Constructed wetlands are typically models as plug flow reactors
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A constructed
wetland or wetpark is an artificial wetland created as a new or restored habitat for native and migratory wildlife, for anthropogenic discharge such as wastewater, stormwater runoff, or sewage treatment, for land reclamation
after mining, refineries, or other ecological
disturbances such as required mitigation for natural areas lost to a development.
Natural wetlands act as a biofilter, removing sediments and pollutants such as heavy metals from the water, and constructed
wetlands can be designed to emulate these features.
The plug
flow reactor model (PFR)
is a model used to describechemical reactions in
continuous, flowing systems of cylindrical geometry. The PFR model is used to
predict the behavior of chemical
reactors of such design, so that
key reactor variables, such as the dimensions of the reactor, can be estimated.
Fluid going through a PFR may be modeled as flowing through
the reactor as a series of infinitely thin coherent "plugs", each
with a uniform composition, traveling in the axial direction of the reactor,
with each plug having a different composition from the ones before and after
it. The key assumption is that as a plug flows through a PFR, the fluid is perfectly mixed in the radial direction but not in the
axial direction (forwards or backwards). Each plug of differential volume is
considered as a separate entity, effectively an infinitesimally small continuous stirred tank
reactor,limiting to zero volume.
As it flows down the tubular PFR, the residence
time (
) of the plug is a function of its
position in the reactor. In the ideal PFR, the residence time distribution is
therefore a Dirac delta function with a value equal to
.Constructed wetlands have
been shown to be capable of removing a wide variety of contaminants, including
bacterial pollution. Wetlands are known to act as biofilters through a
combination of physical, chemical and biological factors which all participate
in the reduction of the number of bacteria. Treatment efficiency in a constructed
wetland is related in part to the amount of time that a wastewater remains in
the system. Current design methods idealize the system as a plug flow reactor
and use a "residence time" based solely on the volume of the cell and
the flow rate. Under this assumption, every element of wastewater entering the
wetland cell experiences the same residence time. It is understood that this
idealization ignores the existence of longitudinal dispersion, short circuiting
and stagnant regions within the wetland cell. Hydraulic regimes of constructed
wetland systems were investigated at a pilot project site providing tertiary
treatment of a pulp mill wastewater.The fluids to be reacted mix and flow through a pipe that is
stuffed full of catalyst. The length of the pipe depends on the desired
conversion of product (the longer the pipe, the greater the % conversion.) The
pipe diameter depends on the desired capacity of the reactor. A recycle stream
can be added to mix with the incoming feed if desired
An ideal plug flow reactor has a
fixed residence time: Any fluid (plug) that enters the reactor at time
will
exit the reactor at time
, where
is
the residence time of the reactor. The residence time distribution function is
therefore a dirac delta function at
. A real
plug flow reactor has a residence time distribution that is a narrow pulse around
the mean residence time distribution.
A typical plug flow reactor could be
a tube packed with some solid material
(frequently a catalyst). Typically these types of reactors are called packed bed
reactors or PBR's. Sometimes the tube will be a tube in a shell and tube heat exchanger.
Treatment efficiency in a
constructed wetland is related in part to the amount of
time that a wastewater remains in
the system. Current design methods idealize the system
as a plug flow reactor and use a
"residence time" based solely on the volume of the cell
and the flow rate. Under this
assumption,every element of wastewater entering the
wetland cell experiences the same
residence time.It is understood that this idealization ignores the existence of
longitudinal dispersion, short circuiting and stagnant regions within the
wetland cell. The result of these phenomena is a distribution of residence
times. In other words, portions of theeffluent exit the cell earlier than
predicted, resulting in undertreatment, and portions exitlate, resulting in
excess treatment. The average concentration of treated wastewater at theoutlet
is a function of this distribution and the reaction kinetics associated with
the waste.
The overall effect of a distribution
of residence times is reflected ina reduction of
treatment efficiency at the outlet.
Hydraulic regimes of constructed wetland systems were
investigated at a pilot project site
providing tertiary treatment of a pulp mill wastewater.
Two vegetation types, bulrush and
cattail,were investigated and compared to nonvegetated and rock-filter cells
with identical configurations. Tracer studies useda fluorescent dye and were
performedover the course of a year.
the same residence time. In the
nonideal case, there is a distribution of residence times;
some elements are detained in the
reactor longer than expected (stagnant flow),and some
are moved through faster than
expected (short circuiting).The effect of a distribution of
residence times is a decrease in
overall treatment efficiency. In almost all cases, deviations
from ideal flow patterns results in
decreased efficiency.
The level of performance of a
wastewater treatment system is related in part to the
residence time of the particular
reactor. At present, the design of constructed wetlands for
wastewater treatment is based on the
premise of plug flow. In this ideal situation, all
elements of the waste stream
entering the wetland experience thesame residence time. In
actuality, the hydraulic pattern
deviates somewhat from the ideal. Longitudinal dispersion,
short circuiting and dead space
within the system contribute to this departure from the
ideal case. Overall conversion of
pollutants (eg. BOD) in the system is directly affected by
the resulting distribution of
residence times. In almost allcases, a distribution of residence
times in a reactor leads to
decreased efficiency. Investigations of the hydraulic regime will
provide an increased understanding
of the importantparameters in the design and physical
configuration of constructed
wetlands.
Constructed wetlands for wastewater
treatment are currently designed based on an
assumption of plug flow. The affects
of short circuiting, dead space, and dispersion in
these systems result in a deviation
from this ideal, a corresponding distribution of
residence times. Since treatment
efficiency is related in part to the residence time of the
wastewater it is of interest to
determine the degree to which these "engineered" systems
deviate from ideal. Tracer studies
of the constructed wetland provide a statistical picture
of what the flow patterns of the
system are like.
First Order
Kinetic Rate Equations Method
Constructed
wetlands are typically modelled as a plug flow reactor using first-order
reaction
kinetics to describe the system.
C / Ci = exp (−Kv,τ)
Where: Ci = initial
pollutant concentration, mg/L
C = pollutant
concentration at time t, mg/L
Kv = volumetric
reaction rate constant, day-1
τ = hydraulic residence
time, day
τ is defined by
equation.
τ = εAh/Q
Where: A = wetland
surface area, m2
ε = porosity of the
wetland (95% for surface flow wetland),
h = average water
depth, m
Q = flow rate through
the wetland, m3/day
To account for
temperature effects, the rate constant kv is adjusted using the
Arhennius equation:
k v T =
K20θ(T-20)
Where: θ = is the
Arhennius coefficient,
T = is the temperature,
°C
k20 = is the
value at 20°C.
Kadlec and Knight
(1996) use a residence time distribution (RTD) to describe the nominal
detention time within the wetland. The RTD is the probability density function
for residence times in the wetland.
The time function is
defined by (Kadlec and Knight, 1996):
f(t)∆t = fraction of
incoming water which stays in the wetland for a length of time between t and t
+ ∆t. where f = RTD function, d-1,t = time, day.
Kadlec and Knight
(1996) recommend using a tracer test to determine the RTD function for a given
wetland. An impulse of dissolved inert tracer material is injected into the
wetland inlet and then the tracer concentration as a function of time is
measured at the wetland outlet.
References
constructed wetlands. Presented at the
International Winter Meeting of the
ASAE at Nashville,
