A freeze wastewater treatment has several advantages: (I) wastewater, including toxic compounds which is difficult to be treated biologically call be treated; (2) the ice crystals produced can be used for cold heat storage; and (3) a smaller facility is required than that for biological wastewater treatment. However freeze wastewater treatment does not currently prevail in the world. This is because low-purity ice crystals are produced, resulting in poor wastewater treatment In this paper; conditions of producing an ice layer with high purity were investigated. An ice layer was formed on an aluminum plate heat exchanger. Model wastewater including polypepton with concentrations from 2000 ppm, to 10000 ppm corresponding to 670 ppm to 3800 ppm ill chemical oxygen demand (COD) was used. A seed ice layer was effective for producing an ice layer of higher purity. The greater the flow rare on the plate heat exchanger, the higher the purity of the ice layer involved, resulting in 10-30 ppm in COD at the superficial flow rate of 0.3 m/s regardless of the initial COD levels. The coolant temperature also affected little to produce an ice layer with high purity, whether the coolant temperature was 268 K or 263 K.