In developing countries, most of the daily produced household wastes are buried in landfill sites without any prior treatment. One of the many problems associated with these landfills is the formation of leachate. Because of its complexity and unpredictable composition, leachate has always constituted a challenging task for effluent managers. Despite the countless techniques that have been developed since then, leachate treatment still raises big concerns about the efficiency and sustainability of a suitable strategy. Accordingly, this study is carried out aiming at the optimization of leachate treatment through biostimulation using carob powder as a co-substrate. The added amount of biostimulant corresponds approximately to the addition of 3, 1, and 0.4 g/L of sucrose, fructose, and glucose, respectively. The adopted experimental system is based on the anaerobic process using an up-flow anaerobic sludge blanket (UASB) reactor, which was operated at HRT and OLR of 24 h and 26.1 kg COD/m³ d, respectively. For carob particle size larger than 2 mm, only a small amount of biogas was produced and the COD removal efficiency was lower than 77%, indicating that the poor quality of the effluent persists. However, 97% of COD was efficiently removed and a volume of 2.06 L/L_leachate of biogas was produced using a carob particle size of 0.250 mm. The obtained results showed also that the amount of produced biogas increases as the carob sugar content increases. Literally, a 1% fraction of total sugar content resulted in an increase of 17.9 mL/L_leahate of the produced biogas. Finally, the use of 10 g/L of carob amendment improved COD removal and exhibited less influence on syntrophs and methanogens since a large amount of biogas was generated.