Environmental Engineering Reference
In-Depth Information
Where q is the sorption density (μgAg/g dry biomass), K a capacity term (μg-Ag/g-
biomass) (L/μg-Ag)
1/n
), C the pseudo-equilibrium concentration of silver in solution and
1/n is the adsorption intensity value (dimensionless). Our group has conducted batch
isotherms with nano-Ag and activated sludge wastewater biomass. K and 1/n values are
approximately 10 and 0.66, respectively, with only small differences between nano-Ag
or ionic silver at pH 7.2 (Benn and Westerhoff, 2008). Therefore, nano-Ag and ionic
silver both partition onto wastewater biomass. Using the Freundlich equation to
describe partitioning, a steady-state model for an activated sludge wastewater treatment
system, including recycling of biosolids, can be described as follows under steady state
conditions:
(
)
KC
1
/
n
XV
(Eq. 16.4)
QC
−
QC
−
e
=
0
0
Θ
where typical values for the flowrate (Q = 0.1 m
3
/s), biomass concentration (
X
= 2 g/L),
reactor volume (
V
= 1000 m
3
) and sludge retention time (
= 5 days) are used along with
K and 1/n. Assuming an influent nano-Ag concentration of 0.2 mg/L (
C
0
), which is
representative of total silver concentrations in raw sewage, the predicted effluent nano-
Ag concentration (
C
) would be 9 μg/L. This equates to > 99% removal of nano-Ag due
to partitioning to biosolids, and subsequent sedimentation of the biosolids. These values
may only represent an approximation, but illustrate how nano-Ag is at only very low
levels in both liquid effluent from a WWTP that is discharged to a river, stream or lake,
and mostly in biosolids that are digested and then disposed to landfills, land-applied or
incinerated. This type of modeling approach can be used to estimate the exposure levels
for nano-Ag in aquatic systems receiving wastewater discharges.
θ
Grit
Chamber
Grit
Chamber
Primary
Clarifier
Primary
Clarifier
Secondary
Clarifier
Secondary
Clarifier
Aeration tank
Aeration tank
Raw sewage
Raw sewage
Effluent discharged to
surface water
Effluent discharged to
surface water
Return Activated Sludge
Return Activated Sludge
Waste Activated Sludge
Waste Activated Sludge
Stabilized Biosolids
Stabilized Biosolids
Anaerobic digestor
Anaerobic digestor
Figure 16.8
Schematic of an activated sludge wastewater treatment plant.
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