The sugarcane ethanol industry is currently generating an intensive amount of biowaste while consuming significant water resources. In this work, sugarcane bagasse fly ash (SCBFA), a major biowaste with high amounts of fixed carbon, is employed as a precursor for activated carbon (SCBFA-AC) production. Here, SCBFA-ACs are valorized as the main component of carbon electrodes employed in capacitive deionization (CDI), an emerging desalination technology. In this way, an abundant and low-cost biowaste could be used as a green alternative to treat the water. Different activation methods of SCBFA were explored obtaining SCBFA-AC with a broad spectrum of structural and chemical properties. The electrochemical characterization of SCBFA-AC showed the positive impact of large surface areas, good combination of micro- and mesopores, and the presence of surface functional groups on specific capacitances (117 F g–1). Subsequently, CDI and membrane CDI experiments showed the importance of ion-exchange membranes on improving charge efficiency values (from 5–30 to 80–95%) and consequently, salt adsorption capacity, SAC, from ≈5 to 22 mg g–1. This SAC value, one of the highest ever obtained with biowaste electrodes, only suffered a slight reduction (19 mg g–1) after 70 CDI cycles. Finally, SCBFA electrodes were successfully tested for water-softening applications, reaching 15 mg g–1 when operating using CaCl2 solutions instead of NaCl. This study represents a great example of the water–energy–food nexus in the framework of the circular economy.

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