U.S. Department of the Interior

U.S. Geological Survey Printed on recycled paper

Fact Sheet 2015–3023

March 2015

Contaminant Removal by Wastewater Treatment Plants in the 

Stillaguamish River Basin, Washington

Introduction

Human activities in most areas of the developed 

world typically release nutrients, pharmaceuticals, personal 

care products, pesticides, and other contaminants into the 

environment, many of which reach freshwater ecosystems. In 

urbanized areas, wastewater treatment plants (WWTPs) are 

critical facilities for collecting and reducing the amounts of 

wastewater contaminants (WWCs) that ultimately discharge 

to rivers, coastal areas, and groundwater. Most WWTPs use 

multiple methods to remove contaminants from wastewater. 

These include physical methods to remove solid materials 

(primary treatment), biological and chemical methods to remove 

most organic matter (secondary treatment), advanced methods 

to reduce the concentrations of various contaminants such as 

nitrogen, phosphorus and (or) synthetic organic compounds 

(tertiary treatment), and disinfection prior to discharge (Metcalf 

and Eddy, Inc., 1979). This study examined the extent to which 

114 organic WWCs were removed by each of three WWTPs, prior 

to discharge to freshwater and marine ecosystems, in a rapidly 

developing area in northwestern Washington State. Removal 

percentages for each WWC were estimated by comparing the 

concentrations measured in the WWTP influents with those 

measured in the effluents. The investigation was carried out in the 

700-mi2

 Stillaguamish River Basin, the fifth largest watershed that 

discharges to Puget Sound (fig. 1). 

Squire Cr

North Fork Stillaguamish River

South Fork Stillaguamish River

Pilchuck Cr

Armstrong Cr

Sauk R

Skagit R

Pilchuck River

Day L

Stillaguamish R 

Sultan R

Jim Cr

Dan Cr

Illabot Cr

Finney Cr

Stillaguamish

R

Boulder R

Tenas Cr

Sauk River

South Fork

Pressentin 

Cr

Skagit R. N Fk

Grade Cr

Helena Cr

Elliott Cr

Straight Cr

Carpenter Cr

Boardman Cr

Black Cr

Murphy

Cr

Cumberland 

Cr

Williamson Cr

Arrow Cr

N. F. Canyon Cr

West

Pass

Canyon Cr

White Chuck 

River

Panther

Cr

Deer Cr

Sauk R, S Fk

Coal Cr

Falls Cr

Deer Cr

Clear Cr

Big Cr

Suiattle R

North Fork

Lake

Cassidy

Basin boundary

WWTP (Wastewater Treatment Plant)

Langley

Lake

Stevens

Granite

Falls

Darrington

Silverton

122°20'W 122°W 121°40'W

48°20'N

48°10'N

48°N

0 1 2 3 4 MILES 5

0 51 2 3 4 KILOMETERS

Modified from Wagner and 

others (2014)

Base map from The National Map.

Transverse Mercator projection, UTM Zone 10,

North American Datum of 1983

WASHINGTON

Location of

study area

Warm Beach

Arlington

Angel of the Winds Casino

Marysville

Stanwood

Everett

Warm

Beach

Possession Sound

Port

Susan

EXPLANATION

ArlingtonArlington

Figure 1. Locations of the wastewater treatment plants sampled in the Stillaguamish River Basin, 2009–11. (Modified from 

Wagner and others, 2014.) 
Study Sites and Methods

The locations of the three WWTPs 

are shown in figure 1. The major watertreatment processes used by the three 

facilities are summarized in table 1. As 

indicated in table 1, prior to the 2011 

sampling, the City of Arlington WWTP 

underwent a major upgrade that included 

the addition of a membrane bioreactor 

(MBR) and biological nutrient removal 

(BNR) to enhance contaminant removal. 

At each of the three WWTPs, paired 

samples of the influent and effluent were 

collected every 6 hours and composited 

over the same 24-hour period by the 

WWTP operators or by the Stillaguamish 

Tribe of Indians. The composited influent and 

effluent samples were filtered and processed 

by the U.S. Geological Survey (USGS), and 

analyzed for their chemical composition at the 

USGS National Water-Quality Laboratory. The 

procedures used to characterize the physical and 

chemical conditions of the sampled waters, and 

to collect, preserve, and chemically analyze the 

samples, are described by Wagner and others 

(2014).

Removal percentages for each WWC were 

computed using the approach described by the 

U.S. Environmental Protection Agency (2010). 

