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Management of wastewater peak flows: Metropolitan Council Environmental Services

Steve Masters
Public Utilities Administrator
City of Lincoln, Nebraska
Member, APWA Water Resources Management Committee

Note: The following article, sponsored by the APWA Water Resources Management Committee, is summarized from a panel discussion at the APWA Congress in Minneapolis on September 9, 2005. Contributors are: Donald Bluhm, Manager of Municipal Services, Metropolitan Council Environmental Services, St. Paul, MN; Rhonda Rae, P.E., Director of Engineering Services, City of Minneapolis, MN; Dan Schluender, P.E., Project Manager, Bonestroo, Rosene, Anderlik & Associates, St. Paul, MN; and Robert Matthews, P.E., DEE, Senior Vice President, CDM, Ontario, CA.

Introduction
How does a community respond when additional treatment capacity cannot be constructed? What can communities do to reduce peak storm flows in their sanitary sewers? Excessive inflow and infiltration (I/I) during wet weather events is a major issue for wastewater operations in North America. "Inflow" and "infiltration" describe the occurrence of surface water and groundwater in a sanitary sewer system. Inflow sources are typically downspouts, area drains and sump pumps. Infiltration is groundwater that seeps into manholes and pipes through defects and cracks.

Treatment of wastewater for Minnesota's largest urban area is managed by the Metropolitan Council Environmental Services (MCES). MCES provides services to 103 communities, including Minneapolis and St. Paul. Service is provided by eight treatment plants and 550 miles of interceptor sewers. Flow monitoring within the seven-county collection system served by MCES is accomplished with 178 meters.

Challenges for MCES
MCES studied service area I/I conditions through the support of a special work group. High I/I flows and system limitations called for an aggressive management approach. A strategy report was published in 2003. The following factors were evaluated:

  • Additional treatment capacity is not feasible. Essentially, treatment facilities are "landlocked" by adjacent land uses and related circumstances.

  • The urban region's long-range economic and growth needs were forecast to 2030.

  • Existing interceptor design peak-to-average flow ratios were reviewed, as applied in the 1970s.

  • Peak flow allocations were assigned as implied by design practice.

  • Anticipated land use guidelines for each community were projected to predict future flows.

  • Long-term peak flow storage in the collection system is not a preferred alternative because it would result in the derating of the wastewater treatment facilities.

MCES determined an allowable I/I of approximately one million gallons per 1,000 acres served. Data collected by trunk sewer meters identified those communities having excess flows. I/I reduction goals were established where recorded flows exceeded peak values. I/I flows that exceed the allowable quantity reduce the growth capacity for the entire region.

MCES Strategies
The strategy now being implemented by the Council is outlined below.

  1. Set I/I goals for all communities served by the metropolitan wastewater treatment system, requiring communities to reduce I/I meeting design flow standards by 2012.

  2. Require each community served to develop and implement an I/I reduction plan.

  3. Institute a surcharge from 2007 to 2011 (proposed at $350,000 per million gallons/day of excess I/I). The surcharge fund is intended to assist affected communities with financing individual solutions.

  4. Require communities meeting their I/I goals to develop maintenance plans, assuring acceptable I/I with aging infrastructure.

  5. In 2013 institute a higher demand charge for communities that fail to meet I/I reduction goals. This fund is intended to be used to construct I/I storage facilities within the system in the event that I/I reduction initiatives have failed.

The Minneapolis Program of Combined Sewer Separation
The City of Minneapolis is the largest single contributor to the MCES treatment facilities. Today, Minneapolis and MCES have mutual obligations to limit overflows into the Mississippi River.

Minneapolis was authorized as a town in 1856. A single sewer transported both raw sewage and stormwater directly into the river. A historical outline of Minneapolis' program of separating sewer systems reflects a growing vibrant community, increasing knowledge about the link between water quality and public health, and a value system that emphasizes aquatic natural diversity for the Mississippi River. Egg-shaped brick conduits were constructed between 1856 and 1933 transporting combined sewer flows to the river. The upper portion of the pipe was configured to provide additional capacity to convey the storm flows.

Growth of the Cities of St. Paul and Minneapolis resulted in high raw sewage loadings in the river. These early systems were the object of a State Health Board order in 1933. The Minneapolis-St. Paul Sanitary District was then created. Continuous interceptors constructed on both sides of the river connected all existing sewers to the treatment plant that opened in 1938. Also constructed were 34 overflow devices (regulators) to direct storm flows to the river.

This system of pipelines and treatment did serve the community satisfactorily for many years. In 1960 an earnest construction program to separate sewers and rebuild residential streets began in Minneapolis. Records report that odors from the river and floating islands of sewage led to an aggressive reconstruction program. Areas previously served by combined sewers were connected to storm flow pipelines with the reconstruction of nearly 600 miles of street.

