Breakthrough research clears a path for wastewater treatment advances

Posted 3 August 2017

LabThe cause of the ‘storminess’ in fluid during sedimentation has long kept the minds of scientists swirling. But new research has settled a debate that could lead to advances in wastewater treatment and industrial practices.

Using theoretical models, Tel Aviv University doctoral student Tomer Goldfriend, along with Professor Haim Diamant of Tel Aviv University and Professor Thomas Witten of the University of Chicago, revealed that the ability of an object to stay suspended is related to its level of symmetry.

He and his fellow researchers demonstrated that objects exhibiting greater symmetry (spherical or ellipsoid objects) will fall more chaotically in fluid, causing the ‘stormy’ appearance. Objects that contain the same centre of mass as their geometric centre will also fall more chaotically. 

But unlike symmetrical objects, asymmetrical objects have an unbalanced mass distribution and are able to twist to orient themselves, enabling them to fall more uniformly. 

“The study was focused on how the rotational motion, induced by the objects' shapes, affects the structure and flow of sedimenting suspension,” Goldfriend said.

“This is related to an old problem involving symmetric objects such as spheres, where swirling of large structures are observed.”

Sedimentation is an important factor in natural processes such as shaping rivers and forming geological deposits. It’s also important in industrial processes, such as the production of polymers including rubber and polyethylene, which rely on a continuous flow of dense suspensions. 

These polymers are created in fluidised bed reactors that force liquid upwards, preventing particles from settling and facilitating the desired chemical reactions. Fluidised beds are commonly used in water and wastewater treatment facilities. 

Greater attention to the shape of powders and granular materials could help increase the efficiency of chemical reactors, as more symmetrical and buoyant particles are available for chemical reactions to occur. 

“In fluidised bed reactors, an external flow from the bottom of the container is injected against the sedimenting objects, causing them to stand still. Then several operations such as chemical reactions can be activated,” Goldfriend explained.
“Our study suggests a mechanism that stabilises the structure of floating objects, allowing chemical reactions to happen in an efficient and optimised way.” 
The model can be tested experimentally using video microscopy and light-scattering techniques. 

Further research is being planned in order to determine the specific shapes that best allow particles to remain suspended and continue to hone the efficiency of industrial reactors.

“The research focuses on general properties of sedimentation of irregular objects, not specialising to specific shapes. The next step is to explore which shapes of objects provide the maximum effect of stabilising the suspension.” Goldfriend said.