For WWCs that were detected in the influent 

of a WWTP but not in the effluent, the removal 

percentages reported here represent minimum 

estimates. When this occurred for the two 

WWCs that were detected in field 

blanks at concentrations exceeding 

their reporting limits (benzophenone 

and camphor), the minimum removal 

percentages were computed using the 

maximum field blank concentration, 

rather than the reporting limit. 

The maximum non-significant 

difference (MNSD) between removal 

percentages for a given WWC (α = 

0.05) was computed using the results 

reported from the analysis of spiked 

replicates of surface-water samples 

for the analytical method used for 

the compound of interest (Furlong 

and others, 2008; Zaugg and others, 

2006, 2014).

Contaminant Removal by the 

Wastewater 

Treatment Plants

Removal percentages are shown 

in figure 2 for the 16 WWCs for 

which removal percentages could 

be computed for all three WWTPs. 

The study results for the remaining 

98 compounds are summarized 

in table 2, which shows the 

removal percentages for individual 

contaminants at the individual 

facilities. Table 2 also identifies 

instances where (1) the WWC was 

not detected in the influent; (2) the 

effluent concentration was greater 

than the influent concentration; 

(3) the chemical analyses were 

inconclusive because interfering 

substances were present, the 

compound was detected in the 

influent at its reporting limit, or 

the compound was not detected in 

the effluent using a reporting limit 

that was greater than the measured 

influent concentration; or (4) the 

analyses were incomplete because 

one of the samples (influent or 

effluent) was not analyzed, the 

maximum analyte recovery was less 

than 35 percent for one or more of 

the WWTPs, or at least one of the 

samples was not filtered prior to 

analysis.

Among the 114 WWCs 

examined for this study, 71 were 

detected in the influent of 1 or more 

of the 3 WWTPs. Of the 52 WWCs 

for which removal percentages could 

be computed at 1 or more of the 

WWTPs, nearly all the compounds 

Table 1. Characteristics of the wastewater treatment plants sampled in the Stillaguamish River 

Basin, 2009–11.

[Wastewater treatment plant: Years of sample collection are in parentheses. mm, millimeter]

Wastewater treatment 

plant characteristic

Wastewater treatment plant

(year of sampling)

City of Arlington Angel of the 

Winds Casino

(2011)

Warm Beach

Camp

(2009) (2009) (2011)

Capacity

Type and size of 

population served

Municipal;

17,711 residents

Municipal;

17,930 residents

Recreation;

3,600 visitors 

per day

Recreation;

800 seasonal 

visitors

WWTP capacity, in million 

gallons per day (number of 

connections)

1.00

(4,347)

2.65

(4,427)

0.18

(3)

0.035

(350)

Treatment processes

Primary (solids removal) 6 mm screen; 

grit removal

3 mm screen; 

grit removal

2 mm screen; 

no grit removal

6 mm screen; 

no grit removal

Secondary (organic matter 

reduction) Aeration basins Aeration basins Aeration basins Two aerated 

lagoons followed 

by two constructed 

wetlands

Tertiary (additional 

contaminant removal) None

Membrane 

bioreactor; 

biological nutrient 

removal

Membrane 

bioreactor

Disinfection Ultraviolet irradiation Chlorination Chlorination/

dechlorination

Point of discharge

Stillaguamish River Groundwater 

recharge Port Susan

City of Arlington WWTP 

(photograph taken by Ken Clarke,

City of Arlington).

Angel of the Winds Casino WWTP 

(photograph montage assembled 

from Google EarthTM by Jack 

Barbash, July 2, 2014).

Warm Beach Camp WWTP 

(photograph from Dawda, 2013).
tac14-0929_fig 02

0 20 40 60 80 100

Triphenyl phosphate

Tris(2-butoxyethyl) phosphate

Phenol

p-Cresol

Triclosan

Methyl salicylate

Triethyl citrate

Camphor

Menthol

D-Limonene

β-Sitosterol

Cholesterol

β-Coprostanol

3-Methyl-1H-indole (skatole)

Minimum removal percentage

Hexahydrohexamethylcyclopentabenzopyran (HHCB)

Tris(dichloroisopropyl)

phosphate (FYROL FR 2)*

Arlington (2009)

Arlington (2011)

Angel of the Winds

Warm Beach Camp

EXPLANATION

Wastewater Treatment Plant

Maximum non-significant 

difference between 

percent removal values 

*Effluent concentration greater than 

 influent concentration at 

 Arlington in 2009

Consumer 

product 

additives

Personal

care

products

Steroids/

metabolites

Figure 2. Minimum removal 

percentages for 16 wastewater 

contaminants at the 3 wastewater 

treatment plants, Stillaguamish River 

Basin, 2009–11.

