Synergistic enhancement of sewage contaminant removal and crop growth using biochar associated with microbial nutrient stimulants from sludge anaerobic digestate

Sludge anaerobic digestate, a nutrient-rich byproduct of anaerobic fermentation contains valuable bioactive compounds, however, its resource recovery is often constrained by elevated levels of heavy metals and other hazardous substances. This study demonstrates the efficacy of wood chip-derived biochar in recovering resources from anaerobic digestate. The porous structure and oxygen-containing functional groups of biochar facilitated efficient adsorption of microbial nutrient stimulants (MNS) from anaerobic digestate, with adsorption efficiency of 93.39% for free amino acids, 81.48% for phenolic compounds, and 76.85% for terpenoids. When applied to sewage treatment under carbon-limited conditions, the biochar-MNS system significantly enhanced pollutant removal, achieving efficiencies of 87.92% for NO₃⁻-N, 75.26% for NH₄⁺-N, and 63.99% for PO₄³⁻-P. Furthermore, in agricultural trials, the biochar-MNS boosted the germination rate of Chinese cabbage to 80% and stimulated cell elongation, as evidenced by a 46.2% increase in fresh weight over the control. Collectively, these results establish a novel circular economy paradigm for anaerobic digestate utilization that simultaneously overcomes limitations in wastewater treatment and advances sustainable agriculture. • Biochar adsorbs 93.39% of amino acids and 81.48% of phenolics from sludge digestate. • Biochar promote MNS adsorption through H bonding, π-π interactions and ligand bonds. • Biochar-MNS enhances NO₃⁻-N removal to 87.9% under carbon-limited conditions. • MNS may promote nitrogen conversion potentially through enhanced microbial activity. • Biochar can release auxins, organic and amino acids to stimulate seed germination.