U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2004–5222

Effectiveness of a Pressurized Stormwater Filtration 

System in Green Bay, Wisconsin: 

A Study for the Environmental Technology Verification 

Program of the U.S. Environmental Protection Agency

In cooperation with the Wisconsin Department of Natural Resources
Effectiveness of a Pressurized 

Stormwater Filtration System in 

Green Bay, Wisconsin: A Study for the 

Environmental Technology Verification 

Program of the U.S. Environmental 

Protection Agency

By J.A. Horwatich, S.R. Corsi, and R.T. Bannerman

In cooperation with the Wisconsin Department of Natural Resources

Scientific Investigations Report 2004–5222

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2004

For sale by U.S. Geological Survey, Information Services

Box 25286, Denver Federal Center

Denver, CO 80225

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

World Wide Web: http://www.usgs.gov/

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

Cover photos (clockwise from top left):

Drainage basin view that includes the parking lot and gaging station

Drainage basin view of the parking lot and rooftop

Top view of the flow splitter

View of the first pressurized stormwater filtration system unit (disk-filter tower)

View of the second pressurized stormwater filtration system unit (sand filters)
iii

Contents

Abstract .................................................................................................................................................. 1

Introduction ........................................................................................................................................... 1

Purpose and Scope ..................................................................................................................... 2

Acknowledgments ....................................................................................................................... 2

Design of the Pressurized Stormwater Filtration System .............................................................. 2

Sampling Design .......................................................................................................................... 4

Unforeseen Monitoring Complications .................................................................................... 5

Precipitation Measurement ....................................................................................................... 7

Calibrations of Flows ................................................................................................................... 7

Inlet .................................................................................................................................. 7

Outlet .................................................................................................................................. 8

Comparison of Particle-Size Distribution in Inlet, Outlet, and Flow-Splitter Samples ..... 8

Precipitation and Stormwater Quantity ................................................................................... 9

Evaluation of the Pressurized Stormwater Filtration System ...................................................... 11

Comparison of Water-Quality Parameters in Inlet and Outlet Samples ........................... 11

Results of Particle-Size Analysis ............................................................................................ 14

Summary and Conclusions ................................................................................................................ 17

References Cited ................................................................................................................................ 18

Figures

 1. Plan view of St. Mary’s Hospital, Green Bay, Wis. ........................................................ 3

 2. Cumulative rainfall distributions for the study period compared to 

historical rainfall records (1949−92) for Green Bay, Wis., based on 

the National Oceanic and Atmospheric Administration precipitation 

gage at Austin Straubel International Airport, Green Bay, Wis. ................................ 10

 3. Map showing the depth of retained sediment in the flow splitter, the 

location of core samples, and contour lines generated using a contour 

method in ArcInfo .............................................................................................................. 16

 4. Particle-size distributions in inlet samples from six events and 

in flow-splitter samples collected after the monitoring period, 

Green Bay, Wis. .................................................................................................................. 17
iv

Tables

 1. Characteristics of the drainage area that contributes runoff to the 

pressurized stormwater filtration system at St. Mary’s Hospital, 

Green Bay, Wis. .................................................................................................................... 4

 2. Constituent list, limits of detection, and analytical methods for 

samples collected at the pressurized stormwater filtration system ........................... 5

 3. Concentrations of blanks and replicate samples for quality control .......................... 6

 4. Downtime of the pressurized stormwater filtration system during 

the monitoring period .......................................................................................................... 7

 5. Comparison of monthly rainfall between the U.S. Geological Survey 

raingage at St. Mary’s Hospital and the National Oceanic and 

Atmospheric Administration precipitation gage at Austin Straubel

International Airport, Green Bay, Wis. ............................................................................ 8

 6. Rainfall and runoff statistics for sampled events at the pressurized 

stormwater filtration system at St. Mary’s Hospital, Green Bay, Wis. ....................... 9

 7. Percentage of runoff and inlet volumes for the pressurized 

stormwater treatment system ......................................................................................... 10

 8. Event mean concentrations of 15 events during testing of the 

pressurized stormwater filtration system ...................................................................... 12

 9. Comparison of average event mean concentrations from several 

studies in Wisconsin and Michigan ............................................................................... 13

 10. Results of ground-water-seepage samples collected in the storm-sewer 

system at St. Mary’s Hospital, Green Bay, Wis., August 8, 2001 ................................ 13

 11. Sum of loads for 15 events during testing of the pressurized stormwater 

filtration system, Green Bay, Wis. ................................................................................... 15

 12. Results of sand/silt split sediment analysis at the inlet and the outlet 

of the pressurized stormwater filtration system for 12 rainfall events ..................... 16

 13. Particle-size distribution determined from samples taken during six 

events at the inlet sampling site ...................................................................................... 16

 14. Particle-size distribution for the sediment samples collected in the 

flow splitter, November 2002 ........................................................................................... 17
v

Conversion Factors and Abbreviations

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

millimeter (mm) .3937 inch (in.)

Area

acre 4,047 square meter (m2

)

square foot (ft2

) 929.0 square centimeter (cm2)

Volume

gallon (gal) 3.785 liter (L) 

gallon (gal) 0.003785 cubic meter (m3

) 

cubic foot (ft3

) 0.02832 cubic meter (m3) 

liter (L) 0.03531 cubic foot (ft3

)

Flow rate

foot per second (ft/s) 0.3048 meter per second (m/s)

cubic foot per second (ft3

/s) 0.02832 cubic meter per second (m3/s)

gallon per minute (gal/min) 0.06309 liter per second (L/s)

Mass

gram (g) 2.20 x 10-3 pound, avoirdupois (lb) 

pound, avoirdupois (lb) 0.4536 kilogram (kg) 

Concentrations of chemical constituents in water are given either in milligrams per liter 

(mg/L) or micrograms per liter (μg/L). Another unit of measurement used in this report is 

micrometers (μm).
vi
Effectiveness of a Pressurized Stormwater Filtration 

System in Green Bay, Wisconsin: A Study for the 

Environmental Technology Verification Program of the 

U.S. Environmental Protection Agency

By J.A. Horwatich1, S.R. Corsi 1, and R.T. Bannerman2

Abstract

A pressurized stormwater filtration system was 

installed in 1998 as a stormwater-treatment practice to 

treat runoff from a hospital rooftop and parking lot in 

Green Bay, Wisconsin. This type of filtration system has 

been installed in Florida citrus groves and sewage treatment plants around the United States; however, this installation is the first of its kind to be used to treat urban runoff 

and the first to be tested in Wisconsin. The U.S. Geological Survey (USGS) monitored the system between November 2000 and September 2002 to evaluate it as part of the 

U.S. Environmental Protection Agency’s Environmental 

Technology Verification Program. Fifteen runoff events 

were monitored for flow and water quality at the inlet and 

outlet of the system, and comparison of the event mean 

concentrations and constituent loads was used to evaluate 

its effectiveness. Loads were decreased in all particulateassociated constituents monitored, including suspended 

solids (83 percent), suspended sediment (81 percent),

total Kjeldahl nitrogen (26 percent), total phosphorus 

(54 percent), and total recoverable zinc (62 percent). Total 

dissolved solids, dissolved phosphorus, and nitrate plus 

nitrite loads remained similar or increased through the 

system. The increase in some constituents was most likely 

due to a ground-water contribution between runoff events. 

Sand/silt split analysis resulted in the median silt content 

of 78 percent at the inlet, 87 percent at the outlet, and 

3 percent at the flow splitter.

Introduction

In 1978, the U.S. Environmental Protection Agency 

(USEPA) established the Nationwide Urban Runoff Program (NURP) to assess the water-quality characteristics of 

urban runoff. When the city of Milwaukee, Wisconsin, was 

chosen by the EPA as a NURP site, a partnership between 

the Wisconsin Department of Natural Resource (WDNR) 

and the U.S. Geological Survey (USGS) was developed 

to evaluate urban runoff in Milwaukee. Since the NURP 

study, the USGS and the WDNR have continued their 

partnership and have completed more than 15 studies in 6 

different cities to assist the State of Wisconsin in characterization of urban stormwater runoff. The study described 

herein will provide additional information to meet the 

partnership goals of understanding urban runoff.

