Wastewater Treatment Facilities Plan City of Wenatchee, Washington

Transcription

1 Wastewater Treatment Facilities Plan City of Wenatchee, Washington 210 North Worthen Street Wenatchee, WA Contact: Steve Brewer (509) Prepared for City of Wenatchee November 2008 Prepared by

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3 Contents 1.0 Summary, Conclusions, and Recommendations Summary Conclusions Recommendations Introduction Description of Discharge Standards Summary of Water Quality Standards National Pollutant Discharge Elimination System Permit Requirements Biosolids Reclassification Background Information Existing Environment Climate Geology and Soils Surface Water Resources Sensitive Areas Endangered and Threatened Species Public Health Prime or Unique Farmland Archaeological and Historical Sites Federally Recognized Wild and Scenic Rivers Existing Wastewater Flows and Wasteloads Population and Land Use Flows and Wasteloads Determination of Excessive Infiltration/Inflow Capacity of Existing WWTP Pro2D Calibration Activated Sludge Process Capacity Future Conditions Population and Land Use Forecasted Flow and Wasteloads Water Conservation and Flow Reduction Future Industrial Wastewater Forecasted Flow and Wasteloads Future Environment Without Project Capacity Visual Environment Odor Control Effluent Discharge Page iii

4 CONTENTS 6.0 Alternatives Improvements to Existing WWTP Expansion of the Activated Sludge Process Anaerobic Digestion Improvements Drying Bed Expansion Visual Aesthetic Improvements Odor Control Improvements Reclaimed Water Facilities Miscellaneous Maintenance Improvements Cost Estimates for Evaluated Facilities Cost Estimate for New WWTP Evaluation of Alternatives and Conceptual Design Drying Bed Improvements Influent Pump Station Existing Influent Pump and Configuration Replacement Pump Requirements Pumping System Modifications Required Replacement Pump Installation Financial Analysis Other Water Quality Management Plan Conformance SEPA Environmental Checklist SERP Compliance List of Required Permits References Appendixes A B C D E Toxic Substances Criteria NPDES Waste Discharge Permit No. WA Pro2D Model Output WWTP and Preliminary Design of Sludge Drying Beds and Raw Sewage Pumps Improvements Information SEPA Environmental Checklist Tables 1-1 Estimated Cost of Improvements Used to Calculate Residential Rate Impacts Aquatic Life Use Criteria Wenatchee WWTP Effluent Limits Land Use Inventory Existing Wastewater Flows and Wasteloads Calibration of Pro2D Model for Wenatchee WWTP Comparison of Population Served, Wastewater Flow, BOD, and TSS for WWTP in 1980 and iv

5 CONTENTS 4-5 Estimates of BOD and TSS When Average Annual Flow Reaches 5 mgd Users Connected to Wenatchee WWTP Summary of Flow, BOD 5 and TSS Projections for City of Wenatchee WWTP Forecasted Flow and Wasteloads Summary of Improvement Cost Estimates Aggregate Bituminous Surface Course Drying Bed Expansion Design Criteria Existing Raw Sewage Pumps No. 1 and No. 2 Data Existing Raw Sewage Pumps No. 1 and No. 2 Performance Criteria Estimated Costs of Improvements Used to Calculate Residential Rate Impacts Figures 2-1 Facilities Planning Area, City of Wenatchee Effluent CBOD Effluent TSS Effluent Ammonia Effluent Fecal Coliform Location of Existing Drying Beds Activated Sludge Capacity Project Schedule for Maximum Grant and Loan Funding Potential Improvements, Wenatchee WWTP Construction Cost Estimate Accuracy Ranges Sludge Drying Beds Expansion Demolition Plan Sludge Drying Beds Expansion Construction Plan Influent Pump Station Raw Sewage Pump Replacement Plans and Section v

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7 CHAPTER 1 Summary, Conclusions, and Recommendations This Wastewater Treatment Facilities Plan has been prepared to satisfy the requirements established in Washington State Department of Ecology (Ecology) Consent Order DE 02QCR The primary purposes of this plan are as follows: 1. Show that adequate capacity exists to treat estimated flow for the next 10 years 2. Estimate the cost of possible improvements to provide capacity for future wastewater flows after the current wastewater treatment plant (WWTP) capacity is fully used. The planning period for this plan is 2005 to Summary The recently improved WWTP has a maximum capacity for an annual average flow of 5.0 mgd (million gallons per day) based on the wastewater characteristics projected for a 2025 service population of 43,526. Assuming that the population growth rate continues at the rate projected for the time period, the maximum capacity of the WWTP will be reached in approximately There are many assumptions that must be made to make this prediction, and the date that WWTP capacity is reached could be earlier or later than estimated. The minimum capacity is estimated to be 4 mgd, estimated to be reached in Decisions to construct improvements need to occur several years prior to facilities reaching capacity because of the long lead time for financing, design, bidding, and construction. A minimum of 3 years is needed if the City requires no outside financial assistance. A minimum of 7 years is needed if the City desires to obtain State Revolving Fund (SRF) loan funds for planning, design, and construction. Expansion of the WWTP greater than the current design capacity of 5 mgd of wastewater will require additional activated sludge and solids handling facilities. These facilities are assumed to be constructed in 2025 for purposes of estimating the rate impacts. By 2010 two improvements are needed to expand the capacity of existing facilities. First, mixing and heating improvements are needed for the existing secondary anaerobic digester to allow it to operate as a backup to the two existing primary digesters. This increases the rated capacity of the existing anaerobic digesters 100 percent. Second, the sludge drying beds are at capacity in wet weather and additional capacity is needed. A sludge management plan is needed to address Ecology regulation changes in the way that the biosolids that Wenatchee produces are classified. A recent change in regulation reclassifies the biosolids from Class A to Class B. The City needs to study the impacts of the changed regulation on how the City manages biosolids and the type of solids processes that will be needed in the future. City staff is currently working with the farmer who has historically accepted Wenatchee biosolids and with County and Ecology regulatory agencies to obtain the required permits for land application of Class B biosolids. 1-1

