Based on the hypothesis of series multistage continuous stirred reactor（CSTR）

rodamine B tracing experiments were carried out in the biocontact oxidation tank of Dongshen raw water biopretreatment engineering

the largest engineering in the world

in order to get the fitting curve of the outlet concentration coefficient and mixed distance of CSTR stages using Mathlab

and then calculate series number of the reactor. Results show that the length of a singlestage CSTR is 13 m. The whole tank series number of CSTR with a length of 270 m is 21. It is incorrect that the biotank is considered simply as a plug flow reactor. About 71.8% water exchange occurs every 4.5 m at the aeration tube layer. The short flow does not exist in a singlestage CSTR.