CHAMBERS CREEK REGIONAL WASTEWATER TREATMENT PLANT FACILITIES PLAN 4. POPULATION, FLOW, AND LOADING PROJECTIONS

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1 CHAMBERS CREEK REGIONAL WASTEWATER TREATMENT PLANT FACILITIES PLAN 4. POPULATION, FLOW, AND LOADING PROJECTIONS The Chambers Creek Regional Wastewater Treatment Plant Facilities Plan (Facilities Plan) provides a roadmap to prepare for future needs of the Chambers Creek Regional Wastewater Treatment Plant (Plant). Consequently, it is necessary to understand the current and future demands on the Plant to best anticipate what improvements will be necessary and when they will be needed. For a wastewater treatment plant, the demands are largely a function of the number and type of customers, condition of the system, and an understanding of the regulatory framework. This chapter specifically addresses the number and type of connections through population and service projection, and indirectly addresses the system condition by estimating the inflow and infiltration. The Facilities Plan is developed using population and employment projections for the service area coupled with historical flow records to generate a flow and loading forecast for the Plant. These population, flow, and loading projections correlate with the Pierce County (County) Unified Sewer Plan, (projected to be updated in 2010) and was generated based upon flow data collected through Data Sources Projections of the future service population are based upon the Pierce County Environmental Services Planning Division s service area map, which was updated in January 2007 for use in the new Unified Sewer Plan. These projections were combined with population and employment projections published by the Puget Sound Regional Council (PSRC) in Estimates of the current service population and sewered area were drawn from the County sewer model, and a GIS-based shapefile of sewered parcels. The Pierce County Sewer Utility (Sewer Utility) has estimated base sewer flow for each parcel within the service area. Historical Plant flow data have been provided by the County. 4.2 Existing Service Area Population and Employment The County sewer model identifies 48,840 sewered parcels within the Plant service area (Figure 4-1). This does not include the residential parcels belonging to the Steilacoom Sewer Utility, which feed the Plant via a dedicated pump station through a wholesale agreement with the County. These connections are managed by the Steilacoom Sewer Utility. This also does not include the area of southwest Tacoma referred to as the Western Slopes Service Area in the Unified Sewer Plan. The Western Slopes service area discharges to the Plant via a pair of dedicated pump stations under a contracted flow agreement. The PSRC publishes population and employment projections for 10-year intervals between 2000 and 2040 based on the economic and population models developed and maintained by the Washington State Office of Financial Management (OFM). OFM has been charged with developing accurate and fair population estimates since the 1940 s for the allocation of state revenues and program administration. The population and employment figures represent the resident population/employment of an area as defined by the federal Census Bureau (forecast analysis zones or FAZ). The figures include all persons usually residing in an area including military personnel and dependents, persons living in correctional institutions, and persons living in nursing homes or other care facilities. 4-1 November 2010

2 CHAPTER 4 Chambers Creek Regional Wastewater Treatment Plant Facilities Plan Figure 4-1. Sewered parcels, Future Service Area Population and Employment Future population and employment projections were based upon the PSRC data, which are forecast through A gradual expansion of the current sewered area to the extents of the future service area was assumed, with the rate of expansion dependent upon location relative to existing infrastructure and current sewer utility plans to extend service (Figure 4-2). By 2040, the entire service area is assumed to be sewered. The 2006 PRSC publication details this information by FAZ. Combining the map of sewered parcels with the PSRC data, the existing service population of the Plant is estimated at 155,500 residents plus 46,000 employees as indicated in Table 4-1. November

3 Chambers Creek Regional Wastewater Treatment Plant Facilities Plan CHAPTER 4 Figure 4-2. Expansion of sewering, Table 4-1. Service Population and Employment Projection, Year Population Total employment Manufacturing WTCU Retail Services Gov./Ed ,531 46,014 2,947 3,644 9,779 17,321 12, ,622 50,605 3,336 3,953 10,843 19,191 13, ,812 59,068 4,042 4,639 12,687 23,290 14, ,003 67,531 4,749 5,325 14,531 27,388 15, ,992 82,781 5,883 6,576 17,899 35,281 17, , ,695 7,333 8,414 21,949 45,710 19,289 Category codes: Manufacturing = SIC WTCU = Wholesale trade, transportation services, communication, and utilities, SIC 40 42, Retail Trade = SIC Services = SIC 07, 60 67, 70, 72 74, 75 76, 78 81, 83 84, 86, 89. Government/Education = SIC 43, 82, November 2010