8 The City is planning for improvements to reduce WWTP maintenance and enhance the WWTP as a neighbor. These improvements do not increase the WWTP capacity or improve effluent quality. These improvements can be made at the City s discretion and are not required by Ecology. New influent pumps can be installed to eliminate the need to remove rags from the impellers on a daily basis. New fine screens with washer-compactors can be installed to eliminate primary sludge pump plugging. Visual mitigation and odor control are proposed to improve the appearance of the WWTP and to control odors from the facility. Costs for visual mitigation and odor control approaches selected by City staff are modest. Visual mitigation and odor control studies are recommended to identify the approaches suitable for Wenatchee and to develop more accurate cost estimates for these improvements. Costs for effluent filtration facilities are included to allow the City to develop facilities to reuse 1.0 mgd of effluent for irrigation or other suitable use of Class A reclaimed water. Table 1-1 summarizes the estimated costs and approximate year the improvements are needed. The costs are in January 2007 dollars. The cost estimates are Class 5 estimates as defined by the American Association of the Advancement of Cost Engineering International with an accuracy of +100 to -50 percent of actual costs. The costs are total project costs and include Washington State sales tax and an allowance for engineering, legal, and administration. These costs have a low level of accuracy because the level of project definition and engineering is low. City staff requested that the facilities plan estimate the costs of possible improvements using representative improvements rather than evaluate alternatives because the need for the improvements was many years from now and technology would change, thus making the evaluations outdated before they were implemented. As a result, except for sludge drying bed and raw sewage pumps, this facility plan does not evaluate alternatives or present a conceptual design of recommended improvements to satisfy WAC for engineering reports. To satisfy WAC , this facility plan would be amended with specific evaluations of alternatives and conceptual designs prior to proceeding with improvements to the WWTP. TABLE 1-1 Estimated Cost of Improvements Used to Calculate Residential Rate Impacts Improvement New influent pumps $240,000 New fine screens $1,300,000 Expansion of activated sludge process $17,000,000 Anaerobic digestion Heating and mixing of secondary digester $740,000 Future primary digester $5,100,000 Drying bed expansion $1,100,000 Visual aesthetic improvements $3,600,000 Odor control improvements $4,200,000 Reclaimed water facilities (1.0 mgd) $2,700,000 Sludge management plan $150,000 Visual mitigation study $200,000 Odor control plan $100,

9 TABLE 1-1 Estimated Cost of Improvements Used to Calculate Residential Rate Impacts Improvement Total Capital Costs $14,330,000 $22,100,000 Increased Annual O&M Expansion of activated sludge process $320,000 Anaerobic digestion Future primary digester $53,000 Increased Annual O&M Costs $0 $373,000 All costs expressed in January 2007 dollars. The rate increases for residential customers to pay for the improvements shown in Table 1-1 depend on the method of financing. The improvements are assumed to be constructed in either 2010 or Using 20-year revenue bonds with 50-percent coverage, the rate increase is projected to be $9.58 per month in 2010 and increase to $26.43 per month in Using SRF loans for all expenses, the rate increase is projected to be $5.40 per month in 2010 and increase to $15.80 per month in Conclusions The existing activated sludge and anaerobic digestion processes control the capacity of the WWTP to accept additional service population. The WWTP has 11 mgd peak flow capacity, and with use of the flow equalization basin located on the other side of Worthen Street, has adequate hydraulic capacity. There is space on the existing WWTP site to add an additional anaerobic digester to add capacity to approximately 7 mgd. Additional anaerobic digester capacity could be provided if a taller digester design is used. The existing activated sludge process has a maximum average annual capacity of 5 mgd when operated in the step-feed mode. Space is available on the existing WWTP site to add a parallel 1 mgd of annual average capacity using a membrane bioreactor (MBR) type of process. The addition of the anaerobic digester, the parallel 1-mgd MBR facility, and water reclamation facilities completely fill the existing WWTP site. This would increase the capacity to 6 mgd average annual flow. The cost of a new 8-mgd WWTP located within 5.5 miles of the existing WWTP is estimated to be approximately $160 million. The City decided, for now, that it will expand the WWTP on a site it owns across Worthen Street to the west of the existing WWTP site. Potentially, another 4 mgd average annual of MBR facilities could be located on this site which could increase the maximum average annual flow capacity of the existing WWTP site to 10 mgd. 1.3 Recommendations 1. City investigate and confirm the accuracy of flow measurement and sampling with particular attention to waste activated sludge (WAS) and influent and primary effluent biochemical oxygen demand (BOD) and total suspended solids (TSS). WEN_FAC_PLAN_11_17_08.DOC 1-3

