality water from various impaired high-salinity streams while effectively mitigating volatile contaminants, organics, and bacteria/viruses. Additional advantages of MD include low energy requirements and operating temperatures, enabling the use of low-grade heat (LGH) and renewable energy sources. Despite its potential benefits, MD has not yet reached commercial application. This is partly due to technical performance challenges, such as polarization, scaling, fouling, and wetting, as well as economic limitations. Extensive research has aimed to improve knowledge of technical aspects and address operational challenges to aid the implementation of MD on a commercial scale. Even so, implementing MD for desalination and concentrate management remains uncertain. This uncertainty is increased by a gap in knowledge of the economic and environmental implications of scaling up MD particularly in comparison to conventional concentrate management and desalination alternatives as current techno-economic and environmental impact studies are often limited to seawater desalination applications.This study presents comprehensive techno-economic and life cycle assessments of MD compared to conventional concentrate management strategies, such as evaporation ponds, deep well injection, and mechanical concentrators for a variety of impaired streams. Additionally, the study evaluates the potential of MD to integrate renewable energy sources by assessing the economic and environmental implications of integrating a hybrid solar energy system that combines concentrating solar power for thermal energy supply and photovoltaic collectors to meet electric energy requirements with MD and comparing the results gathered to a conventional MD system where the thermal energy requirements are supplied via steam and electricity from the grid. The overarching goals of this study include evaluating whether implementing MD for concentrate management offers advantages over conventional alternatives, identifying economic and environmental stressors to highlight improvement opportunities, identifying best-case scenarios that result in the lowest cost and impact, and assessing alignment between economic and environmental results. Results from the comparative techno-economic assessment of MD to conventional concentrate management support its cost competitiveness when waste heat is available as the main thermal energy source, achieving a levelized cost of water recovery of 0.90 USD/m³. This outperforms levelized costs of concentrate disposal by deep-well injection (1.09 USD/m³), evaporation ponds (1.47 USD/m³), and water recovery costs by mechanical concentrators (6.2 USD/m³) for the best scenarios of operating at the lowest feed salinity modeled. The key drivers of cost advantages for the MD system are the utilization of waste heat and optimized operating conditions, such as module length and circulating flow rate, which enable effective concentrate management at reduced operational costs, especially as salinity increases. Key findings indicate that cost-effective operating conditions vary depending on the thermal energy source and feed salinity which is of particular importance for the application of MD across industries. These results indicate that MD can provide an economically feasible solution for concentrate management, particularly in regions prioritizing resource recovery and reduced disposal costs. In the comparative life cycle assessment, scores were obtained as environmental impact points per unit of concentrate managed. The best environmental impact scenario was defined by operating conditions that resulted in the lowest environmental impact score for each concentrate management system (pts/m3). Costs for the best environmental impact scenarios were obtained from the previously developed cost models. The main environmental impact stressors across all systems are energy-related, either in the construction or operational phase. The evaporation pond system had the lowest environmental impact between 0.01 and 0.02 pt/m³ followed by MD with an impact range of 0.02 to 0.04 pts/m³. However, the large variation in the cost of evaporation ponds according to feed salinity (1.05 to 6.77 USD/m³) compared to MD (0.81 to 1.52 USD/m³) indicates a discrepancy between environmental impact and cost. Underscoring that the lowest cost does not always align with the lowest impact and emphasizing the need for tailored solutions. In this assessment MD emerges as a preferable alternative for concentrate management, balancing environmental impact with economic viability. Finally, results for the comparative economic and environmental assessment of the hybrid solar MD system and the conventional MD system (energy supply via steam and electric grid) indicate the environmental impact of the hybrid solar system (0.07-0.57 pts/m3) to be up to 13 times lower than the conventional steam-electric gris option. However, the hybrid solar MD system costs are up to 3.5 times higher. These results highlight the trade-off between sustainability and cost, suggesting that hybrid solar MD systems may be more suited for projects where environmental impact reduction is prioritized over immediate cost savings, supporting long-term goals in sustainable water management. The techno-economic and life cycle assessments developed in this study support the economic and environmental viability of MD, establishing it as viable and in some cases preferable concentrate management and desalination alternative. Furthermore, the results of the analyses highlight the importance of system selection that aligns with project-specific goals, such as minimizing environmental impact, maximizing cost efficiency, promoting resource recovery, and optimizing energy use. The framework developed in this study can be adapted to assess various energy systems and feed streams, offering critical decision-making insights that support a shift toward a water-energy circular economy.