Rationale for Re-evaluating the POTW Typical Pollutant Concentrations Used in the Nutrient Model

Background

The Water Pollutant Loading Tool Nutrient Model (Nutrient Model) (USEPA, 2017) calculates total nitrogen (TN) and total phosphorus (TP) loadings based on the type of facility (i.e., SIC code) and availability of discharge monitoring report (DMR) data. For facilities that do not have nitrogen and/or phosphorus DMR data available in the program system of record (Integrated Compliance Information System-National Pollutant Discharge Elimination System (ICIS-NPDES)), typical pollutant concentrations (TPCs) are used in the loading calculations. Currently, the Nutrient Model uses two methodologies to calculate TPCs.¹

The first methodology groups the nitrogen and phosphorus concentration data available in ICIS-NPDES by SIC code, season, and facility flow class (<1 MGD, 1-5 MGD, and >5 MGD) and identifies the median TN and TP concentration for each group. For publicly owned treatment works (POTWs) (defined by SIC code 4952), the twelve TPCs for TN range from 6.71 mg/L to 8.96 mg/L, and the twelve TPCs for TP range from 0.78 mg/L to 1.61 mg/L. The Nutrient Model uses these TPCs in calculating TN and/or TP loads for POTWs having flow data available in ICIS-NPDES but no nitrogen and/or phosphorus DMR data.

The second methodology determines the median TN and TP concentration for each SIC code. For POTWs, the median TN and TP concentrations are 6.1 mg/L and 0.8 mg/L, respectively¹ . The Nutrient Model uses these TPCs in calculating TN and/or TP loads for POTWs having no flow data and no nitrogen and/or phosphorus DMR data in ICIS-NPDES.

¹Note that the TPCs and median concentrations calculated using the Nutrient Model methodology change as data are added to ICIS-NPDES. The values shown above reflect those available on the Water Pollutant Loading Tool Nutrient Model website when accessed on October 19, 2017.

Need for Re-Evaluation

In a December 14, 2012, letter to the Natural Resources Defense Council (NRDC), USEPA noted: "where nutrient discharge monitoring data [for POTWs] do exist… generally it is at facilities that employ not just secondary treatment technology but also more advanced treatment technologies" (USEPA, 2012). Because lower TN and TP concentrations are anticipated from POTWs employing advanced treatment technologies, such as denitrification and phosphorus removal, TPCs derived using a dataset more reflective of POTWs employing advanced treatment may underestimate TN and TP concentrations for those POTWs without additional treatment for nutrients.

Earlier efforts by Pait (1993), Tetra Tech (1999), and USEPA (2007) considered treatment level in determining TPCs for TN and TP at POTWs (see Table 1 below). The TPCs derived using the current Nutrient Model methodologies are within the ranges calculated for POTWs employing advanced treatment, according to these earlier TPC derivation efforts.

A state-specific analysis of TN and TP effluent data recently completed by the Iowa Department of Natural Resources (DNR) also shows that TPCs calculated using the current Nutrient Model methodologies underestimate TN and TP concentrations for POTWs only employing secondary treatment. Under Iowa’s Nutrient Reduction Strategy, major POTWs are required to perform weekly influent and effluent monitoring for TN and TP. The Iowa DNR calculated the TPCs, shown in Table 2 below, using 2016 monthly average TN and TP effluent data from 54 major Iowa POTWs grouped by flow class.

While recognizing the inherent uncertainty in deriving TPCs for a subset of POTWs lacking TN and/or TP data in ICIS-NPDES, a re-evaluation of the Nutrient Model’s methods for calculating TPCs was recommended to provide TPCs for POTWs that are not likely to have additional treatment for nutrients.

