•Biochar as an amendment for sewage sludge mixtures prior to vermicomposting.•Biochar as a source of micro and macronutrients to the earthworms.•Precomposted sewage sludge mixture amended with biochar as a growing medium for earthworms.•Biochar to support the growth and reproduction of earthworms.•Increased reproduction rate of the earthworms in the precomposted mixtures with higher biochar ratio. The obtained results showed that the addition of biochar to the mixture of sewage sludge and wheat straw prior to composting facilitated the growth and reproduction of Eisenia fetida during laboratory vermicomposting of this mixture. After 4 weeks of vermicomposting the average number of produced cocoons in the mixtures amended with biochar increased by 13% for SS+ST+4%B and 66% for SS+ST+8%B. Also, the number of juvenile earthworms was higher in the mixtures amended with biochar. However, the total weight of earthworms declined with the depletion of organic matter after 18 weeks of the experiment. Amending sewage sludge mixtures with biochar for vermicomposting resulted in higher reproduction rates, and thus could allow faster and more efficient conversion of sewage sludge into vermicompost.