Analysis of Particulate Matter Removal Efficiency Using the STC900 Unit Operation
Karl Seltzer
Department of Environmental Engineering Science, University of Florida, Gainesville, Florida
Abstract— While the built environs continue to grow, the amount of impervious surfaces that invade our natural
environments will proportionally enlarge as well. Not only do these impervious surfaces change the look of the predeveloped areas but, they also alter the hydrologic and pollutant loading found in surface runoff as well as the phase
transport of all these pollutants. This increase of pollutants puts a strain on the receiving water bodies and the
animals that live in these effluent ponds and streams. Eutrophication, fish kill and ground water contamination are
consequences of such polluted rainfall-runoff. As a result of these unnatural changes, many new types of Unit
Operations and Processes (UOPs) have been developed that seek to protect these receiving water bodies from high
pollutant loadings. They can be installed both on site, in centralized locations or in-situ (at the source) and lower the
amount of pollutants that eventually make their way off site. For this study, the integrity and efficiency of the
STC900, a UOP that provides particle separation, was examined for removal of particulate matter, which carries a
large amount of the total pollutant load. The unit is a hydrodynamic separator that specializes in removing mass
limited species that are carried in stormwater. It was determined that the particulate matter removal efficiency
increased as flow rate into the STC900 decreased but, was able to maintain a 56.17% capture of particles for
maximum experimental flow rate into the unit. The significance of these values and the steps leading to them are
discussed in the following report.
1.
of Florida campus shows that a large mass of
phosphorus and nitrogen eventually makes it way off
site.
Introduction
Particulate matter in stormwater runoff has
increasingly become problematic as expansion and
development continues. When a rainfall-runoff event
occurs, the sprawling impervious surfaces, which
have now taken over a large part of the previously
natural environments, transports pollutants to
receiving water bodies. To put the amount of
impervious surfaces in Florida into perspective,
almost 4 million acres of land in this state have been
“urbanized” in the last 42 years (Bergstrom 2001).
Whether or not the land is transformed into a
commercial setting or industrial site, damaging
effects can be seen in the rainfall-runoff from these
places.
Figure 1. Breakdown of Nutrients and Particulate
Matter on a University of Florida parking lot.
The pollutants associated with runoff from these sites
can potentially have a large diversity in the type and
amount of pollutant washed into the natural
environment. Some experience an abundance of
organic matter, such as leaves and dirt, while others
may contain many inorganic materials such as metals
and oils. One local site that experiences both organic
and inorganic constituents is the Reitz Union Parking
Lot on the University of Florida campus. The
impervious lot receives a large amount of nutrient
and particulate matter pollution from the surrounding
trees while inorganics pile up from the cars that are
constantly being driven on its surface. A breakdown
of the organic constituents can be seen below in
Figure 1. This breakdown of particulate bound and
soluble nutrients from a parking lot on the University
As seen in Figure 1, only a small portion of the total
phosphorus content in the runoff is made up of
phosphate (a soluble form of phosphorus). The
balance of the total phosphorus content is made up of
particulate bound Phosphorus. Therefore, a UOP that
specializes in removing particulate matter from
runoff would in turn remove a significant portion of
the phosphorus in the influent water stream. On the
other hand, nitrogen is comprised of more soluble
species, making it much more difficult to treat.
The particulate matter breakdown in Figure 1 also
displays a large portion of PM falling into the
sediment category, which is greater than 75 microns.
1
These larger particles have larger mass and are
therefore easier to settle and filter. Unlike their
smaller counterparts, sediment-size particles tend to
remain in the UOP once removed and much more
difficult to scour.
2.
The STC900, which is made by Imbrium Systems
Corporation and a part of their Stormceptor System
division of Units, is capable of holding 900 US
Gallons. The diameter of the Unit is 6 feet and the
height is about 4.5 feet. Also, according to Imbrium,
the maximum flow rate that the Unit can handle is
approximately 300 US gallons per minute (GPM).
STC900
In this study, the Stormceptor 900 (STC 900) was
examined and the removal of particulate matter
through this system was quantified. This UOP
requires no energy input and relies on head
differentials to push the influent water through the
system. When installed in the field, these units are
typically buried underground but, for this study, an
above ground STC900 was used. This allowed for
much easier access to various ports that helped in the
mass balance of the experiments and an overall view
of the system in action.
3.
Flow Determination and Conveyance System
For any given storm, the runoff flow throughout the
duration of the storm varies. In order to accurately
depict these flow features that the STC900 will
naturally see in the field, it was necessary to vary the
duration and intensity of the surrogate storms that
was passed through the system. For that reason, it
was important to test a number of different
hydrographs (runoff flow vs. time charts) and
determine the particle removal efficiency from each.