Table 1. TPCs derived with consideration of POTW treatment level.
Treatment Level	Source	TPC
Treatment Level	Source	TN (mg/L)	TP (mg/L)
Secondary	Pait, 1993	11.2	7.0
	Tetra Tech, 1999	11.2	2.0
	USEPA, 2007	15	4
Advanced	Pait, 1993	5.6	3.5
	Tetra Tech, 1999	5.6	0.8
	USEPA, 2007	11.2	2.0

Table 2. Iowa-specific TPCs.
Statistic	Flow Class	Sample Size²	Count of POTWs³	TPC
Statistic	Flow Class	Sample Size²	Count of POTWs³	TN (mg/L)	TP (mg/L)
Median	<1 MGD	216	29	13.1	1.9
	1-5 MGD	322	37	12.2	1.9
	>5 MGD	102	11	21.5	2.4
Average	<1 MGD	216	29	15.8	3.2
	1-5 MGD	322	37	13.3	2.6
	>5 MGD	102	11	24.7	2.9

²Sample size refers to the number of monthly average results.
³Some POTWs have monthly average results associated with more than one flow category. Total universe = 54 POTWs.

Why use average values?

The TPCs for TN and TP from POTWs or facilities with SIC code 4952 – sewerage systems – have been adjusted to reflect average values of monthly average data reported. Depending on individual permit requirements, some facilities may monitor daily (or multiple times a day then averaged to generate a daily sample), weekly (or multiple times in a given week then averaged to generate a weekly sample), or monthly (often several samples in a month averaged together, or the average of weekly or daily samples). Effluent concentrations of TN and TP at POTWs may be expected to vary due to several factors including influent concentration fluctuations and technology performance factors. A monthly average of TN and TP effluent concentration data from a facility captures those fluctuations while providing a typical effluent concentration value often based on several daily or weekly samples. Thus, a monthly average value can be a more accurate representation of any facility’s effluent concentration than a median value that discounts outliers while being less variable than daily or weekly samples and closer to a facility’s long term average concentration.

Additionally, in the Iowa DNR analysis previously noted, 2016 annual TN and TP loads were calculated for a subset of 51 Iowa POTWs using actual facility-specific monthly average flow and concentration data. The same load calculations were then performed except that the Iowa-specific TPCs, shown in Table 2 above, were used in place of the actual facility-specific monthly average concentration data. The loads calculated using the average TPCs were closer to those loads calculated using actual facility-specific concentration data than were the loads calculated using the median TPCs (Table 3).

Table 3. Comparison of 2016 TN and TP Loads (lb) for 51 Iowa POTWs calculated using:
	Facility-specific Mo Avg Conc Data	Iowa-specific Average TPCs	Iowa-specific Median TPCs
TN	12,903,407	13,758,347	12,032,294
TP	1,691,574	1,798,032	1,428,357

How the Hypoxia Task Force (HTF) TPCs were calculated

The following steps identify the process followed to generate the TPCs for TN and TP from major municipal wastewater treatment plant effluent in the twelve Hypoxia Task Force (HTF) Mississippi/Atchafalaya River Basin (MARB) states not likely to have additional treatment for nutrients.

