For fear that conditions for an electrodialysis in seawater concentration will be worsened in future due to reduction in production cost, the author studied a method to determine the probability of scale deposition under anticipated worse conditions from scale deposition under present conditions. For this study, the amount of solute ions removed when they passed through a desalting compartment (the desalting range) was assumed to be a normal distribution, and the ratio of the standard deviation to the desalting range a, was used as an indicator of the distribution character in the unit.
The probability of scale deposition was determined by comparing the distribution with limited desalting ratio for scale deposition which the author had previously reported. Data of the scale deposition obtained with an experimental apparatus of 5 dm² in the effective area well agree with the author's calculated values, which proved to be applicable to practical use. The probability of the scale deposition under larger desalting ratio (i. e. utilization ratio of sea water) was calculated from observed values for a practical scale apparatus.
As a result, the probability of CaCO₃ scale deposition showed a rise from 0.4% to nearly 17%, when the desalting ratio increased from 23% to 40%. When the desalting was divided into two steps to avoid scale deposition, there were optimum values in the allotment of the desalting range to the first and second steps.