PURPOSE: To stably remove phosphorus in waste water by injecting a soln. contg. iron ion eluted by an iron ion generator into a aerobic reaction tank, contact aeration tank and batch tank respectively as a waste water treating tank.

CONSTITUTION: When the process is conducted in the batch tank 1 of a batch- operated waste water treating device, an iron ion generator 2 is fixed to the upper part of the side wall of the batch tank 1, and a circulating pump 3 is fixed to the batch tank 1. A part of the waste water is supplied to the iron ion generator 2 through a pipeline 4, and the waste water contg. iron ion is supplied to the batch tank 1 from the iron ion generator 2 through a pipeline 5. A supernatant water discharge device 9 and a treated water pipe 8 are provided to the batch tank 1, a pipeline 6 is furnished to receive raw waste water into the batch tank 1, and air is blown off from a diffuser 11 to supply necessary oxygen for aeration. A DC current is applied between the anode and cathode made of iron in the iron ion generator 2 to bring about electrolysis, and an active iron ion is eluted into the water.