Effects of silver nanoparticles on biological nitrogen
removal processes
E. Jeong, S. R. Chae, S. T. Kang and H. S. Shin
ABSTRACT
The effects of silver (Ag) nanoparticles (NPs) on activated sludge in a biological nitrogen removal
(BNR) process were investigated under aerobic and anoxic conditions. We show that nitrification was
more vulnerable to Ag NPs exposure than denitrification at the same Ag NPs concentration. In
continuous operation of the BNR process, a higher inhibitory effect on nitrification was attributed to a
smaller size of Ag NPs. About 70–90% of the Ag NPs supplied were embedded in the sludge matrix
but 10–30% of the Ag NPs remained in the supernatant. This indicates that significant amounts of Ag
NPs could be discharged from wastewater treatment plants and potentially impact on aquatic
ecosystems.
S. R. Chae (corresponding author)
School of Chemical and Biomolecular Engineering,
The University of Sydney, NSW 2006,
Australia
E-mail: soryong.chae@sydney.edu.au
E. Jeong
H. S. Shin
Department of Civil and Environmental
Engineering,
Korea Advanced Institute of Science
and Technology (KAIST),
373-1 Guseong-dong, Yuseong-gu,
Daejeon 305-701,
Republic of Korea
S. T. Kang
Department of Civil Engineering,
Kyung Hee University,
1 Seocheon-dong, Giheung-gu,
Yongin-si, Gyeonggi-do,
Republic of Korea
Key words | activated sludge, denitrification, inhibitory effect, nitrification, silver nanoparticles
INTRODUCTION
With the rapid development of nanotechnology, various
nanomaterials have been implemented in consumer pro-
ducts, such as household products, clothing, cosmetics,
electronic devices, pharmaceuticals, and biomedicals. Nano-
technology is expected to become a US$1 trillion industry
within the next decade ( Jin et al. ). In 2011, a report on
Engineering Nanotechnologies from the Woodrow Wilson
International Center compiled a list of more than 1,317 con-
sumer products including engineered nanoparticles. Silver
nanoparticles (Ag NPs) ranked top (24%, 313 products) in
the consumer products among six engineered nanoparticles.
For instance, fabrics and textiles, health supplements, cos-
metics, cleaning and personal care products, detergents,
and home appliances containers that contain Ag NPs are
currently available in the market, and have great potential
for a broad range of future applications (Fabrega et al. ).
Despite the wide application of Ag NPs, little infor-
mation is known about the environmental impacts of the
Ag NPs. Studies have demonstrated the release of Ag NPs
from different consumer products (Benn & Westerhoff
; Benn et al. ; Kaegi et al. ), suggesting that
released Ag NPs would enter municipal wastewater treat-
ment plants (WWTPs) via sewage. As WWTPs provide
potential pollutant pathways into the aquatic ecosystem,
WWTPs are considered to be key intermediate stations in a
recent risk assessment for Ag (Nowack & Bucheli ;
Kiser et al. ). A recent study showed that the presence
of Ag was identified in sewage sludge, and the total concen-
tration of Ag ranged from 1.94 to 856 mg/kg (Kim et al. ).
However, the effects of Ag NPs on biological wastewater
treatment processes are largely unknown. Recently, Choi &
Hu () showed an 86% decrease in nitrifying bacterial
activity at 1 mg/L Ag NPs, while other researchers reported
41% nitrification inhibition in activated sludge at the same
level of Ag NPs exposure (Liang et al. ). Therefore,
this study investigates the effect of Ag NPs on the perform-
ance of biological nitrogen removal (BNR) processes along
with the fate of Ag NPs in the BNR process. Also, the size
effects of Ag NPs on biomass activity were compared
under different operating conditions.
METHODS
Synthesis and characterization of Ag NPs
Ag NPs were synthesized by reducing silver nitrate (AgNO
3
)
with sodium borohydrate (NaBH
4
) in the presence of
1298 © IWA Publishing 2012 Water Science & Technology | 65.7 | 2012
doi: 10.2166/wst.2012.005