Chapter 8-Sanitary Sewer

Transcription

1 Chapter -Sanitary Sewer Executive Summary Background 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 sanitary sewer 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 MCES. In March, 2005 the Metropolitan Council adopted a revised Water Resources Management Policy Plan (WRMPP). The 20 WRMPP includes the metropolitan wastewater system plan with which local comprehensive plans must conform. The method Stillwater has chosen to demonstrate its conformance is through this sanitary sewer chapter. This chapter updates the City of Stillwater s Comprehensive Sanitary Policy Plan or CSPP. The City last updated its CSPP in 1995 and described the expansion of the City s trunk system (in particular within the annexation areas extending west to County Highway ) and the demands this expansion places on the Metropolitan Disposal System (MDS) operated by Metropolitan Council Environmental Services (MCES). MCES also uses the CSPP to determine whether capacity upgrades will be needed at the Metropolitan Wastewater Treatment Plant (WWTP). This update is necessary to reflect land use changes that have occurred since the 1995 CSPP was prepared and to reflect land use changes proposed in the Comprehensive Plan for the 20 period. Stillwater s sewer system connects to the Met Council Interceptor 91 at meter number 0 on the southern border. Interceptor 91 travels south and arrives at the St Croix Valley WWTP in Oak Park Heights. Forecasts Table 1 present s projections of sewered population, households, and employees for the City of Stillwater, as prepared by Metropolitan Council and presented in its 20 WRMPP. 1

2 Table 1 -- Metropolitan Council Projections for Sewered Areas Year Sewered Population Sewered Households Sewered Employment 20 1,0 7,500 11, ,425 7,00, ,550,0, ,75,350 13, ,200,00 13,00 1 Metropolitan Council Water Resources Management Policy Plan, May, Values Interpolated The City of Stillwater has its own sewered population projections which are shown in Table 2. These projections are based on vacant, underutilized and undeveloped parcels remaining in the City. Table 2 -- City of Stillwater Projections 1 Year Sewered Population Sewered Households Sewered Employment 20 1,0 7,350 11, ,050 7,50, ,200 7,950, ,50,250 13, ,0,550 13,00 The City arrives at its projections based upon an inventory of existing underutilized lots and a mix of development densities to achieve the Metropolitan Land Planning Act obligations. The City s projections closely align with Metropolitan Council projections. The City s lower projection of households (only 500 units) will provide the City flexibility in density in the remaining undeveloped areas and further redevelopment or infill that has occurred in the downtown. 2

3 Table 3 presents projected sewer flows for the entire service area represented in Figure 1. Both Stillwater and Metropolitan Council Projections are provided. Stillwater has based average flow projections on the 2007 meter data and adding the proposed flow from the small portion of future development remaining in the City. The flow from the future development areas was generated by applying the unit rates from Table 4 to the proposed land uses. Based on the Proposed Land Use Plan, the City will be fully developed by ; therefore this flow is carried through to 20. Metropolitan Council s projected flows for the years 20, 2020, and 20 are from its Water Resources Management Policy Plan (updated 2005). The City s projections generally correspond with the Met Council projections. Table 3 -- Wastewater Flow Projections Year MCES Projected Average Flow (MGD) 1 City Projected Average Flow (MGD) MGD MGD 2.53 MGD MGD MGD 2. MGD MGD MGD 2. MGD 1. Metropolitan Council s Water Resources Management Policy Plan (May 2005). 2. Based on metered data The projected flows in Table 3 vary from the 1995 CSP due to a decrease in the 20 population projections by Met Council and the City. A detailed description of the changes is described earlier in this Comprehensive Plan. Sanitary Sewer Design Criteria 3

