Final Fact Sheet – May 5, 2010 Final 

R. Koch/Eastern Regional Office 

FACT SHEET FOR NPDES PERMIT WA-004496-2 

CITY OF PASCO WASTEWATER TREATMENT WORKS 

PURPOSE of this Fact Sheet 

This fact sheet explains and documents the decisions Ecology made in drafting the proposed 

National Pollutant Discharge Elimination System (NPDES) Permit for the City of Pasco 

Treatment Works. 

This fact sheet complies with Section 173-220-060 of the Washington Administrative Code 

(WAC), which requires Ecology to prepare a draft permit and accompanying fact sheet for 

public evaluation before issuing an NPDES permit. 

Ecology makes the draft permit and fact sheet available for public review and comment at least 

30 days before issuing the final permit. Copies of the fact sheet and draft permit for the City of 

Pasco Wastewater Treatment Works, NPDES Permit WA-0044962, are available for public 

review and comment from May 12, 2010 until June 11, 2010. For more details on preparing and 

filing comments about these documents, please see Appendix A - Public Involvement. 

The City of Pasco’s wastewater utility staff reviewed the draft permit and fact sheet for factual 

accuracy. Ecology corrected any errors or omissions regarding the facility’s location, history, 

discharges, or receiving water. 

After the public comment period closes, Ecology will summarize substantive comments and 

provide responses to them. Ecology will include the summary and responses to comments in this 

Fact Sheet as Appendix D - Response to Comments, and publish it when issuing the final 

NPDES permit. Ecology will not revise the rest of the fact sheet, but the full document will 

become part of the legal history contained in the facility’s permit file. 

SUMMARY 

The City of Pasco Wastewater Treatment Works discharges to the Columbia River. The 

treatment facilities generally produce an excellent quality effluent. Past exceptions were usually 

related to illegal discharges into the sewer collection system resulting in minor inhibitory or toxic 

impacts on the biological community in the aeration basin. The wastewater treatment plant staff 

has been optimizing solids handling to better mitigate impacts from recycled solids. The facility 

has added a solids thickening drum and improvements to the digesters have been made. Changes 

in the proposed permit from the previous permit are primarily a compliance schedule requiring 

submission of an engineering report suitable for approval. The engineering report must evaluate 

the hydraulic and organic loading capacity of the City of Pasco’s wastewater facilities as they 

currently exist and if necessary recommend upgrades appropriate for the wastewater facilities to 

provide twenty years of additional service to the community. 

 
Fact Sheet for NPDES Permit WA-004496-2 

Pasco Wastewater Treatment Works 

Final Fact Sheet – May 5, 2010 Final 

R. Koch/Eastern Regional Office 

TABLE OF CONTENTS

I. INTRODUCTION ...............................................................................................................1

II. BACKGROUND INFORMATION ....................................................................................2

A. Facility Description ..................................................................................................3

History......................................................................................................................3

Collection System Status .........................................................................................3

Treatment Processes.................................................................................................3

Discharge Outfall .....................................................................................................4

Solid Wastes.............................................................................................................4

B. Permit Status ............................................................................................................5

C. Summary of Compliance with Previous Permit Issued ...........................................5

D. Wastewater Characterization ...................................................................................5

E. Description of the Receiving Water .........................................................................5

F. SEPA Compliance ...................................................................................................6

III. PROPOSED PERMIT Limits ..............................................................................................7

G. Design Criteria .........................................................................................................7

H. Technology-Based Effluent Limits ..........................................................................8

I. Surface Water Quality-Based Effluent Limits .........................................................9

Numerical Criteria for the Protection of Aquatic Life and Recreation ....................9

Numerical Criteria for the Protection of Human Health ..........................................9

Narrative Criteria ...................................................................................................10

Antidegradation......................................................................................................10

Mixing Zones .........................................................................................................11

J. Designated Uses and Surface Water Quality Criteria ............................................16

K. Evaluation of Surface Water Quality-Based Effluent Limits for Numeric 

Criteria ...................................................................................................................17

L. Whole Effluent Toxicity ........................................................................................21

M. Human Health ........................................................................................................21

N. Sediment Quality ...................................................................................................22

O. Ground Water Quality Limits ................................................................................22

P. Comparison of Effluent Limits with the Previous Permit Issued on July 7, 

2004........................................................................................................................22

IV. MONITORING REQUIREMENTS ..................................................................................22

Q. Lab Accreditation...................................................................................................23

V. OTHER PERMIT CONDITIONS .....................................................................................24

R. Reporting and Record Keeping ..............................................................................24

S. Prevention of Facility Overloading ........................................................................24

T. Operation and Maintenance (O&M) ......................................................................24

U. Pretreatment ...........................................................................................................25

Duty to Enforce Discharge Prohibitions ................................................................25

Federal and State Pretreatment Program Requirements ........................................25

Routine Identification and Reporting of Industrial Users ......................................26
Fact Sheet for NPDES Permit WA-004496-2 

Pasco Wastewater Treatment Works 

Final Fact Sheet – May 5, 2010 Final 

R. Koch/Eastern Regional Office 

Requirements for Performing an Industrial User Survey ......................................26

V. Solid Waste Control ...............................................................................................26

W. General Conditions ................................................................................................26

VI. PERMIT ISSUANCE PROCEDURES .............................................................................27

X. Permit Modifications .............................................................................................27

Y. Proposed Permit Issuance ......................................................................................27

VII. REFERENCES FOR TEXT AND APPENDICES ...........................................................28

APPENDIX A—PUBLIC INVOLVEMENT INFORMATION ..................................................29

APPENDIX B—GLOSSARY .......................................................................................................30

APPENDIX C—TECHNICAL CALCULATIONS ......................................................................34

APPENDIX D—RESPONSE TO COMMENTS ..........................................................................35
Fact Sheet for NPDES Permit WA-004496-2 

Pasco Wastewater Treatment Works 

Final Fact Sheet – May 5, 2010 Page 1 

R. Koch/Eastern Regional Office 

I. INTRODUCTION

The Federal Clean Water Act (FCWA, 1972, and later amendments in 1977, 1981, and 1987) 

established water quality goals for the navigable (surface) waters of the United States. One 

mechanism for achieving the goals of the Clean Water Act is the National Pollutant Discharge 

Elimination System (NPDES), administered by the federal Environmental Protection Agency 

(EPA). The EPA authorized the State of Washington to manage the NPDES permit program in 

our state. Our state legislature accepted the delegation and assigned the power and duty for 

conducting NPDES permitting and enforcement to Ecology. The legislature defined Ecology's 

authority and obligations for the wastewater discharge permit program in 90.48 RCW (Revised 

Code of Washington). 

The following regulations apply to municipal NPDES permits: 

• Procedures Ecology follows for issuing NPDES permits (chapter 173-220 WAC) 

• Technical criteria for discharges from municipal wastewater treatment facilities (chapter 

173-221 WAC) 

• Water quality criteria for surface waters (chapter 173-201A WAC) and for ground waters 

(chapter 173-200 WAC) 

• Sediment management standards (chapter 173-204 WAC)

• Submission of Plans and Reports for Construction of Wastewater Facilities (Chapter 173-

240 WAC) 

These rules require any treatment facility operator to obtain an NPDES permit before 

discharging wastewater to state waters. They also help define the basis for limits on each 

discharge and for requirements imposed by the permit. 

Under the NPDES permit program and in response to a complete and accepted permit 

application, Ecology must prepare a draft permit and accompanying fact sheet, and make them 

available for public review before final issuance. Ecology must also publish an announcement 

(public notice) telling people where they can read the draft permit, and where to send their 

comments, during a period of thirty days (WAC 173-220-050). (See Appendix A - Public 

Involvement for more detail about the public notice and comment procedures). After the public 

comment period ends, Ecology may make changes to the draft NPDES permit. Ecology will 

summarize the responses to comments and any changes to the permit in Appendix D.
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it may not be reproduced without consent of the 

copyright holder. 

Removed pages can be viewed at the USEPA 

Docket Center, Public Reading Room. 

Address: 1301 Constitution Ave, NW 

Room 3334 

Washington, DC 20004 

Telephone: 202-566-1744 

Email: docket-customerservice@epa.gov
Fact Sheet for NPDES Permit WA-004496-2 

Pasco Wastewater Treatment Works 

Final Fact Sheet – May 5, 2010 Page 3 

R. Koch/Eastern Regional Office 

A. Facility Description 

The City of Pasco owns and operates an activated sludge process domestic wastewater 

treatment plant designed to oxidize, nitrify and disinfect wastewater with ultraviolet light 

radiation. Ecology provided financial assistance with the expectation that the treatment plant 

would remove ammonia and chlorine from the receiving environment and eliminate noncompliance issues. The facility is classified as an EPA major discharger and discharges to 

the Lake Wallula reach of the Columbia River. 

History 

Pasco originally built the wastewater treatment plant in 1954 as a primary treatment facility. 

In 1970, it constructed a trickling filter so that the plant could meet secondary treatment 

requirements. The 1970 addition of a secondary treatment process increased the plant's 

design population equivalent to 30,000. The most recent upgrade (1998) increased the 

design population equivalent to 43,500. During the last permit cycle, the City upgraded the 

solids handling facilities, digestion and solids thickening. These improvements likely 

increased the overall treatment works capacity. 

