From Waste to Resource: Integrated CaO–Fenton Treatment of Digestate for Nutrient Recovery and Hydroponic Lettuce Cultivation

Abstract

An integrated CaO–Fenton waste-to-resource strategy was evaluated for converting anaerobic sewage digestate into a microbiologically safe and nutritionally functional solution for hydroponic lettuce (Lactuca sativa L.) cultivation. Lime stabilization at dosages up to 35% achieved near-complete pathogen inactivation. Beside substantial decreases in total solids (TS), total volatile solids (TVS), chemical oxygen demand (COD), NH₄⁺ ions, and heavy metals.  Subsequent Fenton oxidation using hydrogen peroxide (H₂O₂) and Fe²⁺ ensured complete microbial sterilization and further improved effluent quality, achieving additional reductions in TS (87.7%), TVS (100%), COD (52.5%), and BOD (52.4%). When applied in a deep-water culture (DWC) hydroponic system, CaO–Fenton-TD supported plant growth with moderate biomass reductions but significantly increased the accumulation of bioactive compounds with potential nutritional value, including ascorbic acid, total carotenoids, total phenolics, and total flavonoids, relative to the control. One-way ANOVA followed by Tukey’s HSD confirmed that both CaO stabilization and Fenton oxidation had significant effects (P ≤ 0.05) on the physicochemical and microbial properties of the digestate. Overall, the results demonstrated the feasibility of using CaO–Fenton-treated anaerobic digestate in hydroponic systems. The proposed approach supports circular economy by promoting nutrient recovery and beneficial reuse of digestate, thereby reducing waste-disposal burdens and dependence on conventional mineral fertilizers.