Objectives Biogas is an important and effective way in deal with the waste from intensive animal farming in China, however, the recycling use of biogas residue has become a bottleneck of restricting the development of biogas projects. This paper attempted to use the biogas residue to make safe compost.

Methods The tested biogas residue was from a chicken farm, the biochar was made of fruit wood through pyrolysis at 550℃ for 2 h. The biogas residue was the main raw material, the pig manure and maize straw of 1 cm long were used to adjust the C/N to 25∶1, the moisture contents was controlled at 65%−70% during the composting. Biochar was added in rate of 0, 2%, 5% and 10% of the fermentation materials (dry-base ratio), and recorded as CK, F1, F2, and F3, respectively. The composting lasted 30 days at room temperature. The physical and chemical properties and the quality of composts were determined during the aerobic composting process.

Results During the composting process, the highest temperature in all the treatments reached above 55.0℃, and the high temperature lasted for 6 days in F1 and F2, which met the harmless requirements. The variation of pH and EC was the same, all showed the trend of increasing first and then decreasing, and finally tending to be flat. The pH values in all the four treatments were in range from 8.55 to 8.80, and the pH of F2 treatment was always higher than those of the other three groups (>8.7) with the maximum pH of 9.03 during the heating period. The electrical conductivity of all treatments was below 1 mS/cm. Compared with CK, the organic matter contents in F1, F2 and F3 were respectively reduced by 13.0%, 9.3% and 7.4%, nevertheless, they were all higher than 45%; the total nutrient contents were increased by 6.5%, 4.3% and 2.2%; the seed germination indices were all higher than 85%. The addition of biochar could increase the number of microbes, but the effects on bacteria, fungi and actinomycetes were different. With the increase of biochar addition proportion, the number of bacteria were reduced, the number of actinomycetes showed a concave trend and the greatest inhibitory effect was in F2, and the number of fungi was increased. The humus contents in composts of all treatments decreased first and then increased, showing the "V" shape, reaching the lowest value on the 11th day. The humic acid content in F1 treatment was higher than those in the other three groups, the highest value was 24.08%, the lowest was 17.92%. Compared with CK, the humus contents of the final product were increased by 8.12%, 7.23% and 7.43% in F1, F2 and F3 treatments, respectively.

Conclusions The addition of biochar could prolong the high temperature duration, change the physical and chemical properties of biogas residue compost, promote the maturity and improve the total nutrient contents. Among the three addition proportions tested, the addition of 2% biochar (dry basis ratio) performs the best for the composting process.