 The NURP project and other projects have provided 

information needed to set cost-effective and achievable 

goals for the USEPA’s National Pollutant Discharge 

Elimination System Permit Program (NPDES) as amended 

in the Clean Water Act. The NPDES Phase I program 

requires municipalities with populations greater than 

100,000 to apply for a NPDES permit, which requires 

stormwater-management controls to reduce pollutant discharge (U.S. Environmental Protection Agency, 2000). The 

NPDES Phase II program was developed to reduce pollutant discharges in urban areas of populations with less than 

100,000 and at smaller construction sites. For the NPDES 

Phase II program, individual states establish specific pollutant-reduction criteria to minimize adverse effects from 

urban stormwater on water quality of receiving streams. 

To meet these requirements, NPDES permit holders may 

choose any available stormwater-treatment practice; 

however, data on selection, efficiency, and maintenance of 

these management practices is currently insufficient. 

1 U.S. Geological Survey, Middleton, Wisconsin

2 Wisconsin Department of Natural Resources, Madison, Wisconsin
In 1999, the USEPA established the Environmental 

Technology Verification Program (ETV), setting a national 

focus on verifying manufacturer’s claims for commercially available stormwater-treatment practice efficiencies; 

this study described herein is the first of such studies to 

be completed. The USEPA cooperated with the National 

Sanitation Foundation International (NSF International) as 

its verification partner, and NSF is in charge of the following tasks: (1) create a national protocol to test wet-weather 

flow technologies, (2) contract independent groups to 

evaluate the effectiveness of the stormwater-treatment 

practices of interest, (3) review and implement the verification testing plans, and (4) make study results available to 

the general public (U.S. Environmental Protection Agency, 

2002). Municipalities and other interested parties will then 

have access to all ETV program results to help them make 

informed decisions on the choice of stormwater-treatment 

practices for their stormwater-management programs. 

Results from this study were forwarded to ETV personnel 

for their final verification reporting (U.S. Environmental 

Protection Agency, 2004).

In 1998, a pressurized stormwater filtration system (the Arkal Filtration System, manufactured by Zeta 

TechnologyTM) was installed by private contractors at 

St. Mary’s Hospital in Green Bay, Wis., to treat roof and 

parking lot runoff. This particular PSF system consists 

of two types of filtering units. After a 50-μm disk system 

prefilters particulates, an array of sand filters then removes 

particles of smaller size. This type of filtration system has 

been installed in Florida citrus groves and sewage treatment plants around the United States; however, this installation is the first of its kind to be used to treat urban runoff 

and the first to be tested in Wisconsin.

This pressurized stormwater filter is one of ten such 

stormwater-treatment practices that the WDNR and USGS 

have examined to evaluate water-quality effects. Three of 

these studies are in cooperation with the ETV program.

Purpose and Scope

The primary objective of this report is to describe 

the effectiveness of the PSF system in removing a suite 

of inorganic and physical water-quality constituents from 

stormwater runoff. This report also describes methods and 

techniques used to determine the effectiveness of the PSF 

and presents results from data collection and analysis of 15 

runoff events (greater or equal to 0.2" of rainfall) monitored at the inlet and outlet to the system between November 2000 and September 2002. Detailed data concerning 

water quality, flow, constituent loadings, and efficiencies 

of removal are presented.

A revision to the Clean Water Act in 1987 recognized 

stormwater discharges as a point source. Since then the 

U.S. Geological Survey (USGS) and Wisconsin Department of Natural Resources (WDNR) have conducted 

projects to understand the water-quality characteristics of 

stormwater in the State of Wisconsin. Another primary 

objective of this report is to add to the understanding of 

stormwater quality and quantity in the urban environment. 

The USGS role in monitoring stormwater treatment practices is to collect and evaluate how these systems could 

reduce pollutant discharges to nearby waterways.

Acknowledgments

The authors thank the following people, without 

whose help this project could not have been completed:

• The staff of St. Mary’s Hospital, Green Bay, Wis., 

for assistance in maintaining the filtration system;

• James Bachhuber and Jay Kemp of Earth Tech Inc., 

for their aid in writing the monitoring protocol and 

assisting with filter requirement and maintenance;

• David Owens of the USGS, Middleton, Wis., for 

his instrumentation and programming expertise; 

and

• John Shenk at NSF for overall project collaboration.

Design of the Pressurized 

Stormwater Filtration System

The PSF was manufactured by ZetaTM Technology 

and installation was completed at St. Mary’s Hospital 

in December 1998. The shelter for the system is set 4 ft 

below ground; the roof is 2 ft above ground, covering 

300 ft2. The system was designed to minimize the amount 

of aboveground space needed. Runoff from the site drains 

to a municipal storm sewer, and subsequently into Beaver 

Dam Creek (a tributary of Duck Creek) and eventually 

discharges into Green Bay (Lake Michigan). 

Stormwater runoff at the hospital passes through an 

18-in. inlet pipe to a 6-ft-diameter flow splitter (fig. 1). 

The flow splitter has two possible exits: a 15-in. PVC pipe 

that drains to a 9,200 ft3

 holding tank, and an 18-in. pipe 

that serves as a high-flow bypass. The 15-in. pipe is 1.8 ft 

lower in elevation than the inlet pipe, whereas the bypass 

2 Effectiveness of a Pressurized Stormwater Filtration System in Green Bay, Wisconsin
Figure 1. Plan view of St. Mary’s Hospital, Green Bay, Wis. (Arkal Filtrations System).

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Design of the Pressurized Stormwater Filtration System 3
is at the same elevation as the inlet pipe. The 15-in pipe is 

elevated 30 in. above the bottom of the flow splitter, which 

enables larger particles to be retained in the flow splitter. 

The holding tank volume of 9,200 ft3 is designed to hold 

runoff from a 2-year, 30-minute duration event, which is 

equivalent to 0.54 in. of runoff depth over the drainage 

area. A design with this capacity will capture approximately 76 percent of the total average annual rainfall 

depth (Bachhuber and others, 2001). The holding tank is 

equipped with an electric pump that feeds the filtration 

system. The pump is triggered when the water level in 

the holding tank exceeds a preset high-flow water-surface 

elevation, and it is turned off when the water level drops 

below a low-flow elevation. A detailed description of the 

PSF system is included in Bachhuber and others (2001). 

The system pumps stormwater through two pressurized filter units at an approximate rate of 450 gal/min, or 

1 ft3

/s. The first unit consists of four filter towers with 

three disk-filter units per tower (Bachhuber and others, 

2001). Each disk-filter unit has 40−50 rings designed to 

remove particles greater than 50 μm. An individual tower 

automatically goes through a backwashing cycle when the 

pressure differential across the filter rings exceeds a preset 

level; the other three towers continue to process stormwater to the second filtration unit. The backwash water flows 

to a backwash tank that drains to the sanitary sewer.

The stormwater is passed to a second filtration unit 

that has a series of five 48-in. diameter sealed filter tanks 

filled with sand. These filter tanks are designed to remove 

particles greater than 5 μm. Each of the filter tanks undergoes a similar backwashing cycle as the first unit while the 

other four filter tanks process stormwater. The manufacturer states that 80 percent of suspended solids greater than 

5 μm will be removed (Bachhuber and others, 2001).

The filtration system treats runoff from 5.5 of the 

21 acres of St. Mary’s Hospital (table 1). The largest percentage of the treated drainage area is a parking lot 

for patients and staff, with a capacity of approximately 

200 cars. This parking lot is swept in the spring and an 

additional 1−2 times each year. Sand and salt are applied 

periodically during the cold weather for snow and ice control. Fertilizer and pesticides are applied to the lawn area 

during spring and fall (Bachhuber and others, 2001).

Sampling Design

Fifteen nonconsecutive runoff events greater than or 

equal to 0.2 inches of rainfall were sampled over a 2-year 

monitoring period. The inlet, the outlet, and the bypass 

pipes were equipped for monitoring flow and water quality 

into and out of the system (fig. 1). Electromagnetic velocity meters were used to measure velocity in the inlet and 

bypass pipes, and pressure transducers were used to measure water level. A separate backup pressure transducer 

was installed at the inlet site in case of malfunctions with 

the area-velocity meters. Flow at each site was calculated 

by multiplying the velocity by the cross-sectional area 

based on the water level. A Doppler-type area-velocity 

meter between two pressurized valves on the 6-in. outlet 

pipe was used for measuring pipe-full conditions. Flow 

was calculated by multiplying the measured velocity by 

the entire cross-sectional area of the 6-in. pipe. Continuous 

water levels were also recorded by pressure transducers 

inside the holding tank and flow-splitter.