4 CHAPTER 4 Chambers Creek Regional Wastewater Treatment Plant Facilities Plan 4.4 Current Base Sanitary Flow Plant flow is a combination of a base sanitary flow (BSF) plus infiltration and inflow (I/I). BSF represents the direct wastewater contribution by the customers (also referred to the billable flow from which revenues can be generated) and I/I represents indirect contribution from customers and the system in response to changing hydrologic conditions (also referred to as unbillable flow since the costs to manage this material must be recovered through other means). BSF to the Plant has varied from approximately 13.2 million gallons per day (mgd) in 2000 to 15.7 mgd in While BSF by definition does not include any I/I, measurement of a pure BSF is rarely achieved. Even the driest weeks of the year will be influenced by long-term factors such as the groundwater level, and other less seasonal sources of I/I such as artesian springs and natural lakes, ponds, and rivers. Figure 4-3 depicts the 7-day average flow recorded at the Plant. The BSF generally increases approximately 3.3 percent a year and follows a line drawn between the relatively dry years of 2000, 2001, and 2005, resulting in a 2006 BSF of 15.7 mgd. The Sewer Utility has estimated the relative BSF for each of the sewered parcels highlighted on Figure 4-1. These estimates are based upon the County s standard of 8.3 hundred cubic feet (ft 3 ) of flow per month (204.5 gallons per day (gpd)/household unit), with nonresidential units estimated on a case-by-case basis. The sewer model is designed to be somewhat conservative, and outputs a total BSF of mgd. To calibrate to the actual measured BSF at the Plant (15.70 mgd), the flow estimates for each parcel were scaled by a factor of From the sewer model, a total of six commercial sources generating flows in excess of 30,000 gpd were identified. These are listed in Table 4-2. Table 4-2. Major Commercial and Industrial Sources Point source Flow (gpd) Western State Hospital 330,700 James Hardie Building Products, Inc. 132,783 Pacific Lutheran University 120,904 Boeing Skin and Spar 59,762 Clover Park Technical College 32,875 St. Claire Hospital 32,530 Table 4-3 summarizes the breakdown of 2006 residential and employment-related wastewater flows. Table 4-3. Wastewater Flow Generation Profiles Base sanitary flow (gpd) Population gpd/capita Residential 13,305, , Employment 2,394,292 46, Total 15,700,000 Figure day average flow recorded at the Plant Parshall flume, The employment-related flow was broken down into five major categories, matching the five commercial categories which the PSRC uses for its employment forecasts. These flows, plus the corresponding population and generation rates, are outlined in Table 4-4. BSF projections for the Plant are shown in Table 4-5 through November

5 Chambers Creek Regional Wastewater Treatment Plant Facilities Plan CHAPTER 4 Table 4-4. Nonresidential Wastewater Flow Generation Profiles Type a Base sanitary flow (gpd) b Population c gpd/capita Manufacturing 228,541 2, WTCU 98,608 3, Retail trade 848,936 9, Services 773,649 17, Government/education 444,557 12, Total 2,394,292 46, a. Category codes: Manufacturing SIC WTCU Wholesale trade, transportation services, communication, and utilities, SIC 40 42, Retail Trade SIC Services SIC 07, 60 67, 70, 72 74, 75 76, 78 81, 83 84, 86, 89. Government/Education SIC 43, 82, b. Flow is based upon Pierce County S ewer Model, adjusted to BS F of 15.7 mgd. c. Population is from PSRC projections, published October Year Table 4-5. Plant BSF Projections, Population Total employment Base sanitary flow (mgd) ,531 46, ,622 50, ,812 59, ,003 67, ,992 82, , , Infiltration and Inflow I/I can represent a substantial portion of the Plant influent total wastewater flowrate. Estimating I/I is necessary to establish the peak hydraulic conditions for the Plant. Hourly influent flow records and rainfall data were used to construct a precipitation-based model of Plant I/I (Figure 4-4). Hourly Plant flow records were not available for the period August 2004 through December Once calibrated, the model was applied to a 50-year record of hourly precipitation to output the following risk-based I/I projections (Table 4-6). The I/I quantities listed in Table 4-6 represent the sewer system as it currently exists. As new pipes are added to the system, new sources of I/I will develop. In general, new pipes are expected to admit lower amounts of I/I than old pipes. This is due to modern building codes which prohibit connection of roof drains, and installation of leakresistant piping (PVC, HDPE). In this case, the amount of I/I per linear foot of new pipe added to the system is assumed to be one-half the amount of I/I per linear foot of existing pipe (Table 4-7). This reduction is supported by data from the nearby LOTT Clean Water Alliance collection system. Note that the average amount of pipe added per new customer is assumed to be inchdiameter-mile (IDM). This value is based upon the average IDM per person in the Plant service area calculated for the 2005 Re-Rating and Capacity Study, and based upon the pipe and customer inventories presented in the 2001 Unified Sewer Plan. Over the long term, the average I/I in the existing system is expected to improve as proportion of new pipes increases and as the County conducts preservation to repair/replace portions of the existing system. Given the I/I estimates outlined above, the Plant influent wastewater flow rate projections for different design conditions are summarized in Table November 2010

