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HomeMy WebLinkAbout2040 Chapter 08 - Wastewater City of Prior Lake Wastewater 2040 Comprehensive Plan Page 612 CHAPTER 8, WASTEWATER 8.1 Introduction The Metropolitan Land Planning Act (amended 1995) requires local governments to prepare comprehensive plans and submit them to the Metropolitan Council to determine their consistency with metropolitan system plans. The local comprehensive plan is to include a wastewater plan element covering the collection and disposal of wastewater generated by the community. Similarly, the Metropolitan Sewer Act requires local governments to submit a Comprehensive Sewer Plan (CSP) which describes the current and future service needs required from Metropolitan Council Environmental Services (MCES). The Metropolitan Council’s 2040 Water Resources Policy Plan (2040 WRPP) lays out requirements for the wastewater plan element of the comprehensive plan as well as the Comprehensive Sewer Plan. For simplicity, the Metropolitan Council has combined the required elements of both plans into a single set of criteria, contained in the 2040 WRPP. This 2018 Prior Lake CSP addresses the criteria of the 2040 WRPP. It provides growth projections, design parameters for the existing sanitary sewer system, proposed improvements to or expansion of the system, and strategies for addressing excessive infiltration and inflow in the system. The CSP updates previous sewer planning efforts and describes in detail the expansion of the City’s sanitary sewer system to serve projected urban development. The Prior Lake CSP focuses on the municipal conveyance facilities required to collect and transport wastewater to the MCES interceptor system and does not specifically address the elements that constitute the MCES system. The local elements of conveyance are the sewer services, laterals, trunks, manholes, lift stations and generators, force mains, and any other facilities required to collect and transport wastewater. As land develops, the location and alignment of sewer laterals and service lines, as opposed to trunk lines and interceptors, are dictated by platting. Prior to platting, those smaller facilities cannot be accurately sized and located, and thus are excluded from the CSP. Trunk sewers, however, are largely dependent on topography, soil conditions, physical features, and manmade barriers and can be adequately addressed in the context of a CSP. The CSP lays out a plan for the trunk system, which includes, at a minimum, all sanitary sewer pipes 10 inches in diameter and larger and other facilities (such as lift stations and force mains) that are a vital part of the trunk sewer system. Since the trunk sewer design determines the ultimate service area for the system, it is essential that an overall trunk plan be available as a guide for future development. Such a plan should be flexible enough to absorb some changes in planning and development patterns. Since planning goals may change, periodic review and update of facilities and costs is required. 8.2 Background The City of Prior Lake is located in the southwestern portion of the Twin Cities Metropolitan Area, within Scott County, as shown in Figure 8.1. Major transportation routes in and out City of Prior Lake Wastewater 2040 Comprehensive Plan Page 613 of the City include State Trunk Highway 13, County 42, County 82, County 21, County 83, and County 17. Prior Lake has experienced steady growth over the past 30 years. Development within Prior Lake has, over the years, moved outward from the lake. Another factor in determining development patterns, other than proximity to the lake, has been access to the major transportation corridors. For areas currently within the City limits, future development is expected to occur along County Roads 21 and 42. Treatment and disposal of wastewater generated by the City of Prior Lake is presently accomplished by the MCES at the Blue Lake Wastewater Treatment Plant. The MCES Prior Lake Interceptor services the entire City of Prior Lake. It starts at the intersection of Eagle Creek Avenue NE (County Road 21) and 154th Street NW (County Road 82) and runs north, generally paralleling County Road 21. There are several trunk branches that connect to the main interceptor and collect the east and west reaches of the City. The Prior Lake Interceptor conveys Prior Lake’s wastewater flow to the Blue Lake Wastewater Treatment Plant. Figure 8.1: Location Map City of Prior Lake Wastewater 2040 Comprehensive Plan Page 614 8.3 Land Use and Population A. General The sizing of sanitary sewer facilities is dependent on the hydraulic capacity required for each part of the system. Municipal wastewater is generally a mixture of domestic sewage, commercial and industrial wastes, groundwater infiltration, and surface water inflow. With proper design, construction, and maintenance, groundwater infiltration and surface water inflow are reduced to a minor percentage of the total flow, leading to hydraulic discharges that depend predominantly on land use. The City of Prior Lake has experienced a steady growth rate over the past few years to reach a 2010 census population of 22,206, with an anticipated 2040 population of 37,600. Since properly designed and constructed sanitary sewer pipes have long life expectancies, it is reasonable to assume that the full-development population will be reached before facility replacement becomes necessary. None of the facilities shown on Figure 8.5 - Map 1 or described in this