Automatic samplers were programmed to collect 

flow-weighted samples when the water level exceeded a 

threshold level. In this respect, the sampling frequency 

increased or decreased to reflect the magnitude of flow. 

Flow-weighted sampling allowed for the collection of one 

composite runoff-event sample consisting of numerous 

subsamples throughout the hydrograph. This approach 

resulted in a single average or “event mean” concentration 

for each runoff event. The data logger in the monitoring 

station was programmed to initiate a subsample for a predefined volume of flow; consequently, more subsamples 

were collected for large-volume events than for small-volume events. The volume between subsamples was determined such that a minimum of five 1-L subsamples were 

collected for each event, and the sampler capacity was 40 

1-L subsamples. 

The initial constituent list was based on constituents 

that the device may reduce, according to the manufacturer. 

Concentrations of additional constituents (table 2) were 

measured to help urban land managers base stormwatermanagement decisions on scientific data.

Two blank samples were collected at the inlet and 

outlet and analyzed for the same constituents as those 

from runoff-event samples (table 3). The first sample 

Table 1. Characteristics of the drainage area that 

contributes runoff to the pressurized stormwater filtration 

system at St. Mary’s Hospital, Green Bay, Wis.

Land use Acres Percent of total 

runoff contributed

Rooftops 1.45 26

Parking lots 3.50 64

Lawn 0.55 10

4 Effectiveness of a Pressurized Stormwater Filtration System in Green Bay, Wisconsin
was collected at the beginning of the project before event 

sampling began, and the second blank was collected 

in the middle of the project to validate clean sampling 

procedures. Values from the blank samples were all below 

detection limits, indicating that sampling methods did not 

introduce contamination (table 3). Replicate samples were 

collected during events 9 and 13 to quantify variability in 

the sampling process. Replicates of samples from event 9 

resulted in relative percent difference (RPD) of less than 

15 percent for 8 of 9 constituents at the inlet (the exception 

was total Kjeldahl nitrogen (TKN), at 24 percent) and 7 of 

9 constituents at the outlet (the exceptions were total suspended solids (TSS), 36 percent; and TKN, at 29 percent). 

Replicates from event 13 samples resulted in RPDs of less 

than 15 percent for all but two constituents at the inlet (calcium, at 20 percent; and magnesium, at 20 percent) and all 

but one constituent at the outlet (TKN, at 17 percent).

Unforeseen Monitoring Complications

Several unforeseen complications in maintaining the 

filtration system arose during this project. These complexities prevented sampling of consecutive runoff events and 

resulted in extension of the project by a year. Some of 

these problems are described below. Periods of downtime 

during the study are listed in table 4. 

A bypass pipe in the flow splitter was designed to 

allow high flow (that which exceeded the treatment capacity of the system) to discharge directly to the storm sewer 

system (fig. 1). The low elevation of the bypass pipe, however, allowed filtered water that had been discharged to the 

receiving storm sewer to drain back into the flow splitter. This backflow could possibly skew results from the 

outlet during a runoff event. Therefore, in January 2001, 

Table 2. Constituent list, limits of detection, and analytical methods for samples collected at the pressurized 

stormwater filtration system.

[mg/L, milligrams per liter; μg/L micrograms per liter; NA, nonapplicable]

Constituent or characteristic Unit Limit of 

detection

Limit of

quantification Method

Total dissolved solids mg/L 5 NA SM2540C1

Total suspended solids3 mg/L 5 NA EPA 160.22, 3

Total Kjeldahl nitrogen mg/L 0.14 0.4 EPA 351.22

Nitrate plus nitrite as N mg/L as N .01 .031 EPA 353.22

Dissolved phosphorus mg/L as P .002 .005 EPA 365.12

Total phosphorus mg/L as P .005 .016 EPA 365.12

Total recoverable calcium4 mg/L .02 .06 EPA 200.71, 4

Total recoverable magnesium4 mg/L .03 .1 EPA 200.71, 4

Total recoverable zinc4 μg/L 19 62 EPA 200.91, 4

Suspended sediment concentration mg/L .5 NA Knott, 1993

Sand-silt split NA NA NA Guy, 1977

Five-point Sedigraph (fall diameter) NA NA NA USGS, 1993

Sand fractionation NA NA NA Guy, 1977

1 American Public Health Association and others, 1989 SM (Standard Methods).

2 U.S. Environmental Protection Agency (EPA) (1979).

3 Manufacturer states that 80 percent of the particles larger than 5 μm can be controlled.

4 Samples for the first four events were analyzed by Method sw846, 6010B; in the spring of 2001.

Design of the Pressurized Stormwater Filtration System 5
Table 3. Concentrations of blanks and replicate samples for quality control.

[mg/L, milligrams per liter; μg/L, micrograms per liter; <, less than]

Quality -

control 

sample

Date

Dissolved 

solids,

total 

(mg/L)

Suspended 

solids,

total 

(mg/L)

Kjeldahl 

nitrogen, 

total 

(mg/L)

Nitrate plus 

nitrite as 

nitrogen 

(mg/L)

Phosphorus, 

dissolved 

(mg/L)

Phosphorus,

total 

(mg/L)

Zinc, 

total 

(μg/L)

Calcium, 

total 

(mg/L)

Magnesium, 

total 

(mg/L)

Inlet Outlet Inlet Outlet Inlet Outlet Inlet Outlet Inlet Outlet Inlet Outlet Inlet Outlet Inlet Outlet Inlet Outlet

 Blank 1 11/09/00 <7 <7 <5 <5 <0.14 <0.14 <0.010 <0.010 <0.005 <0.005 <0.005 <0.005 <19 <19 0.05 0.34 <0.03 0.17 Blank 2 06/25/01 <20a <20a <5 <5 .24 <.14 <.010 <.010 <.005 <.005 <.005 <.005 <19 <19 .22 .05 .12 <.03

 Replicate 1a (event 9)

 04/27/02 174 448 25 9 .33 .35 .393 .839 .022 .021 .069 .051 35 26 13.20 33.30 5.4 14.90

 Replicate 1b(event 9)

 04/27/02 170 470 25 13 .42 .47 .386 .858 .021 .021 .068 .049 37 25 13.00 32.70 5.2 14.70

 Replicate 2a (event 13)

 06/21/02 122 574 88 8 .84 .57 .623 1.340 .016 .008 .114 .034 73 21 21.10 51.90 9.8 25.60

 Replicate 2b(event 13)

 06/21/02 110 582 85 7 .79 .48 .637 1.360 .015 .007 .114 .034 69 21 17.90 51.50 8.0 25.40

aThe limit of detection for total dissolved solids was increased by the Wisconsin State Laboratory of Hygiene in June of 2001.

6 Effectiveness of a Pressurized Stormwater Filtration System in Green Bay, Wisconsin
a backflow preventer (a door flap in front of the pipe) was 

installed to prohibit treated water from draining back into 

the flow splitter.

The system was sampled during a test period of 

the monitoring equipment in March 2000. Large black 

particles were observed in the sampled filtered water. It 

was determined that the particles were passing through the 

sand media of the secondary filtering unit. The sand was 

replaced in accordance with the 2-year life expectancy of 

the media.

The filter was not operational from April 2001 

through May 2001 because the sanitary pipe that services 

the backwash tank was clogged. To repair this problem, 

hospital maintenance constructed an access door through 

the roof of the filter house. The backwash tank was then 

dewatered, the sanitary pipe flushed out, and the valve 

leading to the sanitary pipe was enlarged to prevent future 

clogging. 

In June 2001, the disk filters in the first filtering unit 

became fouled by a buildup of biomass film. The backwash system could not completely flush the disc filters free 

of the film, which then fostered biomass for regrowth after 

the system was turned off. This continual regrowth on the 

filters increased the frequency with which the system went 

into the backwash mode. In October 2001, the system was 

sanitized with a chlorinated disinfecting rinse. After the 

rinse, biomass did not build up again during the monitoring period.

