A low-cost activated carbon derived from lignin-based black liquor was prepared and has been applied as an adsorbent for tripolyphosphate ion. Lignin-based black liquor was coagulated using polyaluminum chloride (PAC), producing black liquor sludge that was used as the precursor for preparing the activated carbon. Black liquor sludge-based activated carbon (BLS-AC) was treated by calcination at 725 °C with inert N2 gas flow and then continued by chemical treatment using 0.5 N hydrochloric acid in order to remove the impurities and to increase its surface area. BLS-AC was characterized physically and its ability to adsorb tripolyphosphate ion was evaluated using batch system with varied contact time. The characterization using FTIR reveals that BLS-AC has some functional groups that were typical with commercial activated carbon, such as C-H, C=C and C-O. The BET method analysis indicates that the BLS-AC is a mesoporous material with pore size of 19.74 nm and surface area of 302.498 m2/g. Based on the EDX analysis, it is known that BLS- AC dominantly contains 34.22 % carbon, 18.35 % aluminium, and 45.67 % oxygen. The XRD analysis exhibits that BLS- AC has amorphous phase. The experimental results demonstrate that BLS-AC was quite effective to adsorb tripolyphosphate ion with adsorption efficiency of 90.79 % and adsorption capacity of 0.39 mg/g within 60 minutes. This study emphasizes the byproduct resulted from black liquor sludge which can be utilized to reduce phosphate concentration in wastewater.