10 2. City review influent waste loads annually to confirm loading increases are within planning assumptions. 3. Preparation of facilities plan amendments for WWTP improvements that evaluate alternatives and include conceptual designs and obtain Ecology approval prior to proceeding with improvements. 4. Preparation of an odor control plan to determine the odor control facilities appropriate to Wenatchee and their cost. 5. Preparation of visual mitigation plan to determine visual mitigation facilities appropriate to Wenatchee and their cost. 6. Anaerobically digest primary and thickened waste activated sludge (TWAS) together in the two existing primary digesters. 7. Recommend modifying the overflow piping to allow operation of Primary Digester No. 2 at the increased elevation of Preparation of a sludge management plan that evaluates biosolids management alternatives and solids processing prior to making solids improvements. 9. Use of Hidrostal influent pumps to eliminate ragging problems. 10. Use of 1/4-inch-diameter-opening perforated plate escalator screens and washer compactors to eliminate primary sludge pump plugging. 1-4

11 CHAPTER 2 Introduction This Wastewater Treatment Facilities Plan (Facilities Plan), City of Wenatchee, Washington, has been prepared to satisfy the requirements established in Washington State Department of Ecology (Ecology) Consent Order DE 02WQCR The primary purpose of the Facilities Plan is to show that adequate WWTP capacity exists for the next 10 years and develop a capital improvement program based on possible improvements needed to provide capacity for future wastewater flows. The planning period for the facilities plan is 2005 to The majority of improvements are not needed for another 10 years or more, and technology is likely to change in this time interval. As a result, City staff has requested that this plan estimate the costs of possible improvements using representative improvements rather than evaluate alternatives. Representative improvements are based on facilities that have been successfully used at one or more WWTPs to satisfy similar requirements in other jurisdictions. Costs for the representative improvements are based on the actual costs experienced, increased using general cost escalation factors, and adjusted using accepted scaling factors to the Wenatchee facilities. When the City decides to proceed with improvements to the WWTP, amendments to this Facilities Plan will be prepared that evaluate alternative approaches and develop design criteria for the potential improvements. Figure 2-1 shows the facilities planning area, which consists of the area within the current city limits of the City of Wenatchee, plus an unincorporated area in Chelan County north of the Wenatchee River referred to as the Sunnyslope area. The planning area is bounded by the Columbia River on the east and foothills to the west. The planning area is within the Wenatchee urban growth boundary established by Chelan County. The following information is provided in this Facilities Plan: The capacity of the existing WWTP was estimated and projections were made to estimate when this capacity will be reached. Costs for increasing the capacity of the existing WWTP were developed including: Adding secondary treatment capacity Improving the performance and capacity of the anaerobic digesters Expanding the existing sludge drying beds Improvements to the existing WWTP to improve visual aesthetics and reduce odors Additional treatment to allow use of the effluent on the Riverfront Park and for other irrigation purposes For purposes of comparison, rough estimates were prepared for constructing a new WWTP at another location. 2-1

12 A series of studies and WWTP improvements are recommended on a schedule to maintain WWTP capacity. This information provides the basis for a capital improvements program. 2-2

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15 CHAPTER 3 Description of Discharge Standards Facility planning for wastewater treatment plants (WWTPs) is based on the level of treatment required and the quantity of wastewater to be treated. This chapter examines current and potential future requirements for level of treatment. The WWTP discharges to the Columbia River, and the treatment level is determined by the water quality in this receiving stream. The Wenatchee River is also addressed because it passes through the northern end of the planning area and has the potential to be a future receiving water body. 3.1 Summary of Water Quality Standards This section summarizes the current and anticipated water quality standards for the Columbia River in the vicinity of the WWTP discharge point. Water quality standards can change as a result of water quality improvement projects or Total Maximum Daily Load (TMDL) studies conducted by Washington State Department of Ecology (Ecology) and the U.S. Environmental Protection Agency (EPA). Discharge requirements for the City of Wenatchee are not anticipated to change in the next 10 to 20 years because there are no nutrient or dissolved oxygen (DO) TMDLs active or planned for the Columbia River. There is a temperature TMDL currently on hold, although this is not likely to affect the Wenatchee WWTP discharge standards. However, growth is occurring along the Wenatchee River. TMDL studies are being conducted on the Wenatchee River for DDT and temperature. A DO, ph, and phosphorus TMDL study was published in April 2006 for the Wenatchee River, and a fecal coliform TMDL was completed in Based on these studies, the water quality standards for the Wenatchee River are likely to change and become much more stringent. These water quality standards could affect any new WWTP discharging to the Wenatchee River. Current water quality standards for the Columbia River are listed in the Washington Administration Code (WAC) A-200, which describes the water quality standards for all surface waters within the state. The standards for the Columbia River at the City of Wenatchee are summarized below. The Columbia River has an aquatic life use designation of salmon/trout spawning, noncore rearing, and migration. This classification determines aquatic life temperature, dissolved oxygen, turbidity, total dissolved gas, and ph criteria. These criteria are listed in Table 3-1. The Columbia River is also classified as Primary Contact Recreation. This classification requires that 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 10 sample points exist) obtained for calculating the geometric mean value exceeding 200 colonies/100 ml. The Columbia River is listed as an appropriate water supply for domestic, agricultural, industrial, and stock watering uses as well as the miscellaneous uses of wildlife habitat, 3-1