From 1960 to present, cumulative efforts to remove stormwater and groundwater from the sanitary system have been the target of the Minneapolis construction program. Difficult and costly projects tied with scheduled street improvements or flood mitigation objectives have been included with separation efforts.

Strategies for Minneapolis
Currently, the City collects wastewater from over 100,000 occupied buildings that drain to the MCES wastewater treatment facility in St. Paul. Minneapolis contributes an average flow of 60 million gallons per day. In 2002, the Minneapolis City Council approved a five-year initiative to further address I/I needs, emphasizing capital improvements, engineering and inspection. Studies demonstrate that today's major source of overflow problems are inflow, rather than infiltration. A 2003 City Ordinance added language authorizing building inspections and required identified sources of inflow to disconnect discharges from the sanitary sewer system.

Outlined below are the implementation actions underway for the City:

  1. Disconnect private-sector inflow sources (building roof rain leaders, area property drains, sump pumps, or "clearwater" connections to sanitary sewer system).

  2. Disconnect public-sector inflow sources, remaining storm drain structures connected to sanitary sewer system, and cross-connections between storm drains and sanitary sewer systems.

  3. Study and implement storage or conveyance to reduce peak flow delivery to treatment facilities.

Forest Lake: A Success Story
Peak flows for Forest Lake had been recorded at nearly seven times the dry weather flow. Reduction of I/I was essential to allow continued growth of the community. Foundation drains and residential and commercial sump pumps improperly connected to the sanitary sewer pipe were found to be the sources of high peak flows. Forest Lake has approximately 4,500 connections that discharge to the regional sewer system. Inflow reduction was a condition for approval of Forest Lake's Comprehensive Plan by MCES. A five-step program was adopted to eliminate excess peak flows:

  1. Eliminate sump pump cross connections through inspections and removal.

  2. Investigate known or suspected areas of leaking clean-outs and leaking services.

  3. Inspect and repair defective manholes, rings and covers.

  4. Manage the repair of pipeline structural deficiencies through cleaning and televising.

  5. Enforce stringent requirements for new sanitary sewer and home building service construction.

Forest Lake's first step was a public information campaign, targeting homes and businesses. Multiple mailings and public meetings were provided. Of the properties inspected, nearly 10% were found to be directing clear water to the sanitary sewer. Just six months after launching their I/I initiative, adequate capacity was regained to serve nearly 2,000 additional homes. Surprisingly, not only have wet weather contributions been reduced, but also dry weather flow reductions have been achieved.

Blending
Historically, many publicly-owned treatment facilities have struggled with how to deal with I/I. The concept of fully treating normal flows and partially treating peak flows and "blending" them prior to discharge was often viewed as a technically feasible and economical alternative. An agreed-upon, comprehensive application of a criteria governing blending is not in place within the United States. The result is a differing approach across the country, in some cases leading to high-cost construction programs with limited-dollar return. An approach to "blending" provisions in discharge permitting has been proposed to EPA. This proposal can help to resolve peak wet weather treatment permitting for some communities.

Conclusions and "Lessons Learned"

  1. The Metro Council's wastewater service area has an urgent need to reduce the amount of I/I collected and transported to regional treatment facilities. Virtually no land is available at wastewater treatment sites to build additional facilities to treat peak flows. Continued growth in the region requires the reduction of I/I flows. Inflow is the predominant component of extraneous water.

  2. Reductions in peak wet-weather flows provide the added benefit of optimizing costs of chemicals, electricity and equipment replacement.

  3. Metro has assigned base flows for each contributing community adjusted from a base year of 1970. Trunk sewer discharges that exceed the assigned flow values will pay a significant surcharge for those higher volumes beginning in 2007. The surcharge fund may be used as a funding source for project improvements within systems.

  4. Wastewater systems have traditionally balanced three options in addressing peak flow reduction and treatment: collection system peak storage; conveyance of peak flows with treatment; and service/system source elimination.

  5. Minnesota's history of wastewater conveyance away from area lakes and developing areas represents a proactive approach to safeguarding groundwater quality. Citizens, community councils, their staff, and contractors continue a united goal to increase water quality and public health for the region and, ultimately, the Mississippi watershed.

  6. A functional criteria for blending is needed having consistent application.

  7. An increasing number of communities in North America are providing dedicated pipelines as a component of their storm systems. These dedicated systems receive foundation and sump pump discharges.

Steve Masters can be reached at (402) 441-7588 or SMasters@ci.lincoln.ne.us.

APWA Congress panelists Donald Bluhm, Rhonda Rae, Dan Schluender and Robert Matthews can be reached, respectively, at (651) 602-1116 or donald.bluhm@metc.state.mn.us; (612) 673-3627 or Rhonda.Rae@ci.minneapolis.mn.us; (651) 604-4808 or dschluender@bonestroo.com; and (239) 839-2141 or matthewsrl@cdm.com.