report’s appendices are interim facilities. The facilities shown on the map and described in this report all support full-development of the community. B. Land Use The 2040 Land Use Plan (Figure 8.2) served as the basis for the development of the sanitary sewer flow projections and guided development of the trunk system. In order to estimate the volume of wastewater flow anticipated for each subdistrict, the Land Use Plan was used to divide the City into land use types. Detailed descriptions of the various land uses are found in the City of Prior Lake Comprehensive Plan. Areas of each land use by subdistrict are presented in Appendix A. C. Population, Household and Employment Tables 8-1 and 8-2 present the Metropolitan Council’s projections of total and sewered population, households, and employees for the City of Prior Lake from the 2040 WRPP. The numbers represent the most up-to-date figures from the Metropolitan Council regarding population, households, and employment within the City’s current corporate boundary, which is included within the study area for this CSP. Table 8-1: Metropolitan Council City-Wide Projections Year Population Households Employment 2010 22,206 8,210 3,766 2020 27,500 10,500 5,000 2030 32,500 12,600 7,000 2040 37,600 14,700 8,100 City of Prior Lake Wastewater 2040 Comprehensive Plan Page 615 Table 8-2: Metropolitan Council Sewered Projections Year Population Households Employment 2010 21,586 7,998 3,766 2020 26,930 10,300 5,000 2030 31,950 12,400 7,000 2040 37,050 14,500 8,100 The facilities described in this report are designed to serve the entire study area under conditions of ultimate development. The projections in Tables 8-1 and 8-2 reflect estimates of the timing of growth in the City. Actual growth rates will affect only the timing of trunk sewer construction and not the actual design of the system. City of Prior Lake Wastewater 2040 Comprehensive Plan Page 616 Figure 8.2: 2040 Future Land Use Map City of Prior Lake Wastewater 2040 Comprehensive Plan Page 617 8.4 Design Criteria A. Average Wastewater Flows Average wastewater flows for ultimate conditions are presented in Appendix B. The ultimate wastewater flows were calculated by applying unit flow rates to each of the land use areas in Appendix A. The unit wastewater flow rates are presented in Table 8-3. Lot counts and per-unit flow generation rates were used in determining expected wastewater flow from existing residential developments, while per-acre flow generation rates were used for proposed residential and existing and proposed commercial and industrial land uses. The unit flow rates presented in Table 8-3 are in accordance with standard engineering practice and are generally considered conservative. The Urban Low-Density Residential land use designation includes future residential development both within and outside the existing City limits. As shown in Table 8-3, the City anticipates that this land will develop at an average net density of 1 to 3.6 dwelling units per net developable acre. Since the locations where future Urban Low-Density Residential land use might approach 3.6 units per net acre have not been established, it was necessary to provide excess sanitary sewer capacity in the entire proposed system. To do this, the proposed trunk sanitary sewer system is designed based on 3.6 units per net acre for future Urban Low- Density Residential Land Use. Table 8-3: Unit Wastewater Flows Land Use Type Persons/ Unit Gal/Cap /Day (GCD) Gal/Unit /Day (GUD) Units/ Acre Gal/Acre/D ay (GAD) Existing: Mixed Residential 2.56 100 256 -- N/A* Future: Rural Density Residential 2.56 100 256 1.0 256 Future: Urban Low Density Residential 2.56 100 256 3.6 922 Future: Urban Medium Density Residential 2.56 90 230.4 6.0 1,382 Future: Urban High Density Residential 2.56 80 204.8 9.0 2,048 Existing and Future: Commercial/Industrial -- -- -- -- 1,200 * Existing residential flows are based on lot counts and thus are not consistently convertible to a per-acre rate. B. Design Peak Wastewater Flows The trunk sanitary sewer system must be capable of handling not only the average flows, but also anticipated peak flows, including diurnal variations and infiltration/inflow. These peak flows are obtained by multiplying average flows by a factor. This factor, called the Peak Flow Factor, was generated by determining the total average wastewater flows for the City and using Figure 8.3, Peak Flow Factors. After the Peak Flow Factor was determined for the City, a diurnal pattern was established in InfoSWMM to represent the City of Prior Lake Wastewater 2040 Comprehensive Plan Page 618 City’s Peak Flow Factor. Although a diurnal pattern was established for the entire City, the City model is dynamic in its routing, so the timing of peak flows may vary throughout the City. The design peak flows for each segment of pipe are presented in Appendix C. This information includes the reference points of the segment involved, the average flow from Appendix B, the design Peak Flow Factor, and the resulting design flow. Figure 8.3: Peak Flow Factors City of Prior Lake Wastewater 2040 Comprehensive Plan Page 619 8.5 Full-Development System Description A. General The full-development trunk sanitary sewer system layout for the City of Prior Lake is presented on Figure 8.5 - Map 1 at the back of this report. This map shows major district and subdistrict boundaries, existing and proposed trunk sanitary sewers, lift stations, and force mains. In addition, sizes of all trunk sewers are