By weighting the particulate matter based on
incremental volume from each hydrograph, total
removal efficiency over any time period can be
determined.
Stormwater enters the STC900 through the inlet (seen
in Figure 2) and utilizes the internal flow conveyance
system to create a hydrodynamic separation process
that keeps particles in the unit while allowing a much
cleaner effluent to leave through an upflow orifice, as
seen in Figure 3.
A maximum flow rate of 290 gallons per minute was
used as the stepping stone for all the surrogate storms
in this research. . This value is the maximum inflow
value for this particular UOP as specified by its
manufacturer, Imbrium. As seen in Figure 4 below, a
hydrograph was built around this maximum flow
rate. As can be seen, the flow value starts at zero and
rises to the maximum value over a near 15 minute
span. From there, the flow decreases back down to
zero flow. To create a number of different
hydrographs, certain percentages of this peak flow
were then determined and a delineated hydrograph of
each were composed. Figure 5 shows the hydrograph
that was used for a 50% peak flow analysis. Figure 6
shows the hydrograph that was used for a 25% peak
flow analysis.
Figure 2. STC900 Inlet
Figure 4. Hydrograph for 100% Peak Flow
Figure 3. STC900 View from Outlet Port
2
Figure 5. Hydrograph for 50% Peak Flow
Figure 7. Tanks Used for Experimental Water
Storage
Figure 6. Hydrograph for 25% Peak Flow
The water was then pulled from the storage tanks by
the pumping system seen in the figure below. The
water then traveled through an 8 inch PVC pipe
conduit system and eventually makes its way to the
STC900 unit.
As seen above, each of these hydrographs represents
unsteady flow conditions, which is how a real storm
occurs. As the runoff values at each time interval
change, the amount of water entering a UOP also
changes, increasing the difficulty in treating
stormwater due to increased turbulence.
Figure 8. Pumping System
In order to produce these surrogate hydrographs, a
varying flow rate system was needed. To do this, a
pumping station and a piping conduit made of 8-inch
PVC pipe was used.
Before any experimental run took place, it was
necessary to have an adequate supply of water. The
water towers seen below in Figure 7 served as storage
units and housed the potable water that would
eventually be pushed through the STC900 Unit. It
was essential that tap was be used in the
experimentation process. If random particles were to
be present in the influent that were not defined by the
influent particle size distribution, an inaccurate mass
balance would result and an incorrect removal
efficiency would be determined. Potable water
provided the quality control of knowing that the
influent water was clean and particle free.
3
Figure 9. Hydraulic Conveyance System
4.
Influent PSD
In order to determine the efficiency of the STC900, it
is necessary to define what enters the system and
compare it to the amount and type of particles that
leave in the effluent. A set particle size distribution
(PSD) was used for each slurry injection and
maintained throughout the course of each individual
hydrograph. A particle size distribution graph depicts
the percentage of particles by mass that are below a
given size. In the case of this study, the sizes were
defined as microns (μm).
Figure 11 below graphs the influent PSD that was
used for each injection. This graph follows a NJDEP
(New Jersey Department of Environmental
Protection) gradation and encompasses all phases of
the particulate matter spectrum (sediment, settleable
and suspended).
Figure 11. Influent PSD
% finer by mass
100
Finally, once the water made its way through the
conveyance system, it reached the STC900 inlet. A
visual of this area can be viewed in Figure 3 and
Figure 10.
80
60
Influent PSD
40
20
NJCAT gradation
0
10000
Figure 10. Entrance to the STC900
1000
100
10
1
0.1
Particle diameter, d (m)
By utilizing the PSD above and comparing it to the
PSD from the effluent, a total removal efficiency can
be determined.
As discussed earlier, the removal efficiency of the
unit is dependent on the PSD, flow and velocity of
the water as it travels through the STC900. The larger
the flow, the faster the water makes its way through
the Unit and the more difficult it is to settle out
particles that are in the influent. This is due to the
turbulence within the tank. To obtain a thorough
understanding of the Units ability to remove
particles, the weighted effluent PSD for a 100% flow,
50% flow and 25% flow rates were measured.
5.
Particle Removal Efficiency for 100% Run
When looking at the 100% flow hydrograph
presented in Figure 4, there are two clear trends in the
4
graph. There is a portion in which the flow steadily
increases to the peak and the rest of the hydrograph,
which presents the flow decreasing from the peak to
zero.