Continuous ground-water flow entering the storm 

sewer system filled the tank every 2–3 days, triggering 

the pump and filter operation during nonevent periods. 

To prevent sample collection during nonevents, the filter 

was put into a nonevent mode through data logger phone 

communication from the USGS office in Middleton. A 

few runoff events were missed because communications 

difficulties prevented event samplers from being turned on. 

These difficulties included power to the site being tripped 

by a power surge or lightning and cellular phone communication problems.

Precipitation Measurement

A tipping-bucket rain gage was used for continuous measurement of rainfall. A data logger recorded the 

number of bucket tips (0.01 in. per tip) every 60 seconds. 

This gage was not designed to record frozen precipitation 

so values during periods of snowfall and freezing rain 

were not used. Calibration data showed there was no need 

to adjust rainfall data. A comparison of the data collected 

during this study to data from the National Oceanic and 

Atmospheric Administration rain gage at Green Bay (about 

3 mi southwest of St. Mary’s Hospital) on a monthly basis 

also indicated that the rain gage was working properly 

throughout the period (table 5). A comparison of rainfall 

during the study period to the 10-year average rainfall in 

Green Bay shows that rainfall was below average during 

the study period (table 5). 

Calibration of Flows

Inlet

Stage measurements were adjusted by applying corrections that reflect differences between water-surface elevations that were measured manually and those measured 

by the area-velocity meter. Flow measurements at the 

inlet were calibrated by computing a water mass-balance 

between the calculated flow at the inlet and the holding 

tank during periods when the filter pump was not operating. The change in volume in the holding tank during these 

periods was compared against the summation of volume 

calculated from velocity and stage measurements at the 

inlet. Of the 29 events used to complete the calibration, the 

maximum difference between the calculated volumes was 

-12 percent. The average difference for the inlet calibration 

was 0.1 percent.

Table 4. Downtime of the pressurized stormwater filtration 

system during the monitoring period (March 2001–September 

2003).

Beginning

 Date

End 

date

Number

of days Problem

03/27/01 04/04/01 8 Media replacement on 

second unit

04/28/01 05/31/01 33 Backwash tank clogged

06/18/01 06/20/01 2 Biomass clog on disk 

filters

08/09/01 08/22/01 13 Biomass clog on disk 

filter

09/08/01 09/10/01 2 Biomass clog on filter 

disk

11/02/01 12/05/01 34 Maintenance

05/06/02 05/09/02 3 Plastic bag clogging 

filter

08/27/02 09/03/02 7 Maintenance

Design of the Pressurized Stormwater Filtration System 7
Outlet 

For the purpose of collecting volume-weighted samples, the flow calculated from velocity measurements was 

used. However, the inlet flow measurement was considered 

to be more accurate than the outlet flow measurement, so 

the inlet flow volumes were used to calculate final mass 

loadings. 

The calibration of the outlet flow measurement was 

similar to that described for the inlet site. A total of 54 dryweather periods were used, during which the water level 

in the holding tank exceeded the pump threshold because 

of constant ground-water contributions. The volume of 

water pumped from the holding tank was computed from 

water-level measurements in the tank; this was done by 

taking the difference between water levels before and after 

the pump cycling and multiplying by the area of the tank. 

This volume was compared to the volume computed from 

measurements by the outlet velocity probe. The relation 

between these two volumes changed over time. Five different periods were identified that resulted in distinctly different linear regressions between the two volumes. Correlation coefficients (R2

) values for these regressions ranged 

from 0.44 to 0.96. When final volumes were computed for 

the 15 sampled events, the average percentage difference 

between the inlet and outlet volumes was -17 percent (the 

outlet had less flow), with a maximum difference of 

-62 percent. There are several possible causes for the 

changing relation between volume pumped from the holding tank and volume computed, but it is uncertain exactly 

why these relations changed. Some possible explanations 

include the following: 

1. The Doppler velocity meter did not always measure 

velocity properly because of the low suspendedsediment concentrations in effluent water. The 

specifications state that a minimum of 25 mg/L of 

suspended sediment with particle sizes 30 μm or 

greater is necessary for proper operation (Dynasonics, rev 4/00); only 3 of 15 events had sufficient 

concentrations to meet these requirements.

2. Backflushing of the filters and subsequent discharge to the sanitary sewer system reduce effluent 

to the storm sewer that was measured by the outlet 

velocity meter. Frequency of backflushing changed 

throughout the study period due to a buildup of 

biomass and subsequent manual removal of this 

biomass several times. The volume of water that 

was backflushed to the sanitary sewer was not 

measured.

3. Instrumentation problems were not detected, but it 

is possible that irregularities in the velocity probe 

could have affected the measurements. 

For these reasons, the outlet flow measurements and 

volume calculations were considered to be unreliable, and 

the inlet volumes were used for mass loading calculations. 

Comparison of Particle-Size Distribution in 

Inlet, Outlet, and Flow-Splitter Samples

Particle-size analysis of runoff-event samples was 

conducted in three different ways. The first level particlesize definition was the “sand/silt split,” which was used 

to determine the percentage of sediment, by mass, with 

Table 5. Comparison of monthly rainfall between the U.S. Geological Survey raingage at St. Mary’s Hospital and the National 

Oceanic and Atmospheric Administration precipitation gage at Austin Straubel International Airport, Green Bay, Wis. 

[Rainfall is presented in inches; ASIA, Austin Straubel International Airport; NOAA National Oceanic and Atmospheric Administration; --, no data]

Month

USGS

rain gage 

2001 water 

year

USGS

 rain gage 

2002 water 

year

NOAA 

ASIA 2001

NOAA

ASIA 2002

Percent 

difference 

2001

Percent 

difference 

2002

NOAA

Normal

 1991−2000

October -- 1.5 0.5 1.7 -- -14 2.2

November 0.8 1.7 1.3 1.7 -34 2 2.3

December 0.0 1.2 1.2 1.2 -100 -4 1.4

Winter months not included

April 3.6 3.5 3.7 3.0 -1 16 2.6

May 3.8 2.8 4.7 2.8 -21 -1 2.8

June 4.4 3.8 5.2 4.7 -15 -20 3.4

July 0.9 2.3 0.9 2.2 7 7 3.4

August 4.2 2.8 3.4 4.0 21 -31 3.8

September 1.8 2.4 2.4 2.7 -22 -10 3.1

Totals 22.25 24.2 25.9 28.2 -14 -14 29.2

8 Effectiveness of a Pressurized Stormwater Filtration System in Green Bay, Wisconsin
a diameter greater than 62 μm (defined as sand) and less 

than 62 μm (defined as silt). These two fractions were analyzed further to provide additional particle-size definition. 

To define the sand fraction of the sample further, a visualaccumulation (VA) tube analysis was done (Guy, 1977). 

This analysis determines the percentage of sediment by 

mass with diameters less than 1,000, 500, 250, 125, and 

62 μm. To determine the silt fraction of the sample with 

more definition, a pipet analysis was done (Guy, 1977). 

This analysis determined the percentage of sediment by 

mass with diameters less than 31, 16, 8, 4, and 2 μm. 

Precipitation and Stormwater Quantity

Fifteen events during June 2, 2001, through 

September 9, 2002, were monitored for water quality 

and water quantity. The rainfall depth of these sampled 

events ranged from 0.20 to 1.27 in., and 15- and 30-minute 

maximum intensities were 1.12 and 0.70 in/hr, respectively (table 6). Based on a drainage area of 5.5 acres, total 

rainfall volumes ranged from 3,990 ft3

 to 25,310 ft3, and 

the volume of stormwater that passed through the filtration 

system ranged from 2,370 ft3 to 16,630 ft3 (table 7). On 

average, 70 percent of the rainfall resulted in direct runoff 

from the site. The remaining rainfall volume could have 

been lost by interception storage on the roof and parking 

lot or by infiltration into the parking lot cracks and lawn 

areas. 

