Valorizing liquid digestate by incorporating into hydroponics enhances lettuce growth, nutrient recovery, and reduces environmental emissions

Hydroponic lettuce production requires efficient nutrient management strategies to enhance crop productivity while reducing environmental impacts. Liquid digestate, a nutrient-rich by-product from anaerobic digestion, has emerged as a promising alternative nutrient source for hydroponics; however, the optimal incorporation strategy and concurrent environmental benefits have not been well investigated. This study thoroughly evaluated lettuce growth, nutrient removal and uptake, and environmental emissions of LD-integrated hydroponics. Results showed that optimal LD supplementation was achieved when substituting 30% of mineral nitrogen. The lettuce fresh weight and photosynthetic pigments content were increased by 1.8-fold and 1.2-fold compared to the inorganic fertilizer control. Moreover, the soluble sugar and amino acids contents increased by 2.4-fold and 1.3-fold, respectively, suggesting the improved nutritional quality, which has been rarely reported in LD-based hydroponic systems. The nutrient assimilation efficiencies reached 78.6% for nitrogen and 41.4% for phosphorus, outperforming the inorganic control and previously reported values. In addition, over 90% of nitrogen and phosphorus in the hydroponic effluent were reduced during the rapid growth period, suggesting excellent nutrient removal efficiency of this system. Furthermore, life cycle assessment demonstrated reduced global warming potential (by 39–45%) and eutrophication potential (by 27–39%) of the LD-integrated hydroponics compared to the conventional system applying only synthetic fertilizer. Overall, optimized LD integration can simultaneously improve crop productivity and environmental performance, providing a sustainable strategy for nutrient recycling in hydroponic systems.