From Waste to Value: A Circular Approach for the Sustainable Recovery of Resources from Maize Lime-Cooking Wastewater

Maize lime-cooking wastewater (MLCWW), a Ca²⁺-rich, high-strength effluent from maize-processing industries, poses treatment challenges due to its complex composition. This study applied a sustainable, sequential approach to recover mineral (nanohydroxyapatite, nHAp), energetic (CH₄), and hydric (treated water) resources from MLCWW. About 72% of residual Ca²⁺ was removed as nHAp (∼11.5 g/L), with potential biomedical applications due to its size (<50 nm), surface area (∼45 to 70 m²/g) BET, and morphology (rod-like). Ca²⁺-depleted MLCWW presented an improved biodegradability index (∼0.27), enhancing anaerobic digestion, which achieved maximum CH₄ production rates of 5.1 ± 0.2 mL CH₄/h and soluble COD (COD_sol) removal of 17.0 ± 2.6 mgO₂/L·h. Further treatment by microalgae-bacteria processes achieved N depletion as well as enabled satisfactory P elimination. Furthermore, the treated effluent reached non detectable of Ca²⁺ (≤0.05 mg/L), volatile solids (≤0.1 mg/L), as well as low levels of total COD (∼0.42 gO₂/L, 97% removal). Cost analyses revealed that nHAp could offset 40% or more of MLCWW treatment costs, with potential profits of 19.5 to 137.2 USD/L MLCWW treated. This study highlights how resource recovery and green chemistry principles can transform MLCWW treatment into a sustainable and economically viable solution.