To assess how rainfall during the 15 monitored events 

compared to long-term rainfall patterns, the distribution of 

monitored rainfall depths from this study was compared to 

the distribution of rainfall depths from the historical record 

monitored by the National Oceanic and Atmospheric 

Administration (NOAA). Probability distributions for 

both data sets were constructed using the Weibull plotting 

position (Helsel and Hirsch, 1992). Rainfall depths for 

individual events were computed for both data sets. Events 

that were greater than or equal to 0.2 in. (the minimum 

event depth sampled during this project) were ranked 

from smallest to largest depth. A cumulative probability 

distribution was then computed for both data sets using the 

formula PR=iR/(n+1) where R is the rainfall event referred 

to, PR is the probability of a storm having a rainfall depth 

less than that of event R, iR is the ranking of event R, and 

n is the total number of events in the data set. Results 

indicate that the distribution of rainfall depths of the events 

that were monitored for this study compare well to the 

distribution of historical rainfall events (fig. 2). 

Table 6. Rainfall and runoff statistics for sampled events at the pressurized stormwater filtration system at St. Mary’s Hospital, 

Green Bay, Wis.

[P15, maximum 15-minute rainfall intensity; P30, maximum 30-minute rainfall intensity; hr, hour; min, minute; in. inch; in/hr, inches per hour; ft3, cubic feet]

Event

Rainfall start

(date and 

24-hour time)

Rainfall end

(date and 

24-hour time)

Rain duration 

(hr:min)

Total rain

(in.)

P15

(in/hr)

P30

(in/hr)

Total rainfall 

volume

 (ft3

)

1 06/02/2001 0342 06/02/2001 1108 07:26 0.81 0.20 0.16 16,140

2 06/10/2001 1216 06/10/2001 1510 02:54 .41 .40 .26 8,170

3 06/11/2001 2213 06/12/2001 0002 01:49 .20 .20 .16 3,990

4 06/15/2001 1013 06/15/2001 1203 01:50 .38 .40 .34 7,570

5 08/25/2001 0242 08/25/2001 0934 06:52 .34 .32 .20 6,780

6 12/12/2001 2154 12/13/2001 0049 02:55 .39 .28 .24 7,770

7 04/18/2002 0414 04/18/2002 0746 03:32 .40 .28 .24 7,970

8 04/24/2002 1453 04/24/2002 1832 03:39 .63 .76 .46 12,560

9 04/27/2002 2102 04/28/2002 0735 11:33 1.13 .40 .34 22,520

10 05/01/2002 2155 05/02/2002 0042 02:47 .22 .12 .10 4,380

11 05/25/2002 0811 05/27/2002 0751 47:40 1.27 .20 .20 25,310

12 06/13/2002 2045 06/14/2002 1046 14:01 .31 .56 .44 6,180

13 06/21/2002 1714 06/21/2002 1819 01:05 .36 .88 .52 7,170

14 07/25/2002 1729 07/25/2002 1837 01:08 .40 1.12 .70 7,970

15 09/19/2002 0448 09/19/2002 0636 01:48 .21 .28 .22 4,190

Design of the Pressurized Stormwater Filtration System 9
Figure 2. Cumulative rainfall distributions for the study period compared to historical rainfall records (1949–92) for Green Bay, 

Wis., based on the National Oceanic and Atmospheric precipitation gage at Austin Straubel International Airport, Green Bay, Wis.

          

#5-5,!4)6%&2%15%.#9&/2!,,2!).&!,,'2%!4%24(!.).#(%3

2!).&!,,).).#(%3

































(ISTORICAL

3TUDYPERIOD

Table 7. Percentage of runoff and inlet volumes for the pressurized stormwater treatment system.

[ft3, cubic feet]

Event

Inlet start

(date and

24-hour time)

Inlet end

(date and 

24-hour time)

Percent

runoff

Inlet volume

(ft3

)

1 06/02/2001 0345 06/02/2001 1123 99 16,070

2 06/10/2001 1226 06/10/2001 1536 77 6,310

3 06/11/2001 2238 06/12/2001 0027 59 2,370

4 06/15/2001 1020 06/15/2001 1236 71 5,370

5 08/25/2001 0245 08/25/2001 1047 66 4,470

6 12/12/2001 2218 12/13/2001 0133 71 5,500

7 04/18/2002 0427 04/18/2002 0843 62 4,960

8 04/24/2002 1507 04/24/2002 1904 64 8,040

9 04/27/2002 2015 04/28/2002 0807 74 16,630

10 05/01/2002 2219 05/02/2002 0100 58 2,560

11 05/25/2002 0831 05/27/2002 0808 64 16,110

12 06/13/2002 2348 06/14/2002 1133 75 4,640

13 06/21/2002 1715 06/21/2002 1857 69 4,980

14 07/25/2002 1739 07/25/2002 1956 72 5,730

15 09/19/2002 0448 09/19/2002 0709 61 2,790

10 Effectiveness of a Pressurized Stormwater Filtration System in Green Bay, Wisconsin
Evaluation of the Pressurized 

Stormwater Filtration System

Event mean concentrations from inlet and outlet 

samples for each runoff event are summarized in table 8. 

Concentrations of total suspended solids and total zinc 

inlet samples were lower than those typically reported 

in previous studies of parking lot runoff (table 9). Lower 

concentrations in this study may be due to dilution of parking-lot runoff by runoff from the hospital rooftop, which 

constitutes 26 percent of the drainage area. Concentrations 

of nitrate plus nitrite, and dissolved and total phosphorus 

concentrations were similar to those reported in other studies (table 9). 

Concentrations of individual constituents at each 

site were tested for normality/log-normality by use of 

the Shapiro-Wilk test statistic (Helsel and Hirsch, 1992). 

Concentrations from inlet samples were log-normally 

distributed. Concentrations from outlet samples for TSS, 

total dissolved solids (TDS), nitrate plus nitrogen, total 

magnesium, and total calcium were normally distributed. 

Concentrations of total phosphorus, dissolved phosphorus, 

TKN, and total zinc were log-normally distributed. 

Because the inlet and outlet concentrations were 

generally not of the same statistical distribution, the nonparametric Wilcoxon rank-sum test was used to explore 

differences between inlet and outlet concentrations. The 

Spearman’s rho statistic was used to test for positive correlations between concentrations at the two sites (Helsel 

and Hirsch, 1992). Test conclusions were made using 

a confidence interval of 95 percent. For the Wilcoxon 

rank-sum test, the null hypothesis was that the inlet and 

outlet concentration distributions are the same. A direct 

comparison of inlet and outlet constituent concentrations 

from individual runoff events is problematic due to mixing 

of stormwater and ground water stored in the holding tank 

between events. Ground water continuously leaks into the 

storm sewer and therefore into the PFS system during dry 

weather. For this reason, the holding tank contains an initial amount of ground water at the beginning of each event. 

Because the tank is upstream from the outlet, the ground 

water stored in the tank at the beginning of each event 

is sampled as part of the event. For suspended constituents and some dissolved constituents, this ground-water 

influence may reduce the concentrations at the outlet. For 

dissolved constituents such as calcium and magnesium, the 

concentrations at the outlet may be increased as a result of 

the ground-water influence.

Inlet samples had detectable concentrations of all 

constituents analyzed except for dissolved phosphorus in 

three samples (table 8). Generally, outlet samples also had 

detectable concentrations of all constituents except TSS in 

three samples, dissolved phosphorus in three samples, total 

phosphorus in one sample, and total zinc in one sample. 

For values that were reported as below the detection limit 

(censored values), one-half of the detection limit was used 

as an estimate of the concentration for statistical testing and for calculation of event loads. To verify that this 

estimate did not bias reported results, the load efficiencies were also computed by ignoring the events that had 

censored values or replacing these censored values with a 

zero value. Resulting differences were only + 2 percent, 

suggesting that using one-half of the detection limit as 

an estimate of concentration does not substantially bias 

results.

Comparison of Water-Quality Parameters in 

Inlet and Outlet Samples

Of the 10 constituents analyzed for in samples collected throughout this study, those that are associated with 

particulate matter had higher mean concentrations in inlet 

samples than in outlet samples, dissolved constituents that 

are associated with ground-water seepage entering the system had higher mean-concentrations in outlet samples, and 

dissolved phosphorus concentrations were not significantly 

different between the two monitoring locations.

