Antibiotic mycelial dreg is a hazardous pollutant that is difficult to deal with because of its high contents of moisture and biomass pectin. Our previous work has demonstrated that the hydrothermal treatment (HTT) provides a feasible way to produce safe solid biofuel by reducing its moisture and nitrogen contents and decomposing its residual antibiotics. This study is devoted to further investigating the HTT through characterizing the HTT produced solid biofuels using oxygen bomb calorimeter, XPS, FTIR, XRF, TGA, DTA and so on. The results showed that the higher heating value (HHV) of the solid biofuel can reach about 26.5kI/g, which was much higher than the HHV of dried raw antibiotic mycelial dreg made by conventional drying (only 19.3 kJ/g). Moreover, the N content was lowered from 7.7 wt.% in the dried raw antibiotic mycelial dreg to 5.6 wt.% in the 200 degrees C HTT produced solid biofuel, while most alkaline metals and chlorine in ash of antibiotic mycelial dreg were leached out with centrifugation after HTT. The atomic ratios of H/C and O/C in the solid biofuels both decreased with increasing the temperature of HTT, indicating the occurrence of dehydration, decarboxylation and hydrolysis of solid matters in the dreg. The HTT also greatly improved the combustion reactivity of the solid matters in the antibiotic mycelial dreg. All of these show that HTT provides indeed an effective way to convert the antibiotic mycelial dreg into high-quality solid biofuel. (C) 2015 Elsevier B.V. All rights reserved.