Identify from the HTF Point Source committee’s 2016 Report on Point Source Progress in Hypoxia Task Force States a list of a minimum of 10 (where available) randomly selected major POTWs from each of the 12 states with required monitoring for nitrogen and phosphorus parameters but without effluent limitations. The state of Iowa had done its own analysis on 54 Iowan facilities, with 12 months of data for the year 2016; that data was included in the analysis.
Collect from EPA’s Environmental Compliance History Online (ECHO) portal the monthly discharge monitoring reports from facilities identified in step 1 for the months April 2014 – July 2017.
From the DMR data downloaded, classify each facility into one of three categories based on the average of the facility’s reported monthly average flow in millions of gallons discharged per day (MGD): small (<1 MGD), medium (1-5 MGD), and large (>5 MGD).
Create a master excel file with all the facility DMR data for TN and TP, linked through a data model identifying each facility’s flow class.
1. Create an Excel 2016 PowerPivot pivot table to calculate the average TN and phosphorus concentrations.
  1. Filters:
    1. Perm_feature_nmbr: any, other than "influent" or "INF"
    2. Perm_feature_type_code: "EXO"
    3. Perm_feature_type_desc: "External Outfall"
    4. Parameter_desc: "Nitrogen, Total [as N]" and "Phosphorus, Total [as P]"
    5. Monitoring_location_desc: "Effluent Gross," "Effluent Option 1," Effluent Option 2," and "See Comment."
    6. Standard_unit_desc: "mg/L"
    7. Statistical_base_short_desc: "30DA AVG," "MO AVG", and "QTR AVG"
    8. Flow_Class: small, medium, large (user created)
  2. Columns:
    1. Flow Class (user created)
  3. Rows:
    1. npdes_id
  4. Values:
    1. Average of dmr_value_nmbr
    2. Median of dmr_value_nmbr
2. Create slicers to easily toggle and filter out data:
  1. Flow Class
  2. State
Analyze data
1. a. Calculate an average value from monthly average TN and TP data points for all facilities and calculate the same for the three distinct flow classes.
2. Caveats:
  1. Initial review indicated that all facilities within the "large" flow class from Indiana had average monthly average TP effluent concentrations well below 1 mg/L, indicative of treatment beyond national secondary treatment standards for POTWs. Due to time constraints, data from all Indiana facilities were removed from the analysis as staff was unable to splice out data from only the large Indiana facilities.
  2. TP values from the states of Minnesota and Wisconsin were also removed as those states have state laws requiring 1 mg/L effluent limit concentrations for TP at major POTWs.
  3. Facilities from Arkansas reported only nitrite + nitrate values which is not a full measure of TN (TN = nitrite + nitrate plus total kjeldahl nitrogen) and thus the analysis did not incorporate that data. Facilities from Indiana only reported daily maximum TN data, not 30-day or monthly average values so those data were also not incorporated in the analysis. Most facilities from Ohio and Minnesota reported nitrite + nitrate and total kjeldahl nitrogen separately. Due to time constraints, the analysis did not include the manual addition of those monthly average values to generate TN values for inclusion in the analysis.
  4. EPA did not verify, other than under the circumstances listed above in (5)(a)(i), whether the facilities it included in the analysis had received effluent limits for TP, TN, or both since the publishing of its 2016 Report on Point Source Progress in Hypoxia Task Force States. Also due to time and resource constraints, EPA did not independently verify that any of the facilities had effluent limits at any time from April 2014 – July 2017, or that the facilities are designed to treat beyond national secondary treatment standards.

Results:

Table 4. Results of Total Nitrogen TPC Calculations
Total Nitrogen
Flow Class	Avg. of Monthly Avg. (mg/L)	Median of Monthly Avg. (mg/L)	Number of Distinct NPDES facilities	Number of Samples	95% Confidence Range
All	13.343	11.58	152	3129	13.147 - 13.539
Large	13.586	10.9	25	533	13.145 - 14.027
Medium	12.75	11.3	84	1760	12.497 - 13.003
Small	14.436	12.945	43	836	14.022 - 14.85

Table 5. Results of Total Phosphorus TPC Calculations
Total Phosphorus
Flow Class	Avg. of Monthly Avg. (mg/L)	Median of Monthly Avg. (mg/L)	Number of Distinct NPDES facilities	Number of Samples	95% Confidence Range
All	2.439	1.86	199	4860	2.402 - 2.476
Large	2.039	1.698	39	1030	1.991 - 2.087
Medium	2.599	1.936	109	2690	2.545 - 2.653
Small	2.422	1.8	51	1140	2.342 - 2.502

