Conventional coagulation or enhanced coagulation water treatment produced mass floc sludge, which contained plenty of metals and organic matter. Floc sludge disposal and resource recycling was vital relevance and urgently needed. Here, we reported on the waste flocs recycling through in-situ manganese-aluminum-iron biochar (MAFBC) synthesized via one-step pyrolysis using potassium permanganate (KMnO4) oxidation- aluminum-iron composite flocculant coagulation for humic acid removal floc sludge as raw material. Compared with ex-situ loaded or similar types of biochar, MAFBC showed faster and efficient activation effect of peroxymonosulfate (PMS) to degrade tetracycline (TC). TC was rapidly removed within 1 min in MAFBC/PMS system with degradation rate constant Kobs was 1.825 min−1, which mainly attributed to the multiple promoting effects of manganese-aluminum-iron. Aluminum oxides facilitated TC adsorption and synergistic Fe/Mn further significantly promoted electron transfer on the surface of the MAFBC. Additionally, bimetallic structure of Fe/Mn and the CO functional group on MAFBC accelerated the formation of reactive species. The redox cycles of Fe2+/Fe3+ and Mn2+/Mn3+/Mn4+ promoted the formation of free radicals OH and SO4−. Moreover, O2– and 1O2 also assumed an important role for TC removal during active species evolution drived by Fe/Mn. This research provides a new perspective for in-situ design of economical and stable metal carbon-based water treatment activator, and make contributions to the utilization of waste flocs in water treatment.