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Posted: November 13, 2008
EU study finds that wastewater treatment fails to remove all nanoparticles
(Nanowerk News) Nanomaterials are being used increasingly in the manufacturing industry, but questions remain
about the best way to efficiently remove these nanoparticles from industrial wastewater
processes or sewage treatment plants. Recent research suggests that some nanoparticles
escape from treatment plants and are discharged into water.
Studies have suggested that nanoparticles can remain in the environment for long periods and can be toxic to
aquatic life. Treated water from sewage plants and waste industrial processes is frequently discharged into aquatic
systems, and there is concern that residual nanoparticles present in these effluents could harm aquatic life.
Swiss researchers studied what happened to a common nanomaterial, cerium oxide, when it was added to a
laboratory-scale model of a water treatment plant ("Removal of Oxide Nanoparticles in a Model Wastewater Treatment Plant: Influence of Agglomeration and Surfactants on Clearing Efficiency"). The model plant contained activated sludge taken from an actual wastewater treatment plant in Zurich. Cerium oxide is used to polish silicon wafers in the computer industry and
optical lenses for mobile phones.
The researchers anticipated that the nanoparticles would cluster together to form larger bodies and sink into the
sludge, allowing them to be easily removed from the wastewater. However, analysis of both the sludge and
wastewater collected from the model plant revealed that a significant number of the nanoparticles had not been
removed from the treated sludge. Six percent of the cerium oxide was found in treated effluent that had been
washed through the sludge. The researchers suggest this might be because other ingredients found in the
wastewater could act like ‘surfactants’, substances that bind to the outside of the nanoparticles and prevent them
from clustering together.
These surfactants could come from the industrial process itself, where they are used to disperse the cerium oxide
particles, for example, from proteins found in the sludge or from the bacteria living in the sludge which naturally
secrete chemicals to avoid forming clusters with one another.
In the study, higher concentrations of cerium oxide were used than would normally be found in wastewater from
industrial plants. This implies that in normal circumstances there would be less likelihood of smaller concentrations
of nanoparticles coming together and sinking in the sludge of commercial plants.
Further studies are required to understand how different nanoparticles behave in water treatment sludge and how
they can effectively be removed from industrial wastewater and sewage treatment plants.