16 harvesting, commerce and navigation, boating, and aesthetics. For these uses, Table A-1 in Appendix A details the criteria for toxic, radioactive, and deleterious materials. TABLE 3-1 Aquatic Life Use Criteria Parameter Aquatic life temperature Aquatic life dissolved oxygen (DO) Aquatic life turbidity criteria in fresh water Aquatic life total dissolved gas criteria in fresh water Aquatic life ph criteria in fresh water Criterion The highest 7-day average of the daily maximum temperatures (7-DADMax) is 17.5 Celsius (63.5 Fahrenheit). The lowest 1-day minimum is 8.0 mg/l. 5 NTU (Nephelometric Turbidity Unit) over background when the background is 50 NTU or less; or a 10 percent increase in turbidity when the background turbidity is more than 50 NTU. Total dissolved gas shall not exceed 110 percent of saturation at any point of sample collection. ph shall be within the range of 6.5 to 8.5, with a humancaused variation within the range of less than 0.5 units. 3.2 National Pollutant Discharge Elimination System Permit Requirements Effluent from the treatment plant is discharged into the Columbia River. To protect the water quality of the river, the effluent must meet treatment standards identified by Ecology. The plant's current National Pollutant Discharge Elimination System (NPDES) permit, issued in 2005, describes the discharge standards for the plant and is provided in Appendix A. Effluent limits for the plant are summarized in Table 3-2. The existing standards described in Section 3.1 are achieved by meeting the requirements of the NPDES permit as described in this section. The City is not requesting any changes to the effluent limits shown in Table 3-2 which are from the current NPDES permit. Figures 3-1 through 3-4 show effluent CBOD, TSS, total ammonia, and fecal coliform for September 2005 through September The most recent WWTP improvements began operation in September Effluent CBOD, TSS, fecal coliform, and total ammonia are much lower than the maximum permitted discharge concentrations. 3.3 Biosolids Reclassification Ecology revised biosolids regulations for the State of Washington that eliminated Alternative 4 of CFR503 for achieving Class A biosolids in the sludge drying beds. The biosolids from the drying beds will now be classified as Class B and be subject to different regulatory requirements. The City is working with the farmer who has historically accepted Wenatchee biosolids and with County and Ecology personnel to obtain permits and regulatory approvals required to land-apply Class B biosolids. 3-2

17 TABLE 3-2 Wenatchee WWTP Effluent Limits Parameter Average Monthly Average Weekly 5-Day Carbonaceous Biochemical Oxygen Demand (CBOD 5) Total Suspended Solids (TSS) 25 mg/l; 1,147 lb/day 85% removal of influent CBOD 5 30 mg/l, 1,376 lb/day 85% removal of influent TSS 40 mg/l; 1,835 lb/day 45 mg/l, 2,064 lb/day Fecal Coliform Bacteria 200/100 ml 400/100 ml ph Daily minimum is equal to or greater than 6.0 and the daily maximum is less than or equal to 9.0. Parameter Average Monthly Maximum Daily a Total Ammonia (as NH 3-N) 25 mg/l; 1,147 lb/day 47 mg/l; 2,156 lb/day Note: The average monthly and weekly effluent limitations are based on the arithmetic mean of the samples taken with the exception of fecal coliform, which is based on the geometric mean. a The maximum daily effluent limitation is defined as the highest allowable daily discharge. The daily discharge means the discharge of a pollutant measured during a calendar day. For pollutants with limitations expressed in units of mass, the daily discharge is calculated as the total mass of the pollutant discharged over the day. 3-3

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23 CHAPTER 4 Background Information This chapter establishes the existing wastewater flows and wasteloads being treated by the Wenatchee WWTP and determines the capacity of the WWTP in its current configuration. The flows and wasteloads are based on the existing environment, the population and land use in the service area, and infiltration and inflow to the collection system. The existing flows, wasteloads, and plant capacity are used in Chapter 5 to determine future wastewater flows and wasteloads and estimate future facility requirements. 4.1 Existing Environment The City of Wenatchee is located in central Washington on the east side of the Cascade Mountain Range. The City lies in the Columbia River Valley, just south of the confluence of the Wenatchee and Columbia Rivers. The Columbia River forms much of the eastern boundary of the Wenatchee city limits Climate The climate of the area is heavily influenced by the Cascade Mountain Range. The prevailing westerly flow of air across the Cascades loses much of its moisture before reaching the Wenatchee area. The result is a relatively dry and mild climate pattern. The area experiences average precipitation of around 9 inches, with snowfall averaging 30 to 35 inches in the winter. Precipitation patterns are characterized by infrequent rainfalls of high intensity. Temperatures range from an average of 26 degrees Fahrenheit (ºF) in January to 73ºF in July Geology and Soils The major geologic formation underlying the area is the Wenatchee Formation. This formation is composed of medium- to course-grained sandstone that is cross-bedded with pebbly sandstone. Stream deposits (alluvium) consisting of uncemented silt, sand, or gravel overlie the Wenatchee Formation. Flooding during the Ice Age is thought to have deposited a layer of clay in the valley that has rendered much of the area unsuitable for septic tanks. Available City records and existing borings drilled at the existing wastewater treatment plant site in 1972 for a previous plant improvements project indicate that the site has been used as a solid waste disposal area. Refuse has not been accepted at the site since Much of the refuse was excavated and removed from the site during construction of the secondary clarifiers and aeration basins in 1975 and during construction in Additional refuse remains within the WWTP site and in areas surrounding the WWTP site. The WWTP site is greatly disturbed and soils at the site are not considered prime or unique soils. The soils existing at the site of the existing drying beds are PrB-Pogue gravelly fine sand loam, which is on the prime soils list, and PrC-Pogue gravelly fine sandy loam, which is on 4-1

