Alum sludge, as a byproduct of processing river water into drinking water, are often discarded and thrown into the river. This can cause the accumulation of aluminum in waters that can pollute the environment and harmful to health. Whereas in fact alum sludge can be used as auxiliary coagulant substance, which can reduce the turbidity of water by way of re-circulation it into the coagulation-flocculation unit. In an effort to run an environmentally friendly production processes and to improve effectiveness in reducing water turbidity, it indispensable to reduce the use of alum, which is a chemical-based coagulant, and mixed it with alum sludge. Hence, it is necessary to ascertain the optimal dose of the coagulant mixture and also the influence of other factors on water turbidity. This study aims to analyze the influence of coagulant dose (i.e., a mixture of alum and alum sludge), stirring speed and stirring time on the water turbidity, which can then be determined the optimal combination of these factors by using Response Surface Method.