2.1. Study area and problem encountered

The Surabaya River is located in Surabaya City, Indonesia. The river is a source of raw water for two water treatment plants (WTPs) in Surabaya City. It is downstream of the Brantas River, and residential and industrial activities occur along the river. The pollutant load contained in the river is thus large, and fluctuates every day. At present, Surabaya WTP treats wastewater using physical and chemical processes that consist of three stages: sedimentation, coagulation and flocculation, and filtration including disinfection. The processed water from this WTP does not meet drinking water quality standards, especially for biological parameters.