Inlet concentrations were significantly higher than 

outlet concentrations (p less than 0.05) for TSS, TKN, total 

phosphorus, and total zinc. In addition, inlet and outlet 

concentrations were positively correlated for TSS, TKN, 

and total phosphorus (p less than 0.05). This implies that, 

in general, outlet concentrations for these three constituents increased with increasing inlet concentrations. For 

concentrations of TKN and total zinc, the pattern appears 

similar, but tests do not show this relation to be statistically 

significant.

Concentrations of TDS, calcium, magnesium, and 

nitrate plus nitrite—all dissolved constituents—were 

higher (p less than 0.05) in the outlet. The increase in 

TDS, calcium, and magnesium is most likely explained 

by ground-water contributions to the holding tank. The 

initial ground water stored in the tank was pumped through 

the PSF system at the beginning of each runoff event. 

Because the stormwater was combined with this initial 

ground water in the holding tank, it was included in samples from the outlet but not in samples from the inlet. On 

Evaluation of the Pressurized Stormwater Filtration System 11
Table 8. Event mean concentrations of 15 events during testing of the pressurized stormwater filtration system.

[mg/L, milligrams per liter; μg/L, micrograms per liter; --, no data; <, less than; Min, minimum; Max, maximum]

Event

Suspended 

solids,

total

(mg/L)

Dissolved 

solids,

total 

(mg/L)

 Suspended 

sediment 

concentration 

(mg/L)

Kjeldahl 

nitrogen,

total

(mg/L)

Nitrate plus 

nitrite as 

nitrogen

(mg/L)

Phosphorus,

dissolved

(mg/L)

Phosphorus, 

total

(mg/L)

Zinc,

total

(μg/L)

Calcium,

total

(mg/L)

Magnesium,

total

(mg/L)

Inlet Outlet Inlet Outlet Inlet Outlet Inlet Outlet Inlet Outlet Inlet Outlet Inlet Outlet Inlet Outlet Inlet Outlet Inlet Outlet

1 10 <3 38 186 12 -- 0.32 0.39 0.286 0.674 0.005 <0.005 0.023 0.015 24 19 6.5 19.0 2.3 8.32 38 7 62 610 47 4 .51 .59 .641 1.700 <.005 <.005 .070 <.005 50 20 11.0 60.0 4.3 29.03 20 <2 106 504 -- -- 1.01 .55 .889 1.490 <.005 <.005 .059 .028 36 24 13.0 49.0 4.6 23.04 23 4 74 367 25 4 .62 .57 .557 1.100 .020 .014 .061 .037 45 25 11.0 37.0 4.1 17.05 32 <2 160 250 31 2 1.34 .53 1.690 1.630 .012 .021 .061 .025 72 19 21.8 25.7 8.5 11.06 17 3 54 380 14 3 .42 .43 .357 .931 .021 .025 .043 .033 45 <16 7.7 41.6 3.1 23.07 153 45 282 646 141 43 2.05 1.71 1.340 1.800 .038 .028 .165 .087 134 56 33.6 47.2 12.0 20.08 426 61 546 768 340 67 2.22 1.04 .743 .906 .026 .021 .316 .087 208 50 63.6 35.3 30.0 14.09 25 9 174 448 28 19 .33 .35 .393 .839 .022 .021 .069 .051 35 26 13.2 33.3 5.4 14.9

10 15 8 204 950 -- -- .56 .43 .697 1.880 .024 .017 .053 .034 36 18 17.4 77.2 7.4 36.011 14 4 122 282 13 5 .84 .51 .599 .789 .014 .015 .056 .033 53 22 12.0 25.0 4.5 11.012 19 3 190 344 16 4 .51 .75 .587 .935 .015 .011 .050 .032 31 18 23.8 34.1 10.0 15.013 88 8 122 574 122 7 .84 .57 .623 1.340 .016 .008 .114 .034 73 21 21.1 51.9 9.8 25.614 176 25 68 808 238 12 1.06 .78 .839 2.090 .017 .025 .213 .067 132 22 34.6 75.9 16.0 41.015 21 6 208 556 43 2 .78 .46 .839 1.240 .168 .119 .255 .134 52 17 22.5 47.4 11.0 28.0

Min 10 <2 38 186 12 2 .32 .35 .286 .674 <.005 <.005 .023 <.005 24 17 6.5 19.0 2.3 8.3

Max 426 61 546 950 340 67 2.22 1.71 1.690 2.090 .168 .119 .316 .134 208 56 63.6 77.2 30.0 41.0

Median 23 8 122 504 31 5 .78 .55 .641 1.240 .020 .021 .061 .034 50 22 17.4 41.6 7.4 20.0

Mean 72 15 161 512 82 14 .89 .64 .739 1.290 .031 .027 .107 .050 68 26 20.9 44.0 8.9 21.1

12 Effectiveness of a Pressurized Stormwater Filtration System in Green Bay, Wisconsin
August 8, 2001, grab samples of ground water that had 

seeped into the storm-sewer system were collected at four 

different locations in the system. The concentrations for 

TDS, calcium, and magnesium in ground water were much 

higher than concentrations in the runoff (table 10), providing evidence that the ground-water contribution influenced 

the composition of water at the outlet. These ground-water 

seepage samples were not analyzed for nitrate plus nitrite, 

so the cause of the higher nitrate plus nitrite concentrations 

at the outlet has not been determined. Because nitrate plus 

nitrite is also a dissolved constituent, the ground-water 

seepage is a likely source. Another potential source of 

added nitrate plus nitrite at the outlet could be transformation of organic nitrogen and ammonium to nitrate and 

nitrite. 

 Inlet concentrations for TSS ranged from 10 to 

426 mg/L, and the median was 23 mg/L. The outlet maximum concentration was 61 mg/L, the median value was 

8 mg/L, and three samples had concentrations below the 

detection limit. Concentrations of TSS exceeded Wisconsin 

standards for sewage-treatment-works effluent (30 mg/L) 

at the inlet for six events and at the outlet for two events 

Table 9. Comparison of average event mean concentrations from several studies in Wisconsin and Michigan.

[mg/L, milligrams per liter; μg/L micrograms per liter; --, no data available]

Study

Total

suspended 

solids

(mg/L)

Total

Kjeldahl

nitrogen

(mg/L)

Nitrate plus 

nitrite

as N

(mg/L)

Dissolved

phosphorus 

(mg/L)

Total

phosphorus 

(mg/L)

Total zinc

(μg/L)

St. Mary’s Hospital parking lot, 

Green Bay 72 0.89 0.74 0.031 0.106 68

City maintenance yard, Madison 

(Waschbusch, 1999) 214 -- .48 .16 .44 217

City garage and parking lot,

 Milwaukee (Corsi and others, 1999) 313 -- .39 .02 .28 164

Commercial strip, Madison 

(Waschbusch and others, 1999) 82 -- -- .012 .09 --

Commercial strip, Marquette, Mich., 

(Steuer and others, 1997) 134 1.25 .39 .031 .20 148

Shopping center, Madison 

(Waschbusch, 1995) 47 .90 .96 .054 .09 182

Industrial, Madison

(Bannerman and others, 1993) 91 -- -- .07 .26 261

Commercial strip, Milwaukee 

(Bannerman and others, 1983; 

Post Office)

212 .108 .708 -- .108 145

Shopping center, Milwaukee 

(Bannerman and others, 1983; 

Rustler)

202 .105 .781 -- .105 156

Evaluation of the Pressurized Stormwater Filtration System 13

Table 10. Results of ground-water-seepage samples 

collected in the storm-sewer system at St. Mary’s Hospital, 

Green Bay, Wis., August 8, 2001.

[Concentrations in milligrams per liter]

Sample

Total

dissolved

solids

Total

calcium

Total 

magnesium

1 774 81.5 49.0

2 836 73.2 47.6

3 780 78.0 46.2

4 782 80.9 48.0

Mean 793 78.4 47.7
(Wisconsin Administrative Code NR 210, 1997). The total 

event loadings of TSS monitored for the study were 453.7 

lb at the inlet and 79.4 lb at the outlet, resulting in an overall removal efficiency of 83 percent (table 11).

