Conventional water treatment plant (WTP) typically involves coagulation-flocculation processes to remove suspended particles and colloids in raw water. The process generates large volume of alum sludge with high aluminum content, which is discharged into a river with improper treatment. The sludge may cause potential risk to human health, and disrupt river biota’s life. Aims of this study were to determine the physical and chemical characteristics of alum sludge from Surabaya WTP, and to compare with those of other Indonesian’s and developing countries’. The alum sludge sample was obtained from Surabaya WTP in Indonesia. The results showed that the alum sludge had a pH value of 7.47, volatile solids of 12,696 mg/L, total suspended solids of 12,511 mg/L, Chemical Oxygen Demand (COD) 9666.7 mg/L, Biochemical Oxygen Demand (BOD) 1082.5 mg/L, and sludge volume index 114.18 mL/g. The sludge had high aluminum and iron concentrations. Aluminum content of the sludge was 1194 mg/L, iron 515 mg/L, chromium 0.217 mg/L, and copper 0.559 mg/L. Having a BOD/COD ratio of 0.1, the alum sludge contained high level of nonbiodegradable organic matter.