Human population growth and urbanization are aggravating water quality problems in many regions, and wastewater volumes and quantities of pollutants are increasing due to greater industrial and urban activity. Thus, it is necessary to find efficient, sustainable and simple methods to separate miscellaneous impurities from wastewaters. One potential separation methods is freeze crystallization, because of its non-selective nature. However, previous research investigating freeze separation using real wastewaters has been rather marginal. This study examines natural freeze crystallization in purification of urban origin wastewaters, that is, municipal wastewater and landfill leachate of various organic and inorganic matter concentration. The effect of different freezing conditions on ice growth and separation efficiency in terms of ice impurity relative to initial solution impurity was investigated with a laboratory scale winter simulator. The results showed air flow velocity to have an almost as significant an influence on ice mass growth as air temperature. Although separation efficiencies decreased linearly with increased ice growth rates, no clear correlation was found between the impurity concentration of the wastewater and the ice mass growth rate. This finding notwithstanding, the separation efficiency of freeze crystallization of concentrated wastewater (landfill leachate) was noted to decrease more clearly with increased ice growth rate. Purification efficiencies of 95% to nearly 100%, determined by indicators such as chemical oxygen demand (COD), were achieved in treatment of municipal wastewater when using low ice growth rates. These findings indicate that the approach can meet future legislative requirements for treatment plants and that further research of the utilization of freezing techniques for wastewater purification is warranted.