Collection System Status 

The City of Pasco submitted a Comprehensive Sewer Plan to Ecology in December 1992 and 

updated it in 1999. The City submitted a facility plan with supplements in 1993, which was 

approved in February 1994. The plan proposed improvements to the City's collection 

system, wastewater treatment facility and pumping stations. The City has replaced the 

pumping stations at the intersection of 9th and Washington and at the intersection of 5th and 

Ainsworth with a single pump station at 9th and Washington resulting in the elimination of an 

overflow. It also replaced the old “Navy” pump station in the industrial area east of the 

airport. Areas in West Pasco and East Pasco have been added to the sewerage system. The 

Port of Pasco abandoned the wastewater stabilization lagoons serving its facilities on 

Ainsworth and the City now serves the area by a trunk sewer connected to its sewerage 

system. The City’s collection system consists of gravity sewers from 8 inches to 36 inches in 

diameter, pump stations and pressure piping. 

Treatment Processes 

The current wastewater treatment facility includes: 

• A headworks building with grit removal. 

• Screening equipment and inlet piping. 

• An operations building and laboratory. 

• A renovated primary clarifier and trickling filter (new distributor). 

• Secondary process elements designed to provide nitrification - aeration basins. 

• Secondary clarifiers with lime addition for alkalinity and pH control. 

• Ultraviolet disinfection. 

• Anaerobic digesters. 

• Renovated and expanded sludge digestion processes with the addition of a solids 

drum thickener and a sludge drying bed. 
Fact Sheet for NPDES Permit WA-004496-2 

Pasco Wastewater Treatment Works 

Final Fact Sheet – May 5, 2010 Page 4 

R. Koch/Eastern Regional Office 

The City’s rate of growth is reflected in the increase in flows. The wastewater treatment 

works design is based on the presumed need to nitrify and avoid toxic effects from ammonia 

in the Columbia River. At the time the City’s outfall was an end of pipe in the river. While 

the wastewater treatment plant was built to nitrify, consideration was also given to extend 

the outfall further into the river and add a multi-port diffuser. The installation of the outfall 

negated the need to nitrify in terms of water quality based effluent limitations. In August 

 2005, the City commissioned an engineering evaluation of the wastewater treatment 

facilities capacity. Until recently, the wastewater treatment works was being operated in a 

nitrifying mode in order to minimize the generation of biosolids. 

The treatment facilities generally produce an excellent quality effluent. Past exceptions were 

usually related to illegal discharges into the sewer collection system resulting in minor 

inhibitory or toxic impacts on the biological community in the aeration basin. Some 

violations were related to how recycle of solids was handled. The wastewater treatment plant 

staff has been optimizing solids handling to better mitigate impacts from recycled solids. 

The facility has added a solids thickening drum and improvements to the digesters have been 

made. The 2005 assessment of the wastewater treatment facilities capacity pointed that 

solids handling was a critical capacity limiting factor. With the new digester it is time to 

reassess the capacity of the facility and its ability to serve the community. 

The WWTP is an activated sludge treatment plant with a flow between 1 MGD and 10 MGD. 

As such, it is a class III facility according to 1WAC 173-230-140; Classification of 

Wastewater Treatment Plants.

Discharge Outfall 

The effluent is discharged from the facility via a 24-inch steel multiport diffuser outfall into 

the Columbia River. The outfall has three 8” diameter diffuser ports. At this location the 

river flows west to east south east. Leaving the plant site, the outfall follows a southerly line 

defined by Gray Street continuing approximately 900 feet offshore from the north bank and 

at a depth of approximately 30 feet. 

Solid Wastes 

The treatment facilities remove solids during the treatment of the wastewater at the 

headworks (grit and screenings), and at the primary and secondary clarifiers (biosolids). In 

addition to incidental solids (rags, scum, and other debris) removed as part of the routine 

maintenance of the equipment, Scum is routed to the anaerobic digester for biological 

oxidation. 

Biosolids removed from the clarifiers are thickened (dissolved air floatation), anaerobically 

digested, dried in the drying beds and land applied under a permit issued by Ecology’s Waste 

2 Resources Program. The biosolids produced at the treatment works are of exceptional 

quality and beneficial use options are available. Grit, rags, and screenings are drained and 

disposed of as solid waste at the local landfill. The proposed permit includes a condition 

requiring the City to properly handle residual solids so that no leachate enters ground or 

surface water. 
Fact Sheet for NPDES Permit WA-004496-2 

Pasco Wastewater Treatment Works 

Final Fact Sheet – May 5, 2010 Page 5 

R. Koch/Eastern Regional Office 

B. Permit Status 

Ecology issued the previous permit for this facility on July 7, 2004. The previous permit 

placed effluent limitations on 5-day Biochemical Oxygen Demand (BOD5), Total Suspended 

Solids (TSS), pH, and Fecal Coliform Bacteria. 

The City of Pasco submitted an application for permit renewal on January 5, 2009. Ecology 

accepted it as complete on January 30, 2009. 

C. Summary of Compliance with Previous Permit Issued 

Ecology staff last conducted a non-sampling compliance inspection on June 12, 2008. The 

City of Pasco WWTP has complied with the effluent limits and permit conditions 

throughout the duration of the permit issued on July 7, 2004. Ecology assessed compliance 

based on its review of the facility’s discharge monitoring reports (DMRs) and on inspections 

conducted by Ecology. 

D. Wastewater Characterization 

The concentration of pollutants in the discharge was reported in the NPDES application and 

in discharge monitoring reports (DMRs). The tabulated data represents the quality of the 

effluent discharged from the Pasco WWTP using DMR data from January 2005 through 

March 2009. The effluent is characterized as follows: 

Table 2: Wastewater Characterization 

Parameter Average 

Concentration

Maximum 

Concentration

Minimum 

Concentration 

Flow, MGD 3.23 3.62 3.01 

Influent BOD5, mg/L 237.3 313 191 

Influent TSS, mg/L 246 327 203 

Effluent BOD5, mg/L 12.5 57 4 

Effluent TSS, mg/L 15.7 100 6 

Effluent NH3, mg/L 13.6 35.1 0.41 

Effluent Temperature, January,

 deg F 

59 60 58 

Effluent Temperature, August, 

 deg F 

78 79 77 

E. Description of the Receiving Water 

The City of Pasco WWTP discharges to the Columbia River. Other nearby point source 

outfalls include the City of Kennewick WWTP and the City of Richland WWTP. 

The ambient background data used for this permit includes the following from environmental 

Assessment Program monitoring station 36A070 on the Columbia River near Vernita: 
Fact Sheet for NPDES Permit WA-004496-2 

Pasco Wastewater Treatment Works 

Final Fact Sheet – May 5, 2010 Page 6 

R. Koch/Eastern Regional Office 

Table 3: Ambient Background Data 

Parameter Value used 

Temperature (highest annual 1-DADMax) 21 o C 

Temperature (**some waterbodies have specific 

temperature criteria as assigned in Table 602) 

See Footnote 1 

pH (Maximum / Minimum) 8.45/8.03 

Dissolved Oxygen (Maximum / Minimum) 14.08/9.38 mg/L 

Total Ammonia-N 0.01 mg/L 

Fecal Coliform (Maximum) 11/100 mL 

Turbidity 1.2 NTU 

Hardness 65 mg/L as CaCO3 

Lead 0.1 μg/L 

Copper 0.67 μg/L 

Zinc 5 μg/L 

Footnote: 

1. From Washington-Oregon border (river mile 309.3) to Priest Rapids Dam (river mile 

397.1). Temperature shall not exceed a 1-DMax of 20.0°C due to human activities. 

When natural conditions exceed a 1-DMax of 20.0°C, no temperature increase will be 

allowed which will raise the receiving water temperature by greater than 0.3°C; nor shall 

such temperature increases, at any time, exceed t = 34/(T + 9) 

F. SEPA Compliance 

Regulation exempts reissuance or modification of any wastewater discharge permit from the 

SEPA process as long as the permit contains conditions that are no less stringent than state 

rules and regulations. The exemption applies only to existing discharges, not to new 

discharges. 

The City prepared SEPA and State Environmental Review Process (SERP) documents 

during the planning phase of the wastewater treatment works upgrade completed in July 

1998. The City prepared SERP documents to comply with Washington Water Pollution 

Control Revolving Fund requirements. 
Fact Sheet for NPDES Permit WA-004496-2 

Pasco Wastewater Treatment Works 

Final Fact Sheet – May 5, 2010 Page 7 

R. Koch/Eastern Regional Office 

III. PROPOSED PERMIT LIMITS 

Federal and state regulations require that effluent limits in an NPDES permit must be either 

technology- or water quality-based. 

• Technology-based limits are based upon the treatment methods available to treat specific 

pollutants. Technology-based limits are set by the EPA and published as a regulation, or 

Ecology develops the limit on a case-by-case basis (40 CFR 125.3, and chapter 173-220 

WAC). 

• Water quality-based limits are calculated so that the effluent will comply with the Surface 

Water Quality Standards (chapter 173-201A WAC), Ground Water Standards (chapter 173-

200 WAC), Sediment Quality Standards (chapter 173-204 WAC) or the National Toxics 

Rule (40 CFR 131.36). 

• Ecology must apply the most stringent of these limits to each parameter of concern. These 

limits are described below. 

The limits in this permit reflect information received in the application and from supporting 

reports (engineering, hydrogeology, etc.). Ecology evaluated the permit application and 

determined the limits needed to comply with the rules adopted by the state of Washington. 

Ecology does not develop effluent limits for all reported pollutants. Some pollutants are not 

treatable at the concentrations reported, are not controllable at the source, are not listed in 

regulation, and do not have a reasonable potential to cause a water quality violation. 

Nor does Ecology usually develop limits for pollutants that were not reported in the permit 

application but that may be present in the discharge. The permit does not authorize discharge of 

the non-reported pollutants. If significant changes occur in any constituent of the effluent 

discharge, the City of Pasco is required to notify Ecology (40 CFR 122.42(a)). The City of 

Pasco may be in violation of the permit until Ecology modifies the permit to reflect additional 

discharge of pollutants. 

