The CARMEn project: A sustainable and circular brine treatment chain

The increasing demand for water and critical minerals is currently largely met through linear processes, such as desalination and saltworks, which generate highly saline waste streams typically discharged to the sea. These brines, however, are rich in valuable components. The Italian-funded CARMEn project proposes a circular approach to valorize such streams by producing magnesium hydroxide, freshwater, and energy through an integrated treatment chain. As proof of concept, real reverse osmosis (RO) brine and saltworks bitterns were processed at laboratory scale through a sequence of nanofiltration, reactive precipitation, softening, electrodialysis with bipolar membranes, membrane distillation, and reverse electrodialysis. Experimental data supported the development of a preliminary techno-economic analysis to assess the viability of a pilot plant targeting 50 tons/year of Mg(OH)2. Results show that feed composition and operating pH strongly influence performance. Bitterns, with Mg2+ concentrations ~20 times higher than RO brine, led to the lowest CAPEX and EnEx and enabled >99.7 % product purity, but required external water input, reducing circularity. RO brine and NF retentate achieved full circular operation without external inputs, though with higher costs and slightly lower product purity (approximately between 85 and 93 %). Across all scenarios, EDBM and MD dominated energy demand, but coupling MD with waste heat reduced power consumption lowering the levelized cost of Mg(OH)2.