List of facilities

Table 6. List of Facilities⁴
Name	NPDES ID	Flowclass	Flow
PLANO STP, CITY OF	IL0020052	Small	0.736
MILAN STP, VILLAGE OF	IL0020214	Small	1
FLORA STP, CITY OF	IL0020273	Medium	1.14
LITCHFIELD STP, CITY OF	IL0020621	Medium	2.269
DANVILLE SD	IL0020788	Large	7.9
MARSEILLES WWTP, CITY OF	IL0021059	Medium	1.417
MORRIS - MAIN STP	IL0021113	Medium	2.31
CREVE COEUR WWTP, VILLAGE OF	IL0021237	Medium	1.149
BONNIE BRAE FOREST MANOR SD	IL0021261	Small	0.813
GREATER PEORIA SANITARY AND SEWAGE DISTRICT	IL0021288	Large	22.97
O'FALLON STP, CITY OF	IL0021636	Medium	3.397
CHARLESTON STP, CITY OF	IL0021644	Small	1
JACKSONVILLE STP, CITY OF	IL0021661	Medium	4.6
STREATOR STP, CITY OF	IL0022004	Medium	3.288
WATSEKA STP, CITY OF	IL0022161	Medium	1.491
BENTON NORTHWEST STP, CITY OF	IL0022365	Medium	1.52
ST CHARLES EASTSIDE WWTF, CITY OF	IL0022705	Medium	4.38
GALESBURG SANITARY DISTRICT	IL0023141	Large	8.07
MENDOTA STP	IL0023221	Medium	1.99
MURPHYSBORO STP, CITY OF	IL0023248	Medium	1.48
FREEPORT STP, CITY OF	IL0023591	Medium	4.71
CLINTON SD STP	IL0023612	Medium	1.44
JERSEYVILLE STP, CITY OF	IL0024465	Medium	2.06
ALTON STP, CITY OF	IL0027464	Large	9.35
CARBONDALE NORTHWEST WWTP	IL0027871	Medium	1.61
EAST PEORIA STP #1, CITY OF	IL0028576	Medium	4.25
HERRIN STP, CITY OF	IL0029165	Medium	2.03
HIGHLAND STP, CITY OF	IL0029173	Medium	1.49
SPRING VALLEY WWTP	IL0031216	Small	0.95
SYCAMORE STP - NORTH PLANT	IL0031291	Medium	2.88
TROY STP, CITY OF	IL0031488	Medium	1.65
STOUGHTON CITY WWTF	WI0020338	Small	0.9315
SUSSEX VILLAGE WWTF	WI0020559	Medium	1.9013
ARCADIA CITY WWTF	WI0023230	Medium	1.46715
OREGON	WI0020681	Medium	1.2748
BEAVER DAM CITY WWTF	WI0023345	Medium	4.157
NEW ALBANY POTW	MS0020044	Medium	1.9
LAUREL POTW NUMBER 2	MS0020176	Medium	3
PASCAGOULA POTW	MS0020249	Medium	4.7
BELZONI POTW	MS0020371	Small	0.47
WEST POINT POTW, WEST	MS0020788	Medium	1.83
HATTIESBURG NORTH LAGOON	MS0020826	Medium	1.98
WINONA POTW	MS0021024	Small	0.8
ESCATAWPA WWTP	MS0021521	Medium	1.1
VICKSBURG POTW	MS0022381	Medium	3.41
BROOKHAVEN POTW	MS0024147	Medium	2
WAYNESBORO POTW	MS0024228	Small	0.975
BATESVILLE POTW	MS0024627	Medium	1.104
WAVELAND POTW	MS0027847	Medium	2.51
HCUA, WEST BILOXI POTW	MS0030333	Medium	4.04
VERONA POTW	MS0042048	Small	0.754
JCUA, WEST JACKSON COUNTY POTW	MS0045446	Large	5.1
DIAMONDHEAD WATER AND SEWER DISTRICT	MS0046078	Small	0.88
GULFPORT POTW, NORTH	MS0051756	Large	5.95
ABERDEEN POTW, EAST	MS0055581	Medium	2.65
BEATTIES BLUFF WWTF	MS0057517	Medium	3.91
MCCOMB POTW	MS0061077	Medium	2.75
S5 KILN NORTHERN REGIONAL WWTP	MS0061239	Small	0.33
MONETT WASTEWATER TREATMENT PLANT	MO0021440	Medium	2.2
BOLIVAR WWTF	MO0022373	Medium	1.57
KC BLUE RIVER WWTF	MO0024911	Large	27.5
MSD LEMAY WWTP	MO0025151	Large	103.5
MSD COLDWATER CREEK WWTF	MO0025160	Large	24.5
CABOOL WWTF	MO0026301	Medium	1.37
SAVANNAH WWTF	MO0026336	Small	0.454
CALIFORNIA	MO0023272	Medium	1.55