G. Design Criteria 

In accordance with WAC 173-220-150 (1)(g), flows or waste loadings must not exceed approved 

design criteria. However Ecology-is requiring an update of the design criteria to reflect recent 

upgrades of the solids handling treatment train which limiting hydraulic capacity and organic 

loading capacity. 

Table 4: Design Criteria for the City of Pasco WWTP 

Influent Loadings in the Design Criteria Year 

Population To be determined 

(TBD)

Average Annual Flow (MGD) TBD 

Maximum Month Flow (MGD) TBD
Fact Sheet for NPDES Permit WA-004496-2 

Pasco Wastewater Treatment Works 

Final Fact Sheet – May 5, 2010 Page 8 

R. Koch/Eastern Regional Office 

Influent Loadings in the Design Criteria Year 

Maximum Day Flow (MGD) TBD

Maximum Hour Flow (MGD) TBD

BOD5 Loading– Average Annual (lbs/day) TBD 

BOD5 Loading – Maximum Month (lbs/day) TBD 

TSS Loading – Average Annual (lbs/day) TBD 

TSS Loading– Maximum Month (lbs/day) TBD 

Ammonia Loading – Average Annual (lbs/day) TBD 

Ammonia Loading – Maximum Month (1,000 

lbs/day) 

TBD 

H. Technology-Based Effluent Limits 

Federal and state regulations define technology-based effluent limits for municipal 

wastewater treatment plants. These effluent limits are given in 40 CFR Part 133 (federal) 

and in chapter 173-221 WAC (state). These regulations are performance standards that 

constitute all known, available, and reasonable methods of prevention, control, and 

treatment (AKART) for municipal wastewater. 

Chapter 173-221 WAC lists the following technology-based limits for pH, fecal coliform, 

BOD5, and TSS: 

Table 5: Technology-based Limits 

Parameter Limit 

pH The pH must measure within the range of 6.0 to 9.0 standard 

units. 

Fecal Coliform Bacteria Monthly Geometric Mean = 200 organisms/100 mL 

Weekly Geometric Mean = 400 organisms/100 mL 

BOD5 

(concentration) 

Average Monthly Limit is the most stringent of the following: 

 - 30 mg/L 

 - may not exceed fifteen percent (15%) of the average 

 influent concentration 

Average Weekly Limit = 45 mg/L 

TSS 

(concentration) 

Average Monthly Limit is the most stringent of the following: 

 - 30 mg/L 

 - may not exceed fifteen percent (15%) of the average 

 influent concentration 

Average Weekly Limit = 45 mg/L 
Fact Sheet for NPDES Permit WA-004496-2 

Pasco Wastewater Treatment Works 

Final Fact Sheet – May 5, 2010 Page 9 

R. Koch/Eastern Regional Office 

The technology-based mass limits are based on WAC 173-220-130(3)(b) and 

173-221-030(11)(b). From the previous permit the effluent mass was as follows and will be 

used temporarily until the required engineering report is approved. 

The maximum month effluent mass (lbs/day) was calculated as the maximum monthly design 

flow (4.52 MGD) x Concentration limit (30 mg/L) x 8.34 (conversion factor) = mass 

limitation of 1131 lb./day of BOD5 or TSS. 

Monthly effluent mass loadings (lbs/day) = maximum monthly influent design loading 

(10,690 lbs./day) x 0.15 = 1603.5 lbs./day, BOD5 or . (8,720 lbs./day) x 0.15 = 1308 

lbs./day TSS 

The weekly average effluent mass loading = 1.5 x monthly loading. 

1.5 x 1603.5 lbs./day =2405 lbs/day BOD5 or 1.5 x 1308 lbs/day = 1962 lbs/day TSS 

The calculation of mass based on flow x concentration x conversion faction above generates 

the more restrictive effluent mass limit. 

I. Surface Water Quality-Based Effluent Limits 

The Washington State Surface Water Quality Standards (chapter 173-201A WAC) are 

designed to protect existing water quality and preserve the beneficial uses of Washington's 

surface waters. Waste discharge permits must include conditions that ensure the discharge 

will meet the surface water quality standards (WAC 173-201A-510). Water quality-based 

effluent limits may be based on an individual waste load allocation or on a waste load 

allocation developed during a basin wide total maximum daily load study (TMDL). 

Numerical Criteria for the Protection of Aquatic Life and Recreation 

Numerical water quality criteria are listed in the water quality standards for surface waters 

(chapter 173-201A WAC). They specify the maximum levels of pollutants allowed in 

receiving water to protect aquatic life and recreation in and on the water. Ecology uses 

numerical criteria along with chemical and physical data for the wastewater and receiving 

water to derive the effluent limits in the discharge permit. When surface water quality based 

limits are more stringent or potentially more stringent than technology based limits, the 

discharge must meet the water quality based limits.

Numerical Criteria for the Protection of Human Health 

The U.S. EPA has published 91 numeric water quality criteria for the protection of human 

health that are applicable to dischargers in Washington State (EPA 1992). These criteria are 

designed to protect humans from exposure to pollutants linked to cancer and other disease, 

based on consuming fish and shellfish and drinking contaminated surface waters. The water 

quality standards also include radionuclide criteria to protect humans from the effects of 

radioactive substances. 
Fact Sheet for NPDES Permit WA-004496-2 

Pasco Wastewater Treatment Works 

Final Fact Sheet – May 5, 2010 Page 10 

R. Koch/Eastern Regional Office 

Narrative Criteria 

Narrative water quality criteria (e.g., WAC 173-201A-240(1); 2006) limit the toxic, 

radioactive, or other deleterious material concentrations that the facility may discharge to 

levels below those which have the potential to: 

• Adversely affect designated water uses. 

• Cause acute or chronic toxicity to biota. 

• Impair aesthetic values. 

• Adversely affect human health. 

Narrative criteria protect the specific designated uses of all fresh waters (WAC 173-201A200, 2006) and of all marine waters (WAC 173-201A-210,; 2006) in the State of 

Washington. 

Antidegradation 

The purpose of Washington's Antidegradation Policy (WAC 173-201A-300-330; 2006) is 

to: 

• Restore and maintain the highest possible quality of the surface waters of 

Washington. 

• Describe situations under which water quality may be lowered from its current 

condition. 

• Apply to human activities that are likely to have an impact on the water quality of 

surface water. 

• Ensure that all human activities likely to contribute to a lowering of water quality, 

at a minimum, apply all known, available, and reasonable methods of prevention, 

control, and treatment (AKART). 

• Apply three tiers of protection (described below) for surface waters of the state. 

Tier I ensures existing and designated uses are maintained and protected and applies to all waters 

and all sources of pollutions. Tier II ensures that waters of a higher quality than the criteria 

assigned are not degraded unless such lowering of water quality is necessary and in the 

overriding public interest. Tier II applies only to a specific list of polluting activities. Tier III 

prevents the degradation of waters formally listed as "outstanding resource waters," and applies 

to all sources of pollution. 

A facility must prepare a Tier II analysis when all three of the following conditions are met: 

• The facility is planning a new or expanded action. 

• Ecology regulates or authorizes the action. 
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Pasco Wastewater Treatment Works 

Final Fact Sheet – May 5, 2010 Page 11 

R. Koch/Eastern Regional Office 

• The action has the potential to cause measurable degradation to existing water quality at 

the edge of a chronic mixing zone. 

This facility must meet Tier I requirements. 

• Dischargers must maintain and protect existing and designated uses. Ecology must not 

allow any degradation that will interfere with, or become injurious to, existing or 

designated uses, except as provided for in chapter 173-201A WAC. 

• The facility must include a Tier II analysis in the engineering report demonstrating that 

accommodating an additional 20 years of capacity will not cause measurable 

degradation to existing water quality at the edge of a chronic mixing zone. 

Mixing Zones 

A mixing zone is the defined area in the receiving water surrounding the discharge port(s), 

where wastewater mixes with receiving water. Within mixing zones the pollutant 

concentrations may exceed water quality numeric standards, so long as the discharge does 

not interfere with designated uses of the receiving water body (for example, recreation, 

water supply, and aquatic life and wildlife habitat, etc.) The pollutant concentrations 

outside of the mixing zones must meet water quality numeric standards. 

State and federal rules allow mixing zones because the concentrations and effects of most 

pollutants diminish rapidly after discharge, due to dilution. Ecology defines mixing zone 

sizes to limit the amount of time any exposure to the end-of-pipe discharge could harm 

water quality, plants, or fish. 

The state’s water quality standards allow Ecology to authorize mixing zones for the 

facility’s permitted wastewater discharges only if those discharges already receive all 

known, available, and reasonable methods of prevention, control, and treatment (AKART). 

Mixing zones typically require compliance with water quality criteria within a specified 

distance from the point of discharge and use no more than 25% of the available width of the 

water body for dilution. Ecology uses modeling to estimate the amount of mixing within the 

mixing zone. Through modeling, Ecology determines the potential for violating the water 

quality standards at the edge of the mixing zone and derive any necessary effluent limits. 

Steady state models are the most frequently used tools for conducting mixing zone analyses. 

Ecology chooses values for each effluent and for receiving water variables that correspond 

to the time period when the most critical condition is likely to occur (see Ecology’s Permit 

Writer’s Manual). Each critical condition parameter, by itself, has a low probability of 

occurrence and the resulting dilution factor is conservative. The term “reasonable 

worst-case” applies to these values. 