shown with reference points along each pipe. Appendix C contains design flows and pipe capacities for all trunk sewers (shown on Figure 8.5 - Map 1). Appendix D has trunk sewer sizes, lengths, and slopes, as well as capacities. Appendix I contains lift station information. The proposed trunk sanitary sewer alignments are preliminary and should be reviewed at the time of final design to ensure conformance with existing and proposed development. In most cases, the alignments closely follow natural drainageways. Major changes in alignment are not recommended because these could lead to excessive pipe depths and thus increased construction costs. It should be emphasized that private wastewater treatment facilities are prohibited in the City of Prior Lake. B. Wastewater Flow Projections Table 8-4 presents projected wastewater flows for the City of Prior Lake, as provided by the Metropolitan Council. It is assumed that Metropolitan Council projections pertain to Prior Lake only and do not include areas outside the City. Table 8-4: Wastewater Flow Projections Year Prior Lake Projected Average Flow (MGD)1 2010 1.62 2020 1.91 2030 2.42 2040 2.78 1From the Metropolitan Council’s 2040 WRPP Appendix B provides average flows by sewer subdistrict and totals them for all the districts within the Prior Lake CSP service area. The Appendix B total average flow is 5.64 MGD for full development of Prior Lake. This number is larger than the Metropolitan Council projection in Table 8-4. Reasons for the differences include: (1) Appendix B is for full- development conditions, which could extend beyond the 2040-time frame in Table 8-4; (2) Appendix B includes proposed future annexation areas; and (3) flows in Appendix B are based on conservative unit flow rates for system design purposes. As was noted in Section 4, since the locations where future Urban Low-Density Residential land use might approach its greatest density have not been established, it was necessary to provide excess sanitary sewer capacity in the entire proposed system. In order to do this, a City of Prior Lake Wastewater 2040 Comprehensive Plan Page 620 conservative land use density and corresponding unit wastewater flow rate were used across the system. C. Intercommunity Flows Wastewater flow from the City of Prior Lake is metered by the MCES at their Meter Number 404. This meter measures and records the flow from the entire City and includes intercommunity flows from small areas of the City of Savage, Spring Lake Township, and the Shakopee Mdewakanton Sioux Community (SMSC). Figure 8.6 on the attached Appendix G identifies the neighboring communities served by Prior Lake utilities. The City of Prior Lake has entered into one intercommunity service agreement since 2008. In 2014, the Cities of Prior Lake and Savage entered into a Cooperative Agreement Regarding Utilities Sharing (attached as Appendix H). The agreement identifies how the communities reimburse each other for the municipal wastewater charges that each will incur by receiving flow from the adjacent community. Utility costs for properties served by City of Prior Lake utilities within the Vergus Avenue and South Shore Drive area of Spring Lake Township and the SMSC properties served within the City of Prior Lake boundaries are invoiced by the City of Prior Lake directly to the individual properties being served. The City of Prior Lake is billed for this flow; the Metropolitan Council does not make an adjustment in the wastewater flow filling between the communities. D. Individual Sewage Treatment Systems (ISTS) There are 146 utilized individual on-site treatment systems for wastewater disposal in Prior Lake, as shown in Figure 8.4. These systems lie in older portions of the City where sewer infrastructure is not yet available and on agricultural lands. The percentage of on-site systems in Prior Lake will fall as municipal sewer service is extended throughout the City. At the present time, no significant problems are known to exist in areas served by on-site systems, although temporary repairs of systems may be required until municipal sewer service becomes available. In addition, there are several scattered individual disposal systems within areas currently serviced by trunk sewer facilities. Permitting and maintenance of ISTSs is handled by Scott County due to the small number of on-site systems. The program instituted by Scott County can be found in Individual/Community Sewage Treatment System Ordinance No 4. E. Publicly and Privately-Owned Wastewater Treatment Plants There are no publicly or privately-owned wastewater treatment plants within Prior Lake. F. Prior Lake Interceptor (MCES 7120) The Prior Lake Interceptor will ultimately serve all of Prior Lake municipal limits and proposed annexation areas. It starts at the intersection of Eagle Creek Avenue NE (County Road 21) and 154th Street NW (County Road 82). The gravity trunk conveys Prior Lake's wastewater flow north generally paralleling County Road 21, eventually into Shakopee and discharging into the Blue Lake Wastewater Treatment Plant. City of Prior Lake Wastewater 2040 Comprehensive Plan Page 621 G. City Trunks Inside Prior Lake, several City trunk lines convey flow to the Prior Lake Interceptor. Two existing trunk lines roughly follow the north and south edge of Lower Prior Lake to reach subdivisions in the northeast portion of the City. These are the North 21 Trunk and South 42 Trunk, respectively. Trunk lines also travel west along County Road 82 to service the