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.

Dissipation of Three Veterinary Antimicrobials in Beef Cattle Feedlot Manure Stockpiled over Winter

Dissipation of veterinary antimicrobials is known to occur during aerated windrow composting of beef cattle manure. However, it is unclear if a similar dissipation occurs during stockpiling. Chlortetracycline, tylosin, and sulfamethazine are three of the most commonly used veterinary antimicrobials in beef cattle production in western Canada. Their dissipation in stockpiled manure was investigated over 140 d during winter in Alberta, Canada. Beef cattle housed in pens were administered 44 mg of chlortetracycline kg feed (dry weight), 44 mg of chlortetracycline + 44 mg sulfamethazine kg feed, 11 mg of tylosin kg feed, or feed without antimicrobials (control). Manure samples were extracted using pressurized liquid extraction, and the extracts were analyzed for chlortetracycline, sulfamethazine, and tylosin by LC-MS-MS. Dissipation of all three antimicrobials in the manure was explained by exponential decay kinetics. Times for 50% dissipation (DT) were 1.8 ± 0.1 d for chlortetracycline alone or 6.0 ± 0.8 d when mixed with sulfamethazine, 20.8 ± 3.8 d for sulfamethazine, and 4.7 ± 1.2 d for tylosin. After 77 d, <1% of initial chlortetracycline and <2% of sulfamethazine remained. Tylosin residues were more variable, decreasing to approximately 12% of initial levels after 28 d, with 20% present after 77 d and 13% after 140 d. Temperatures within stockpiles reached maximum values within 6 d of establishment and varied with location (bottom, 62.5°C; middle, 63.8°C; and top, 42.9°C). Antimicrobials in the manure did not inhibit microbial activity, as indicated by temperature and mass losses of carbon (C) and nitrogen (N). The C/N ratio in the manure decreased over the stockpiling period, indicating decomposition of manure to a more stable state. Dissipation of excreted residues with DT values 1.8 to 20.8 d showed that stockpiling can be as effective as windrow composting in mitigating the transfer of these three veterinary antimicrobials into the environment during land application of processed manure.