4 The land use plan for the City of Stillwater served as the basis for the development of the sanitary sewer flow projections and analysis of the trunk system. Using the land use plan, the area of each land use was determined for each sewer district. Existing land uses used in this plan include low density, low/medium density, medium density, and high density residential, commercial, research and development(r&d)/industrial, public and open space. Several types of commercial and residential land use are proposed. For the purposes of generating sewer flows, these are lumped into the general categories. Detailed descriptions of the various land uses are found earlier in this Comprehensive Plan. Municipal wastewater is made up of a mixture of domestic sewage, commercial and industrial wastes, groundwater infiltration, and surface water inflows. With proper design and construction, groundwater infiltration and surface water inflows, often called infiltration/inflow (I/I), can be minimized. The flows due to I/I are accounted for in the analysis and design of the trunk sewer system. The anticipated average wastewater flows from the various subdistricts were determined by applying unit flow rates to each of the land use categories. The system design unit flow rates are presented in Table 4. For all land uses unit rates/acre were used to generate average flow projections. The units per acre assumptions for Low, Low/Medium, Medium, and High Density Residential, Commercial, R&D/Industrial were based in part on information from the 1995 CSP and City staff regarding projected number of units for each land use. Open Space was assumed to not generate any sewer flows. Table 4 -- System Design Wastewater Unit Flow Rates Land Use Type Gal/Unit/Day Units/Acre Gal/Acre/Day Low Density Residential ,020 Low/Medium Density Residential Medium Density Residential , ,0 High Density Residential 190 2,50 Commercial ,0 Institutional

5 Land Use Type Gal/Unit/Day Units/Acre Gal/Acre/Day Downtown/MU ,000 R&D/Industrial ,000 Stillwater s system design flow projections originate from the land use statistics based directly on the land use plan. Certain reductions in land use area are made to account for wetlands, steep slopes etc. and a net developable acreage for each land use category is thus created. The net acreage is multiplied by standard unit flow rates to obtain an average flow for each sewershed. Sanitary Sewer Trunk System The trunk sewer system layout for the City of Stillwater is presented on the Trunk Sewer System Map, Figure 1. This map shows the main sanitary sewer districts, existing and proposed trunk sanitary sewers, and existing and proposed lift stations and force mains. The modeling of the sanitary sewer system was based on a variety of parameters, such as: land use, population density, standard wastewater generation rates, topography, and future land use plans. Based on the topography of the undeveloped areas, the sewer subdistricts were created and the most cost-effective locations for future trunk line facilities were determined. The location of smaller sewer laterals and service lines are dependent upon future land development plats and cannot be accurately located from a study of this type. Both the existing and proposed pipe systems were evaluated and broken up into design segments. Each end of a design segment has a node assigned to it. The nodes were designated for the following reasons: 1. Flow from a subdistrict entering the pipe network. 2. Significant grade change has occurred. 3. Change in pipe size. 4. Two or more trunks connect. 5. Manmade elements (roads, railroads, etc.) affecting location and installation costs for the trunk system or lateral service of the subdistricts. 5

6 The proposed alignments shown on the Trunk Sewer System Map generally follow the natural drainage of the land to minimize the use of lift stations and consequently provide the City with the most economical ultimate design sanitary sewer system. Minor adjustments in the routing and size of the trunk facilities will take place as determined by the specific land use and development conditions at the time of final design. Any such adjustments are expected to deviate minimally from this plan. Each subdistrict contains at least one collection point where the subdistrict's sewage is defined to enter the pipe network. Upstream of that collection point, a lateral network of gravity lines can serve unserviced areas. Table 5 represents capacities of existing and proposed trunk pipes shown on Figure 1. Table 5 Capacity and Design Flows for Trunk System Node Pipe Size Pipe Slope 1 Capacity Average Flow Peak Flow 2 From To (in) (%) (MGD) (MGD) (MGD) S-1 S S-3 S-4 FM S-4 S S-5 S S-2 S S- M ) Where data was not available, minimum pipe slopes have been assumed based on Ten States Standards 2) After applying a standard peak flow factor As shown in Table 5, all sanitary sewer trunk pipes have adequate capacity for the peak flows. The majority of the sewer system for Stillwater was originally designed to accommodate both sanitary sewage and stormwater runoff. Sewer separation was accomplished in 1972 and 1973 throughout the original city. The remaining sewer was designed to serve sanitary sewage only. Since the older sewer was designed as a part of the combined system, the majority of the sanitary sewer has excess capacity as illustrated in Table 5. The City of Stillwater is committed to the proper design, location, installation and ongoing maintenance of ISTS. The Stillwater Code requires that all new systems be installed according to Minnesota Pollution Control Agency (MPCA) rule 700 permit requirements. Groundwater conditions, soil borings, distance to surface water, percolation tests, and design and type of ISTS are further factors included in the developer s site evaluation. Permitting and maintenance is

