Biogas slurry purification-lettuce growth nexus: Nutrients absorption and pollutants removal

As a main by-product of anaerobic digestion in biogas plants, biogas slurry contains a high concentration of mineral elements (such as ammonia‑nitrogen and potassium) and chemical oxygen demand (COD). So determining how to dispose the biogas slurry in a harmless and value-added ways is crucial from the perspective of ecological and environmental protections. This study explored a novel nexus between biogas slurry and lettuce, in which the biogas slurry was concentrated and saturated with carbon dioxide (CO₂) to serve as a hydroponic solution for lettuce growth. Meanwhile, the lettuce was used to purify the biogas slurry through removing pollutants. Results showed that when concentrating the biogas slurry, the total nitrogen and ammonia nitrogen contents in the biogas slurry decreased with the increase of concentration factor. The CO₂-rich 5-time-concentrated biogas slurry (CR-5CBS) was screened as the most suitable hydroponic solution for lettuce growth after comprehensively considering the nutrient element balance, energy consumption of concentrating the biogas slurry and CO₂ absorption performance. The quality of lettuce cultivated in CR-5CBS was comparable to that of the Hoagland-Arnon nutrient solution in terms of physiological toxicity, nutritional quality, and mineral uptake. Obviously, the hydroponic lettuce could effectively utilize the nutrients in CR-5CBS to purify CR-5CBS, meeting the standard of reclaimed water quality for agricultural reuse. Interestingly, when the same yield of lettuce is targeted, using CR-5CBS as the hydroponic solution to cultivate lettuce can save about US $151/m³-CR-5CBS for lettuce production compared to the Hoagland-Arnon nutrient solution. This study might provide a feasible method for high-value utilization and harmless disposal of biogas slurry.