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Impact of Silver Nanoparticles Effects at Threshold Levels
Silver nanoparticles (AgNPs) are a frequently used nanomaterial with
a wide range of industrial and consumer applications, including fiber
coating, detergents, and hydrogels and plastics to prevent bacterial and
fungal growth. Nanoparticles released from various
nanotechnology-enhanced consumer products will inevitably enter our
sewers and wastewater treatment plants (WWTPs). What effects, if any,
they have on treatment processes is the subject of an ongoing WERF
research project, Impact of Silver Nanoparticles on Wastewater
Treatment (U3R07).
Researchers from the University of Missouri are evaluating how silver
nanoparticles will affect bacterial growth during wastewater treatment.
The team, led by Dr. Zhiqiang Hu, assistant professor of civil and
environmental engineering, has set up several lab-scale wastewater
treatment modular units using activated sludge processes designed to
remove organic matter and nutrients in wastewater. After a prolonged
period (>300 days) of operation, the researchers took the activated
sludge samples for nanotoxicity testing.
Results to date demonstrate that nitrifying bacteria are especially
susceptible to inhibition by silver nanoparticles. Researchers found
that at a concentration of 0.4 mg/L total Ag, a mixture of positively
charged silver ions and AgNPs (50:50 in mass ratio, average size =15-21
nm) inhibited the growth of nitrifying bacteria from the modified
Ludzack-Ettinger bioreactor by 11.5 percent. In an experiment on shock
loading of 100% AgNPs (lasting for 12 hours), a peak concentration of
0.75 mg/L total Ag in the activated sludge basin (more than 95%
associated with biomass) was detected, and about 50% nitrifying
bacterial growth inhibition (or nitrification inhibition) accompanied
with a slight accumulation of nitrite concentration in wastewater
effluent was observed.
The results suggest that the accumulation of silver could have a
detrimental effect on wastewater treatment, if the concentration reaches
threshold levels. Preliminary results from parallel studies of anaerobic
digestion, a commonly used solid stabilization process in wastewater
treatment plants, indicate that AgNPs at concentrations of 19 mg/L
(19,000 ppb) or above in biomass started to inhibit anaerobic microbial
activities. Because most of the silver particles are in the activated
sludge or biosolids, the researchers will continue to examine the impact
of these nanoparticles on anaerobic digestion.
The U.S. Environmental Protection Agency has instituted a secondary
drinking water standard for silver of 100 ppb. Although typical silver
effluent concentrations from sewage plants are low (at ppb levels or
below), increased use of nanosilver products poses new threats to
sensitive water bodies. U.S. EPA has set water quality criteria values
for silver in salt and fresh water at 1.9 and 3.2 ppb, respectively. The
research team will provide a summary in the final project report that
identifies threshold levels of silver nanoparticles in various treatment
processes. Awareness of these threshold levels will directly benefit
wastewater treatment operators and state regulatory officials in making
informed decisions with regard to silver nanoparticles in wastewater
treatment plants.
Look for the published report for Impact of Silver Nanoparticles on
Wastewater Treatment (U3R07) in May 2010.
February 4, 2010
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