Untreated alum sludge from Surabaya water treatment plant (WTP), which contained high concentration of alum was directly discharged into Surabaya River. It might caused problems because of the accumulation of aluminum in the lower part of the river. Alum could be recovered from the drinking water sludge using electrolysis method. Aims of this study are to determine the optimum pH and electrical current for electrolysis using carbon-silver electrodes to recover aluminum coagulant from the sludge, and to determine the amount of the recovered alum. The sludge was acidified prior to electrolysis. Acidification was done by adding sulfuric acid at pH 3 and 4. Polarization test was conducted at 100, 200, and 300 mA, in order to determine optimum electrical current. The electrolysis was performed in one compartment batch recirculation reactor, using silver as cathode and carbon as anode for 10 hours. pH values were measured every hour. Precipitated matter in the cathode was weighed, and analysed by Inductively Coupled Plasma. The optimum condition of the electrolysis was achieved at initial pH 3 and electrical current 300 mA. The electrolysis resulted in highest precipitate of 2.6112 g in the cathode.