Spring Lake and Stemmer Districts. One of these is an existing 12-inch line. The other is the larger proposed Spring Lake Trunk. The Spring Lake Trunk system starts at the intersection of Baseline Avenue and 170th Street West and continues along 170th Street to Spring Lake. Two additional branches originate at the south line of Sections 17 and 18 and travels north to intersect the main line at 170th Street West and Langford Road. The other branch is south of County Road 72, starts near Baseline Avenue and runs east to Marschall Road and continues south on Marschall Road to where it ties into the 170th Street trunk. From 170th Street West the City trunk line travels east to intersect Shoreline Boulevard and Sunset Avenue, where it will ultimately be pumped from a lift station to County Road 82. There is also a length of existing trunk line, labeled the South 21 Trunk, along the south edge of Upper Prior Lake to service subdivisions in South 21 District. Based on unit flow rates and full development of the current municipal boundary and annexation area, Prior Lake’s contribution to the Prior Lake Interceptor is estimated at 5.64 MGD average flow and 12.02 MGD peak flow by 2040. To ease discussion and recall of the various trunk alignments, each has been given a name. These names are summarized in Table 8-5 (see also Figure 8.5 - Map 1). Table 8-5: City Trunk Naming From Point To Point City Trunk Name S42-7.1 S42-2.1 South 42 Trunk W1.1 W4.1 Wilds Trunk N211.1 N21-1.1 North 21 Trunk S21-1.1 S21-12.1 South 21 Trunk SL1.1 S42-1.1 Spring Lake Trunk The following sections describe in detail each sanitary sewer district and the trunks that serve them. H. Sanitary Sewer Districts There are twelve major sanitary sewer districts in the City, each defining the limits of service for a separate trunk system. These districts are further subdivided into smaller subdistricts that were used to develop design flows and to determine cumulative design flows in the various sewer segments. The major sanitary sewer districts, their corresponding prefix abbreviations, and the number of subdistricts in each major district are presented in Table 8-6. City of Prior Lake Wastewater 2040 Comprehensive Plan Page 622 Table 8-6: Sanitary Sewer Districts Sewer District Abbreviation No. of Subdistricts South 21 S21 71 South 42 S42 34 North 42 N42 20 Wilds W 21 Northwood N 15 Estate E 13 Frost Point FP 9 Candy Cove CC 4 Red Oaks RO 5 North 21 N21 18 Stemmer ST 2 Spring Lake SL 12 The boundaries of all the major and minor districts are shown on Figure 8.5 - Map 1 at the back of this report. Recent development and currently proposed phasing have prompted revisions to the subdistrict boundaries over those shown in previous reports, though the major district boundaries remain essentially the same. A summary of the areas and the existing and anticipated residential units in each major district and subdistrict is presented in Appendix A. 1. South 21 District (S21) This district generally lies south of CSAH 21 and is intersected by Trunk Highway 13. The area includes Blind Lake and is adjacent to Spring Lake and Upper Prior Lake. It encompasses 2,131 gross acres. The majority of the South 21 District is fully developed with infill lots generally being the only remaining land that has yet to be developed. The majority of this remaining land is typically adjacent to CSAH 21 and Trunk Highway 13 and has been slated as commercial use based on the future land use associated with the properties. Most of this area is serviced by the South 21 Trunk that follows the south edge of Upper Prior Lake. The existing South 21 Trunk ends at node S21-1.1. Even with full development of S21-1, this trunk will likely have reserve capacity to serve potential future annexation. Lateral sanitary sewer lines are fully built out within the district to service current City Limits. If the City annexes future areas that can be serviced by the South 21 District, it will be necessary to construct additional lateral sewer lines to convey flows to the Prior Lake system. Several small existing lift stations pump flows to the South 21 Trunk from depressions along Upper Prior Lake. No additional trunk service will need to be constructed in this district. City of Prior Lake Wastewater 2040 Comprehensive Plan Page 623 2. South 42 District (S42) The South 42 District lies north of Lower Prior Lake and encompasses 1,316 acres. It is generally bounded by Wilds Ridge Court on the west and County 42 on the north. The land here generally slopes to the south to Lower Prior Lake while existing sanitary sewer pipe generally runs to the west and north near the intersections of CSAH 21 and 42. Several lift stations have been constructed in this area to serve low areas adjacent to the lake. Most of the South 42 District is already developed as urban low density residential. A small portion of the area remains to be developed south of CSAH 42 and adjacent to both sides of CSAH 21. Development is slated to be a mixture of commercial and high- density residential uses. The majority of the South 42 District is served by the South 42 Trunk that connects to the Prior Lake Interceptor at node N42-15.1 near CSAH 21. No additional trunk service will need to be constructed in this district. 