The mixing zone analysis produces a numerical value called a dilution factor (DF). A 

dilution factor represents the amount of mixing of effluent and receiving water that occurs at 

the boundary of the mixing zone. For example, a dilution factor of 10 means the effluent is 

10% and the receiving water is 90% of the total volume of water at the boundary of the 

mixing zone. Ecology uses dilution factors with the water quality criteria to calculate 

reasonable potentials and effluent limits. 
Fact Sheet for NPDES Permit WA-004496-2 

Pasco Wastewater Treatment Works 

Final Fact Sheet – May 5, 2010 Page 12 

R. Koch/Eastern Regional Office 

Water quality standards include both aquatic life-based criteria and human health-based 

criteria. The former are applied at both the acute and chronic mixing zone boundaries; the 

latter are applied only at the chronic boundary. The concentration of pollutants at the 

boundaries of any of these mixing zones may not exceed the numerical criteria for that zone. 

Each aquatic life acute criterion is based on the assumption that organisms are not exposed 

to that concentration for more than one hour and more often than one exposure in three 

years. 

Each aquatic life chronic criterion is based on the assumption that organisms are not 

exposed to that concentration for more than four consecutive days and more often than once 

in three years. 

The two types of human health-based water quality criteria distinguish between those 

pollutants linked to non-cancer effects (non-carcinogenic) and those linked to cancer effects 

(carcinogenic). The human health-based water quality criteria incorporate several exposure 

and risk assumptions. These assumptions include: 

• A 70-year lifetime of daily exposures. 

• An ingestion rate for fish or shellfish measured in kg/day. 

• An ingestion rate of two liters/day for drinking water 

• A one-in-one-million cancer risk for carcinogenic chemicals. 

This permit authorizes a small acute mixing zone, surrounded by a chronic mixing zone 

around the point of discharge (WAC 173-201A-400). The water quality standards impose 

certain conditions before allowing the discharger a mixing zone: 

1. Ecology must specify both the allowed size and location in a permit. 

The mixing zones will accommodate the geometric configuration and flow restriction for 

mixing zones in Chapter 173-201A WAC and are defined as follows: 

The chronic mixing zone shall extend no more than 330 feet downstream from the point of 

discharge and the acute mixing zone shall extend no more than 33 feet downstream. 

Ecology used the mixing zone dilution using as-built record drawing information for the 

new outfall, the defined mixing zone geometry and the CORMIX model to verify that the 

outfall provides adequate dilution to ensure that the discharge meets water quality standards. 

Ecology determined the following dilution factors using CORMIX in preparation for issuing 

the 1999 NPDES permit. 

Temperature scenario Acute mixing zone dilution Chronic mixing zone dilution

 Summer 1:150 1:182 effluent:river 

 Winter 1:139 1:173 effluent:river 
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Ecology used the following critical conditions to model the discharge: 

• The seven-day-average low river flow with a recurrence interval of ten years 

(7Q10); was 38,000 cfs when the outfall was designed and modeled. 

Parameter Value Value

Velocity 0.17 m/s 0.17 m/s

Depth 9.1 m 9.1 m

Width 915 m 915 m

Roughness (Manning) 0.025 0.025

Temperature 4 C 21 C

Copper (August 2002 result) 35 ug/L in effluent 35 ug/L in effluent

Before reissuing the NPDES permit in 2004 Ecology rechecked dilution using Visual 

Plumes and modeled the discharge from each diffuser ports separately. The plumes are 

separate and distinct within the allowed mixing zone, particularly in winter. In summer, 

when the effluent is slightly more buoyant than the receiving water, the plumes begin to 

merge at the edge of the chronic mixing zone. However, the dilution at the edge of the 

chronic mixing zone as calculated by Visual Plumes is 649.8. Using Visual Plumes the 

dilution at the edge of the acute mixing zone is 27.9. 

No changes have occurred generating new input parameters. Therefore, Ecology has used 

the mixing analysis completed for the 2004 permit for this proposed permit. 

2. The facility must fully apply “all known, available, and reasonable methods of 

prevention, control and treatment” (AKART) to its discharge. 

 

 Ecology has determined that the treatment provided at the Pasco WWTP meets the 

requirements of AKART (see “Technology based Limits”). 

3. Ecology must consider critical discharge conditions. 

Surface water quality-based limits are derived for the waterbody’s critical condition 

(the receiving water and waste discharge condition with the highest potential for 

adverse impact on the aquatic biota, human health, and existing or designated 

waterbody uses). The critical discharge condition is often pollutant-specific or 

waterbody-specific. 

Critical discharge conditions are those conditions that result in reduced dilution or 

increased effect of the pollutant. Factors affecting dilution include the depth of water, 

the density stratification in the water column, the currents, and the rate of discharge. 

Density stratification is determined by the salinity and temperature of the receiving 

water. Temperatures are warmer in the surface waters in summer. Therefore, density 

stratification is generally greatest during the summer months. Density stratification 

affects how far up in the water column a freshwater plume may rise. 
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The rate of mixing is greatest when an effluent is rising. The effluent stops rising when 

the mixed effluent is the same density as the surrounding water. After the effluent 

stops rising, the rate of mixing is much more gradual. Water depth can affect dilution 

when a plume might rise to the surface when there is little or no stratification. 

Ecology’s Permit Writer’s Manual describes additional guidance on criteria/design 

conditions for determining dilution factors. The manual can be obtained from 

Ecology’s website at http://www.ecy.wa.gov/biblio/92109.html. 

4. Supporting information must clearly indicate the mixing zone would not: 

• Have a reasonable potential to cause the loss of sensitive or important habitat. 

• Substantially interfere with the existing or characteristic uses. 

• Result in damage to the ecosystem. 

• Adversely affect public health. 

Ecology established Washington State water quality criteria for toxic chemicals using 

EPA criteria. EPA developed the criteria using toxicity tests with numerous organisms 

and set the criteria to generally protect the species tested and to fully protect all 

commercially and recreationally important species. 

EPA sets acute criteria for toxic chemicals assuming organisms are exposed to the 

pollutant at the criteria concentration for one hour. They set chronic standards assuming 

organisms are exposed to the pollutant at the criteria concentration for four days. 

Dilution modeling under critical conditions generally shows that both acute and chronic 

criteria concentrations are reached within minutes of being discharged. 

The discharge plume does not impact drifting and non-strong swimming organisms 

because they cannot stay in the plume close to the outfall long enough to be affected. 

Strong swimming fish could maintain a position within the plume, but they can also 

avoid the discharge by swimming away. Mixing zones generally do not affect benthic 

organisms (bottom dwellers) because the buoyant plume rises in the water column. 

Ecology has additionally determined that the effluent will not exceed 33 degrees C for 

more than two seconds after discharge; and that the temperature of the water will not 

create lethal conditions or blockages to fish migration. 

Ecology evaluates the cumulative toxicity of an effluent by testing the discharge with 

whole effluent toxicity (WET) testing. 

Ecology reviewed the above information, the specific information on the characteristics 

of the discharge, the receiving water characteristics and the discharge location. Based 

on this review, Ecology concluded that the discharge does not have a reasonable 

potential to cause the loss of sensitive or important habitat, substantially interfere with 

existing or characteristics uses, result in damage to the ecosystem, or adversely affect 

public health if the permit limits are met. 
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5. The discharge/receiving water mixture must not exceed water quality criteria 

outside the boundary of a mixing zone. 

Ecology conducted a reasonable potential analysis using procedures established by the 

EPA and by Ecology for each pollutant and concluded that the discharge/receiving 

water mixture will not violate water quality criteria outside the boundary of the mixing 

zone if permit limits are met. 

6. The size of the mixing zone and the concentrations of the pollutants must be 

minimized. 

At any given time, the effluent plume uses only a portion of the acute and chronic 

mixing zone, which minimizes the volume of water involved in mixing. The plume 

may rise through the water column as it mixes depending on the temperature, therefore 

much of the receiving water volume at lower depths in the mixing zone may not be 

mixed with discharge. Similarly, because the discharge may stop rising at some depth 

due to density stratification, waters above that depth will not mix with the discharge. 

Ecology determined it is impractical to specify in the permit the actual, much more 

limited volume in which the dilution occurs as the plume rises and moves with the 

current. 

Ecology minimizes the size of mixing zones by requiring dischargers to install diffusers 

when they are appropriate to the discharge and the specific receiving waterbody. Pasco 

has a multi-port diffuser, 3 ports. When a diffuser is installed, the discharge is more 

completely mixed with the receiving water in a shorter time. 

Ecology also minimizes the size of the mixing zone (in the form of the dilution factor) 

using design criteria with a low probability of occurrence. For example, Ecology uses 

the expected 95th percentile pollutant concentration, the 90th percentile background 

concentration, the centerline dilution factor, and the lowest flow occurring once in 

every ten years to perform the reasonable potential analysis. 

Because of the above reasons, Ecology has effectively minimized the size of the mixing 

zone authorized in the proposed permit. 

7. Maximum size of mixing zone. 

 The authorized mixing zone does not exceed the maximum size restriction. 

8. Acute mixing zone. 

• The discharge/receiving water mixture must comply with acute criteria as 

near to the point of discharge as practicably attainable.

Ecology determined that the acute criteria will be met at 10% of the distance or of 

the chronic mixing zone at the ten year low flow. 
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• The pollutant concentration, duration, and frequency of exposure to the 

discharge will not create a barrier to migration or translocation of indigenous 

organisms to a degree that has the potential to cause damage to the ecosystem. 

As described above, the toxicity of any pollutant depends upon the exposure, the 

pollutant concentration, and the time the organism is exposed to that concentration. 

Authorizing a limited acute mixing zone for this discharge assures that it will not 

create a barrier to migration. The effluent from this discharge will rise as it enters 

the receiving water, assuring that the rising effluent will not cause translocation of 

indigenous organisms near the point of discharge (below the rising effluent).