Suspended-sediment concentrations were determined 

in samples from 13 events at the inlet and 12 events at 

the outlet. Sample volume was not sufficient to perform 

analysis of suspended-sediment concentration for several 

events. The general relation between suspended-sediment 

concentration and TSS concentrations is positive; linear 

regression results in R2

 values of 0.92 and 0.93 for the inlet 

and the outlet data, respectively. However, the ratio of TSS 

to suspended-sediment concentration ranged from 0.49 to 

1.25 at the inlet with a median of 0.92. The ratio ranged 

from 0.47 to 3.0 at the outlet with a median of 1.0. The 

total event loadings of suspended-sediment concentration 

for the sampled events were 443.9 lb at the inlet and 

84.1 lb at the outlet, resulting in a removal efficiency of 

81 percent.

Total nutrients (total phosphorus and TKN) were also 

reduced through the PSF system. The median concentrations of total phosphorus were 0.061 at the inlet and 0.034 

at the outlet (table 8). The total monitored loading of phosphorus was reduced by 54 percent. Median concentrations 

of TKN were 0.78 and 0.55 at the inlet and outlet, respectively; a total loading reduction of 26 percent.

Total recoverable zinc concentrations were also 

reduced between the inlet and the outlet. The median concentration at the inlet was 50 μg/L and the median at the 

outlet was 22 μg/L. This resulted in an overall 62-percent 

reduction of zinc loading. The Wisconsin State Standard 

for Surface Water Quality Criteria for Toxicity (Wisconsin Administrative Code NR 105, 1997) was exceeded at 

the inlet for only two events and was not exceeded at the 

outlet. 

Concentrations of dissolved nutrients were not significantly reduced through the PSF system. The median 

concentrations of dissolved phosphorus were nearly identical (0.20 and 0.21 mg/L at the inlet and outlet, respectively). The loading, however, was reduced by 9 percent. 

Most likely, nitrate plus nitrite concentrations increased 

between the inlet and outlet monitoring sites because of 

the ground-water influence to the PSF system. Median 

nitrate plus nitrite concentrations were 0.641 mg/L at the 

inlet and 1.24 mg/L at the outlet. The total monitored loading of nitrate plus nitrite was increased by 76 percent.

Other constituents monitored were TDS, calcium, and 

magnesium. All three of these constituents were influenced by the ground-water contribution to the PSF system. 

Median TDS concentration increased from 122 to 

504 mg/L between the inlet and the outlet, resulting in an 

increased total TDS loading of 193 percent. Median calcium concentrations increased from 17.4 mg/L at the inlet 

to 41.6 mg/L at the outlet, and median magnesium concentrations increased from 7.4 to 20.0 through the system. 

Corresponding inlet and outlet loadings of calcium and 

magnesium increased by 98 and 121 percent, respectively.

Results of Particle-Size Analysis

Results of particle-size analysis for the 15 monitored 

events, samples from 12 events at the inlet and outlet were 

analyzed for a sand/silt split (table 12). Of those 12 events, 

6 samples at the inlet had sufficient sediment content and 

sample volume for the VA tube analysis (table 13). Of 

those six samples, three samples also had sufficient sediment content for the pipet analysis, resulting in a full definition of the particle-size distribution. All outlet samples 

did not contain enough sediment and sample volume for 

the VA tube and pipet analyses. 

At the end of the monitoring period, core samples 

were collected from the flow splitter (a catch basin that 

separated the inlet pipe and the bypass pipe from the holding tank). All sediment was removed from the flow splitter 

in May 2001, and the coring was done in November 2002. 

The depth of sediment in the flow splitter ranged from 

14 in. near the inlet to 1 in. towards the bypass (fig. 3). 

Three composite samples of three subsamples each were 

collected and analyzed for particle-size distribution. These 

samples were used to define the percentage of sediment by 

mass that had diameters less than 2,000, 1,000, 500, 250, 

125, and 62 μm.

Results from the sand/silt split analyses demonstrated 

that the particle-size distribution in samples from the inlet 

and the outlet contained a higher percentage of silt-sized 

particles than sand-sized particles. The median silt content 

in samples was 78 percent and 87 percent for the inlet and 

outlet, respectively. Sediment in samples from only 1 of 12 

events at the inlet and 1 of 12 events at the outlet had more 

sand than silt-sized particles (table 12). Sediment particlesize results also showed that 83 percent of the sediment 

load for these 12 events consisted of silt-sized particles, 

and 92 percent of the outlet sediment load was silt. Linear 

regression did not indicate any obvious trends between 

sand or silt percentage and rainfall depth or intensity. 

Results from the VA tube and pipet analyses illustrate 

that there can be large between-event differences in the 

particle-size distribution at the inlet (table 13, fig. 4). For 

example, 60 percent of sediment in the sample collected 

during event 15 was greater than 1,000 μm, whereas all 

14 Effectiveness of a Pressurized Stormwater Filtration System in Green Bay, Wisconsin
Table 11. Sum of loads for 15 events during testing of the pressurized stormwater filtration system, Green Bay, Wis.

[lbs, pounds; SOL, sum of loads; --, no data]

Event

Suspended 

solids,

total

(lbs)

Dissolved

solids,

total

(lbs)

 Suspended 

sediment 

concentration 

(lbs)

Kjeldahl 

nitrogen,

total

(lbs)

Nitrate plus 

nitrite

(lbs)

Phosphorus,

dissolved

(lbs)

Phosphorus,

total 

(lbs)

Zinc,

total

(lbs)

Calcium,

total

(lbs)

Magnesium,

total

(lbs)

# Inlet Outlet Inlet Outlet Inlet Outlet Inlet Outlet Inlet Outlet Inlet Outlet Inlet Outlet Inlet Outlet Inlet Outlet Inlet Outlet

1 10.0 1.5 38.1 186.6 12.0 -- 0.32 0.39 0.287 0.676 0.005 0.005 0.023 0.015 0.024 0.019 6.5 19.1 2.3 8.32 15.0 2.8 24.4 240.3 18.5 1.6 0.20 0.23 0.253 0.670 0.001 0.001 0.001 0.001 0.020 0.008 4.3 23.6 1.7 11.43 3.0 0.1 15.7 74.6 -- -- 0.15 0.08 0.132 0.220 0.000 0.000 0.009 0.004 0.005 0.004 1.9 7.2 0.7 3.44 7.7 1.3 24.8 123.0 8.4 1.3 0.21 0.19 0.187 0.369 0.007 0.005 0.020 0.012 0.015 0.008 3.7 12.4 1.4 5.75 8.9 0.3 44.6 69.8 8.6 0.6 0.37 0.15 0.472 0.455 0.003 0.006 0.017 0.007 0.020 0.005 6.1 7.2 2.4 3.16 5.8 1.0 18.5 130.5 4.8 1.0 0.14 0.15 0.123 0.320 0.007 0.009 0.015 0.011 0.016 0.003 2.6 14.3 1.1 7.97 47.4 13.9 87.3 200.0 43.7 13.3 0.63 0.53 0.415 0.557 0.012 0.009 0.051 0.027 0.042 0.017 10.4 14.6 3.7 6.28 213.8 30.6 274.0 385.5 170.6 33.6 1.11 0.52 0.373 0.455 0.013 0.011 0.159 0.044 0.104 0.025 31.9 17.7 15.1 7.09 26.0 9.3 180.6 465.1 29.1 19.7 0.34 0.36 0.408 0.871 0.023 0.022 0.072 0.053 0.036 0.027 13.7 34.6 5.6 15.5

10 2.4 1.3 32.6 151.8 -- -- 0.09 0.07 0.111 0.300 0.004 0.003 0.008 0.005 0.006 0.003 2.8 12.3 1.2 5.811 14.1 4.0 122.7 283.6 13.1 5.0 0.84 0.51 0.602 0.794 0.014 0.015 0.056 0.033 0.053 0.022 12.1 25.1 4.5 11.112 5.5 0.9 55.0 99.6 4.6 1.2 0.15 0.22 0.170 0.271 0.004 0.003 0.014 0.009 0.009 0.005 6.9 9.9 2.9 4.313 27.4 2.5 37.9 178.4 37.9 2.2 0.26 0.18 0.194 0.417 0.005 0.002 0.035 0.011 0.023 0.007 6.6 16.1 3.0 8.114 63.0 8.9 24.3 289.0 85.1 4.3 0.38 0.28 0.300 0.748 0.006 0.009 0.076 0.024 0.047 0.008 12.4 27.2 5.7 14.715 3.7 1.0 36.2 96.8 7.5 0.3 0.14 0.08 0.146 0.216 0.029 0.021 0.044 0.023 0.009 0.003 3.9 8.3 1.9 4.9

Total 453.7 79.4 1,016.7 2,974.6 443.9 84.1 5.33 3.94 4.173 7.339 0.133 0.121 0.600 0.279 0.429 0.164 125.8 249.6 53.2 117.4SOL Efficiency 83 -193 81 26 -76 9 54 62 -98 -121

Evaluation of the Pressurized Stormwater Filtration System 15




























"YPASSPIPE )NFLOWPIPE









      

    

   



 

      

    

   



 

%80,!.!4)/.

