Pretreatment of wastewater by freezing under vacuum in a cool-thermal storage process was investigated. Analyses of vacuum freezing and zone melting were made for the estimation of the ice thickness and the solute distribution of ice crystals from which optimum solidification thickness and operating time were determined. It was found that ice sublimation rate played an important role in determining the efficiency and the pay-off of the process. (C) 2002 Elsevier Science B.V. All rights reserved.