3. North 42 District (N42) The North 42 District generally lies north of County Road 42 and encompasses approximately 1,913 acres. The district abuts Shakopee city limits on the north and contains a portion of SMSC trust land that wasn’t evaluated as part of this study due to SMSC having their own sanitary sewer system. The entire area lies within the current municipal boundary of Prior Lake. Under existing conditions, the majority of the land use is agricultural. The 2040 Land Use Plan shows this area developing as a mixture of urban low density residential and commercial uses. Pike Lake and the outlet channel for Jeffers Pond are within the North 42 District. The final length of the Prior Lake Interceptor before entering Shakopee runs through this district. The Interceptor follows County 42 east to Pike Lake Trail NE. There it runs north to Shakopee. An additional gravity Trunk line will need to be installed as development occurs to the east of the Prior Lake Interceptor to accommodate flows from districts N42-2, N42-3, and N42-4. 4. Wilds District (W) The Wilds District encompasses approximately 862 acres in northwest Prior Lake and is comprised mainly of residential development around The Wilds Golf Club. This area generally drains to several large wetlands – Mystic Lake, Haas Lake, and Jeffers Pond. The existing City trunk system includes an 18-inch and 30-inch pipe along County 82 that services the majority of the City west of CSAH 21. The City’s trunk line discharges into the Prior Lake interceptor at node S42-18.1 in the South 42 District. A small portion of the SMSC property is in the southwest corner of this district but has been excluded from this plan since the SMSC operates their own independent sewer treatment plant. No additional trunk service will need to be constructed in this district as it is fully developed. City of Prior Lake Wastewater 2040 Comprehensive Plan Page 624 5. Northwood District (N) The Northwood District encompasses approximately 707 acres west of Upper Prior Lake and north of Spring Lake. This area generally drains towards the lakes. Due to the numerous depressions in this district, most of the area is serviced by several lift stations. The lift stations pump the wastewater south towards County 12 at node N- 1.1 and into the South 21 Trunk at node S21-17.1. The area is designated as Urban Low Density Residential and is generally fully developed. No additional trunk service will need to be constructed in this district. 6. Estate District (E) The Estate District is located in the far northeast corner of Prior Lake and encompasses 444 acres. The North 21 Trunk starts on the northern edge of the Estate District and then follows Lower Prior Lake towards TH 13 and the Frost Point District. The majority of the district is fully developed as medium/low density residential. Roughly 70 acres, north of County 42, remains to be developed as a mixture of low and medium density residential according to the 2040 Land Use Plan. Sanitary sewer has been extended to the north to accommodate this along Kensington Avenue Northeast. Additional lateral sewer lines will need to be constructed to service this remaining area. The existing North 21 Trunk line has sufficient capacity to handle the additional flow from full development of the districts outlined on Figure 8.5 - Map 1. 7. Frost Point District (FP) The Frost Point District is directly north of Candy Cove. It encompasses 231 acres. It is similar to both Candy Cove and Red Oaks – fully-developed residential and low elevations. The Frost Point District utilizes three lift stations that pump wastewater to the North 21 Trunk. Sanitary sewer improvements to remove I/I were performed within this district from 2003 to 2005. This district is fully built out and additional trunk service is not needed. 8. Candy Cove District (CC) The Candy Cove District is the smallest of all the sewer districts and encompasses 42 acres. Similar to Red Oaks District, the district is served by three lift stations that pump flow to the North 21 Trunk. The area is fully developed and won’t need additional trunk lines. 9. Red Oaks District (RO) The Red Oaks District encompasses approximately 212 acres south of Lower Prior Lake. The entire district is served by four lift stations due to low elevations. Three of the lift stations are adjacent to Lower Prior Lake and pump into the fourth. The fourth, at node RO-1.1, pumps into the North 21 Trunk. A large portion of the Red Oaks District is park and therefore has minimal sewer flow. This area is fully developed and won’t need additional trunk lines. City of Prior Lake Wastewater 2040 Comprehensive Plan Page 625 10. North 21 District (N21) The North 21 District encompasses approximately 1,055 acres. Trunk Highway 13 forms part of its northern boundary, and the City Limits forms its eastern and southern boundaries. Land use in this region is characterized by low density residential, medium density residential, commercial property, and industrial property. Although the district is mainly developed, there are infill lots from the industrial area south of CSAH 21 that have yet to be built. There is also an area of low-density residential development in the southwest corner of the district that is not developed. This low density residential would tie into the lateral sewer service in sewer subdistrict N21-11, where it is pumped by a lift station located at N21-11.1 and eventually ties into the City’s North 21Trunk. The North 21 Trunk follows TH 13 and picks up wastewater from the Estate, Frost Point, Candy Cove, and Red Oaks Districts. It connects to the Prior Lake Interceptor at node N21-1.1. Only lateral sewer service is expected in the southwest corner of the district for future development. No expansion of this trunk is expected. 