6 CHAPTER 4 Chambers Creek Regional Wastewater Treatment Plant Facilities Plan Figure 4-4. Precipitation-based I/I model: actual flow in blue, model flow in red, rainfall in green Table 4-6. Risk-Based I/I Model (all flows in mgd) Period Flow I/I Base sanitary flow Average day Average summer a Average winter b year peak summer c year peak month c year peak day c year peak hour c a. Summer is defined as June, July, and August. b. Winter is defined as December, January, and February. c. 10-year return period between flows means that in any given year, there is a 10 percent chance of a flow greater than this amount (10 percent risk). Table 4-7. I/I per IDM of New Pipe added to the System (gpd/idm) a Period gpd/idm Average day 237 Average summer b 63 Average winter c year peak summer d year peak month d year peak day d 1, year peak hour d 1,878 a. New pipe I/I per IDM is estimated by taking one-half the measured I/I per IDM of existing pipe. b. Summer is defined as June, July, and August. c. Winter is defined as December, January, and February. d. 10-year return period between flows means that in any given year, there is a 10 percent chance of a flow greater than this amount (10 percent risk). Table 4-8. Plant Total Flow Projections for Design Conditions (mgd), year peak summer c 10-year peak month c Year Base sanitary flow Average day Average summer a Average winter b 10-year peak day c 10-year peak hour c a. Summer is defined as June, July, and August. b. Winter is defined as December, January, and February. c. 10-year return period between flows means that in any given year, there is a 10 percent chance of a flow greater than this amount (10 percent risk). November

7 Chambers Creek Regional Wastewater Treatment Plant Facilities Plan CHAPTER Historical Plant Loadings The Plant capacity is a function of the influent wastewater hydraulic flowrate and mass loading of organic and inorganic material. The next sections discuss the historical and projected Plant influent loadings. The daily recorded Plant influent loadings of biochemical oxygen demand (BOD) and total suspended solids (TSS) are plotted on Figure 4-5. While both parameters demonstrate a steady increase over time, the TSS shows considerably more variation, including spikes of over 160,000 pounds per day (lb/d). Figure 4-6 shows 30-day average loadings. From this data, there does not appear to be a repeatable seasonal pattern to the Plant loadings (i.e., peak month loadings just as likely to occur during the summer and the winter.) Table 4-9 lists the recorded annual loads alongside the peak 30-day average loadings for each year since Table 4-10 summarizes the current Plant influent loadings and recommended peaking factors based upon the historical records. Figure 4-5. Daily influent loadings of BOD and TSS, Figure day average influent loadings, Table 4-9. Historical Plant Loadings and 30-day Maximum Peaking Factors Annual average 30-day maximum Peaking factor Year TSS BOD TSS BOD TSS BOD ,625 31,484 45,500 52, ,547 31,800 44,000 37, ,665 37,599 56,400 49, ,654 37,469 65,000 57, ,393 32,081 42,000 38, ,063 35,510 51,000 42, average 37,370 35,020 52,667 46, November 2010

