Separation efficiencies of freeze crystallization in wastewater purification

Abstract

In a changing world, it is of paramount importance to ensure the sufficiency of clean water for all. Wastewater purification will face unprecedented challenges as a result of growing human population and accelerating industrialisation. Thus, novel methods and applications are needed to manage multifarious pollutants and increasing wastewater volumes. The new generation of the wastewater treatment should focus on developing techniques that allow efficient water and material reclamation for recycling and reuse.

The freeze crystallization method possesses the main features required for sustainable wastewater purification, i.e. the potential for the simultaneous separation of water (ice) and material, and the high separation efficiency of impurities of all kinds – energy efficiently without added chemicals. Freeze crystallization has previously been the subject of rather extensive research in various fields of application, i.e. the food industry, desalination, and wastewater purification. However, much of the research has been done on a laboratory scale and using model solutions, as the pioneering research interests have focused on the phenomenon of freeze separation and the theoretical models to explain them. Thus, real wastewaters have rarely been used for research, and the overall impurity removal efficiencies of freeze purification are still incompletely known.

The research of this thesis aimed to demonstrate the purification efficiencies of freeze crystallization in wastewater treatment by appraising the freezing conditions and the wastewater characteristics affecting the process performance. Freeze crystallization in the separation of impurities was studied through experiments based on convenient engineering practices and using wastewaters from various origins, i.e. a municipal wastewater treatment plant, landfill, peat production lands, and the mining industry. The experiments were implemented both in laboratories and on naturally frozen ice layers in wastewater basins. In addition, the design and functionality testing of a pilot-scale suspension freeze crystallizer was presented.

Based on the experiments, freeze crystallization was found to be a suitable purification method for all wastewaters tested. Highly efficient impurity removal, even close to 100%, was achieved with low ice growth rates in the freezing of municipal wastewater. The increased ice growth rate was found to decrease the separation efficiency. The decrease in separation efficiency was much stronger in more concentrated wastewater, even though no substantive differences in ice growth rates could be noticed between the wastewaters of different concentrations under the same freezing conditions. The exploration of ice layers in wastewater basins showed separation efficiencies of 65-90%, which can be consider high under unpredictable water and weather conditions. The pilot-crystallizer achieved over 95% purification efficiencies determined by an extensive impurity analysis when landfill leachate was used in tests. These promising results of high efficiencies achievable by the freeze purification of wastewaters are an important motivator for further development of energy-efficient water treatment applications with an option for material recovery.