Evaluation of the image analysis method using statistics for determining the floc size and settling velocity in ballasted flocculation

Abstract

The global climate changes lead to more severe floods and droughts recently. Since the increase of non-point water contaminants, it is highly required the technology for the treatment of the high turbidity and the increased water volume in the water such as ballasted flocculation. The purpose of this study is to determine the floc size and settling velocity among the properties of the ballasted flocculation process. The appropriate number of floc samples which can be reliable in the use of the image analysis method was determined by comparing the statistical margin of error with the maximum experimental and measurement error. It was confirmed that a certain degree of reliability can be obtained at a confidence level of 95% when 40 samples are measured whereas it is identified that when the confidence level is raised to 99%, the number of samples must be 80 or more. On the other hand, it was confirmed that even if the number of samples is changed through the normal distribution density function curve using the average value calculated according to the number of samples, there was not much difference in the average value and standard deviation. In the ballast flocculation experiments of 25 and 120 mu m using the number of samples set based on the reliability analysis, the floc size, and the settling velocity increased with the increase of the injection amount of the 25 mu m ballast. In the case of the 120 mu m ballast, the floc size did not increase significantly, but the average settling velocity increased as much as that of 25 mu m ballast. This result implies the probability of the change of floc density.