Enhancing Digestate-Based Bioponics through Ceramsite Addition with and without Biochar: Effects on Water Quality, Nutrient Recovery, Heavy Metal Removal, and Microbial Community Composition

Digestate is a nutrient-rich substrate for bioponics, and ceramsite is a natural adsorbent for heavy metal (e.g., Cu and Zn) removal. This study primarily examined the potential of incorporating ceramsite with and without biochar into a digestate-based bioponic system for growing Green Oak lettuce. The amendment of digestate enhanced initial total nitrogen and phosphorus levels and organic matter while increasing the Cu and Zn levels in the bioponic water and plant biomass. The incorporation of ceramsite reduced organic matter and Cu and Zn levels in both water and plants, achieving higher efficiency when combined with biochar. The combination of ceramsite and biochar reduced Cu and Zn concentrations in bioponic water by 40.3% and 64.6%, respectively, compared to the digestate-only system, while ceramsite alone achieved 13.5% and 10.5% reductions, indicating a synergistic effect. Nitrogen and phosphorus recovery did not differ significantly among all conditions. Plant roots, digestate, ceramsite, and biochar harbored different microbial communities. In digestate-based bioponics, keystone microbes were identified in each medium, such as Bacillus (digestate), SH-PL14 (ceramsite), Prosthecobacter (biochar), and Ideonella (plant roots), indicating their key roles in organic degradation, heavy metal removal, and nutrient recovery. Microbial functional profiling using FAPROTAX revealed distinct roles, including aerobic ammonia oxidation in ceramsite and nitrate respiration and fermentation in biochar, supporting nitrogen cycling and organic degradation. Therefore, the use of ceramsite, with and without biochar, as an adsorbent offers a sustainable, nature-based solution for maintaining water quality and enhancing nutrient recovery in bioponics, leveraging digestate as a nutrient source.