(51) exhibited removal percentages greater than zero, and most compounds 

(49) showed removal percentages greater than 50 percent at 1 or more of the 

facilities. (Confidence in the removal percentages at the WWTPs examined for 

this study, however, was constrained because the influent and effluent for each 

facility were sampled only once in each year of interest.) The data shown in 

figure 2 and table 2 also indicate that after the MBR and BNR were added to 

the City of Arlington WWTP, significantly higher removal percentages were 

observed for eight compounds, and none showed a significantly lower removal 

percentage. (It was not possible to identify increases or decreases in removal 

percentages for a given compound after the MBR and BNR were installed if 

the removal percentages before and after the MBR and BNR installation were 

both minimum values—as was the case for β-sitosterol—or if only one removal 

percentage was a minimum value, but less than the other, as was the case for 

benzophenone.) The results from this study indicate that the water treatment 

processes used by these three WWTPs reduce the concentrations of many 

WWCs in the wastewater from the communities served by these facilities prior 

to discharge into the hydrologic system.

Acknowledgments

The authors wish to express their 

appreciation to Jennifer Sevigny, Jody 

Pope (Stillaguamish Tribe of Indians), Erik 

Thornburgh (Operator of the Angel of the Winds 

Casino and Warm Beach Camp WWTPs), Fred 

Rapelyea (Arlington WWTP Supervisor), and 

Raegan Huffman (USGS Washington Water 

Science Center) for their valuable assistance on 

this project.

References Cited

Dawda, Mike, 2013, Fact sheet for NPDES permit 

WA0029904—Warm Beach Christian Camps and 

Conference Center: Bellevue, Wash., Washington 

Department of Ecology Fact Sheet, 53 p.

Furlong, E.T., Werner, S.L., Anderson, B.D., and 

Cahill, J.D., 2008, Determination of human-health 

pharmaceuticals in filtered water by chemically 

modified styrene-divinylbenzene resin-based 

solid-phase extraction and high-performance 

liquid chromatography/mass spectrometry: U.S. 

Geological Survey Techniques and Methods, 

book 5, chap B5, 56 p.

Metcalf and Eddy, Inc., 1979, Wastewater 

engineering—Treatment, disposal, reuse, 2nd 

ed., (revised by George Tchobanoglous): San 

Francisco, McGraw-Hill, 920 p.

U.S. Environmental Protection Agency, 2010, 

Treating contaminants of emerging concern—A 

literature review database: Washington, D.C., U.S. 

Environmental Protection Agency report EPA820-R-10-002, 27 p., plus 4 appendixes.

Wagner, R.J., Moran, P.W., Zaugg, S.D., Sevigny, 

J.M., and Pope, J.M., 2014, Contaminants of 

emerging concern in the lower Stillaguamish River 

Basin, Washington, 2008–11: U.S. Geological 

Survey Open-File Report 2014-1028, 8 p.

Zaugg, S.D., Phillips, P.J., and Smith, S.G., 2014, 

Analysis of pharmaceutical and other organic 

wastewater compounds in filtered and unfiltered 

water samples by gas chromatography/mass 

spectrometry: U.S. Geological Survey Open-File 

Report 2013-1297, 24 p.
Table 2. Removal percentages for 92 wastewater contaminants for which removal percentages could not be computed for one or more of the wastewater 

treatment plants studied in the Stillaguamish River Basin, 2009–11.

[Shading—Tan, not detected in influent; green, effluent concentration greater than influent concentration. Not detected in influent or effluent of any WWTP: 5-Methyl‐1H‐benzotriazole; 

9,10-Anthraquinone; Carbazole; 4-Cumylphenol; 4-n-Octylphenol; Dehydronifedipine. INC, chemical analysis inconclusive; MNSD, maximum difference between removal percentages 

considered non-significant (α = 0.05); WWC, wastewater contaminant; WWTP, wastewater treatment plant; >, greater than; –, chemical analyses incomplete or MNSD value not needed]

WWC

WWC removal percentage at WWTP

City of Arlington Angel of the 

Winds Casino

(2011)

Warm Beach 

Camp

(2009)

MNSD

(2009) (2011)

Consumer product additives

1,4-Dichlorobenzene 60 – –

3-tert-Butyl-4-hydroxyanisole – – – INC –

Acetophenone > 50 > 7 –

Acetyl hexamethyl tetrahydro 

naphthalene (AHTN)