• Comply with size restrictions.

The mixing zone authorized for this discharge complies with the size restrictions 

published in chapter 173-201A WAC. 

 9. Overlap of Mixing Zones. 

This mixing zone does not overlap another mixing zone. 

J. Designated Uses and Surface Water Quality Criteria 

Applicable designated uses and surface water quality criteria are defined in chapter 

173-201A WAC. In addition, the U.S. EPA set human health criteria for toxic pollutants 

(EPA 1992). Criteria applicable to this facility’s discharge are summarized below in 

Table 6. 

• Aquatic Life Uses are designated based on the presence of, or the intent to provide 

protection for, the key uses. All indigenous fish and non-fish aquatic species must be 

protected in waters of the state in addition to the key species. The Aquatic Life Uses 

for this receiving water are identified below. 

Table 6: Aquatic Life Uses & Associated Criteria 

Salmonid Rearing and Migration Only

Temperature Criteria – Highest 7DAD MAX From the Washington-Oregon border (river 

mile 309.3) to Priest Rapids Dam (river mile 

397.1) temperature shall not exceed a 1-

DMax of 20.0°C due to human activities. 

When natural conditions exceed a 1-DMax of 

20.0°C, no temperature increase will be 

allowed which will raise the receiving water 

temperature by greater than 0.3°C; nor shall 

such temperature increases, at any time, 

exceed t = 34/(T + 9). 

Dissolved Oxygen Criteria – Lowest 1-Day 

Minimum 

6.5 mg/L 

Turbidity Criteria • 10 NTU over background when the 

background is 50 NTU or less; or 

• A 20 percent increase in turbidity when the 
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background turbidity is more than 50 NTU 

Total Dissolved Gas Criteria Total dissolved gas shall not exceed 110 

percent of saturation at any point of sample 

collection 

pH Criteria pH shall be within the range of 6.5 to 8.5 with 

a human-caused variation within the above 

range of less than 0.5 units 

• The recreational uses are extraordinary primary contact recreation, primary contact 

recreation, and secondary contact recreation. The recreational uses for this receiving 

water are identified below. 

Table 7: Recreational Uses and Associated Criteria

Recreational Use Criteria 

Primary Contact 

Recreation 

Fecal coliform organism levels must not exceed a geometric mean value 

of 100 colonies/100 mL, with not more than 10 percent of all samples (or 

any single sample when less than ten sample points exist) obtained for 

calculating the geometric mean value exceeding 200 colonies/100 mL 

• The water supply uses are domestic, agricultural, industrial, and stock watering. 

• The miscellaneous freshwater uses are wildlife habitat, harvesting, commerce and 

navigation, boating, and aesthetics. 

K. Evaluation of Surface Water Quality-Based Effluent Limits for Numeric Criteria 

Pollutants in an effluent may affect the aquatic environment near the point of discharge 

(near-field) or at a considerable distance from the point of discharge (far-field). Toxic 

pollutants, for example, are near-field pollutants—their adverse effects diminish rapidly 

with mixing in the receiving water. Conversely, a pollutant such as biological oxygen 

demand (BOD) is a far-field pollutant whose adverse effect occurs away from the discharge 

even after dilution has occurred. Thus, the method of calculating surface water 

quality-based effluent limits varies with the point at which the pollutant has its maximum 

effect. 

With technology-based controls (AKART), predicted pollutant concentrations in the 

discharge exceed water quality criteria. Ecology therefore authorizes a mixing zone in 

accordance with the geometric configuration, flow restriction, and other restrictions imposed 

on mixing zones by chapter 173-201A WAC. 

Chronic Mixing Zone 

WAC 173-201A-400(7)(a) specifies that mixing zones must not extend in a downstream 

direction from the discharge ports for a distance greater than 300 feet plus the depth of water 

over the discharge ports or extend upstream for a distance of over 100 feet, not utilize 

greater than 25% of the flow, and not occupy greater than 25% of the width of the water 

body.
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The maximum boundaries of the mixing zones are defined as follows: The chronic mixing 

zone shall extend no more than 330 feet downstream from the point of discharge. The acute 

mixing zone shall extend no more than 33 feet downstream of the point of discharge. 

Acute Mixing Zone 

WAC 173-201A-400(8)(a) specifies that in rivers and streams a zone where acute toxics 

criteria may be exceeded must not extend beyond 10% of the distance towards the upstream 

and downstream boundaries of the chronic zone, not use greater than 2.5% of the flow and 

not occupy greater than 25% of the width of the water body. 

Ecology determined the dilution factors that occur within these zones at the critical 

condition using the reasonable potential spreadsheet. The dilution factors are listed in Table 

7. The dilution factors below use a 7Q10 from the most recent recorded data from USGS; 

the current 7Q10 flow is 61,200 cfs. 

Table 8: Dilution Factors (DF) 

Criteria Acute Chronic 

Aquatic Life 1:181 1:2198 

Human Health, Carcinogen 1:8477 

Human Health, Non-carcinogen 1:3077 

Ecology determined the impacts of temperature, pH, ammonia, and copper as described 

below, using the dilution factors in the above table. The derivation of surface water qualitybased limits also takes into account the variability of pollutant concentrations in both the 

effluent and the receiving water. 

BOD5--With technology-based limits, this discharge results in a small amount of BOD 

loading relative to the large amount of dilution in the receiving water at critical conditions. 

Technology-based limits will ensure that dissolved oxygen criteria are met in the receiving 

water. 

Temperature--The state temperature standards (WAC 173-201A-200-210 and 600-612) 

include multiple elements: 

• Annual summer maximum threshold criteria (June 15 to September 15) 

• Supplemental spawning and rearing season criteria (September 15 to June 15) 

• Incremental warming restrictions 

• Protections against acute effects 

Ecology evaluates each criterion independently to determine reasonable potential and derive 

permit limits. 

• Annual summer maximum and supplementary spawning/rearing criteria 
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Each water body has an annual maximum temperature criterion [WAC 173-201A200(1)(c), 210(1)(c), and Table 602]. These threshold criteria (e.g., 12, 16, 17.5, 

20°C) protect specific categories of aquatic life by controlling the effect of human 

actions on summer temperatures. 

Some waters have an additional threshold criterion to protect the spawning and 

incubation of salmonids (9°C for char and 13°C for salmon and trout) [WAC 173-

201A-602, Table 602]. These criteria apply during specific date-windows. 

The threshold criteria apply at the edge of the chronic mixing zone. Criteria for 

most fresh waters are expressed as the highest 7-Day average of daily maximum 

temperature (7-DADMax). 

The 7-DADMax temperature is the arithmetic average of seven consecutive 

measures of daily maximum temperatures. Criteria for marine waters and some 

fresh waters are expressed as the highest 1-Day annual maximum temperature (1-

DMax). 

• Incremental warming criteria 

The water quality standards limit the amount of warming human sources can cause 

under specific situations [WAC 173-201A-200(1)(c)(i)-(ii), 210(1)(c)(i)-(ii)]. The 

incremental warming criteria apply at the edge of the chronic mixing zone. 

At locations and times when background temperatures are cooler than the assigned 

threshold criterion, point sources are permitted to warm the water by only a defined 

increment. 

These increments are permitted only to the extent doing so does not cause 

temperatures to exceed either the annual maximum or supplemental spawning 

criteria. 

At locations and times when a threshold criterion is being exceeded due to natural 

conditions, all human sources, considered cumulatively, must not warm the water 

more than 0.3°C above the naturally warm condition. 

When Ecology has not yet completed a TMDL, our policy allows each point source 

to warm water at the edge of the chronic mixing zone by 0.3°C. This is true 

regardless of the background temperature and even if doing so would cause the 

temperature at the edge of a standard mixing zone to exceed the numeric threshold 

criteria. Allowing a 0.3°C warming for each point source is reasonable and 

protective where the dilution factor is based on 25% or less of the critical flow. 

This is because the fully mixed effect on temperature will only be a fraction of the 

0.3°C cumulative allowance (0.075°C or less) for all human sources combined. 
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• Temperature Acute Effects 

Instantaneous lethality to passing fish: The upper 99th percentile daily maximum 

effluent temperature must not exceed 33°C; unless a dilution analysis indicates 

ambient temperatures will not exceed 33°C 2-seconds after discharge. 

General lethality and migration blockage: Measurable (0.3°C) increases in 

temperature at the edge of a chronic mixing zone are not allowed when the 

receiving water temperature exceeds either a 1DMax of 23°C or a 7DADMax of 

22°C. 

Lethality to incubating fish: Human actions must not cause a measurable (0.3°C)

warming above 17.5°C at locations where eggs are incubating. 

Annual summer maximum, supplementary spawning criterion, and incremental 

warming criteria: Ecology calculated the reasonable potential for the discharge to exceed 

the annual summer maximum, the supplementary spawning criterion, and the incremental 

warming criteria at the edge of the chronic mixing zone during critical condition(s). 

Reasonable potential for temperature was checked for early summer when a higher 

difference between effluent temperature and river temperature was noted. The incremental 

increase was marginally greater in June and both June and August (critical condition) had 

incremental increase much less than the criteria. The incremental increase (0.04 C) for this 

discharge is within the allowable amount. Therefore, the proposed permit does not include a 

temperature limit. 

The proposed permit will require monitoring of effluent and influent temperatures. USGS 

data supplies the ambient temperature. 

pH--Ecology predicts no violation of the pH criteria under critical conditions. Therefore, 

the proposed permit includes technology-based effluent limits for pH. 

Fecal Coliform--Under critical conditions, modeling predicts no violation of the water quality 

criterion for fecal coliform. Therefore, the proposed permit includes the technology-based 

effluent limit for fecal coliform bacteria. 