11. Stemmer District (ST) The Stemmer District lies on the western portion of Prior Lake and encompasses 251 acres. The district is comprised of Spring Lake Regional Park, SMSC land, and low density residential just south of CSAH 82. This district will be picked up by the Spring Lake Trunk and routed east along County 82. The district is fully developed with potential redevelopment in the low-density residential areas. Existing trunk lines provide service to this district and future expansion of sanitary sewer lines is not needed. 12. Spring Lake District (SL) The Spring Lake District is the largest of the sanitary sewer districts and encompasses roughly 42,856 gross acres of undeveloped land in the annexation area. The 2040 Land Use Plan shows future development in this region as primarily low density residential, medium density residential, commercial, and industrial. The planned medium density residential, industrial, and commercial development is concentrated in the area west of Baseline Avenue and Langford Avenue (see Figure 8.5 - Map 1). The Spring Lake District is serviced by the Spring Lake Trunk. All the trunk lines in this district will need to be constructed and are being planned as part of this CSP. The trunk line starts at the intersection of Dairy Lane and County Road 17 and continues east along County Road 17 to a proposed lift station located along Langford Boulevard between Marschall Road and Spring Lake Circle. An additional branch originates near Vergus Avenue and continues west along Langford Road to the same proposed lift station. The lift station will then pump the wastewater to the high point along Langford Avenue and tie into the proposed 170th Street East trunk line. The 170th Street East trunk line will start near Baseline Avenue, where a lift station will be needed to pump wastewater to a point where it can gravity flow into the lift station located near the 170th Street East and Sunset Avenue intersection. Another trunk line will be necessary just south of County Road 72. This will begin east of Baseline Avenue and will consist of a lift station that pumps wastewater from low lying ground east to a high point until it can be fed by gravity and eventually tie into the 170th Street East trunk line via the City of Prior Lake Wastewater 2040 Comprehensive Plan Page 626 Marschall Road corridor. All of these trunk lines have been preliminarily laid out and are subject to change based on development needs for this district. All wastewater from this district will outfall into the pump station (node SL-1.1) near Sunset Avenue and 170th Street East and discharge into the Stemmer District into the Spring Lake Trunk. The Spring Lake Trunk was designed with adequate capacity to carry the entire Spring Lake District. I. Analysis of Trunk Capacities The full-development trunk sanitary sewer system for the City of Prior Lake is presented in Figure 8.5 - Map 1. Appendix C shows the existing and proposed trunk capacities, and the design flow for each segment of trunk based upon the computed average flow and the estimated peak flows from the InfoSWMM models. In general, the system has sufficient capacity to handle design flows with a conservative peak flow factor. Generally, the projected flows utilize a majority but not all the capacity of the new trunks proposed in this report. As mentioned, this reserve capacity allows for instances of higher density. City of Prior Lake Wastewater 2040 Comprehensive Plan Page 627 Figure 8.4: Individual Sewage Treatment Systems City of Prior Lake Wastewater 2040 Comprehensive Plan Page 632 8.6 Infiltration and Inflow A. General The Metropolitan Council instituted an Inflow and Infiltration (I/I) Surcharge Program (IISP) in 2006. The fundamental policy statement summarizing this program is that the Metropolitan Council “will not provide additional capacity within its interceptor system to serve excessive inflow and infiltration.” The Council establishes inflow and infiltration thresholds for each of the communities that connect to its system. Communities that exceed the thresholds are required to eliminate the excess flow within a reasonable timeframe or pay a surcharge fee. The City of Prior Lake has not been identified by Metropolitan Council as a municipality with excessive I/I. However, the City has identified places where improvements could be made. B. Sources and Extents of Infiltration and Inflow Infiltration/Inflow (I/I) is clear water that enters the sanitary sewer system through a variety of sources including pipe and manhole defects and private property sources like sump pumps and foundation drains. Infiltration is typically related to ground water and it normally occurs over an extended period of time at a relatively low rate. In contrast, Inflow is typically associated with a rainfall or snowmelt event, and it normally occurs over a short period of time at a relatively high rate. The combination term Infiltration/Inflow (I/I) is used for discussion of the overall problem without differentiation between the two possible sources. In many cases, it is difficult to differentiate between infiltration and inflow since some defects can contribute both infiltration and inflow depending on the conditions. The EPA Guide for Estimating Infiltration and Inflow (June 2014) was used to estimate the proportion of I/I contribution in the City’s wastewater system. Monthly flow data were obtained from Metropolitan Council Environmental Services, for the period from 1991 to 2016. 