Finite energy resources and their rapidly waning imprint necessitate a sustainable wastewater treatment method. Nature could be exploited to freeze wastewater in locations which experience subzero temperatures during winter. The two most vital components that influence the efficiency of natural freezing are the ambient temperature and air velocity. The turbulent and unsteady air-cooled natural freezing is simulated for ice crystallization from 0.1 wt % and 1 wt % NiSO4 (aq) solutions. The efficiency of natural freezing is tested for different air velocities (2 ms(-1), 5 ms(-1)) and levels of undercooling (T=0.5 degrees C, 1 degrees C) from the freezing temperature of the corresponding solution. The airflow in the winter simulator is modeled by computational fluid dynamics to investigate its behavior and to assess its effect on freezing. (c) 2017 American Institute of Chemical Engineers AIChE J, 63: 200-208, 2018