MSD MISSOURI RIVER	MO0004391	Large	23.7
KENNETT WWTF	MO0028568	Large	7.45
FARMINGTON WEST WWTF	MO0040312	Medium	1.6
MISSOURI RIVER WWTF	MO0058351	Medium	3.98
FESTUS-CRYSTAL CITY STP	MO0080632	Medium	1.96
WARRENTON WWTP	MO0087912	Medium	1.452
NEVADA	MO0089109	Medium	1.33
PCRSD BRUSH CREEK FAC	MO0119474	Small	0.69
CHISHOLM	MN0020117	Small	0.997
MONTEVIDEO	MN0020133	Small	0.908
LUVERNE WWTF	MN0020141	Medium	1.005
NEW PRAGUE	MN0020150	Small	0.804
DETROIT LAKES	MN0020192	Small	0.888424
THIEF RIVER FALLS	MN0021431	Large	8.6885
MARSHALL	MN0022179	Medium	2.5
WINDOM WWTF	MN0022217	Small	0.791
MADELIA WASTEWATER PLANT	MN0024040	Small	0.68
NORTHFIELD WWTP	MN0024368	Medium	2.08
PRINCETON	MN0024538	Small	0.37
RED WING WWTP	MN0024571	Medium	2
ROCHESTER WWTF	MN0024619	Large	12.6
SAINT JAMES	MN0024759	Small	0.803
WORTHINGTON WWTF	MN0031186	Medium	1.7
BUFFALO	MN0040649	Medium	1.003
ALEXANDRIA LAKE AREA SSD	MN0040738	Medium	2.726
ST CLOUD WWTF	MN0040878	Large	9.17
WORTHINGTON INDUSTRIAL	MN0031178	Medium	1.65
GRAND RAPIDS	MN0022080	Medium	3.145
EAST GRAND FORKS	MN0021814	Large	10.833
ALBERT LEA WASTEWATER FACILITY	MN0041092	Medium	3.8
BROUSSARD, CITY OF	LA0020613	Small	0.87
ST. CHARLES PARISH COUNCIL - LULING OXIDATION POND	LA0032131	Medium	1.68
HAMMOND, CITY OF- SOUTH SLOUGH WETLAND WASTEWATER ASSIMILATION PROJECT	LA0032328	Medium	4.6
OAKDALE	LA0033430	Medium	1.37
THIBODAUX, CITY OF- WASTEWATER TREATMENT FACILITY	LA0032948	Medium	3.29
BREAUX BRIDGE, CITY OF	LA0033014	Medium	1.15
MANDEVILLE, CITY OF	LA0038288	Medium	1.31
TERREBONNE PH GOVT-HOUMA NORTH	LA0040207	Large	6.83
DENHAM SPRINGS, CITY OF	LA0045730	Medium	2.44
WALKER, TOWN OF	LA0059951	Small	0.834
TALLULAH, CITY OF- WASTEWATER TREATMENT PLANT	LA0086576	Medium	2.7
CHARLESTOWN MUNICIPAL WWTP	IN0020508	Medium	1.01
TELL CITY WWTP	IN0021016	Medium	1.88
INDIANAPOLIS BELMONT & SOUTHPORT ADVNCD WTP	IN0023183	Large	65.8
JEFFERSONVILLE WWTP	IN0023302	Large	5.5
NEW ALBANY WWTP	IN0023884	Large	8.9
NEWBURGH WWTP	IN0023892	Medium	2.62
SOUTH DEARBORN RSD	IN0024538	Medium	3.51
MADISON WWTP	IN0025666	Medium	2.2
EVANSVILLE WESTSIDE WWTP	IN0032956	Large	13.8
SPEEDWAY WWTP	IN0032972	Medium	4.01
EVANSVILLE EASTSIDE WWTP	IN0033073	Large	13.1
MOUNT VERNON WWTP	IN0035696	Medium	2.08
ERWIN STP	TN0023001	Medium	1.013
MORRISTOWN STP	TN0023507	Medium	4.4
ELIZABETHTON STP	TN0023515	Medium	2.34
MCMINNVILLE STP	TN0023591	Medium	1.7
OAK RIDGE STP	TN0024155	Medium	3.7
SHELBYVILLE STP	TN0024180	Medium	3.1
CHATT.-MOCCASIN BEND STP	TN0024210	Large	66.2
JOHNSON CITY KNOB CREEK STP	TN0024236	Medium	2.43
JOHNSON CITY STP	TN0024244	Large	7.22
SOUTH PITTSBURG STP	TN0024295	Small	0.5
ROANE COUNTY STP	TN0024473	Small	0.27
JACKSON ENERGY	TN0024813	Large	8.131
CROSSVILLE STP	TN0024996	Medium	2.43
BELLS LAGOON	TN0026247	Medium	1.56
WHITEVILLE STP	TN0026590	Small	0.57
LEBANON STP	TN0028754	Large	6.74
