To reduce the waste water aeration load of an aeration section by providing the system with a waste water treatment equipment having an aeration section into which waste water flows and a sludge treatment equipment having a dehydrator with which sludge is dehydrated and carried outward and feeding dehydrated cakes carried out of the dehydrator into the aeration section.

A water treatment system is constituted of a waste water treatment equipment 1 with an aeration section and a sludge treatment equipment 2 with a dehydrator 7. Mixed sludge fed into the dehydrator 7 together with cleaning water is dehydrated and carried out of a cake hopper 7a. A part of dehydrator cakes from a cake hopper 7a is carried outward and reused. At that time, the dehydrated cakes from the dehydrator 7 are fed into the aeration tank. The dehydrated cakes of high degree of nitrification are provided with superior adsorptivity for absorbing the oil content or the like. When the dehydrated cakes of the nature mentioned above are fed into the dehydration tank, the activation of waste water is accelerated to reduce the waste water aeration load in the aeration tank and also improve the purifying capability of the aeration tank.