To provide a system for treating waste water, in which waste water is treated stably while reducing the cost required to treat waste water without separately doing the work of recovering residual sludge mainly containing sand, and in addition, an excellent sludge treatment environment can be created.

The system for treating waste water is characterized in that stored waste water 6 is pumped up from a temporary waste water storage tank 2 by a waste water pumping-up pump 5 together with sludge so that the sludge is not accumulated/retained in the temporary waste water storage tank, the pumped-up waste water is supplied to a sludge separation apparatus 15 to obtain sludge-separated raw water and the separated sludge is recovered in a sludge recovery tank 19 before the raw water is sent to a raw water tank 9. As a result, it is not necessary to recover the accumulated/retained sludge in vacuum by using the conventional system for recovering precipitated sludge.

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