67 78 – > 90 3.7

Benzophenone > 78 91 > 92 2.9

Isoborneol > 67 – > 64 –

Isophorone 53 – –

Isopropylbenzene – –

Isoquinoline – –

N,N-diethyl-m-toluamide 

(DEET)

95 98 94 4.7

Tetrachloroethene – – – –

Tributylphosphate 75 – > 70 –

Tris(2-chloroethyl) phosphate 

(FYROL CEF)

0 25 63 0.77

Consumer product metabolites

1,7-Dimethylxanthine > 99 > 99 – > 99 –

2-Ethyl-2-phenylmalonamide – – –

4-Nonylphenols (sum of all 

isomers)

INC > 67 > 74 –

4-Nonylphenol diethoxylates 

(sum of all isomers)

75 93 – 90 1.9

4-tert-Octylphenol diethoxylate – –

4-tert-Octylphenol monoethoxylate

INC > 62 –

4-tert-Octylphenol 80 > 81 3.8

Cotinine – > 85 – – –

Natural biochemicals

β-Stigmastanol INC > 67 – > 44 –

Indole – > 99 – –

Pesticides

Bromacil – –

Carbaryl – –

Chlorpyrifos – –

Diazinon – – –

Metalaxyl – –

Metolachlor – –

Piperonyl butoxide – > 80 > 87 – –

Prometon – –

Petroleum-derived compounds

1-Methylnaphthalene INC – –

2,6-Dimethylnaphthalene INC – –

2-Methylnaphthalene INC – –

Anthracene – –

Benzo[a]pyrene – – –

Fluoranthene – –

Naphthalene INC 84 – –

Phenanthrene – INC –

Pyrene – –

Pharmaceuticals

Acetaminophen – > 99.9 – – –

Albuterol – – –

WWC

WWC removal percentage at WWTP

City of Arlington Angel of the 

Winds Casino

(2011)

Warm Beach 

Camp

(2009)

MNSD

(2009) (2011)

Pharmaceuticals—Continued

Amitriptyline – – – –

Antipyrine – – –

Butalbital – – –

Caffeine – 99.8 – – –

Carbamazepine – – – –

Carisoprodol – – – –

Celecoxib – – – –

Chloroxylenol – 99 – –

Chlorpheniramine – – –

Citalopram – 71 58.4 – 2

Codeine – –

Dextromethorphan – – – –

Diazepam – –

Dihydrocodeine – – – –

Diltiazem – –

Diphenhydramine – – –

Efavirenz – – – –

Fluconazole – – –

Griseofulvin – – – –

Hydrocodone 2.00 – –

Ibuprofen – INC – – –

Iminostilbene – > 72 – –

Lidocaine – – –

Meperidine – – – –

Meprobamate – > 88 – –

Metaxalone – – –

Methadone 78 – –

Methocarbamol – – 83.3 – –

Methylphenidate – – –

Oxcarbazepine – – – –

Oxycodone 67 79 77.9 – 1.9

Pentobarbital – – –

Pentoxifylline – – – –

Phendimetrazine – – –

Phenobarbital – – –

Phenytoin – – – –

Primidone – 57 59 – 85

Propofol – 76 – – –

Sulfamethoxazole –

Temazepam – – –

Thiabendazole – – – –

Ticlopidine – – –

Tramadol – 68 – –

Trimethoprim > 63 – > 87 –

Venlafaxine – – –

Verapamil – – –

Warfarin – – INC –

Pharmaceutical metabolites

Chirald – – –

Norpropoxyphene – – – –

Trihalomethanes

Tribromomethane – –

Zaugg, S.D., Smith, S.G., and Schroeder, M.P., 2006, Determination 

of wastewater compounds in whole water by continuous liquidliquid extraction and capillary-column gas chromatography/mass 

spectrometry: U.S. Geological Survey Techniques and Methods, 

book 5, chap. B4, 30 p.

Authors: Jack E. Barbash, Patrick W. Moran, Richard J. Wagner (U.S. 

Geological Survey), and Mike Wolanek (City of Arlington Wastewater 

Treatment Plant)

For Information Contact:

U.S. Geological Survey

Director, Washington Water Science Center

934 Broadway, Suite 300

Tacoma, WA 98402 ISSN 2327-6932 (online)

http://dx.doi.org/10.3133/fs20153023

Publishing support provided by the U.S. Geological Survey

Tacoma Publishing Service Center