Degradation of antibiotics in activated sludge by ionizing radiation: Effect of adsorption affinity of antibiotics

•Degradation of four antibiotics in sludge was investigated by ionizing radiation.•Degradation of antibiotics in sludge needs higher doses than in aqueous solution.•The higher adsorption ability of antibiotic to sludge, the lower removal rate.•Removal of SMX/PEG, NOF and OTC in sludge was more than 97 %, 40–52 % at 10 kGy.•Lower removal efficiency of OTC and NOF was due to higher affinity to sludge.