,INEOFEQUALSEDIMENTDEPTHININCHES

3EDIMENTCOREMEASURINGPOINT	

DEPTHININCHES





IN



IN

Figure 3. Map showing the depth of retained sediment in the flow splitter, the location of core samples, and contour lines 

generated using a contour method in ArcInfo.

Table 12. Results of sand/silt split sediment analysis at the 

inlet and the outlet of the pressurized stormwater filtration 

system for 12 rainfall events.

[μm, micrometer; %, percent by mass; >, greater than; <, less than]

Inlet % Outlet %

Event 

number

>62

(μm)

<62

(μm)

>62

(μm)

<62

(μm)

2 44.0 56.0 29.1 70.9

4 23.8 76.2 21.7 78.3

5 33.0 67.0 54.8 45.2

6 10.0 90.0 20.6 79.4

7 1.9 98.1 2.3 97.7

8 0.9 99.1 8.8 91.2

9 19.4 80.6 7.6 92.4

11 10.0 90.0 1.4 98.6

12 18.5 81.5 24.7 75.3

13 42.5 57.5 17.7 82.3

14 37.9 62.1 3.0 97.0

15 67.9 32.1 2.4 97.6

Table 13. Particle-size distribution determined from samples 

taken during six events at the inlet sampling site.

[mm, millimeter; --, insufficient sample amount for determination of smaller 

particle sizes; %, percent by mass; NA, not analyzed; <, less than]

Event number

Particle

size 

(mm)

7

(%)

8

(%)

9

(%)

13

(%)

14

(%)

15

(%)

<1,000 100 100 87.6 79 94.3 40.2

<500 100 100 86.8 72.2 85.1 NA

<250 99.6 100 82.2 60.4 75.2 36

<125 98.7 99.8 80.8 57.5 68.8 32.5

<62 98.1 99.1 80.6 57.5 62.1 32.1

<31 94.4 95.5 -- -- 44.4 --

<16 81.4 78.8 -- -- 32.6 --

<8 58.9 56.4 -- -- 25.2 --

<4 35.3 38.9 -- -- 17.3 --

<2 17.9 23.9 -- -- 13.5 --

16 Effectiveness of a Pressurized Stormwater Filtration System in Green Bay, Wisconsin
sediment in samples from events 7 and 8 had diameters 

less than 1,000 μm. In the flow splitter, 97 percent of the 

sediment had diameters greater than 62 μm, 49 percent had 

diameters greater than 1,000 μm, and only 3 percent had 

diameters less than 62 μm (table 14). These larger particles 

consisted of sand, gravel, pavement, leaves, cigarette butts, 

and vegetation. Data from the flow splitter suggested that 

some of the larger particles were selectively removed by 

settling before water was drained to the holding tank but 

that the smaller particles remained in suspension.

Summary and Conclusions

The primary purposes of this project were to evaluate 

the performance of the pressurized stormwater filter (PSF) 

system for the U.S. Environmental Protection Agency’s 

(USEPA) Environmental Technology Verification Program 

(ETV) program and to characterize runoff from the roof 

and parking lot for comparison to other sources monitored 

in WDNR and USGS urban-runoff studies. The ETV 

program was established to provide information to help 

guide municipalities in selecting treatment practices for 

reduction of stormwater pollutants in urban runoff. This 

is the first of three systems that the USGS and WDNR 

      

0!24)#,%3):%).-)#2/-%4%23

0%2#%.4!'%3%$)-%.4,%334(!.!')6%.0!24)#,%3):%"97%)'(4























%VENT

%VENT

%VENT

%VENT

%VENT

%VENT

&LOW
SPLITTER

MEAN

Figure 4. Particle-size distributions in inlet samples from six events and in flow-splitter samples collected after the monitoring 

period, Green Bay, Wis.

Table 14. Particle-size distribution for the sediment 

samples collected in the flow splitter, November 2002.

[mm, millimeter; %, percent by mass; <, less than]

Sample number

Particle 

size 

(mm)

1

(%)

2

(%)

3

(%)

Mean

(%)

<2,000 70 68 71 70

<1,000 51 49 54 51

<500 34 34 39 36

<250 16 16 21 18

<125 6 7 8 7

<62 3 3 4 3

Summary and Conclusions 17
collaborated on in cooperation with the USEPA ETV 

program. Since 1978, the U.S. Geological Survey (USGS) 

and Wisconsin Department of Natural Resources (WDNR) 

have conducted several projects to understand the waterquality characteristics of stormwater in Wisconsin.

A pressurized stormwater filter system was installed 

at St. Mary’s Hospital in Green Bay, Wisconsin to treat 

runoff from 5.5 acres of rooftop and parking lot in 1998. 

The pressurized stormwater filtration system consists of 

a flow splitter, which directs runoff either to the underground holding tank or to the bypass pipe. The water that 

enters the holding tank is then pumped through a dual 

filter unit and discharged to the storm sewer. The system 

was designed to treat event runoff up to 0.54 inches, which 

would account for approximately 76 percent of the total 

average annual rainfall depth in Green Bay. 

From November 2000 until September 2002, 15 

nonconsecutive runoff events greater than or equal to 

0.2 inch of rainfall were monitored for flow and water 

quality. All runoff from these 15 events was treated 

without the occurrence of bypassing. Samples from the 

inlet and the outlet to the system were analyzed for water 

quality for all events. At the end of the monitoring period, 

sediment core samples were collected from the flow splitter and were analyzed for sediment mass and sieved for 

particle-size distribution. The quantity of water passing 

through the treatment system for the 15 monitored events 

ranged from 2,370 to 16,630 ft3 for individual events and 

was, on average, 70 percent of the total rainfall volume 

in the contributing watershed. Significant reductions in 

concentration were achieved between the inflow and the 

outflow in constituents such as TSS (83 percent), suspended sediment (81 percent), total phosphorus 

(54 percent), TKN (26 percent), and total recoverable zinc 

(62 percent). Concentrations of TSS exceeded Wisconsin 

Standards for sewage-treatment-works effluent (30 mg/L) 

at the inlet for six events and at the outlet for two events. 

The Wisconsin State Standard for Surface Water Quality 

Criteria for Toxicity for total recoverable zinc (Wisconsin 

Administrative Code NR105, 1997) was exceeded at the 

inlet during two events and was not exceeded at the outlet. 

The dissolved fraction of phosphorus was not significantly 

removed. Concentrations of other dissolved constituents 

were increased between the inlet and outlet due to groundwater contribution to the holding tank between monitored 

events. Although the amount of total sediment was reduced 

through the system, the sand/silt fractions did not change 

appreciably from the inlet to the outlet. Final results were 

consistent with the manufacturer’s design for the removal 

of 80 percent of suspended solids greater than 5 μm for all 

water passing through the PSF system. Results from this 

study were forwarded to ETV personnel for their verification reporting (U.S. Environmental Protection Agency 

2004).

References Cited

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methods for the examination of water and wastewater, 

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18 Effectiveness of a Pressurized Stormwater Filtration System in Green Bay, Wisconsin
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[variously paged]. 

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water phase II final rule: U.S. Environmental Protection 

Agency EPA 833−F−00−001, Fact Sheet 1.0, 4 p.

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technologies: EPA Draft 4.1, 25 p.

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Madison, Wisconsin, 1993−94: U.S. Geological Survey 

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References Cited 19
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