Thermophilic pig manure compost as an additive for sewage sludge composting: humification efficiency and changes in heavy metals, antibiotics, and antibiotic resistance genes.

Thermophilic livestock manure compost harbors microbes with vigorous metabolic activity, making it a promising natural inoculant for composting systems. However, there has been no documented application of thermophilic livestock manure compost as an additive in sewage sludge composting. The impacts of this strategy on shifts in heavy metal speciation and the removal of antibiotics and antibiotic resistance genes (ARGs) during sludge composting remain poorly understood. In this study, thermophilic pig manure compost materials (TM) were used as additives and incorporated into sewage sludge composting to evaluate its effects and underlying mechanisms on humification efficiency, heavy metal immobilization, and the removal of antibiotics and ARGs. Results indicate that TM significantly improved organic matter degradation rate, germination index, and humification index by 26.9%, 18.9%, and 35.8%, respectively, while reduced the composting maturation period by at least five days. TM significantly decreased both the concentrations and exchangeable fractions of Zn, Pb, As, and Ni in sludge compost, as well as their accumulation in corn seedlings (P < 0.05), by modulating key environmental factors (moisture content, carbon-to-nitrogen ratio, and pH) and influencing the microbial community structure. TM facilitated the removal of tetracycline, sulfonamide, and quinolone antibiotics, and reduced the prevalence of ermA, ermF, sul1, sul2, tetM, and tetX by decreasing the abundance of ARGs-hosting microorganisms (e.g., Pseudomonas, Streptomyces, Flavobacterium, Bordetella, Escherichia, and Klebsiella) and mobile genetic elements (e.g., intl1, intI2, and Tn916/1545). These results offer a scientific basis for the resource recovery and utilization of sewage sludge.