/ Public Works Engineering / Water, Sewer, and Stormwater Utilities / Sewer System Plan Update
Sewer System Plan
Goode Place Sewer Construction
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The City of Issaquah Sewer System Plan Update serves as a guide for planning and designing future sewerage facilities. The plan presents an evaluation of the existing system, and recommends improvements needed to correct existing deficiencies and to accommodate projected growth by providing a detailed six-year capital improvement program. Identified in this Plan are sewer system improvements needed to provide adequate service within the City's service area through the year 2020.
In 2003 a new Sewer System Plan Update was approved by the City Council. The plan document can be viewed or downloaded at the following link:
The Washington State Department of Ecology (DOE) requires the City to periodically update its sewer system plan (WAC 173-240-050), and King County must also review and approve the plan because the City provides sewer collection in unincorporated King County in the area of the Lakemont Triangle (KCC 13.24). The King County Council provides the County approval, following review by the Utility Technical Review Committee. The sewer system plan was last updated in 1986. Plans are updated on about a five year cycle.
Below is a summary of the sewer system and principal conclusion from the Sewer System Plan Update.
Issaquah’s Sewer System
The City of Issaquah Sanitary Sewer system is a municipally owned utility that is operated in conjunction with the City's water and stormwater utilities by the City of Issaquah Public Works Operations and Maintenance Department. The City’s sewer system services approximately 2.7 square miles of developed properties within the current City boundary, plus 46 acres in the “Lakemont Triangle” located in unincorporated King County. (These areas do not include public roads). The total number of sewer utility accounts billed for service is approximately 3,170, of which 73% are single family residential or duplex. Additional developed land in the City limits, including North Issaquah and Providence Point, are served by the Sammamish Plateau Water and Sewer District.
The City’s sewer system is a conveyance system only and functions mostly by gravity flow. Wastewater is sent to King County’s Renton Wastewater Treatment Plan via a system of interceptors and pump stations that are also operated by King County. This map shows the system of pipelines and pump stations that transport sewage over the Eastgate Hill to the main Eastside Interceptor that runs along the east side ot Lake Washington to Renton. Effluent from the Renton plant is discharged into Elliot Bay. For more information on the King County wastewater operations, go to this link.
Up until 1969 the City operated a secondary treatment plant, which was abandoned when Metro constructed the Issaquah Interceptor to convey wastewater flows to Renton. In 1982, Metro constructed a second interceptor (Issaquah Creek Interceptor) to relieve the Issaquah Interceptor that was flowing near capacity during peak flow periods.
Only two significant industrial customers are located within the City. Both the Darigold Farms and Data I/O facilities have waste discharge permits administered by King County.
Service Area Growth Projection
The City’s present service area that covers 2.7 square miles of developed properties could expand in the future to include up to an additional 3.9 square miles of currently unsewered land. The area of expansion includes both undeveloped land and existing developed land currently served by septic systems, located within the City limits and in two potential annexation areas on Cougar Mountain. (Most of Issaquah Highlands, all of Talus, and all of Park Pointe are included in this estimate, because it is based on 2000 land use). Another 2.6 square miles of land in the City will remain permanently unsewered (i.e., park land and open space).
The City of Issaquah population in 2000 was 11,056 in 5,813 households. Within the same area, the 2020 population forecast prepared by the State Office of Financial Management is 25,768 people in 11,713 households.
An Equivalent Residential Units (ERUs) is amount of drinking water used by a “typical” single family residence, and is used to estimate the flow in the sewer system under current and future land use conditions. One ERU in current development is assumed to equal 180 gallons per day (gpd) under average winter consumption, and for future development is assumed to equal 125 gpd. ERUs also represent wastewater flows from multi-family and non-residential land uses by applying conversion factors. For example, multi-family developments contribute 0.65 ERU per dwelling unit.
The amount of wastewater now entering the City’s sewer system is based on a population equivalent of 5,229 ERUs. This could potentially increase to about 14,301 ERUs in the year 2020. This projection is based on zoning and population information for the projected sewer system service area, and is a conservative forecast because it represents the long-term potential buildout for the year 2020.
