Effect of seed ice on formation of tube ice with high purity for a freeze wastewater treatment system with a bubble-flow circulator

A freeze wastewater treatment system with a bubble-flow circulator is advantageous from an industrial point of view because a bubble-how circulation requires much less energy for the mixing of the wastewater to facilitate heat, mass and momentum transfers in the system than that using agitators or pumping circulation. In this work an ice tube was produced in a bubble-flow circulator and the effect of seed ice crystals on tube ice formation with a high purity was investigated. Without seed ice crystals a large supercooling appears before ice crystal formation; and dendritic ice;crystals are suddenly formed. Components in wastewater are contaminated and trapped between dendritic ice crystals, resulting in poor quality in the wastewater treatment. To avoid abrupt ice crystallization it is necessary to keep supercooling small. A seed ice crystal is effective for this purpose because a seed ice crystal generates other ice crystals, resulting in no supercooling. Model wastewater including polypeptone with concentrations of 5000 and 10000 ppm, corresponding to 1290 to 3800 ppm in COD (chemical oxygen demand) was used. Two kinds of experiments with different types of seed ice addition were examined. In each concentration examined more than 30% COD normalized by initial COD was confirmed in the melted ice tube with no seed ice. On the other hand, only a few % of COD of the initial values were detected when seed ice crystals were used to initiate the ice tube formation. (C) 1999 Elsevier Science Ltd. All rights reserved.