Assessment of Anaerobic Digestate as a Nutrient Source for Bioponic Production of Lettuce (Lactuca sativa ‘Claribel’) in a Rooftop NFT System

Abstract

Finding sustainable alternatives to mineral fertilizers is essential for improving food production in urban areas. This study evaluated the use of anaerobic digestate (AD), which is derived from cheese whey and swine wastewater, as a nutrient source for producing lettuce (Lactuca sativa, “Claribel”) in a rooftop NFT hydroponic system. The study focused on designing and validating a modular hydroponic prototype and assessing the plants response to four nutrient treatments: raw digestate (AD), nitrified digestate (ND), enriched nitrified digestate (END), and conventional inorganic control (C). The system operated reliably over a 21-day cycle, maintaining stable flow and effective aeration (> 6 mg/L DO), with no clogging. Initial leaks were corrected, and the turbidity of the digestate effectively suppressed algal growth, compared to water-only pretests and the control. Regarding the second objective, significant differences in plant performance were observed across treatments. The control solution consistently outperformed all digestate-based treatments in terms of plant height, leaf area, leaf number, and fresh weight (p ≤ 0.05). END and AD treatments performed similarly, while the ND treatment showed the weakest results across all metrics, though still close to the other digestate treatments. Partial nitrification was achieved in the END treatment, with a maximum conversion efficiency of 40%. These results suggest that anaerobic digestate, especially when partially nitrified and micronutrient-enriched, can support moderate lettuce growth in hydroponics, though it remains inferior to conventional mineral solutions. Further optimization of the nitrification process and nutrient formulation is recommended for practical application.