24 the unique soils list. However, these are not considered prime or unique soils unless they are irrigated, which they are not. Figure 4-1 shows the location of the drying beds Surface Water Resources The Columbia and Wenatchee Rivers are two major surface water resources in the area. Both are regulated for hydroelectric power generation and irrigation supply. The effluent from the Wenatchee WWTP discharges into the Columbia River in an area where it is designated as a Class A water course by Ecology. Columbia River water quality in the Wenatchee area is generally quite good and influenced more by naturally occurring impurities gathered at times of high runoff than by human activities. To protect the water quality of the receiving water resource from contamination by elements in wastewater discharged by the WWTP, treatment standards must be met as specified in the NPDES permit issued for the plant (see Chapter 3). Reliability and redundancy in the treatment works are also specified in regulations to protect the water quality of surface water resources receiving treated effluent. The designation of Reliability Class II (as defined in Technical Bulletin Design Criteria for Mechanical, Electric, and Fluid System and Component Reliability by U.S. EPA issued 1974 and described in Table G2-6 of Criteria for Sewage Works Design by Ecology in December 1998) applies to works for which discharge or potential discharge as a result of volume and/or character would not permanently or unacceptably damage or affect the receiving waters or public health during periods of short-term operations interruptions, but could be damaging if continued interruption of normal operations were to occur (on the order of several days). Tables G2-7 and 8 of the Criteria contains specific requirements excerpted from the EPA technical bulletin. The Wenatchee treatment system is designated to meet requirements of Reliability Class II, which include works with a discharge or potential discharge moderately distant from shellfish areas, drinking water intakes, areas used for contact water sports, and residential areas. In accordance with the requirements of Reliability Class II, capabilities must be provided for satisfactory operation of the Wenatchee treatment works during power failures, flooding, peak loads, equipment failure, and maintenance shutdown Sensitive Areas Neither the existing WWTP site nor the existing drying bed site is located in a sensitive area. The WWTP and sludge drying beds are located at higher elevation than the 100-year-flood elevation and are not subject to flooding. Both sites do not contain and are not located adjacent to wetlands Endangered and Threatened Species The Washington Department of Fish and Wildlife (WDFW) has determined the Columbia River to be priority habitat for certain species of resident and anadromous fish. The upper Columbia River spring run of Chinook salmon and the upper Columbia River steelhead are listed as federal threatened species under the Endangered Species Act. WDFW also calls out the City of Wenatchee as being a part of the historical winter range for mule deer. This winter range has been lost due to agriculture, housing development, and game fences. The drying bed site has not yet been mapped by WDFW for priority habitats and/or species. 4-2

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27 4.1.6 Public Health The Chelan-Douglas Health District does not have a system for tracking failing septic tanks. However, according to the Health District, failing septic tanks have become a major issue for many jurisdictions in the area near Wenatchee. The Health District believes that it can be assumed that many of the older septic tanks in the area are failing or will fail in the near future. 1 The Health District believes that planning to expand capacity to allow those with failing septic tanks to connect to the sewer system would be beneficial in the long term for public health Prime or Unique Farmland There is no record of the WWTP site or the sludge drying bed site ever having been used as farmland. Available City records and existing borings drilled at the plant site in 1972 for a previous plant improvement project indicate that the site has been used as a solid waste disposal area. Refuse has not been accepted at the site since Much of the refuse was excavated and removed from the site during construction of the secondary clarifiers and aeration basins in Additional refuse remains within the WWTP site and in areas surrounding the site Archaeological and Historical Sites A field survey of the area in the vicinity of the drying bed site was conducted in January The survey found two separate archaeological sites located a distance away from the drying beds. One site contains 22 or more rock cairns. These cairns are suspected to be Native American burial sites. The other site has low-density lithic scatter. Petrified wood, cryptocrystalline (ccs) flakes, and a cryptocrystalline biface knife fragment were found at this site. Consultation with the Colville Federated Tribes and the Washington Department of Archaeology and Historic Preservation must be conducted before any work is performed in the area Federally Recognized Wild and Scenic Rivers Neither the Columbia River nor the Wenatchee River are federally recognized wild and scenic rivers. However, both rivers are important to the region because they provide habitat for threatened and endangered species. They also provide recreation for local residents and tourists. 4.2 Existing Wastewater Flows and Wasteloads This section describes the population and land use in the Wenatchee service area, the current wastewater flows and wasteloads being treated at the WWTP, and any excess infiltration and inflow included in the existing flows. 1 Septic tanks fail because the drainfield fails for a variety of reasons. This leads to sewage on the ground surface or groundwater contamination. Septic tanks can also cause elevated concentrations of nitrate and sometimes fecal coliform to build up in the groundwater. Failing septic tanks causing health problems due to ponding on the surface and groundwater contamination are the public health issues. 4-5

