Method and apparatus for determination of coagulant injection rate in water treatment process

A method of determining the ratio of the amount of coagulant to be injected to the amount of water to be treated (coagulant injection rate) in a process of water treatment through coagulant injection to water to be treated followed by coagulation and sedimentation, which comprises the following steps:

• (1) a step of collecting a predetermined amount of the water to be treated in one sample tank, then injecting a certain amount, which is predetermined on the basis of the quality of the water to be treated, of a coagulant to the water sample collected in the sample tank, and after the coagulant injection thereto, mixing it to thereby disperse the coagulant therein, and determining the time taken before the start of agglomeration of the particles in the water sample (agglomeration start time);
• (2) when the difference between the measured value of the agglomeration start time as determined in the above and the reasonable value of the agglomeration start time as previously set in accordance with a water treatment plant falls within a predetermined range, a step of determining the coagulant injection rate corresponding to the predetermined amount of the coagulant in the above step (1), as the reasonable coagulant injection rate;
• but when the above difference is larger than the predetermined range, a step of computing the reasonable coagulant injection rate according to the following process of (21) to (23):
• (21) the correlation between the agglomeration start time and the coagulant injection rate is defined by a general formula including two constants, then based on the database previously obtained through experiments, one of those constants is defined based on the quality of the water to be treated, and the general formula including one constant is found;
• (22) based on the general formula including one constant and on the measured value of the agglomeration start time and the coagulant injection rate at that time, the other constant is found, whereby the computational formula for the correlation between the agglomeration start time and the coagulant injection rate is specifically defined;
• (23) based on the specifically-defined computational formula and the reasonable value of the agglomeration start time as previously set in accordance with the water treatment plant, the reasonable coagulant injection rate is computed.