Valorisation of spent activated carbon for accelerated compost stabilisation and enrichment of plant growth-promoting bacterial taxa

The sustainable management of spent activated carbon (SAC) from water treatment remains an environmental challenge because reuse options are limited and disposal still relies on landfilling. This study evaluated SAC as a carbonaceous amendment during co-composting of chicken manure and spent mushroom substrate at 0% (Control), 10% (T1), 20% (T2), and 30% (T3) addition levels. SAC addition shortened the thermophilic phase from 12 days in the Control to 9 days in T2 and T3, and SAC-amended treatments reached operational stabilisation within 30-42 days, as indicated by temperature decline, lower late-stage gas emissions, and Solvita CO₂/NH₃ indices. Increasing SAC dosage raised total organic carbon from 37.94% to 54.12% and compost yield from 41.94% to 54.70%, whereas total nitrogen concentration decreased from 2.28% to 1.34% because of dilution by recalcitrant carbon. Lower NH₃ emissions in the SAC-amended treatments were consistent with reduced volatilisation, although nitrogen retention was not quantified by a mass balance. All SAC-amended treatments achieved germination indices above 100% (111.48-129.87%), indicating the absence of phytotoxicity in the assay. High-throughput 16 S rRNA gene sequencing showed treatment- and stage-dependent shifts in bacterial community structure. At Day 60, T1 recorded the highest observed richness (197 OTUs) and Faith PD (23.14), whereas T2 had the highest combined relative abundance (22.47%) of bacterial taxa previously associated with plant growth-promoting functions, including members of Rhizobiaceae and the genera Mesorhizobium and Ensifer. Because Shannon diversity in the SAC-amended treatments did not exceed that of the Control at maturity, the results indicate selective community restructuring and enrichment rather than a uniform increase in bacterial diversity. Overall, moderate SAC addition accelerated compost stabilisation and promoted favourable bacterial succession, supporting a circular pathway for converting multiple waste streams into biologically enriched compost.