Toxic Pollutants--Federal regulations (40 CFR 122.44) require Ecology to place limits in 

NPDES permits on toxic chemicals in an effluent whenever there is a reasonable potential 

for those chemicals to exceed the surface water quality criteria. Ecology does not exempt 

facilities with technology-based effluent limits from meeting the surface water quality 

standards. 

The following toxic pollutants are present in the discharge: ammonia, and heavy metals. 

Valid ambient background data was available for ammonia and copper. Ecology used all 

applicable data to evaluate reasonable potential for this discharge to cause a violation of 

water quality standards. 
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Ecology determined that ammonia and copper pose no reasonable potential to exceed the 

water quality criteria at the critical condition using procedures given in EPA, 1991 and as 

described above. Ecology’s determination assumes that this facility meets the other effluent 

limits of this permit. 

L. Whole Effluent Toxicity 

The water quality standards for surface waters forbid discharge of effluent that causes toxic 

effects in the receiving waters. Many toxic pollutants cannot be measured by commonly 

available detection methods. However, laboratory tests can measure toxicity directly by 

exposing living organisms to the wastewater and measuring their responses. These tests 

measure the aggregate toxicity of the whole effluent, so this approach is called whole 

effluent toxicity (WET) testing. Some WET tests measure acute toxicity and other WET 

tests measure chronic toxicity. 

WET testing conducted during effluent characterization showed no reasonable potential for 

effluent discharges to cause receiving water acute toxicity. The proposed permit will not 

impose an acute WET limit. 

If this facility makes process or material changes which, in Ecology's opinion, increase the 

potential for effluent toxicity, then Ecology may (in a regulatory order, by permit 

modification, or in the permit renewal) require the facility to conduct additional effluent 

characterization. 

WET testing conducted during effluent characterization showed no reasonable potential for 

the effluent to cause receiving water chronic toxicity. The proposed permit will not impose 

a chronic WET limit. 

• If this facility makes process or material changes which, in Ecology's opinion, increase 

the potential for effluent toxicity, then Ecology may (in a regulatory order, by permit 

modification, or in the permit renewal) require the facility to conduct additional effluent 

characterization. 

M. Human Health 

Washington’s water quality standards include 91 numeric human health-based criteria that 

Ecology must consider when writing NPDES permits. These criteria were established in 

1992 by the U.S. EPA in its National Toxics Rule (40 CFR 131.36). The National Toxics 

Rule allows states to use mixing zones to evaluate whether discharges comply with human 

health criteria. 

Ecology determined the applicant's discharge does not contain chemicals of concern based 

on existing effluent data or knowledge of discharges to their system. Ecology will 

reevaluate this discharge for impacts to human health at the next permit reissuance. 
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N. Sediment Quality 

The aquatic sediment standards (chapter 173-204 WAC) protect aquatic biota and human 

health. Under these standards Ecology may require a facility to evaluate the potential for its 

discharge to cause a violation of sediment standards (WAC 173-204-400). You can obtain 

additional information about sediments at the Aquatic Lands Cleanup Unit website at 

http://www.ecy.wa.gov/programs/tcp/smu/sediment.html . 

Through a review of the discharger characteristics and of the effluent characteristics, 

Ecology determined that this discharge has no reasonable potential to violate the sediment 

management standards. 

O. Ground Water Quality Limits 

The ground water quality standards (chapter 173-200 WAC) protect beneficial uses of 

ground water. Permits issued by Ecology must not allow violations of those standards 

(WAC 173-200-100). 

The City of Pasco Wastewater Treatment Plant does not discharge wastewater to the ground. 

No permit limits are required to protect ground water. 

P. Comparison of Effluent Limits with the Previous Permit Issued on July 7, 2004 

Table 9: Comparison of Effluent Limits 

Basis of 

Limit

Previous Effluent Limits: 

Outfall # 001

Proposed Effluent Limits: 

Outfall # 001

Average 

Monthly

Average 

Weekly

Average 

Monthly

Average 

Weekly

Biochemical Oxygen 

Demand (5-day) Technology 30 mg/L 45 mg/L 30 mg/L 45 mg/L

Total Suspended 

Solids Technology 30 mg/L 45 mg/L 30 mg/L 45 mg/L

Fecal Coliform Bacteria Technology 200/100 mL 400/100 mL 200/100 mL 400/100 mL

pH Technology 6.0 to 9.0 6.0 to 9.0 

 

Parameter Average 

Monthly

Maximum 

Daily

Average 

Monthly

Maximum 

Daily

pH Technology 9.0 9.0 

IV. MONITORING REQUIREMENTS 

Ecology requires monitoring, recording, and reporting (WAC 173-220-210 and 40 CFR 122.41) 

to verify that the treatment process is functioning correctly and that the discharge complies with 

the permit’s effluent limits. 
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The monitoring schedule is detailed in the proposed permit under Condition S.2. Specified 

monitoring frequencies take into account the quantity and variability of the discharge, the 

treatment method, past compliance, significance of pollutants, and cost of monitoring. The 

required monitoring frequency is consistent with agency guidance given in the current version of 

Ecology’s Permit Writer's Manual (Publication Number 92-09) for a municipal wastewater 

treatment plant using the activated sludge process and having an annual average flow of between 

2 and 5 MGD. Ecology’s Permit Writer's Manual also recommends that WWTP with flow over 

1.0 MGD or having a pretreatment program monitor for phenols, cyanide, volatile organic 

compounds, base-neutral compounds and acid-extractable compounds. 

Monitoring of sludge quantity and quality is necessary to determine the appropriate uses of the 

sludge. Biosolids monitoring is required by the current state and local solid waste management 

program and also by EPA under 40 CFR 503. 

As a pretreatment publicly owned treatment works (POTW), the City of Pasco is required to 

sample influent, primary clarifier effluent, final effluent, and sludge for toxic pollutants in order 

to characterize the industrial input. Sampling is also done to determine if pollutants interfere 

with the treatment process or pass-through the plant to the sludge or the receiving water. The 

City of Pasco will use the monitoring data to develop local limits which commercial and 

industrial users must meet. 

Q. Lab Accreditation 

Ecology requires that facilities must use a laboratory registered or accredited under the 

provisions of chapter 173-50 WAC, Accreditation of Environmental Laboratories to prepare 

all monitoring data (with the exception of certain parameters). Ecology accredited the 

laboratory at this facility for the following parameters: 

General Chemistry 

parameter name method reference matrix *

Alkalinity, Total 2320 B SM N 

Ammonia 4500-NH3 D SM 19/20 N 

Biochemical Oxygen Demand, BOD/CBOD 5210 B SM N 

Chlorine Residual, Total 4500-Cl G SM N 

Dissolved Oxygen 4500-O G SM N 

Nitrate 10020 Hach N 

pH 4500-H SM N 

Solids, Total Suspended 2540 D SM N 

Specific Conductance 2510 B SM N 

Microbiology 
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parameter name method reference matrix *

Fecal Coliform - count 9222 D SM N 

* Matrix key: D = drinking water; N = non-potable water; S = solids/chem materials; A = air

V. OTHER PERMIT CONDITIONS 

R. Reporting and Record Keeping 

Ecology based permit condition S3 on our authority to specify any appropriate reporting and 

record keeping requirements to prevent and control waste discharges (WAC 173-220-210). 

S. Prevention of Facility Overloading 

Overloading of the treatment plant is a violation of the terms and conditions of the permit. 

To prevent this from occurring, RCW 90.48.110 and WAC 173-220-150 require the City of 

Pasco to take the actions detailed in proposed permit requirement S.4 to plan expansions or 

modifications before existing capacity is reached and to report and correct conditions that 

could result in new or increased discharges of pollutants. Condition S.4 restricts the amount 

of flow. 

The City of Pasco did exceed 85% of the design maximum month in October, November 

and December 2008. This likely is due to infiltration. Historically the highest flows of the 

year are in the fall of the year. In August 2005, the City requested a capacity analysis of the 

various components. The trickling filter is the limiting unit process. However, the trickling 

filter is used as a roughing filter and is not a limiting critical unit. The next limiting process 

unit at the time of the report was the anaerobic digester. The solids portion of the plant was 

upgraded last year and the solids portion of the plant is now unlikely to be the limiting 

factor. It is not clear what the current capacity and design criteria is. 

The proposed permit will require an evaluation of the hydraulic and organic loading 

capacity of the City of Pasco’s wastewater facilities as they currently exist and if necessary 

make recommendations appropriate for the City’s wastewater facilities to provide twenty 

years of additional service to the community. 

T. Operation and Maintenance (O&M) 

The proposed permit contains Condition S.5 as authorized under RCW 90.48.110, WAC 

173-220-150, chapter 173-230 WAC, and WAC 173-240-080. Ecology included it to 

ensure proper operation and regular maintenance of equipment, and to ensure that the City 

of Pasco Wastewater Treatment Plant takes adequate safeguards so that it uses the 

constructed facilities to their optimum potential in terms of pollutant capture and treatment. 
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U. Pretreatment 

Duty to Enforce Discharge Prohibitions 

This provision prohibits the publicly owned treatment works (POTW) from authorizing or 

permitting an industrial discharger to discharge certain types of waste into the sanitary 

sewer. 

• The first section of the pretreatment requirements prohibits the POTW from accepting 

pollutants which causes “Pass-through” or “Interference”. This general prohibition is 

from 40 CFR §403.5(a). Appendix B of this fact sheet defines these terms. 

• The second section reinforces a number of specific State and Federal pretreatment 

prohibitions found in WAC 173-216-060 and 40 CFR §403.5(b). These reinforce that 

the POTW may not accept certain wastes, which: 

• Are prohibited due to dangerous waste rules. 