28 4.2.1 Population and Land Use Population The City s comprehensive plan (2006) provides the following information: With 29,920 residents [in 2006], Wenatchee is currently the 34th largest city in the state of Washington. Of all cities in Washington State, Wenatchee places 12th in terms of people per square mile. Wenatchee experienced strong growth in the 1990s. Between 1990 and 2000, Wenatchee s population grew 28 percent, similar to the growth rate of Chelan County (27 percent). Wenatchee s population represented 42 percent of the total county population during both census years. For wastewater facility planning, it is necessary to plan for the connected population, which is the population that is currently connected and is forecasted to be connected to the sewer system within a specified time period, and who send wastewater to the City treatment plant. There are still homes in Wenatchee that use on-site septic systems and are not connected to the sewer system. The City s sewer system serves the area within the city limits, plus a few areas outside of the city limits. In year 2006 the City had 8,278 residential sewer accounts, representing approximately 7,903 single family residences and 375 multifamily residences, with a total estimated population served of 29,240 persons. Of this, approximately 27,918 people are within the city limits, representing about 93 percent of the city s population. Outside the city limits, there were about 494 single family and 8 multi-family accounts representing about 1,322 people. There are estimated to be about 792 dwelling units (2,003 persons) on septic systems within the City limits, based on discussions with Chelan-Douglas Health District and the City, combined with review of the number of residential accounts and population estimates. The island of Chelan County land that is surrounded by city limits is almost entirely served by on-site septic systems and is not included in this number (they are outside city limits). The Sunnyslope Urban Growth Area (UGA), located north of the Wenatchee River, currently is located in unincorporated Chelan County and will be served by the City of Wenatchee WWTP. The Chelan County Draft Sunnyslope Subarea Plan (Studio Cascade 2007) lists a current estimated population of about 3,100 people for the Sunnyslope UGA. Currently, there are only three properties at Sunnyslope that are served with sewer: Sunnyslope Elementary School, the U.S. Forest Service building, and a car wash that is under construction Land Use The City of Wenatchee currently encompasses approximately 4,725 acres (City of Wenatchee, 2006). Within the city limits there is a mix of residential land use of varying densities, commercial uses, and warehouse industrial uses. Residential use dominates the land use in Wenatchee, more than 60 percent of the City. Slightly more than 10 percent of the urban area is used for commercial purposes (City of Wenatchee, 2006). Table 4-1 summarizes the land use for the city and the UGA south of the Wenatchee River. 4-6

29 TABLE 4-1 Land Use Inventory City of Wenatchee and Urban Growth Area South of the Wenatchee River Land Use Acres % of Total Civic and Cultural Commercial Industrial Multi-family Parks & Open Space Public Facilities Resource Lands Single Family 2, Undeveloped Source: Table 5 of City of Wenatchee (2006). TOTAL 5, The UGA outside the current city limits is zoned as mostly residential, with areas of commercial and industrial zoning. Within the city limits, there is an island of Chelan County land that has not been annexed into the City. The City and County entered into an interlocal cooperative agreement in 2004 that addresses the Olds Station and Sunnyslope areas. This agreement incorporated the Sunnyslope and Olds Station UGA into the City s UGA. The agreement leaves planning responsibilities for Sunnyslope/Olds Station with the County, but obligates the City to plan for and supply sanitary sewer service within the existing and any expanded Sunnyslope UGA. In November, 2005, the County commissioned a consultant to create the Sunnyslope Long-Range Plan. The City adopted a waterfront sub-area plan in The plan aims to encourage new development, both high-density residential and commercial, along the waterfront from about Chehalis Street on the south end to the Wenatchee River on the north end. This area will receive consideration in planning sanitary sewer and evaluating the impacts on the current system Flows and Wasteloads Wenatchee WWTP flow and wasteload data were analyzed for the years 2000 and 2001 to establish existing wastewater flows and wasteloads. These data are summarized in Table 4-2. Complete data analysis and a graphical presentation of the data are available in Appendix B of Final Facility Plan Amendment for Wenatchee Wastewater Treatment Plant Improvements (CH2M HILL, 2003). The draft Comprehensive Sewer Plan (City of Wenatchee, October 2008) shows influent data for the years 2002 through Data show 5 to 10 percent lower influent BOD and TSS in 2006 and 2007 compared to The City has emphasized control of fats, oil and grease (FOG) from commercial dischargers and 4-7

30 industrial pretreatment since 2002 and these programs along with changes in industrial dischargers may be affecting the amount of BOD and TSS discharged to the sewer system accounting for the observed reduction in BOD and TSS. TABLE 4-2 Existing Wastewater Flows and Wasteloads Plant Flow (mgd) Average annual Maximum 30-day Maximum 7-day Maximum day Peak hour Plant Influent BOD (lb/day) Average annual 6,324 7,003 Maximum 30-day 7,011 7,725 Maximum 7-day 7,656 8,833 Maximum day 9,438 13,532 TSS (lb/day) Average annual 6,984 7,465 Maximum 30-day 8,255 8,588 Maximum 7-day 9,462 10,159 Maximum day 13,544 13,631 Ammonia (lb/day) Average annual Maximum 30-day Maximum 7-day Maximum day Peak flow at the Wenatchee WWTP is different from the peak flow at many WWTPs. In 2005, a new equalization basin was installed west of Worthen Street to store peak flow in excess of 11 mgd. As Table 4-2 shows, peak flows to the Wenatchee WWTP can exceed 11 mgd for short periods of time. These high flows are associated with intense rainfall during thunderstorms or possibly snowmelt. The magnitude of extreme events is not known because they exceed the 15-mgd maximum range of the WWTP flowmeter. The equalization basin is sized to limit the maximum flow to the WWTP treatment processes to 4-8

