Natural Waste Water Purification - Freezing


Microbiologists rarely wash their jeans. The reason is because they know how to keep the color looking new without getting odors - they freeze their pants. 

Freezing can even work in more extreme scenarios, like with waste water. 

When waste water freezes, it is purified through the formation of a cleaner layer of ice. Then the clean layer of ice can be removed from the rest of the waste water, and the remaining waste water is more concentrated, which can be treated as needed with a lot less outside processing. Energy is required only for breaking the ice and transporting it from the waste water pool.

Credit: Lappeenranta University of Technology, LUT

The most practical application is leaving waste water from mines to freeze in special pools and then removing the cleaner part by breaking the ice. Then the treated waste water would be recycled, or undergo further treatment using membrane filtration, reducing the amount of fresh water that is used.

The method is being developed at the Lappeenranta University of Technology for the extractive industry, which produces large amounts of waste water. The freezing of water - crystallization - requires seven times less energy than its evaporation. Equipment developed by LUT chemical technology researchers includes a winter simulator which makes it possible to study how the temperature of cooling air affects freezing. The simulator has been used to study the growth rate of the layer of ice that emerges, and the degree of purity when salt solutions of different concentrations are used. Last winter researchers also took samples on the ice of Lake Saimaa.

"We took samples of both the lake water and the ice and we examined the amount of impurities that they contained. The result was that the lake water contained about ten times more impurities than the ice. Another finding in the research was that the slower the layer of ice grows, the cleaner the ice is. Therefore, the purity of the ice is directly dependent on its rate of growth," says Chemtech Professor Marjatta Louhi-Kultanen, who is specialized in the study of crystallization. According to Professor Louhi-Kultanen, future research will be aimed at an extensive examination of different types of waste water pools and the purity of their layers of ice and the implementation of freezing experiments with waste water samples in mining areas.

The study involves LUT researchers in chemical technology, mechanical engineering, and mathematical modelling. Also conducting research into freezing technology are the so-called icebreakers - the Marine Technology Research Group of Professor Pentti Kujala at Aalto University, and the so-called ice transporters - the mechanical engineering research group of Professor Aki Mikkola. The research group of Professor Jari Hämäläinen at LUT and Associate Professor Joonas Sorvari is involved in the mathematical modelling of freezing.

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