8 CHAPTER 4 Chambers Creek Regional Wastewater Treatment Plant Facilities Plan Table Current Plant Loadings and Peaking Factors Average lb/d Peak month lb/d Peak month factor Peak day lb/d TSS 37,500 52, ,000 BOD 35,000 45, , Plant Loading Projections Future Plant loadings are a function of the service area population and employment similar to the BDF. Using the current loadings and peaking factors from Table 4-10, future Plant loadings are projected to increase at rates proportional to changes of the BSF. These projections are summarized in Table Table Load Projections (lb/d), Average day Peak month Year TSS BOD TSS BOD ,500 35,000 52,500 45, ,800 40,000 59,900 51, ,100 46,700 70,100 60, ,400 53,500 80,300 69, ,200 65,500 98,300 85, ,800 81, , , Build-Out Flow and Loading Projections The projections developed in this analysis span the period from By 2040, the model assumes sewers will be extended to the entire anticipated service area. However, it is unlikely every resident will be connected to the system. To estimate the maximum potential contribution from the service area, an analysis of the developable area is used. An estimate of the available buildable lands was created by subtracting the areas occupied by water bodies, wetlands, and steep slopes from the future service area. A composite zoning map was then created using the County s GIS zoning coverage of unincorporated areas, as well as paper zoning maps obtained from the incorporated municipalities within the Plant s service area (DuPont, Lakewood, Steilacoom, and University Place). The composite zoning map was clipped using the buildable lands map. Clipping is a geoprocessing tool which creates a new GIS coverage by cutting the information in an input layer using the boundaries of a clip layer. Clipping the zoning coverage using the buildable lands map produced a zoning coverage that included only buildable lands within the Plant service area. The overall procedure is illustrated on Figures 4-7 through Figures 4-7 and 4-8 show the procedure for generating the buildable lands GIS coverage using the service area, water bodies, wetlands, and steep slopes maps. Figures 4-9 and 4-10 show the clipped zoning map coverage. An estimate of the build-out population was generated by populating the buildable zoning map with information from the 2002 Pierce County Buildable Lands Report and housing density information drawn from the municipal codes of DuPont, Lakewood, Pierce County, Steilacoom, and University Place. The total area for each zoning classification was multiplied by the dwelling unit densities (in dwelling units/acre) drawn from the appropriate municipal code or the Pierce County Buildable Lands Report, giving a total number of dwelling units for each zoning classification. This was then multiplied by the average number of residents per dwelling unit, taken from the Pierce County Buildable Lands Report, giving a total number of residents for each zoning classification. The dwelling unit densities and residents per dwelling unit for each residential zoning classification are shown in Table November

9 Chambers Creek Regional Wastewater Treatment Plant Facilities Plan CHAPTER 4 Figure 4-7. Unbuildable lands in the Plant service area Figure 4-8. Buildable lands in the Plant service area 4-9 November 2010

10 CHAPTER 4 Chambers Creek Regional Wastewater Treatment Plant Facilities Plan Figure 4-9. Zoning, western Pierce County Figure Zoning, eastern Pierce County November

11 Chambers Creek Regional Wastewater Treatment Plant Facilities Plan CHAPTER 4 Table Build-Out Population Forecast Parameters Residential Zoning Jurisdiction Zone Dwelling unit density (du/acre) Residents/dwelling unit a DuPont R R R R Residential Reserve b Lakewood Air Corridor Arterial Residential Commercial c Multi Family 1 c Multi Family 2 c Multi Family 3 c Mixed Residential 1 c Mixed Residential 2 c R1 c R2 c R3 c R4 c Steilacoom Multi Family d R-7.2 d R-9.6 d University Place Multifamily High e Multifamily Low e R1 e R2 e Pierce County High Density Single Family f Moderate Density Single Family f Moderate- High Density Residential f Single Family f a. Residents per dwelling unit from Pierce County Buildable Lands Report, b. Dwelling unit density from DuPont Municipal Code. c. Dwelling unit density from Lakewood Municipal Code. d. Dwelling unit density from Steilacoom Municipal Code. e. Dwelling unit density from University Place Municipal Code. f. Dwelling unit density from Pierce County Buildable Lands R eport, For mixed-use zoning classifications, a residential fraction, representing the fraction of mixed-use acreage devoted to housing, was multiplied by the total buildable land for each zoning classification. The remainder of the analysis was completed in the same manner as for the residential zoning. The parameters used for mixed-use zoning are shown in Table An estimate of employment at build-out was generated using employment density information from the Pierce County Buildable Lands Report. The report estimates the number of employees per acre for commercial/retail, industrial, and governmental zoning classifications. These values were multiplied by the total area for each zoning classification derived from the buildable zoning map. For mixed-use zonings, a parameter defining the fraction of land devoted to residential use was included. The parameters for the build-out employment forecast are shown in Table November 2010