31 11 mgd by storing flows above that level until the flows to the WWTP are less than that level, and then releasing the stored flow. The firm hydraulic capacity of the WWTP is 11 mgd, and the equalization basin is needed to prevent hydraulic overloading of the WWTP Determination of Excessive Infiltration/Inflow The draft 2008 Comprehensive Sewer Plan prepared by the City of Wenatchee found that infiltration and inflow (I/I) in the City s collection system are not excessive based on available data and U.S. EPA guidelines. The details of the I/I evaluation may be found in the Comprehensive Sewer Plan. 4.3 Capacity of Existing WWTP This section evaluates the capacity of the existing WWTP after completion of improvements in September The purpose of the capacity analysis is to provide an accurate estimate of the capacity of the WWTP using operating data obtained after completion of the improvements to the WWTP. The improvements included the following: Construction of flow equalization basin Replacement of influent gates Replacement of raw sewage pumps and drives for normal flows Replacement of coarse bubble diffused aeration diffusers with fine bubble diffusers in the aeration basins Installation of baffles in aeration basin to create four aeration zones in each aeration basin Installation of mixers in the first aeration zone and mixed liquor pumps in final aeration zone to allow operation of first aeration zone in anoxic conditions Modification of influent and effluent channels to allow operation of the activated sludge process in plug flow and step-feed modes Replacement of secondary clarifier mechanisms Modification of return activated sludge (RAS) pumping control Replacement of chlorine disinfection with ultraviolet disinfection Modification of WWTP stormwater piping to discharge onsite POTW stormwater into the influent pump station instead of Columbia River Change of control system for liquids treatment portion of WWTP to programmable logic controller and personal computer supervisory control and data acquisition (SCADA) systems The improvements to the aeration basins, activated sludge process, secondary clarifiers, and RAS pumping affect the capacity of the WWTP, but the other improvements have no effect 4-9

32 on WWTP capacity or maintain the 11-mgd firm hydraulic capacity of the WWTP. The capacity evaluation focused on the capacity of the activated sludge process, which consists of the aeration basins, aeration system, secondary clarifiers, and RAS pumping. The 11-mgd firm hydraulic capacity was established in the improvements completed in 1992, is not being changed, and therefore is not evaluated in this Facilities Plan. The capacity evaluation was done using Ecology s Criteria for Sewage Works Design (Ecology, 1998), and CH2M HILL s Pro2D wastewater process model. The Wenatchee WWTP is a Class II reliability facility as described earlier in this report, and the evaluation is based on the criteria for Class II reliability. All WWTPs in the state of Washington must comply with criteria in the Ecology manual. Ecology s Criteria for Sewage Works Design allows the use of modeling for evaluation of activated sludge processes. The Pro2D model was developed to calculate loading parameters to confirm conformance with Ecology criteria. It is a spreadsheet-based model that models the entire WWTP, and develops loading criteria for each unit process as well as detailed mass balances. Pro2D uses the ASM 2d model to calculate activated sludge operating parameters. An international group of experts developed the ASM 2d model, which is widely used for evaluation of activated sludge processes and meets the requirements for Criteria for Sewage Works Design. Appendix C contains summaries of the Pro2D modeling results used in this evaluation Pro2D Calibration The Pro2D model was calibrated using Wenatchee WWTP data. Data for July and August 2006 was used to calibrate the model for the summer when the wastewater is warmer. Calibration for the winter when the wastewater is colder was done for January The plant s biological process was modeled as plug-flow reactor of four stages with an initial 12 percent anoxic zone followed by three aerobic zones. Primary effluent and RAS were assumed to discharge to the anoxic zone along with recirculated mixed liquor from the last aerobic zone. Information used to calibrate the model included monthly averages for flow, BOD, TSS, sludge retention time (SRT), ammonia, volatile suspended solids (VSS), ph, temperature, sludge volume index (SVI), aeration basin DO, and TSS removal. Table 4-3 is a summary of the calibration of the Pro2D model. The table shows the parameters that were used to obtain the calibration and compares the results of the Pro2D model with the WWTP data. It was difficult to obtain an exact calibration, and, in the end, the model was primarily calibrated to minimize the difference between the modeled and observed mixed liquor suspended solids (MLSS) and waste activated sludge (WAS) because these parameters are the most important in predicting the capacity of the activated sludge process. As Table 4-3 shows, the calibration of MLSS and WAS was within 11 percent of the observed values. The Pro2D model predicted higher MLSS concentrations and more WAS than were observed in two of the three months modeled, even with the forced calibration. This means that the Pro2D model will tend to underestimate the capacity of the Wenatchee WWTP activated sludge process. The calibration exercise indicates that the WWTP staff may want to perform some checking of their flowmeters and sampling procedures. The calibration parameters used greatly distort other results, such as mixed liquor volatile suspended solids (MLVSS) and oxygen requirements. The MLVSS predicted by the Pro2D model are much less than the observed values, and the oxygen requirements are much too high. As a result, the oxygen 4-10

