Abstract: A method including: introducing a first chemical to a first passage of a mixing apparatus attached to the process liquid flow pipe; introducing an injection liquid into a second passage of the mixing apparatus; introducing to a third passage of the mixing apparatus a second chemical having an electrical charge opposite to the electric charge of the first chemical; injecting the first chemical into the injection liquid by passing the first chemical through a passage between the first passage and the second passage; introducing the mixture of the first chemical and the injection liquid to a process liquid flowing through the process liquid flow pipe, and introducing the second chemical from the third passage to the process liquid in the process liquid flow pipe, wherein the second chemical passes from the third passage to the process liquid a distance from the injection of the first chemical into the injection liquid.