7 handled by Washington County due to the small number of onsite systems. There are no private onsite treatment systems, except in the northeast area of Stillwater. Infiltration and Inflow The City of Stillwater has a proactive program directed at identifying and correcting I/I, including the following: From the City s ordinance first passed in 1973: Sec Prohibiting deposit of stormwater in sewer system. Subd. 1. Discharge of surface water. No person shall discharge or cause to be discharged any stormwater, surface water, groundwater, cooling water or industrial process waters into any sanitary sewer. No rainspout or other form of surface drainage and no foundation drainage shall be connected to any sanitary sewer. Subd. 2. Inspections. The public works director and other authorized employees of the city, bearing proper credentials and identification, shall at reasonable times be permitted to enter upon all properties connected to the city sanitary sewer system or the city storm sewer system for the purpose of inspection, observation, measurement, sampling and testing. Subd. 3. Separate connection required. When separate sewer mains and storm sewer mains have been constructed and are available to property, the owner of the property shall take all action to separate the sewers and connect them separately to the sanitary sewer and the storm sewer in accordance with the provisions of this section within 90 days after the date of mailing or delivery of official notice to do so. The notice shall be given to the owner or occupant in writing by the public works director on the order of the city council. Subd. 4. Interceptors. Grease, oil and sand interceptors shall be provided when they are necessary for the proper handling of any liquid waste containing grease in excess amounts of any flammable waste, sand or other harmful ingredients. Interceptors shall be located so as to be easily accessible for cleaning and inspection. If necessary, in the opinion of the public works director, a suitable control manhole in the building sewer shall be installed to facilitate observation and sampling of the waste. The manhole shall be constructed by the owner in accordance with plans approved by the city engineer. The owner shall maintain the manhole so as to be safe and accessible at all times. The owner shall take action to install the interceptor in accordance with this section within 90 days after the date of mailing or delivery of notice to do so. The notice shall be given to the owner or occupant by the public works director on the order from the city council. Subd. 5. Connection by city. Whenever any owner or occupant fails to comply with the written notice, the city council shall, by resolution, direct that the sewers be separated and installed and connections made with the sanitary and storm sewer systems or that a grease, oil and sand interceptor be installed, and the cost of the installation be paid in the first instance out of the general fund, and then assessed against the property benefitted in accordance with subdivision of this section. Subd.. Assessment. After the installation and connection have been completed pursuant to city council resolution, the clerk shall serve a written notice of the assessment upon the owner or the owner's representative directing the owner to pay the assessment to the treasurer within ten days. If it is not paid, the clerk shall certify the amounts to the county auditor for collection in the same manner as other special assessments and real estate taxes. The city council may, by resolution, spread the assessment over a threeyear period. (Code 190, 29.05; Ord. No. 755, 4--92) 20% of the City sanitary sewer lines are televised annually for potential I/I sources. 7

8 Five total flow meters are set up to identify areas of town with potential inflow. Replacement of vented sanitary manhole lids (estimated -20 replaced annually) Visual inspections of manholes in low lying areas to assure lids are sealed. Installation of external manhole seals on all new sanitary sewer manholes and replacement of seals and castings on street projects (estimated -40 annually). Ongoing annual review of flows to develop the next stage of improvements. The City spent over $20,000 on I/I reduction measures in The Metropolitan Council has instituted its Inflow/Infiltration Surcharge Program since Stillwater s previous plan. The fundamental policy statement summarizing this program is that 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 use its system. Communities that exceed this threshold are required to eliminate this excess flow within a reasonable timeframe. The Metropolitan Council annual I/I surcharge for the City of Stillwater is $4,000. The City has been able to show adequate progress through its numerous programs to reduce I/I over the past two years to not be charged.

9 Ü 3, ,000 1,500 Feet I:\5\507001\GIS\avprojects\sanitarysystem0200.mxd parcels City Boundary Forcemain Existing City Gravity Trunk Pipe Gravity Pipe Sanitary Districts ") Met Council Meter M0 Lift Station Manhole S1 Sanitary District Number Sanitary Node S-1 Sanitary Node ID CSAH CSAH 0th St Boutwell Rd 2nd St CSAH 5 State Hwy 3 State Hwy 9 CR 4 June 200 Trunk Sanitary Sewer Map Figure 1 ") S1 S2 S S3 S7 S S- S-5 S-3 S-4 S-2 S-1