12 CHAPTER 4 Chambers Creek Regional Wastewater Treatment Plant Facilities Plan Jurisdiction Table Build-Out Population Forecast Parameters Mixed-Use Zoning Zone Fraction residential a Dwelling unit density (du/acre) Residents/ Dwelling unit a DuPont Mixed use b Lakewood Central business district c Neighborhood commercial 1 c Neighborhood commercial 2 c Transit oriented commercial c University Place Mixed use d Mixed use office d Pierce County Activity center e Commercial mixed use district e High density residential district e Major urban center e Mixed use district e Office: residential mixed use district e Urban village e a. Fraction residential and residents/dwelling unit from Pierce County Buildable Lands R eport, b. Dwelling unit density from DuPont Municipal Code. c. Dwelling unit density from Lakewood Municipal Code. d. Dwelling unit density from University Place Municipal Code. e. Dwelling unit density from Pierce County Buildable Lands R eport, Jurisdiction Table Build-Out Employment Forecast Parameters Zone Fraction residential Fraction nonresidential Employees/acre DuPont DuPont commercial DuPont industrial DuPont manufacturing research DuPont mixed use DuPont office Lakewood LW Air corridor LW C LW C LW Central business district LW industrial LW industrial LW industrial business park LW neighborhood commercial LW neighborhood commercial LW transit oriented commercial November

13 Chambers Creek Regional Wastewater Treatment Plant Facilities Plan CHAPTER 4 Jurisdiction Table Build-Out Employment Forecast Parameters Zone Fraction residential Fraction nonresidential Employees/acre Steilacoom Steilacoom C general Steilacoom C shoreline Steilacoom industrial University Place UP commercial UP light industrial business park UP mixed use UP mixed use office UP neighborhood commercial UP town center Pierce County Activity center Commercial mixed use district Community center Community employment Employment center Employment services High density residential district Major urban center Mixed use district Neighborhood center Office: residential mixed use district Research office Residential/office: civic Urban village a. All fraction residential and employees/acre from Pierce County Buildable Lands Report, Given the areas and projected densities outlined above, and assuming a value of 80 percent zone density at build-out, the forecasted population and employment at build-out is shown in Table Table Build-Out Population and Employment Forecast Build-out Area Residents Employees DuPont 12,741 10,222 Lakewood 107,710 31,050 Steilacoom 9,432 3,434 University Place 40,660 7,719 Pierce County 168, ,776 Total 339, ,202 Employment projections for build-out were based on an overall employment-based wastewater generation rate. Currently, the average unit employee wastewater generation rate is 52.0 gpd/employee for the service area (Table 4-3). Projecting to 2040, and combining the data from Table 4-3 and Table 4-4, this figure climbs to 53.4 gpd/employee. Using this value alongside the base residential wastewater generation rate (85.6 gpd/capita) a built-out BSF of mgd is projected. The complete set of build-out flow and load projections are outlined in Table The build-out population projection is somewhat lower than the 2040 projection reported in Table 4-8, while the build-out employment projection is considerably higher November 2010

14 CHAPTER 4 Chambers Creek Regional Wastewater Treatment Plant Facilities Plan Table Build-Out Flow and Loadings Projections Point source Flow (gpd) Base sanitary flow, mgd Average day, mgd Average summer, mgd Average winter, mgd year peak summer, mgd year peak month, mgd year peak day, mgd year peak hour, mgd Average day TSS, lb/d 91,000 Average day BOD, lb/d 85,000 Peak month TSS, lb/d 127,500 Peak month BOD, lb/d 110,500 An alternate approach is to use the developable portion of the service area (approximately 59,440 acres) and apply the average build-out residential density for the Pacific northwest of eight residents per acre. This translates to a build-out population of over 475,000 and a BSF of mgd. By comparison, using the PSRC population and employment projections, the model predicts a BSF of mgd by Taken together, the build-out BSF projection is within 5 percent of the 2040 projection, and within 7 percent of the eight-resident-per-acre build-out value. The convergence of three independent build-out flow values between 36 and 40 mgd is encouraging. For the purposes of this Facilities Plan, the flow and loading projections summarized in Tables 4-8 and 4-11 are used to predict conditions from 2009 through The build-out flow projections in Table 4-16 are used to predict conditions at build-out. November