Digestate reuse as an intermittent bio-stimulant for municipal sludge anaerobic digestion: full-scale evidence, biochemical methane potential assessment, and semi-continuous validation

Digestate reuse has been proposed as a strategy for enhancing anaerobic digestion (AD) performance; however, full-scale validation remains limited. This study evaluated digestate reuse for municipal sludge treatment through full-scale monitoring, biochemical methane potential (BMP) assays, semi-continuous continuous stirred-tank reactor (CSTR) experiments, and economic assessment. Operational data from two wastewater treatment plants revealed substantial differences in baseline digestion performance. BMP results showed that digestate reuse increased methane production by up to 28% and improved solids degradation in underperforming sludge, whereas limited benefits were observed in a well-performing system. Semi-continuous CSTR experiments demonstrated that intermittent digestate dosing at approximately 10% (v/v) improved total solids (TS) and volatile solids (VS) removal without compromising process stability. The enhancement effect persisted for 10-20 days following a single addition, suggesting that digestate acted primarily as a transient bio-stimulant rather than a conventional carbon source. Full-scale evaluation indicated that digestate reuse could reduce sludge disposal demand and generate net cost savings of approximately NT$100,000 (∼US$3,125) per month. These findings demonstrate that intermittent digestate reuse is a practical strategy for improving AD performance while supporting resource recovery and circular economy implementation.