The present invention provides a system for sludge treatment which transfers sludge into a dewatering device while maintaining the sludge density at a predetermined level constant suitable for dewatering the sludge even when the density of the precipitated sludge varies, thus improving the sludge treating efficiency and sludge dewatering efficiency, and reducing the amount of a coagulant to be used. In the system for sludge treatment, a sludge concentration device is installed in a mixing flocculation tank. Thus, the sludge is mixed with the coagulant and flocculated in the mixing flocculation tank to produce flocs and a supernatant, while the supernatant is directly discharged from the mixing flocculation tank. Furthermore, the amount of the supernatant is controlled to maintain the desired density of the flocs to be transferred to the dewatering device. Thus, the dewatering device can be stably operated.