JOHNSON CITY REGIONAL STP	TN0028789	Medium	1.94
HALLS LAGOON	TN0057291	Small	0.41
COLLIERVILLE STP	TN0057461	Medium	1.58
LOUDON STP	TN0058181	Large	8.29
WHITE HOUSE STP	TN0059404	Small	0.691
PARIS	TN0061271	Medium	2.55
MUNFORD LAGOON	TN0062499	Medium	1.096
GREENVILLE STP	KY0020010	Medium	1.06
HAZARD STP	KY0020079	Medium	1.87
RWRA MAX RHOADS WWTP	KY0020095	Large	8.735
MAYSVILLE STP	KY0020257	Medium	1.269
HENDERSON STP	KY0020711	Large	5.128
BENTON STP	KY0021172	Small	0.8947
MORGANFIELD WWTP	KY0021440	Medium	1.72
ASHLAND STP	KY0022373	Large	5.93
BOWLING GREEN STP	KY0022403	Large	7.63
PADUCAH/MCCRACKEN JSA PADUCAH	KY0022799	Large	7.87
HARLAN STP	KY0026093	Small	0.951
SOMERSET STP	KY0026611	Medium	2.62
WILLIAMSBURG STP	KY0028347	Medium	1.09
PRINCETON STP	KY0028401	Medium	1.026
MANCHESTER STP	KY0029122	Small	0.948
GREENUP CO ENVIRONMENTAL COMM	KY0048348	Medium	1.85
MOREHEAD STP	KY0052752	Medium	2.55
MIDDLESBORO STP	KY0072885	Medium	1.95
CEDAR CREEK WQTC	KY0098540	Medium	4.54
DEREK R GUTHRIE WQTC MSD	KY0078956	Large	33.37
HONEY BRANCH REGIONAL STP	KY0103578	Small	0.36
ADEL CITY OF STP	IA2503001	Small	0.372
ATLANTIC CITY OF STP	IA1509001	Medium	1.299
BOONE CITY OF STP	IA819001	Medium	2.696
CEDAR FALLS CITY OF STP	IA709001	Large	6.377
CHARLES CITY, CITY OF STP	IA3405001	Medium	1.655
CHEROKEE CITY OF STP	IA1811002	Medium	1.341
CORALVILLE CITY OF STP	IA5208001	Medium	2.600
COUNCIL BLUFFS CITY OF STP	IA7820001	Large	6.969
CRESTON CITY OF STP	IA8816001	Medium	1.530
DAVENPORT CITY OF STP	IA8222003	Large	34.928
DES MOINES METROPOLITAN WRF	IA7727001	Large	66.282
DUBUQUE CITY OF STP	IA3126001	Large	7.477
EAGLE GROVE, CITY OF STP	IA9926001	Medium	1.135
ELDORA CITY OF STP	IA4236001	Small	0.608
ELDRIDGE, CITY OF SOUTH SLOPE	IA8230003	Small	0.560
EMMETSBURG CITY OF STP	IA7428002	Small	0.605
ESTHERVILLE CITY OF STP	IA3218002	Medium	1.383
FOREST CITY, CITY OF STP	IA9525001	Medium	1.024
FORT DODGE CITY OF STP	IA9433003	Large	8.235
GREENFIELD CITY OF STP	IA140001	Small	0.366
GRIMES, CITY OF STP	IA7736001	Medium	1.872
GRINNELL, CITY OF STP	IA7930001	Medium	1.984
GRUNDY CENTER CITY OF STP	IA3833001	Small	0.603
HARLAN CITY OF STP	IA8335002	Small	0.796
HUMBOLDT CITY OF STP	IA4641001	Small	0.952
INDEPENDENCE CITY OF STP	IA1037001	Medium	1.469
IOWA CITY, CITY OF (SOUTH) STP	IA5225002	Large	9.523
IOWA FALLS CITY OF STP	IA4260001	Small	0.956
IOWA GREAT LAKES SD	IA3050901	Medium	2.944
JEFFERSON CITY OF STP	IA3742001	Small	0.937
KNOXVILLE CITY OF STP	IA6342001	Medium	1.428
MAQUOKETA CITY OF STP	IA4950001	Small	0.664
MONTEZUMA CITY OF STP	IA7950001	Small	0.364
MOUNT PLEASANT (MAIN)	IA4453001	Medium	1.341
MOUNT VERNON CITY OF STP	IA5758001	Small	0.453
MUSCATINE CITY OF STP	IA7048001	Medium	4.986
NEW HAMPTON CITY OF STP	IA1970001	Medium	1.333
NEWTON CITY OF STP	IA5059002	Medium	2.242
OELWEIN CITY OF STP	IA3353001	Medium	1.045
OSCEOLA CITY OF STP	IA2038002	Small	0.966
