Simultaneous reductions in antibiotics and heavy metal pollution during manure composting

The co-existence of antibiotics and heavy metal (HM) is common in manure. However, existing strategies for improving antibiotic dissipation or HM immobilization during composting rarely consider their combined pollution. In this study, we used agricultural lime and a newly designed attapulgite-activated carbon composite (AACC) to enhance the stabilization of HMs in a pilot-scale swine manure composting system and assessed the effectiveness of these materials for removing antibiotic residues. Results indicated that the application of either lime or AACC simultaneously enhanced HM immobilization and antibiotic degradation. In particular, the addition of AACC reduced the enrichment of Cr, Cd, Pb, and As during composting and decreased the half-lives of the antibiotics from 10.7 days to 6.3 days, which were more effectively than lime. The physicochemical and microbiological responses to different additives were subsequently studied to understand the mechanisms underlying the fates of HMs and antibiotics. High HM stress in manure inhibited antibiotic dissipation, but metal immobilization alleviated this effect. The AACC accelerated HM immobilization by surface adsorption and metal precipitation, and this enhancement strengthened during the late composting stage due to an increase in pH, whereas lime exhibited a short-term effect. Moreover, the AACC addition enhanced the contribution of bacteria to changes in antibiotic concentrations, while the increase in pile temperature could be a major factor that contributed to the acceleration of antibiotic degradation after the addition of lime. Characterization of the final compost further showed that AACC-treated compost had the lowest residual concentrations of HMs and antibiotics, higher mortality of ascarid egg, improved nitrogen conversation, and reduced phytotoxicity. Thus, co-composting of swine manure with AACC is a promising approach for producing safer compost for use in agriculture.