PURPOSE: To maintain the residual chlorine near the inlet of a settling basin at correct values by settling the injection rate of chlorine per unit time in a purification plant by the product of the sum of a feed forward injection rate and a feedback injection rate and the flow rate of treating water.

CONSTITUTION: A water temp. detector 20 is inserted in a trough 3 to measure the temp. of the raw water. At the same time, the raw water is sampled with a sampling pump 21, and the concn. of ammonium in the raw water is measured by using an ammonium concn. meter 22. A feed forward injection rate is determined from these measured values and the target value of residual chlorine concn. and a feedback injection rate is determined from the difference between the target value of the residual chlorine concn. and the actually measured value of the residual chlorine concn. The injection rate of chlorine per unit time is set by the product of the sum of the feed forward injection rate and the feedback injection rate and the flow rate of the treating water.