OSKALOOSA (NORTHEAST)	IA6273001	Small	0.891
OSKALOOSA (SOUTHWEST)	IA6273002	Small	0.994
RED OAK CITY OF STP	IA6950001	Medium	1.584
SHELDON CITY OF STP	IA7170001	Medium	1.508
SHENANDOAH CITY OF STP	IA3659001	Medium	1.405
SIOUX CENTER CITY OF STP	IA8486002	Medium	1.179
SIOUX CITY CITY OF STP	IA9778001	Large	12.589
VINTON CITY OF STP	IA688001	Small	0.964
WASHINGTON CITY OF STP	IA9271001	Medium	1.538
WAVERLY CITY OF STP	IA990001	Medium	1.342
WEBSTER CITY, CITY OF STP	IA4063001	Medium	1.757
WEST BURLINGTON CITY OF STP	IA2985001	Small	0.728
WEST LIBERTY CITY OF STP	IA7073001	Medium	1.265
WINTERSET CITY OF STP	IA6171001	Small	0.789
TWIN CITY WWTP	OH0020079	Medium	1.74
WEST CARROLLTON WWTP	OH0020133	Medium	1.18
LEXINGTON WWTP	OH0020257	Small	0.484
HAMILTON CO POLK RUN WWTP	OH0020419	Medium	4.56
MILFORD STP	OH0020451	Small	0.67
MASSILLON WWTP	OH0020516	Large	11.14
BROOKVILLE WWTP	OH0020605	Small	0.8
BELPRE WWTP	OH0020621	Small	0.76
EATON WWTP & SEWER SYSTEM	OH0020907	Medium	1.29
LEBANON REGIONAL WWTP	OH0021059	Medium	2.613
HARRISON WWTP	OH0021440	Small	0.95
COLUMBIANA WWTP	OH0021776	Medium	1.15
WELLSTON WWTP NORTH	OH0023507	Small	0.904
ASHLAND WWTP	OH0023906	Medium	4.22
ATHENS WWTP	OH0023931	Medium	2.33
CITY OF CANTON	OH0024350	Large	25.23
CHILLICOTHE WWTP - EASTERLY	OH0024406	Medium	2.22
CITY OF CIRCLEVILLE	OH0024465	Medium	1.57
CITY OF COLUMBUS	OH0024732	Large	72.1
CITY OF COLUMBUS	OH0024741	Large	81.1
DAYTON STP	OH0024881	Large	41.14
EAST LIVERPOOL WWTP	OH0024970	Medium	2.46
ENGLEWOOD WWTP	OH0025011	Medium	1.185
FAIRBORN WATER RECLAMATION CENTER	OH0025062	Medium	4.1
FRANKLIN REGIONAL WWTP	OH0025275	Medium	3.24
HAMILTON WWTP	OH0025445	Large	8.51
MUDDY CREEK WWTP	OH0025470	Large	12.17
LANCASTER WPCF	OH0026026	Large	5.52
MARION WPC	OH0026352	Large	7.96
YOUNGSTOWN WWTP	OH0028223	Large	29.8
SHELBY	OH0023540	Medium	1.805
KENTON	OH0025925	Medium	3.381
BELLEFONTAINE	OH0024066	Medium	2.525
CLARKSVILLE LIGHT & WATER	AR0022187	Medium	1.03
CAMDEN WATER UTILITIES	AR0022365	Medium	2.4
GREENWOOD, CITY OF	AR0022454	Medium	1.004
BLYTHEVILLE, CITY OF-WEST WWTF	AR0022560	Small	0.991
BLYTHEVILLE, CITY OF-SOUTH	AR0022578	Small	0.682
FORT SMITH, CITY OF - "P" STREET WWTP	AR0033278	Large	7.45
PINE BLUFF BOYD POINT WW TREATMENT FAC	AR0033316	Large	11.76
DUMAS, CITY OF-WWTP	AR0033987	Medium	1.148
BRYANT, CITY OF - WWTP	AR0034002	Medium	2.36
STUTTGART WW TREATMENT PLANT	AR0034380	Medium	1.617
MENA WASTEWATER TREATMENT PLANT PLANT	AR0036692	Medium	1.835
SHERWOOD, CITY OF - NORTH FACILITY	AR0037176	Small	0.43
LRWU-FOURCHE CREEK WWTP	AR0040177	Large	9.61
VAN BUREN, CITY OF-NORTH WWTP	AR0040967	Medium	1.103
JONESBORO, CITY OF - CITY WATER & LIGHT EASTSIDE WWTP	AR0043401	Large	8.71
BARLING, CITY OF	AR0048801	Medium	2.38
CLINTON EAST WW TREATMENT FACILITY	AR0048836	Small	1
LITTLE ROCK WASTEWATER-LITTLE MAUMELLE WWTP	AR0050849	Medium	2.13
CONWAY-TUPELO BAYOU WWTP	AR0051951	Large	5.113