For particles in the suspended range (less than 25
µm), the removal efficiency drastically drops and the
removal becomes negligible. This is due to the
turbulence and an overflow rate that is greater than a
settling rate across most of the PSD. Hydrodynamic
separation becomes tough and particles in that small
range just move on through and exit with the effluent.
However, the d50 of this max flow rate should be
noted. Even when water is passing the STC900 at
nearly 300 GPM, the d50 of particles removed is
about 30 microns.
For that reason, two separate PSD charts were made
to show the trend of removal efficiency pre and post
peak flow. Figure 12 below compares the influent
PSD for the 100% peak flow and the effluent PSD
from time 0 till the peak flow.
Figure 12. 100% Flow Effluent PSD Comparison
to Influent PSD from Q = 0 to Q = 0.2
6.
100
For the 50% flow run, the maximum flow is
approximately 140 GPM. As a result, the turbulence
within the STC900 should be drastically less and the
residence time of the water in the system will
proportionally increase. Due to these factors, it
should be expected that a larger amount of particles
are capable of being removed and that there won’t be
as drastic a change between the PSD measurements
throughout the “storm.”
% finer by mass
0<t/tmax<0.2
80
60
Influent
Q+
40
20
0
10000
1000
100
10
1
Particle Removal Efficiency for 50% Run
0.1
Once again, the PSD charts were broken up into two
separate parts; the time leading up to the peak flow
and the time post peak flow that leads back down to a
zero flow. The PSD plots for time zero up until peak
flow are shown below in Figure 14.
Particle diameter, d (m)
As expected, when the flow increases, the velocity
turbulence in the Unit increases and it becomes more
difficult for particles to settle. For that reason, the
PSD over time move further to the left, expressing
that larger particles are leaving the system and that as
a whole, the STC900 is less efficient in removing
particles.
Figure 14. 50% Flow Effluent PSD Comparison to
Influent PSD from Q = 0 to Q = 0.4
100
0<t/tmax<0.4
% finer by mass
Though, after the peak flow occurs, the opposite
effect can be seen. As shown below in Figure 13, the
PSDs start to move back to the right. This signifies a
larger efficiency of the STC900 removing the bigger
size particles. Thus a better total removal efficiency
begins to occur.
Figure 13. 100% Flow Effluent PSD Comparison
to Influent PSD from Q = 0.2 to Q = 1.0
80
60
Influent
Q+
40
20
0
1000
100
10
1
0.1
Particle diameter, d (m)
100
% finer by mass
0.2<t/tmax<1
80
As seen in Figure 14, the PSD trend leading up the
peak for the 50% flow storm follows the same trend
as the 100% flow storm. As the flow picks up, more
turbulence and a smaller particle residence time
occurs in the Unit. Due to these phenomena, the PSD
moves further to the left. Post peak flow, the PSD
measurements start moving back to the right,
indicating higher removal efficiency. This trend can
be seen in Figure 15.
Influent
60
Q-
40
20
0
10000
1000
100
10
1
0.1
Particle diameter, d (m)
5
Figure 15. 50% Flow Effluent PSD Comparison
to Influent PSD from Q = 0.4 to Q = 1.0
Figure 17. 25% Flow Effluent PSD Comparison to
Influent PSD from Q = 0.2 to Q = 1.0
100
100
0.2<t/tmax<1
80
% finer by mass
% finer by mass
0.4<t/tmax<1
Q-
60
Influent
40
20
0
Q-
60
Influent
40
20
0
1000
100
10
1
0.1
1000
Particle diameter, d (m)
100
10
1
0.1
Diameter, d (m)
As expected, the d50 for the maximum flow during
the 50% run (approximately 25 microns) is smaller
than the d50 at the maximum flow during the 100%
run (approximately 30 microns), indicating a higher
removal percentage.
7.
80
As the flow increases, the same trend occurs as
previously shown, with the PSD measurements
becoming steadily coarser. However, one observation
is the very slight change in the difference between
each PSD measurement. At this flow rate level and
storm intensity, only a slight drop in removal
efficiency is seen throughout the entire event.
Particle Removal Efficiency for 25% Run
For the final flow testing, 25% maximum flow was
tested, which is approximately 70 GPM. As was the
case between 100% and 50%, similar trends were
also anticipated and an overall higher removal
efficiency should be seen.
Additionally, after post peak flow, the PSD chart
lines move back to the right, indicating a slightly
better removal efficiency.
