In this study, we present the synthesis of ZnO nanowire by hydrothermal process through reutilization of sludge from soy sauce wastewater electrochemical treatment. The influences of floc content and caramel pigment concentration on the morphologies of ZnO were studied. The products were characterized by XRD and SEM. The results showed that ZnO powder was of hexagonal wurtzite structure and well crystallized with high purity. Floc content and caramel pigment concentration played an important role in the final morphologies of ZnO nanowires. On the basis of these results, a possible growth mechanism for the formation of ZnO nanowires was described. These findings not only may contribute to control the synthesis of ZnO, but also present a novel method for the disposal of wastewater treatment sludge.