⁴Note: this list includes the facilities that were excluded from the analysis as identified in steps 5(a)(i),(ii), and (iii), above.

References

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Pait, A.S. et al. (analysis team). 1993 (December). The National Coastal Pollutant Discharge Inventory, Point Source Methods Document (PDF) (258 pp, 6.5 MB). National Oceanic and Atmospheric Administration (NOAA), National Ocean Service, Office of Ocean Resources Conservation and Assessment, Strategic Environmental Assessments (SEA) Division, Pollution Sources Characterization (PCS) Branch. Silver Spring, Maryland. Retrieved from https://repository.library.noaa.gov/.

Tetra Tech, Inc. 1999 (September 8). Improving Point Source Loadings Data for Reporting National Water Quality Indicators, Final Report. Final Report. Prepared for: U.S. Environmental Protection Agency, Office of Wastewater Management (OWM). Contract #68-C-0014, Work Assignment 1-31. Tetra Tech, Inc., Fairfax, Virginia.

USEPA. 2007. Hypoxia in the northern Gulf of Mexico - An update by the EPA Science Advisory Board (PDF) (333 pp, 4.9 MB). U.S. Environmental Protection Agency, EPA Science Advisory Board. EPA-SAB-08-003. Retrieved from https://www.epa.gov/ms-htf/hypoxia-northern-gulf-mexico-update-epa-science-advisory-board.

USEPA. 2012 (December 14). Letter to the Natural Resources Defense Council in response to a petition requesting that the USEPA publish updated information about secondary treatment nutrient removal capability and establish new technology-based nutrient limits as part of the secondary treatment standards (PDF) (14 pp, 667 K). U.S. Environmental Protection Agency, Washington, D.C.

USEPA. 2017 (accessed October 19). Water Pollutant Loading Tool: About the Nutrient Model. U.S. Environmental Protection Agency, Washington, D. C.