• Are explosive or flammable. 

• Have too high or low of a pH (too corrosive, acidic or basic). 

• May cause a blockage such as grease, sand, rocks, or viscous materials. 

• Are hot enough to cause a problem. 

• Are of sufficient strength or volume to interfere with treatment. 

• Contain too much petroleum-based oils, mineral oil, or cutting fluid. 

• Create noxious or toxic gases at any point. 

40 CFR Part 403 contains the regulatory basis for these prohibitions, with the exception 

of the pH provisions which are based on WAC 173-216-060. 

• The third section of pretreatment conditions reflects state prohibitions on the POTW 

accepting certain types of discharges unless the discharge has received prior written 

authorization from Ecology. These discharges include: 

• Cooling water in significant volumes. 

• Stormwater and other direct inflow sources. 

• Wastewaters significantly affecting system hydraulic loading, which do not 

require treatment. 

Federal and State Pretreatment Program Requirements

Ecology administers the Pretreatment Program under the terms of the addendum to the 

“Memorandum of Understanding between Washington Department of Ecology and the 

United States Environmental Protection Agency, Region 10” (1986) and 40 CFR, part 403. 

Under this delegation of authority, Ecology issues wastewater discharge permits for 

significant industrial users (SIUs) discharging to POTWs which have not been delegated 

authority to issue wastewater discharge permits. Ecology must approve, condition, or deny 

new discharges or a significant increase in the discharge for existing significant industrial 

users (SIUs) (40 CFR 403.8 (f)(1)(i) and(iii)). 
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Industrial dischargers must obtain a permit from Ecology before discharging waste to the 

City of Pasco Wastewater Treatment Plant (WAC 173-216-110(5)). Industries discharging 

wastewater that is similar in character to domestic wastewater do not require a permit. 

Routine Identification and Reporting of Industrial Users 

The permit requires non-delegated POTWs to take “continuous, routine measures to identify 

all existing, new, and proposed significant industrial users (SIUs) and potential significant 

industrial users (PSIUs)” discharging to their sewer system. Examples of such routine 

measures include regular review of water and sewer billing records, business license and 

building permit applications, advertisements, and personal reconnaissance. System 

maintenance personnel should be trained on what to look for so they can identify and report 

new industrial dischargers in the course of performing their jobs. The POTW may not allow 

SIUs to discharge prior to receiving a permit, and must notify all industrial dischargers 

(significant or not) in writing of their responsibility to apply for a State Waste Discharge 

Permit. The POTW must send a copy of this notification to Ecology. 

Requirements for Performing an Industrial User Survey 

The proposed permit is to be issued for 1 year. The requirement for an Industrial User 

Survey has been deleted from the proposed permit. It will be in the following permit. 

V. Solid Waste Control 

To prevent water quality problems the facility is required in permit Condition S7 to store 

and handle all residual solids (grit, screenings, scum, sludge, and other solid waste) in 

accordance with the requirements of RCW 90.48.080 and state water quality standards. 

The final use and disposal of sewage sludge from this facility is regulated by U.S. EPA 

under 40 CFR 503, and by Ecology under chapter 70.95J RCW, chapter 173-308 WAC 

“Biosolids Management,” and chapter 173-350 WAC “Solid Waste Handling Standards.” 

The disposal of other solid waste is under the jurisdiction of the Benton Franklin County 

Health District. 

Requirements for monitoring sewage sludge and record keeping are included in this permit. 

This information will be used by Ecology to develop or update local limits and is also 

required under 40 CFR 503. 

W. General Conditions 

Ecology bases the standardized General Conditions on state and federal law and regulations. 

They are included in all individual municipal NPDES permits issued by Ecology. 
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VI. PERMIT ISSUANCE PROCEDURES 

X. Permit Modifications 

Ecology may modify this permit to impose numerical limits, if necessary to comply with 

water quality standards for surface waters, with sediment quality standards, or with water 

quality standards for ground waters, based on new information from sources such as 

inspections, effluent monitoring, outfall studies, and effluent mixing studies. 

Ecology may also modify this permit to comply with new or amended state or federal 

regulations. 

Y. Proposed Permit Issuance 

This proposed permit meets all statutory requirements for Ecology to authorize a wastewater 

discharge. The permit includes limits and conditions to protect human health and aquatic 

life, and the beneficial uses of waters of the state of Washington. Ecology proposes to issue 

this permit for a term of 1 year with a compliance schedule requiring submission of an 

engineering report suitable for approval. The engineering report must evaluate the hydraulic 

and organic loading capacity of the City of Pasco’s wastewater facilities as they currently 

exist and if necessary recommend appropriate upgrade to the wastewater facilities enabling 

it to provide twenty years of additional service to the community. The report must also 

include a Tier II antidegradation analysis as described earlier in the fact sheet. 
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VII. REFERENCES FOR TEXT AND APPENDICES 

Environmental Protection Agency (EPA) 

1992. National Toxics Rule. Federal Register, V. 57, No. 246, Tuesday, December 22, 1992. 

1991. Technical Support Document for Water Quality-based Toxics Control. EPA/505/2-90-001. 

1988. Technical Guidance on Supplementary Stream Design Conditions for Steady State 

Modeling. USEPA Office of Water, Washington, D.C. 

1985. Water Quality Assessment: A Screening Procedure for Toxic and Conventional 

Pollutants in Surface and Ground Water. EPA/600/6-85/002a. 

1983. Water Quality Standards Handbook. USEPA Office of Water, Washington, D.C. 

Tsivoglou, E.C., and J.R. Wallace. 

1972. Characterization of Stream Reaeration Capacity. EPA-R3-72-012. (Cited in EPA 

1985 op.cit.) 

Washington State Department of Ecology. 

2006. Permit Writer’s Manual. Publication Number 92-109 

(http://www.ecy.wa.gov/biblio/92109.html) 

Laws and Regulations 

(http://www.ecy.wa.gov/laws-rules/index.html) 

 Permit and Wastewater Related Information 

(http://www.ecy.wa.gov/programs/wq/wastewater/index.html) 

Water Pollution Control Federation. 

1976. Chlorination of Wastewater.

Wright, R.M., and A.J. McDonnell. 

1979. In-stream Deoxygenation Rate Prediction. Journal Environmental Engineering 

Division, ASCE. 105(EE2). (Cited in EPA 1985 op.cit.) 

 
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APPENDIX A—PUBLIC INVOLVEMENT INFORMATION 

Ecology proposes to reissue a permit to the City of Pasco Wastewater Treatment Plant. The 

permit includes wastewater discharge limits and other conditions. This fact sheet describes the 

facility and Ecology’s reasons for requiring permit conditions. 

Ecology will place a Public Notice of Draft on May 12, 2010 in the Tri-City Herald to inform the 

public and to invite comment on the proposed draft National Pollutant Discharge Elimination 

System permit and fact sheet. 

The notice – 

• Tells where copies of the draft permit and fact sheet are available for public evaluation. 

• Offers to provide the documents in an alternate format to accommodate special needs. 

• Asks people to tell us how well the proposed permit would protect the receiving water. 

• Invites people to suggest fairer conditions, limits, and requirements for the permit. 

• Invites comments on Ecology’s determination of compliance with antidegradation rules. 

• Urges people to submit their comments, in writing, before the end of the comment period. 

• Tells how to request a public hearing about the proposed NPDES permit. 

• Explains the next step(s) in the permitting process. 

Ecology has published a document entitled Frequently Asked Questions about Effective Public 

Commenting which is available on our website at http://www.ecy.wa.gov/biblio/0307023.html. 

You may obtain further information from Ecology by telephone at (509) 329-3455 or by writing 

to the address listed below. 

Water Quality Permit Coordinator 

Department of Ecology 

Eastern Regional Office 

4601 North Monroe Street 

Spokane, WA 99205-1295

The primary author of this permit and fact sheet is Richard A. Koch, P.E. 

 
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APPENDIX B—GLOSSARY 

1-DMax or 1-day maximum temperature - The highest water temperature reached on any 

given day. This measure can be obtained using calibrated maximum/minimum thermometers 

or continuous monitoring probes having sampling intervals of thirty minutes or less. 

7-DADMax or 7-day average of the daily maximum temperatures - The arithmetic average of 

seven consecutive measures of daily maximum temperatures. The 7-DADMax for any 

individual day is calculated by averaging that day's daily maximum temperature with the 

daily maximum temperatures of the three days prior and the three days after that date.

Acute Toxicity - The lethal effect of a compound on an organism that occurs in a short period of 

time, usually 48 to 96 hours. 

AKART - The acronym for “all known, available, and reasonable methods of prevention, control 

and treatment.” AKART is a technology-based approach to limiting pollutants from 

wastewater discharges which requires an engineering judgment and an economic judgment. 

AKART must be applied to all wastes and contaminants prior to entry into waters of the state 

in accordance with RCW 90.48.010 and 520, WAC 173-200-030(2)(c)(ii), and WAC 173-

216-110(1)(a). 

Ambient Water Quality - The existing environmental condition of the water in a receiving 

water body. 

Ammonia - Ammonia is produced by the breakdown of nitrogenous materials in wastewater. 

Ammonia is toxic to aquatic organisms, exerts an oxygen demand, and contributes to 

eutrophication. It also increases the amount of chlorine needed to disinfect wastewater. 

Annual Average Design Flow (AADF) - The average of the daily flow volumes anticipated to 

occur over a calendar year. 

Average Monthly Discharge Limit - The average of the measured values obtained over a 

calendar month's time.

