chapter 8-sanitary sewer - stillwater, mn · use, population density, standard wastewater...
TRANSCRIPT
1st Draft August 12, 2008
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Chapter 8-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 2030 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 15) 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 2030 period.
Stillwater’s sewer system connects to the Met Council Interceptor 9101 at meter number 606 on the
southern border. Interceptor 9101 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 2030 WRMPP.
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Table 1 -- Metropolitan Council Projections for Sewered Areas
Year Sewered Population Sewered
Households Sewered Employment
2010 18,300 7,500 11,550
20152 19,425 7,800 12,025
2020 20,550 8,100 12,500
20252 19,875 8,350 13,050
2030 19,200 8,600 13,600 1Metropolitan Council Water Resources Management Policy Plan, May, 2005 2Values 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 Projections1
Year Sewered Population Sewered
Households Sewered Employment
2010 18,300 7,350 11,550
2015 19,050 7,650 12,025
2020 20,200 7,950 12,500
2025 19,650 8,250 13,050
2030 19,100 8,550 13,600
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.
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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 2015-2020;
therefore this flow is carried through to 2030.
Metropolitan Council’s projected flows for the years 2010, 2020, and 2030 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)
2007 2.02 MGD2
2010 2.53 MGD 2.53 MGD
2015 --- 2.68 MGD
2020 2.68 MGD 2.68 MGD
2025 --- 2.68 MGD
2030 2.57 MGD 2.68 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 2010 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
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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 340 3 1,020
Low/Medium Density Residential
300 4.4 1,320
Medium Density Residential
280 6 1,680
High Density Residential 190 15 2,850
Commercial -- -- 1,300
Institutional -- -- 500
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Land Use Type Gal/Unit/Day Units/Acre Gal/Acre/Day
Downtown/MU -- -- 2,000
R&D/Industrial -- -- 2,000
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.
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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 8” 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 Slope1 Capacity Average Flow
Peak Flow2
From To (in) (%) (MGD) (MGD) (MGD) S-1 S-2 24 0.08 5.0 1.412 4.236 S-3 S-4 FM -- -- 0.713 2.353 S-4 S-5 27 0.067 6.2 0.207 0.788 S-5 S-6 30 0.058 7.6 0.088 0.353 S-2 S-6 30 0.058 7.6 1.449 4.201 S-6 M606 36 0.046 11.0 0.284 1.052
1) 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 7080 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
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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. 29-5. 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 6 of this section. Subd. 6. 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 three-year period. (Code 1980, § 29.05; Ord. No. 755, 4-8-92) • 20% of the City sanitary sewer lines are televised annually for potential I/I sources.
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• Five total flow meters are set up to identify areas of town with potential inflow.
• Replacement of vented sanitary manhole lids (estimated 10-20 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 30-40 annually).
• Ongoing annual review of flows to develop the next stage of improvements.
The City spent over $206,000 on I/I reduction measures in 2007.
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 $84,000. 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.
Ü3,000 0 3,0001,500 Feet
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City Boundary
Forcemain
Existing City Gravity Trunk Pipe
Gravity Pipe
Sanitary Districts
") Met Council Meter M606
Lift Station
Manhole
S1 Sanitary District Number
Sanitary Node
S-1 Sanitary Node ID
CSAH 12
CS
AH
15
80th StBoutwell Rd
62nd St
CSAH
5
State Hwy 36
State Hwy 96
CR 64
June 2008
Trunk Sanitary Sewer MapFigure 1
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