On a whole results of studies of co-composting of biochar and organic residues suggest a positive role of biochar on both the performance and the quality of the end product of the process but, to date, such trials have been performed only at small or medium scale. Therefore, before utilizing biochar in the composting process, an exhaustive assessment of biochar impact on process performance at full-scale is required. The main aim of this work was to investigate the effects of adding biochar to compost either at the beginning or at the end of the composting process. As far as we know, this is the first study (1) reporting the effect of biochar addition on the composting process in full scale and (2) comparing the effect of adding biochar before or after composting on P fertilizer replacement value of the produced composts