2012 to 2016 monthly data were used to obtain monthly average flows for the five- year period from March to November (representative of a wet portion of the year) and December to February (representative of a dry portion of the year). It was determined that the wet monthly average flow (March-November) was 49.3 City of Prior Lake Wastewater 2040 Comprehensive Plan Page 632 7 mg, and that the dry monthly average flow (December-February) was 28.85 mg. Thus, on average, I/I contributes 4.32 mg monthly (roughly 10% of base flows). The peak flow for the City of Prior Lake is 73.90 mg in June 2014, which is roughly 64% of peak flows. Potential sources of I/I could include: • Groundwater infiltration in low areas around the lakes within the City • Underground springs that may contribute to groundwater infiltration. • The increasing frequency of high-intensity rain events in the region that contribute inflow, especially when the 100-year high water level is exceeded. • Non-compliant residential sump pump connections to the sanitary sewer • Leaky pipes and structures under groundwater conditions • Service line leaks • Inflow from floor drains of flooded structures In general, I/I is not a serious problem in the overall system; however, there are some smaller areas of the City where improvements can be made. The City has been aggressive in improvements to the system to prevent I/I since an “I and I Report” was completed in 2000. All but one of the identified homes were retrofitted to discharge their sump pumps to the surface. One single home is allowed to discharge to the sanitary sewer system but only during the winter months. Furthermore, the City utilized slip lining technology along with manhole techniques to correct poor sections of the sanitary sewer facilities immediately adjacent to Prior Lake (including under the lake) from 2003 to 2005. To understand residential housing stock age within the City of Prior Lake, an analysis was conducted using an existing parcel dataset. According to this dataset, there are 11,909 parcels within the City. 7,628 of those 11,909 parcels have housing age data. 25% of houses in the dataset were constructed prior to 1970, and 75% of those were constructed after 1970. The City of Prior Lake conducts I/I inspections on public mains but does not conduct I/I inspections on private service connections. C. Goals, Policies and Strategies to Address I/I Since excess I/I is a local concern to Prior Lake as well as a regional concern to Metropolitan Council, Prior Lake will continue to work on reducing I/I in its sanitary sewer. The City’s primary goals regarding I/I are to: • Preserve capacity in the local and regional system • Minimize I/I in the system • Prevent excessive I/I, backups, and overflows Sump pump and rooftop drain connections to the sanitary sewer system are prohibited by ordinance (Appendix G), and the City works to prevent I/I through inspection of connections to the sanitary sewer system. The City also uses modern materials and standards in new construction to prevent I/I. The focus of I/I mitigation efforts has been on the public improvements side through pipe rehabilitation and manhole sealing improvements. I/I mitigation on the private side is addressed voluntarily through educational efforts. City of Prior Lake Wastewater 2040 Comprehensive Plan Page 632 D. I/I Implementation Plan In the past five years, the City has made improvements to several old and deteriorated sanitary sewer lines and a “sanitary sewer line and I/I improvement” program has been established to provide funds to identify and replace defective pipes. The maintenance budget includes about $45,000 for rehabilitation of sanitary sewers. The City has initiated an effort to install pump monitors on main lift stations to monitor and record operations around the system in an effort to isolate problem areas and prioritize maintenance efforts for maximum benefits. In new subdivisions, the City is requiring the use of plastic pipe, rubber manhole adapters, and chimney seals to reduce the potential for infiltration and inflow in the City collection system, and new homes are inspected to verify that no sump pump or other types of clear water connections are made to the system. Ongoing implementation activities to prevent or eliminate excessive I/I from the system include: • Annually televise and inspect sewer facilities for degradation and leaks • Inspect sanitary sewer facilities in response to backup notifications and in coordination with the street reconstruction program • Perform repairs or rehabilitation of sewer facilities as needed • Install new sewer facilities in accordance with current standards for materials and construction • Conduct plumbing and other inspections during installation and connection to the system • Work towards disconnecting prohibited/unused connections to the sewer system Table 8-7 presents a summary of budgeted implementation costs to address I/I in the City’s system. Table 8-7: Estimated I/I Implementation Costs I/I Implementation Activity Cost Televise and inspect sewer facilities for leaks $20,000/year Inspect sewer facilities in response to backups and in coordination with street reconstruction program $50,000/year Perform repairs or rehabilitation of sewers $160,000/year Disconnect prohibited/unused connections to sewer $5,000/year Upgrade lift station monitoring equipment (2019) $100,000 8.7 Cost Analysis A. Trunk Sanitary Sewer Costs One of the basic objectives of