33 requirements predicted by Pro2D for this evaluation should not be used. A check of the results was made by measuring the chemical oxygen demand (COD) of the influent wastewater; this check indicated that the parameters used to obtain the best predictions of MLSS and WAS do not predict the COD/BOD ratio of the influent wastewater. This suggests that the WWTP data may have some inaccurate flow measurements or nonrepresentative sampling that is affecting their accuracy. It is recommended that the City investigate and confirm the accuracy of flow measurement and sampling. Particular attention should be paid to the WAS, MLSS, influent and primary effluent BOD, and TSS samples. The Pro2D model results discussed in this Facilities Plan are based on the calibrated model that uses the Wenatchee data. If it is found that there are some inaccuracies in the data and the standard municipal wastewater parameters are found applicable, the impacts are very significant. Use of the standard municipal wastewater parameters will reduce the predicted WAS and MLSS by 15 to 20 percent. This means that the model may be predicting 20 percent less treatment capacity than is actually available. If the influent strength from current sampling is found to be higher than the actual, then the predicted quantity of primary sludge is higher than is actually occurring, and the digester expansion is not required as soon as predicted in this Facilities Plan. TABLE 4-3 Calibration of Pro2D Model for Wenatchee WWTP Calibration Parameter Winter Summer Flow Average Average BOD U/BOD 5 ratio Nonbiodegradable VSS, % of total VSS 10% 17% Yh, aerobic Yh, anoxic fd, all zones COD of VSS, raw influent COD of VSS, aerobic biological b Calibration Parameter Pro2D Model Summer (July 2006) Summer (August 2006) Observed % Difference Pro2D Model Observed % Difference Primary effluent BOD 5 (mg/l) PE TSS, mg/l MLSS concentration (mg/l) 2,056 1, ,903 1, MLVSS concentration (mg/l) 1,398 1, ,281 1, % VSS of MLSS WAS (lb/d) 3,253 3, ,061 3,

34 TABLE 4-3 Calibration of Pro2D Model for Wenatchee WWTP Calibration Parameter Winter Summer Effluent CBOD* (mg/l) Effluent ammonia (mg/l) Effluent alkalinity (mg/l as CaCO 3) Calibration Parameter Pro2D Model Winter (January 2006) Observed % Difference Primary effluent BOD 5 (mg/l) PE TSS, mg/l MLSS concentration (mg/l) 2,951 2, MLVSS concentration (mg/l) 2,163 2, % VSS of MLSS WAS (lb/d) 3,930 3, Effluent CBOD* (mg/l) Effluent ammonia (mg/l) Effluent alkalinity (mg/l as CaCO3) Primary effluent TSS (mg/l) *Pro2D value is BOD 5 concentration; observed value is CBOD concentration. Glossary for Table 4-3: BODu ultimate BOD, typically the BOD after 20 days of incubation in the WWTP application. BOD 5 5-day BOD, biochemical oxygen demand after 5 days of incubation VSS volatile suspended solids, suspended solids that are combusted in a furnace Nonbiodegradable VSS volatile suspended solids that cannot be degraded by biological activity Yh ratio of biological solids produced per unit of BOD by heterotrophic bacteria, which varies for aerobic and anoxic conditions fd rate of endogenous activity used in the ASM2d model COD chemical oxygen demand, measure of the amount of oxygen used to decompose sewage using strong chemicals as oxidizers TSS total suspended solids, measurement of solids removed by filtering PE primary effluent MLSS mixed liquor suspended solids is the concentration of suspended solids in the aeration basins WAS waste activated sludge is the suspended solids removed from the activated sludge process CBOD carbonaceous BOD is the BOD 5 with an inhibitor added to prevent nitrification from occurring during incubation Alkalinity concentration of hydroxide, carbonate and bicarbonate ions and is a measure of the resistance of wastewater to change ph CaCO 3 calcium carbonate is a standard for expressing concentration of alkalinity 4-12

35 Another observed result of the calibration is the disparity between the predicted and observed effluent ammonia concentrations. Normally, this prediction is quite close, but the calibration prediction is 80 to 90 percent lower than the observed effluent ammonia concentration. This suggests that there may be something in the Wenatchee wastewater that is inhibitory to nitrification. Work in Wenatchee a few years ago showed that nitrification can at times occur as predicted, so this calibration result indicates that something is currently interfering with nitrification. The City has observed that the activated sludge process produces lower ammonia when operating in the step-feed mode, which is different than the modeling would predict in the absence of inhibition. It is possible that a toxic material is sufficiently diluted in the step-feed mode of operating the activated sludge process that the effect is reduced compared to the plug flow anoxic mode of operation. The City is continuing to investigate the potential causes and has evidence that the dewatering sidestream may be the cause. The City is considering modification of the schedule of biosolids dewatering to reduce the impacts. It should be noted that while the apparent inhibition is significant, the effect is nowhere near enough to cause ammonia violations of the NPDES permit Activated Sludge Process Capacity The calibrated Pro2D model was used to estimate the capacity of the activated sludge process. The Wenatchee WWTP has been nominally rated for 5 mgd average annual flow since publication of the October 1988 Wenatchee Wastewater Treatment Plant Improvements Engineering Report (CH2M HILL, 1988). In 1992 and 2004, improvements were made to maintain the capacity of the WWTP at this nominal 5-mgd average annual flow. Average annual flow, although simple and easy to understand, is not an accurate description of WWTP capacity. City staff has been effective in removing extraneous flows such as irrigation return flow and stormwater runoff from the sanitary sewer system. As a result, flows have not increased at the same rate as the population served and the BOD and TSS loading. Table 4-4 shows the changes in service population, average annual wastewater flow, and average annual influent BOD and TSS for the Wenatchee WWTP between 1980 and TABLE 4-4 Comparison of Population Served, Wastewater Flow, BOD, and TSS for WWTP in 1980 and 2000 Year Service Population Average Wastewater Flow (mgd) Average Influent BOD (lb/day) Average Influent TSS (lb/day) , ,750 5, , ,660 7,220 Table 4-4 shows that average wastewater flow increased 10 percent, but service population, influent BOD, and influent TSS increased roughly 40 percent four times higher. This means that the wastewater treatment processes sized for the BOD and TSS in wastewater must be larger to treat the wastes contained in an average of 5 mgd of wastewater flow. The activated sludge process and anaerobic digestion processes are both sized to treat BOD and 4-13