Biochar amendment aids in the reduction of antibiotic-resistant bacteria and heavy metals during composting of poultry litter

Poultry litter is a complex mixture comprising heavy metals feed scraps, antibiotics, water, feathers. Although treatment of these wastes has not currently received much attention in Nigeria, poultry litter is widely used as source of organic fertilizer for subsistence farming by rural and urban areas of the country. It is essential to effectively stabilize the organic waste because the continuous use of the poultry manure in agricultural activities poses great threat to the health of the soil environment due to heavy metals and antibiotic-resistant pathogens. Composting is an effective method for stabilizing organic manure toward reducing the heavy metals content. Hence, the present study determined the effect of biochar additives on heavy metals and antibiotic-resistant bacteria removal during composting of poultry litter (PL), vegetable waste (VW), and corn stalk (CS). To this end, three composting trials were set-up comprising control: PL (57.58%) + CS (38.41%) + VW (4.01%), TX: control + 5% biochar, and TY: control + 10% biochar. The composting pile was also spiked with heavy metals (Pb, Cu, Zn, Cr, Cd, and As) and their removal efficiencies were monitored. To enumerate the antibiotic-resistant bacteria in the compost, Nutrient agar was supplemented with antibiotics such as ampicillin, chloramphenicol, amoxicillin, ciprofloxacin, gentamycin, and tetracycline. The outcome of the investigation showed that the highest reduction in total coliform from 3.68 to 1.06 log10 CFU g−1 was observed in TY. The addition of 10% biochar in TY also reduced the concentration of antibiotic-resistant bacteria in all the treatments to an undetectable limit compared to TX (5% biochar) and control (no biochar). The biochar-aided heavy metals removal followed a first-order path. With the exception of Cd removal where TX showed the best removal rate (k = 0.1582), the TY resulted in best removal rates for Pb, Zn, Cu, Cr, and As. The decreasing order of heavy metal removal rates from TY is as follows: Cu (k = 0.2025) > Cr (k = 0.1403) > Pb (k = 0.1300) > Zn (k = 0.1290), > As (k = 0.090). Therefore, biochar additives during composting of poultry litter should be encouraged amongst the rural farmers.