Ultrasound-assisted recovery of humic substances from municipal digestate using response surface methodology

Anaerobic digestion (AD) is widely employed for sewage sludge stabilization, producing substantial volumes of digestate that require further management. This study explores the recovery of high-quality humic substances (HS), specifically humic acids (HA) and fulvic acids (FA), from anaerobically digested sewage sludge (DSS) using ultrasound-assisted extraction (UAE). Implementing this step prior to downstream digestate management positions DSS as a low-cost, sustainable feedstock to produce high-value agronomic biostimulants and soil conditioners. Response surface methodology (RSM) was applied to optimize the UAE parameters, namely ultrasound amplitude (20-100%) and extraction time (5-20 min), to maximise HA yield. A second-order polynomial model demonstrated a strong fit to the experimental data (R² ≥ 0.90), achieving HA and FA yields of 424.67 ± 0.03mg g^-1 and 419.04 ± 0.01 mg g^-1 (dry basis), respectively. Under optimized conditions (97% amplitude, 8.4 min, 0.1 M KOH, 1:40 v/w), HA yields increased by 109% compared to conventional extraction (CE: 200 rpm, 16 h), with a concurrent 85.8% reduction in energy consumption (CE: 0.281 kWh vs. UAE: 0.03 - 0.04 kWh). Elemental profiling highlighted the superior quality of UAE-HAs, characterised by significant carbon intensification (44.64% vs 37.9%), nitrogen recovery (6.69% vs 0.74%) and sulphur enrichment (1.95% vs 0.07%) relative to the commercial standard. Spectrometric analysis further corroborated these findings, revealing greater functional group diversity in DSS-derived samples, positioning them as superior candidates for agricultural applications. These findings demonstrate the potential of UAE as an efficient and economical technology for HS recovery, advancing the circularity of digestate-based biorefineries by transforming urban organic waste streams into value-added agronomic resources.