Modern Alchemy: Making Value Out Of Waste Water

9 Jun 2014 - 11:30

Prof Alison Lewis, of the Department of Chemical Engineering, presented a talk on Modern Alchemy: Making Value out of Waste Water, which proposed a wastewater treatment method to tackle the country’ mine water issue and turn contaminants back into a useful products.

The session looked at the current problems with wastewater purification methods and the way that they actually create more waste in order to recover the water. Reminding us that whilst water covers most of the planet, only 3% is fresh water (and most of that is ice), Prof. Lewis pointed out that in the 20th century, whilst the world’s population tripled, the use of water grew six times. She predicted that we are likely to run out of water before we run out of oil and stressed that H2O will become the most crucial resource of the 21st Century.

South Africa in particular, faces a time bomb in the form of acid mine drainage problems. The current solutions to the countries mining wastewater treatment are not sustainable (e.g. evaporation ponds). Prof Lewis put forward her eutectic freeze crystallisation (EFC) technology as a win-win alternative to the heat-treated sludge and toxic brine pond storage systems used today. Through EFC the contaminated mine water is cooled to a specific temperature. When the water is frozen, the contaminants crystallise into salts. Because salt is denser than ice, it sinks to the bottom, whilst the ice floats to the top.

She explained how freezing is much less energy intensive than evaporation, as the energy needed to freeze a litre of water is one sixth of that required to vaporise or distil it. Each salt has its own specific crystallisation temperature, so by freezing the brine at different temperatures it is possible to recover all the constituent parts of the saline brine, allowing for a number of pure salts to be removed from the waste stream along with potable water. In this way, the mining waste effluent could supply saleable salts such as sodium sulphate for textile and detergent processes, gypsum for the construction industry, as well as platinum and valuable rare earth metals for use in electronics and other applications. The latter are in such demand that they are actively mined in other countries. 

In conclusion, she emphasized that water as a resource is too cheap and not valued enough in the world today. Eutectic freeze technology offers a sustainable treatment process to turn wastewater into a resource with multiple benefits, particularly in water-scarce countries such as South Africa with mining industries which are key to economic development. 

Prof Lewis is the inventor on a UCT patent and holds a number of awards including the DST Distinguished Women Scientists Award, for her work in this field.