Best Management Practices (BMPs) - Schedules of activities, prohibitions of practices,

maintenance procedures, and other physical, structural and/or managerial practices to prevent 

or reduce the pollution of waters of the state. BMPs include treatment systems, operating 

procedures, and practices to control: plant site runoff, spillage or leaks, sludge or waste 

disposal, or drainage from raw material storage. BMPs may be further categorized as 

operational, source control, erosion and sediment control, and treatment BMPs. 

BOD5 - Determining the Biochemical Oxygen Demand of an effluent is an indirect way of 

measuring the quantity of organic material present in an effluent that is utilized by bacteria. 

The BOD5 is used in modeling to measure the reduction of dissolved oxygen in receiving 

waters after effluent is discharged. Stress caused by reduced dissolved oxygen levels makes 

organisms less competitive and less able to sustain their species in the aquatic environment. 

Although BOD is not a specific compound, it is defined as a conventional pollutant under the 

federal Clean Water Act. 

Bypass - The intentional diversion of waste streams from any portion of a treatment facility. 

Chlorine - Chlorine is used to disinfect wastewaters of pathogens harmful to human health. It is 

also extremely toxic to aquatic life. 
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Chronic Toxicity - The effect of a compound on an organism over a relatively long time, often 

1/10 of an organism's lifespan or more. Chronic toxicity can measure survival, reproduction 

or growth rates, or other parameters to measure the toxic effects of a compound or 

combination of compounds. 

Clean Water Act (CWA) - The Federal Water Pollution Control Act enacted by Public Law 

92-500, as amended by Public Laws 95-217, 95-576, 96-483, 97-117; USC 1251 et seq. 

Compliance Inspection - Without Sampling - A site visit for the purpose of determining the 

compliance of a facility with the terms and conditions of its permit or with applicable statutes 

and regulations. 

Compliance Inspection - With Sampling - A site visit for the purpose of determining the 

compliance of a facility with the terms and conditions of its permit or with applicable statutes 

and regulations. In addition it includes as a minimum, sampling and analysis for all 

parameters with limits in the permit to ascertain compliance with those limits; and, for 

municipal facilities, sampling of influent to ascertain compliance with the 85 percent removal 

requirement. Ecology may conduct additional sampling. 

Composite Sample - A mixture of grab samples collected at the same sampling point at different 

times, formed either by continuous sampling or by mixing discrete samples. May be "timecomposite" (collected at constant time intervals) or "flow-proportional" (collected either as a 

constant sample volume at time intervals proportional to stream flow, or collected by 

increasing the volume of each aliquot as the flow increased while maintaining a constant time 

interval between the aliquots). 

Construction Activity - Clearing, grading, excavation, and any other activity which disturbs the 

surface of the land. Such activities may include road building; construction of residential 

houses, office buildings, or industrial buildings; and demolition activity. 

Continuous Monitoring - Uninterrupted, unless otherwise noted in the permit. 

Critical Condition - The time during which the combination of receiving water and waste 

discharge conditions have the highest potential for causing toxicity in the receiving water 

environment. This situation usually occurs when the flow within a water body is low, thus, 

its ability to dilute effluent is reduced. 

Dilution Factor (DF) - A measure of the amount of mixing of effluent and receiving water that 

occurs at the boundary of the mixing zone. Expressed as the inverse of the percent effluent 

fraction, for example, a dilution factor of 10 means the effluent comprises 10% by volume 

and the receiving water 90%. 

Engineering Report - A document which thoroughly examines the engineering and 

administrative aspects of a particular domestic or industrial wastewater facility. The report 

must contain the appropriate information required in WAC 173-240-060 or 173-240-130. 

Fecal Coliform Bacteria - Fecal coliform bacteria are used as indicators of pathogenic bacteria 

in the effluent that are harmful to humans. Pathogenic bacteria in wastewater discharges are 

controlled by disinfecting the wastewater. The presence of high numbers of fecal coliform 

bacteria in a water body can indicate the recent release of untreated wastewater and/or the 

presence of animal feces. 
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Grab Sample - A single sample or measurement taken at a specific time or over as short a 

period of time as is feasible. 

Industrial Wastewater - Water or liquid-carried waste from industrial or commercial processes, 

as distinct from domestic wastewater. These wastes may result from any process or activity 

of industry, manufacture, trade or business; from the development of any natural resource; or 

from animal operations such as feed lots, poultry houses, or dairies. The term includes 

contaminated storm water and, also, leachate from solid waste facilities. 

Major Facility - A facility discharging to surface water with an EPA rating score of > 80 points 

based on such factors as flow volume, toxic pollutant potential, and public health impact. 

Maximum Daily Discharge Limit - The highest allowable daily discharge of a pollutant 

measured during a calendar day or any 24-hour period that reasonably represents the calendar 

day for purposes of sampling. The daily discharge is calculated as the average measurement 

of the pollutant over the day. 

Maximum Day Design Flow (MDDF) - The largest volume of flow anticipated to occur during 

a one-day period, expressed as a daily average. 

Maximum Month Design Flow (MMDF) - The largest volume of flow anticipated to occur 

during a continuous 30-day period, expressed as a daily average. 

Maximum Week Design Flow (MWDF) - The largest volume of flow anticipated to occur 

during a continuous 7-day period, expressed as a daily average. 

Method Detection Level (MDL) - The minimum concentration of a substance that can be 

measured and reported with 99 percent confidence that the pollutant concentration is above 

zero and is determined from analysis of a sample in a given matrix containing the pollutant. 

Minor Facility - A facility discharging to surface water with an EPA rating score of < 80 points 

based on such factors as flow volume, toxic pollutant potential, and public health impact. 

Mixing Zone - An area that surrounds an effluent discharge within which water quality criteria 

may be exceeded. The area of the authorized mixing zone is specified in a facility's permit 

and follows procedures outlined in state regulations (chapter 173-201A WAC). 

National Pollutant Discharge Elimination System (NPDES) - The NPDES (Section 402 of the 

Clean Water Act) is the federal wastewater permitting system for discharges to navigable 

waters of the United States. Many states, including the state of Washington, have been 

delegated the authority to issue these permits. NPDES permits issued by Washington State 

permit writers are joint NPDES/State permits issued under both state and federal laws. 

pH - The pH of a liquid measures its acidity or alkalinity. It is the negative logarithm of the 

hydrogen ion concentration. A pH of 7 is defined as neutral, and large variations above or 

below this value are considered harmful to most aquatic life. 

Peak Hour Design Flow (PHDF) - The largest volume of flow anticipated to occur during a 

one-hour period, expressed as a daily or hourly average. 

Peak Instantaneous Design Flow (PIDF) - The maximum anticipated instantaneous flow. 
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Quantitation Level (QL) - The smallest detectable concentration of analyte greater than the 

Method Detection Limit (MDL) where the accuracy (precision &bias) achieves the 

objectives of the intended purpose. 

Reasonable Potential - A reasonable potential to cause a water quality violation, or loss of 

sensitive and/or important habitat. 

Responsible Corporate Officer - A president, secretary, treasurer, or vice-president of the 

corporation in charge of a principal business function, or any other person who performs 

similar policy- or decision-making functions for the corporation, or the manager of one or 

more manufacturing, production, or operating facilities employing more than 250 persons or 

have gross annual sales or expenditures exceeding $25 million (in second quarter 1980 

dollars), if authority to sign documents has been assigned or delegated to the manager in 

accordance with corporate procedures (40 CFR 122.22). 

Technology-based Effluent Limit - A permit limit that is based on the ability of a treatment 

method to reduce the pollutant. 

Total Suspended Solids (TSS) - Total suspended solids is the particulate material in an effluent. 

Large quantities of TSS discharged to receiving waters may result in solids accumulation. 

Apart from any toxic effects attributable to substances leached out by water, suspended solids 

may kill fish, shellfish, and other aquatic organisms by causing abrasive injuries and by 

clogging the gills and respiratory passages of various aquatic fauna. Indirectly, suspended 

solids can screen out light and can promote and maintain the development of noxious 

conditions through oxygen depletion. 

Solid waste - All putrescible and non-putrescible solid and semisolid wastes including, but not 

limited to, garbage, rubbish, ashes, industrial wastes, swill, sewage sludge, demolition and 

construction wastes, abandoned vehicles or parts thereof, contaminated soils and 

contaminated dredged material, and recyclable materials. 

State Waters - Lakes, rivers, ponds, streams, inland waters, underground waters, salt waters, and 

all other surface waters and watercourses within the jurisdiction of the state of Washington. 

Stormwater - That portion of precipitation that does not naturally percolate into the ground or 

evaporate, but flows via overland flow, interflow, pipes, and other features of a storm water 

drainage system into a defined surface water body, or a constructed infiltration facility. 

Upset - An exceptional incident in which there is unintentional and temporary noncompliance 

with technology-based permit effluent limits because of factors beyond the reasonable 

control of the Permittee. An upset does not include noncompliance to the extent caused by 

operational error, improperly designed treatment facilities, lack of preventative maintenance, 

or careless or improper operation. 

Water Quality-based Effluent Limit - A limit on the concentration of an effluent parameter 

that is intended to prevent the concentration of that parameter from exceeding its water 

quality criterion after it is discharged into receiving waters. 

 
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APPENDIX C—TECHNICAL CALCULATIONS 

Several of the Excel® spreadsheet tools used to evaluate a discharger’s ability to meet 

Washington State water quality standards can be found on Ecology’s homepage at 

http://www.ecy.wa.gov/programs/eap/pwspread/pwspread.html. 

 
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Final Fact Sheet – May 5, 2010 Page 35 

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APPENDIX D—RESPONSE TO COMMENTS 

The public notice that informed the public that a draft permit was available for review was 

published in the Tri-City Herald on May 12, 2010. Ecology did not receive any comments on the 

draft permit following the 30-day public comment period.