this study was to determine the cost of completing the City of Prior Lake’s trunk sanitary sewer system to ensure the area charges will insure availability of sufficient funds for future trunk sewer construction. City of Prior Lake Wastewater 2040 Comprehensive Plan Page 632 The cost estimates presented in this report are based on 2017 construction costs and can be related to the Engineering News Record (ENR) cost index. Future changes in this index are expected to fairly accurately reflect changes in construction costs for the trunk sanitary sewer system. The estimated cost to complete the trunk sewer system proposed in this CSP is $18.1 million. A detailed cost estimate is provided in Appendix E. These costs include all foreseeable expenses associated with the completion of Prior Lake’s trunk sanitary sewer system, including trunk sanitary sewer, lift stations, and force mains. The cost estimates include a 30 percent factor, applied to pipe construction, that accounts for engineering, administrative, and financing costs. Footnotes in Appendix E summarize the cost assumptions. New elements of the trunk sanitary sewer system occur primarily in the Spring Lake District. This district was previously studied under the “Southwest Planning Area Sanitary Sewer Study” (April 2002). Known as the SWPA Study, the study identified two alternatives for connecting the Spring Lake District to the existing municipal trunk and regional interceptor system. Since the last CSP update, dual force mains have been installed, to carry wastewater from lift station SL-1.1 to the Stemmer district and into the Spring Lake Trunk. To service the entire Spring Lake District, the pumps will need to be upgraded and multiple lift stations will need to be constructed to provide service to the Spring Lake District (see Figure 8.5 - Map 1). Other than the Spring Lake District, a gravity trunk line is proposed to service North 42 subdistrict N42-3.1 that will tie directly into the Prior Lake Interceptor. Previously it was part of the Estate District that tied into the North 21 trunk line. All future development outside of the Spring Lake District and subdistrict S42-1.1 can be served by lateral sewer. 8.8 Capital Improvement Program The installation of the City of Prior Lake’s trunk sanitary sewer system has kept pace with development in the City. It is anticipated that additions to this sewer system will provide sufficient sanitary sewer capacity for additional development as it occurs. The City of Prior Lake Capital Improvement Program represents the trunk improvements required to bring service to the remaining undeveloped areas of the City areas as they open to development, as well as costs to repair or upgrade existing sewer lines. The phasing of new trunk elements or repairs/upgrades and their associated costs are presented in Table 8-8. The Capital Improvement Program is intended to serve as a guide for future fiscal planning and should be reviewed annually as more current planning and cost data become available. City of Prior Lake Wastewater 2040 Comprehensive Plan Page 632 Table 8-8: Capital Improvement Program Project Year Estimated Cost ($) Repair/upgrade sewer lines - 2018 2018 $62,407 Repair/upgrade sewer lines - 2019 2019 $214,279 Repair/upgrade sewer lines - 2020 2020 $220,708 Trunkline extension to SL-1, SL-6 2020 $3,801,265 Repair/upgrade sewer lines - 2021 2021 $827,329 Trunkline extension to N42-3 2021 $935,480 Trunkline extension to SL-2, SL-3 2023 $1,063,595 Trunkline extension to SL-8 2026 $883,480 Trunkline extension to SL-4 2028 $618,540 Trunkline extension to service remaining sewer SL districts Future $10,819,932 Total Cost $19,447,015 8.9 Summary and Recommendations This Comprehensive Sewer Plan is intended to serve the City of Prior Lake through buildout of its 2040 Land Use Plan and beyond. This report is intended to meet the Metropolitan Council’s requirements for sewer planning for municipalities intending to expand their sewer systems – as outlined in the 2040 Water Resources Policy Plan. The City of Prior Lake was divided into sewer districts for the purposes of generating existing and future sewage flows. Flow generation was based upon projected land uses and densities within these districts. The areas of land use within the districts are presented in Appendix A. The areas presented in Appendix A include a category for non- developable. This category consists of large wetlands, lakes, and larger open space areas. Unit wastewater flow rates were then applied to the remaining areas that have wastewater flows associated with them. The resultant flows are presented in Appendix B. Figure 8.5 - Map 1 shows the proposed trunk sanitary sewer system for the City of Prior Lake. This map presents district and subdistrict boundaries as well as the existing and proposed trunk sanitary sewer system. Appendix C shows how flows are added within the sewer system model. Appendix D presents the trunk system design information. Appendix E presents cost information for trunk sewers and lift stations. The estimated cost for completing the trunk sanitary sewer system is approximately $18.1 million. A portion of this cost can be recovered from applying over-sizing criteria. Essentially, this means the City pays only for the trunk over-sizing cost with land developers paying a cost equivalent to an 8-inch lateral pipe. The remainder of the cost will be funded through the existing sewer fund balance and application of area charges at the time of development.