Again, the PSD charts were broken up into two
separate figures. Figure 16 presents the PSD trend
from time zero until the maximum flow (about 70
GPM) was achieved. Figure 17 presents the PSD
trend from the maximum flow until the end of the
“storm.”
There is a variety of ways phosphorus and nitrogen
can make their way into a receiving body through
stormwater rainfall-runoff. These nutrients can be
flow limited and mainly depend on their dissolved
constituents or mass limited and proportionally
reliant on particulate matter to transport them.
Though both phosphorus and nitrogen are clumped
together into the same category of “nutrients,” the
limitations associated with each are completely
different.
8.
Figure 16. 25% Flow Effluent PSD Comparison to
Influent PSD from Q = 0 to Q = 0.2
100
% finer by mass
0<t/tmax<0.2
A study on the University of Florida campus that
collected data on the components of stormwater
rainfall-runoff from the summer of 2008 showed that
the approximately 68% of the Total Phosphorus in
runoff is made up of particle bound phosphorus and
about 41% of the Total Nitrogen in runoff is made up
of particle bound nitrogen (FDEP). As these
percentages show, phosphorus tends to be a more
mass limited nutrient species while nitrogen tends to
be a more flow limited species. Additionally, relating
these numbers to the determined total particulate
matter removal efficiency of the STC900 (56.17%), a
80
Q+
60
Influent
40
20
0
1000
100
10
1
Particle Removal in Relation to Pollutants
0.1
Particle diameter, d (m)
6
total nutrient removal efficiency can also be
determined.
UOP at the forefront of such a system would be the
STC900.
Assuming that 56% of the particles entering the
STC900 are removed, that equates to 56% of the
particulate bound phosphorus and nitrogen are
additionally stripped from the runoff. Relating these
percentages, approximately 38% of the Total
Phosphorus content and 23% of the Total Nitrogen
content in this University of Florida runoff can
theoretically be removed through the use of the
STC900 UOP.
Acknowledgements
9.
I would first like to thank Dr. John Sansalone.
Without his mentoring, this study would not have
been possible.
Giusi Garofalo and Annalisa Ciccarello who helped
lead this research.
And the entire University of Florida Stormwater Unit
Operations and Process Lab of students.
Conclusion
The STC900 Unit Operation and Process specializes
in removing particulate matter from a rainfall-runoff
stream through hydrodynamic separation and
sedimentation. While this may seem like a narrow
scope of treatment, the chemistry behind stormwater
enables the STC900 to reach out and treat a variety of
other pollutants that embed themselves in stormwater
stream.
References
(1) Bergstrom, C. (Ed.). (2001). Current Issues
Associates with Land Values and Land Use
Planning: Urbanization and Land Use Change in
Florida and the South: Southern rural
Development Center and Farm Foundation.
(2) FDEP Contract Number WM 910 Management
of Source Area Runoff Using In-situ Controls.
University
of
Florida.
Department
of
Environmental Engineering Sciences.
(3) Kertesz, R., Maccarone, K., Raje, S., Seltzer, K.,
Siminari, M., Simms, P., Wood, B. (2009).
Green Infrastructure and LID Design for Florida
Constructed Environs, Subject to Rainfall Runoff
Loadings. Team Pluvia Munda for Florida Water
Environment Association.
In addition to the 56% removal efficiency of
particulate matter from runoff, particulate bound
nutrients and particulate bound metals are also
maintained by the STC900 through the removal of
particulate matter. As determined previously, about
38% of the Total Phosphorus content and 23% of the
Total Nitrogen content can be removed from runoff
by the simple treatment of particulate matter.
While these numbers may seem low, their economic
value to the treatment of stormwater runoff cannot be
overlooked. The mean cost to remove phosphorus
from runoff using BMPs in FL is about $14,700/lb/yr
and the mean cost to remove nitrogen from runoff
using BMPs in FL is about $3,700/lb/yr (Kertesz et
al. 2009). Therefore, even removing a small portion
of nutrients from a runoff stream can save a large
portion of money, even in the short term.
The STC900 does an excellent job at balancing the
removal of particulate matter from runoff and
minimizing
the
amount
of
extracurricular
maintenance needed on the UOP. It maintains a good
amount of nutrient removal as well but, lacks in
efficiency when it comes to dissolved constituent
removal. As the importance of stormwater runoff
comes to the forefront, more stringent technologies
must be perfected that focus more on the dissolved
portion of pollutants. Various filter designs, ionic ad
mixtures and denitrification can help enhance the
removal of nutrients. These advanced technologies
push the idea of a treatment train and the perfect
7