Inflow and Infiltration
In addition to wastewater flow from connections to residential, commercial, and other buildings, the sewer system intercepts a large amount of inflow and infiltration (I/I). Inflow refers to water that enters the sewers from gaps in manholes or improper stormwater connections to the sewer (such as roof downspouts), and infiltration refers to water that seeps into sewer pipes through cracks, separated joints and other flaws in the underground pipe system. The magnitude of I/I can be very high during wet storm conditions, dwarfing the actual wastewater flow (I/I is predicted to be about 84% of the total sewer flow during a 5-year rainfall event). Soil type, pipe age, pipe material type, quality of construction, and damage from natural and man-made activities are all factors that affect how much I/I enters the sewer system. Current construction and inspection procedures help lower I/I in new systems, but I/I contribution will still remain significant.
To evaluate the magnitude of I/I entering the regional treatment system, King County Wastewater Division conducted an extensive flow monitoring program at over 800 locations within city and county systems between 2000 and 2002. In Issaquah 13 of these flow meters were operated, collecting data on total wastewater flow in sewer mains at key points within the sewer system. These flow records indicate existing residential sewage flow rates vary, due to highly variable I/I conditions. Wastewater flows within the existing sewer system increase during wet weather and particularly in response to rainstorms. These Rain Dependant Infiltration/Inflow (RDII) rates vary, but appear to exceed 5,000 gallons per acre per day (gpad) in some areas of the City. During non-storm conditions, base infiltration ranges from about 100 gpad to over 800 gpad.
The City wastewater agreement with King County includes language based on infiltration and inflow rates not exceeding 1,100 gpad. Other sewer agencies connected to the County system have similar agreements. Few systems currently comply with these agreed rates, as evidenced by the recent collected monitoring data. King County is in the process of developing plans for future wastewater facilities and will be developing an approach to bring required capacity in line with the actual flows expected. This will likely require some degree of sewer rehabilitation by most agencies, including Issaquah.
Future System Capacity
The Sewer System Plan Update evaluated sewer capacity conditions in the year 2020 based on the projected increase in ERUs served by the sewer system. The increase in flows from existing to future conditions is attributed to new sewer service provided to new development and to sewer extensions to areas not currently served. Under the future condition the sewered area more than doubles, from 1,726 acres to 4,268 acres. The existing average daily sewage flow of approximately 1.0 million gallons per day (mgd) also doubles to 2.2 mgd, due to the population increase. The resulting total wastewater flow increases from about 6.9 mgd under a 5-year storm to 13.7 mgd. Substantial surcharges would result in the existing pipe systems as a result. Capacity provided by existing facilities would accommodate dry weather flows; however, even during modest storm events up to five-year storms, minor surcharging occurs at some locations. For five-year events under future conditions, additional capacity will be needed in several trunk lines.
Improvements to Increase System Capacity
The Sewer System Plan Update recommends a Capital Improvement Program to address immediate capacity improvements, sewer extensions, sewer main rehabilitation (including I/I control), and other projects such as inspections, monitoring and plan updates. Total estimated projects costs for these improvements over a 6-year CIP period is estimated at about $5,510,000.
Special emphasis should be placed on I/I control as one strategy to meet wastewater flow capacity requirements. Rehabilitation of sewer systems to reduce I/I flows is most effective when the effort can target those pipe elements with the highest extraneous flows. Review of the base infiltration (BI) and rain dependent infiltration inflow (RDII) rates throughout the City system identified two subbasins with extraneous flow rates that noticeably exceeded other areas of the City. A flow monitoring program or other inspections such as smoke testing would be appropriate to verify these indications. If so, then rehabilitation efforts should be focused in these subbasins first. The resulting effectiveness can then guide future efforts towards achieving conformity with the rest of King County.
The analysis results in this Plan presented for the future full build-out conditions is a projection of possible sewer service requirements at the end of the 20-year planning window. Careful consideration is needed to determine how these results should be used to refine the Capital Improvement Program. Further analysis, including additional monitoring in the sewer system and a higher level of modeling detail, along with verification of the probable build-out land use condition should be performed as part of pre-design studies before specific projects are proposed.
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